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SCHNEIDER - Guide - Electrical Protection | Electric Power System | Capacitor
SCHNEIDER - Guide - Electrical Protection
Protection units continuously monitor the electrical status of power system components and de-energize them (for instance by tripping a circuit breaker) when they are the site of a serious disturbance such as a short-circuit, insulation fault, etc. The choice of a protection device is not the result of an isolated study, but rather one of the most important steps in the design of the power system. Based on an analysis of the behaviour of electrical equipment (motors, transformers, etc.) during faults and the phenomena produced, this guide is intended to facilitate your choice of the most suitable protective devices. Contents of this guide: This guide is intended for those in charge of designing protection for power systems. It comprises two parts: b part 1, Power-system study, b part 2, Solutions for each application. Power-system study This is a theoretical section presenting the information required to carry out a protection-system study covering the following points: b power-system architecture - what are the main architectures used in medium-voltage power systems? b neutral earthing systems - what are the main neutral earthing systems in medium voltage and what are the selection criteria? b short-circuit currents - what are their characteristics, how are they calculated and how do electrical devices react? b measurement sensors - how should instrument transformers for current and voltage be used? b protection functions - what functions do protection units provide and what are their codes (ANSI codes)? b discrimination of protection devices - what techniques must be used to ensure effective fault clearing? Precise determination of protection settings is not dealt with in this guide. Solutions for each application This section provides practical information on the types of faults encountered in each application: b power systems, b busbars, b lines and cables, b transformers, b motors, b generators, b capacitors, and the protection units required for each type of fault, with setting recommendations and application examples.
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A new path for achieving
your electrical installations
The Protection Guide completes the perfectly coordinated
offer of products which meet all medium and low voltage
electrical distribution requirements. All of these products have
been designed to work together: electrical, mechanical and
communication compatibility.
The electrical installation is thus both optimised and has improved
b better service continuity,
b increased personnel and equipment safety,
b guaranteed upgradeability,
b efficient monitoring and control.
You therefore have all the advantages at hand in terms of
knowhow and creativity for achieving optimised, safe, upgradeable
and compliant installations.
Tools for facilitating the design and installation
With Schneider Electric, you have a complete range of tools to
help you get to know and install the products whilst complying
with current standards and good working practices. These tools,
technical sheets and guides, design software, training courses,
etc. are regularly updated.
Schneider Electric is associating itself
with your know-how and your creativity
to produce optimised, safe, upgradeable
and compliant installations
For a real partnership with you
A universal solution doesnt exist because each electrical
installation is specific. The variety of combinations on offer allows
you to truly customise the technical solutions.
You are able to express your creativity and put your know-how to
best advantage when designing, manufacturing and exploiting an
Power-system architecture
Five neutral earthing systems
Low reactance earthing
Compensation reactance earthing
Introduction to short-circuits
Types of short-circuit
Short-circuit across generator terminals
Equipment behaviour during short-circuits
Phase-current sensors (CT)
Phase-current sensors (LPCT)
Residual-current sensors
Voltage transformers (VT)
Time-based discrimination
Current-based discrimination
Logic discrimination
Directional protection discrimination
Differential protection discrimination
Single-incomer power systems
Dual-incomer power systems
Open loop power systems
Closed loop power systems
Types of faults and protection functions
Link (line and cable) protection
Recommended settings and examples of applications
Glossary - Key words and denitions
Denitions of symbols
Protection units continuously monitor
the electrical status of power system
components and de-energize them
(for instance by tripping a circuit breaker)
when they are the site of a serious
disturbance such as a short-circuit,
insulation fault, etc.
The choice of a protection device is not
the result of an isolated study, but rather one
of the most important steps in the design
of the power system.
Based on an analysis of the behaviour of
electrical equipment (motors, transformers,
etc.) during faults and the phenomena
produced, this guide is intended to facilitate
your choice of the most suitable protective
Among their multiple purposes, protection devices:
b contribute to protecting people against electrical hazards,
b avoid damage to equipment (a three-phase short-circuit on medium-voltage
busbars can melt up to 50 kg of copper in one second and the temperature
at the centre of the arc can exceed 10 000 C),
b limit thermal, dielectric and mechanical stress on equipment,
b maintain stability and service continuity in the power system,
b protect adjacent installations (for example, by reducing induced voltage
in adjacent circuits).
In order to attain these objectives, a protection system must be fast, reliable and
ensure discrimination.
Protection, however, has its limits because faults must rst occur before the protection
system can react.
Protection therefore cannot prevent disturbances; it can only limit their effects and
their duration. Furthermore, the choice of a protection system is often a technical
and economic compromise between the availability and safety of the electrical power
The design of protection for a power system can be broken down into two distinct
b denition of the protection system, also called the protection-system study,
b determination of the settings for each protection unit, also called protection
coordination or discrimination.
Denition of the protection system
Fig. 1. Protection system.
This step includes selection of the protection components and a consistent, overall
structure suited to the power system.
The protection system is made up of a string of devices including the following (g. 1):
b measurement sensors (current and voltage) supplying the data required to detect
b protection relays in charge of continuously monitoring the electrical status
of the power system up to and including the formulation and emission of orders
to the trip circuit to clear the faulty parts,
b switchgear in charge of clearing faults, such as circuit breakers or combinations
of switches or contactors and fuses.
The protection-system study determines the devices to be used to protect against
the main faults affecting the power system and the machines:
b phase-to-phase and phase-to-earth short-circuits,
b overloads,
b faults specic to rotating-machines.
b power system architecture and size, as well as the various operating modes,
b the neutral-earthing systems,
b the characteristics of current sources and their contributions in the event of a fault,
b the types of loads,
b the need for continuity of service.
Each protection function must be set to ensure the best possible power system
operation in all operating modes.
The best settings are the result of complete calculations based on the detailed
characteristics of the various elements in the installation.
These calculations are now commonly carried out by specialized software tools
that indicate the behaviour of the power system during faults and provide the
settings for each protection function.
This guide is intended for those in charge of designing protection for power systems.
b part 1, Power-system study,
b part 2, Solutions for each application.
This is a theoretical section presenting the information required to carry out a protectionsystem study covering the following points:
b power-system architecture - what are the main architectures used in medium-voltage
power systems?
b neutral earthing systems - what are the main neutral earthing systems in medium
voltage and what are the selection criteria?
b short-circuit currents - what are their characteristics, how are they calculated
and how do electrical devices react?
b measurement sensors - how should instrument transformers for current
and voltage be used?
b protection functions - what functions do protection units provide
and what are their codes (ANSI codes)?
b discrimination of protection devices - what techniques must be used to ensure
effective fault clearing?
Precise determination of protection settings is not dealt with in this guide.
48 - 51LR
This section provides practical information on the types of faults encountered
in each application:
b power systems,
b busbars,
b lines and cables,
b transformers,
b motors,
b generators,
b capacitors,
and the protection units required for each type of fault, with setting
recommendations and application examples.
Protection of a power system depends on
its architecture and the operating mode.
This chapter compares typical structures
The various components of a power system can be arranged in different ways.
The complexity of the resulting architecture determines the availability of electrical
energy and the cost of the investment.
Selection of an architecture for a given application is therefore based on a trade-off
between technical necessities and cost.
Architectures include the following:
b radial systems
v single-feeder,
v double-feeder,
v parallel-feeder,
v dual supply with double busbars.
b loop systems
v open loop,
v closed loop.
b systems with internal power generation
v normal source generation,
v replacement source generation.
The table below lists the main characteristics of each architecture for comparison.
Illustrations are provided on the next page.
Single-feeder radial
Processes not requiring
E.g. a cement works
Most simple architecture
Double-feeder radial
Continuous processes: steel,
Parallel-feeder
Future expansion is limited
Processes requiring high
Processes with major load
Downtime due to faults may be long
A single fault interrupts supply to the entire
Partial operation of busbars during
Requires automatic control functions
Good continuity of supply
Maintenance possible on busbars
of main switchboard
Flexible operation: no-break transfers Requires automatic control functions
Double busbars
Very large power systems
Major future expansion
Loads concentrated in
different zones of a site
Power system offering high
Internal power generation
Industrial process sites
E.g. paper plants, steel
(source changeover)
Less expensive than closed loop
Faulty segment can be isolated during loop
reconguration
Does not require automatic control
Complex protection system
Cost of energy (energy recovered
from process)
Good continuity of supply for priority Requires automatic control functions
DE55362EN
DE55361
otherwise, all switchgear
is NC.
DE55364EN
DE55363EN
DE55366EN
Local normal source generation
Replacement source generation (source changeover)
DE55367EN
DE55368EN
DE55365EN
The choice of neutral earthing for MV and
HV power systems has long been a topic of
heated controversy due to the fact that
it is impossible to nd a single compromise
for the various types of power systems.
Acquired experience now allows an
appropriate choice to be made according to
the specic constraints of each system.
This chapter compares the different types
of neutral earthing, distinguished by the
neutral point connection and the operating
The neutral potential can be earthed by ve different methods, according to type
(capacitive, resistive, inductive) and the value (zero to innity) of the impedance ZN
of the connection between the neutral and earth:
b ZN = : isolated neutral, i.e. no intentional earthing connection,
b ZN is related to a resistance with a fairly high value,
b ZN is related to a reactance, with a generally low value,
b ZN is related to a compensation reactance, designed to compensate
for the system capacitance,
b ZN = 0: the neutral is solidly earthed.
Difculties and selection criteria
The selection criteria involve many aspects:
b technical considerations (power system function, overvoltages, fault current, etc.),
b operational considerations (continuity of service, maintenance),
b safety,
b cost (capital expenditure and operating expenses),
b local and national practices.
DE57201
Two of the major technical considerations happen to be contradictory:
Fig. 1. Equivalent diagram of a power system
with an earth fault.
Reducing the level of overvoltages
Excessive overvoltages cause the dielectric breakdown of electrical insulating
materials, resulting in short-circuits.
Overvoltages are of several origins:
b lightning overvoltage, to which all overhead systems are exposed, up to the user
b overvoltage within the system caused by switching and critical situations such as
b overvoltage resulting from an earth fault itself and its elimination.
Reducing earth fault current (Ik1) (g. 1)
Fault current that is too high produces a whole series of consequences related to
b damage caused by the arc at the fault point; particularly the melting of magnetic
circuits in rotating machines,
b thermal withstand of cable shielding,
b size and cost of earthing resistor,
b induction in adjacent telecommunication circuits,
b danger for people created by the rise in potential of exposed conductive parts.
Unfortunately, optimizing one of these requirements is automatically to the
disadvantage of the other. Two typical neutral earthing methods accentuate
this contrast:
b isolated neutral, which eliminates the ow of earth fault current through the neutral
but creates higher overvoltages,
b solidly earthed neutral, which reduces overvoltage to a minimum, but causes high
As for the operating considerations, according to the neutral earthing method used:
b continued operation may or may not be possible after a persisting rst fault,
b the touch voltages are different,
b protection discrimination may be easy or difcult to implement.
An in-between solution is therefore often chosen, i.e. neutral earthing
via an impedance.
Summary of neutral earthing characteristics
Damping of transient overvoltages
Limitation of 50 Hz overvoltages
Limitation of fault currents
(no tripping required on rst fault)
Easy implementation of protection discrimination
No need for qualied personnel
DE57202
There is no intentional earthing of the neutral point, except for measurement
or protection devices.
In this type of power system, a phase-to-earth fault only produces a low current
through the phase-to-earth capacitances of the fault-free phases (g. 1).
It can be shown that Ik1 = 3 C V where:
b V is the phase-to-neutral voltage,
b C is the phase-to-earth capacitance of a phase,
b is the angular frequency of the power system dened as = 2 f
DE55203EN
Fig. 1. Capacitive fault current in isolated neutral system.
The fault current Ik1 can remain for a long time, in principle, without causing
any damage since it is not more than a few amperes (approximately 2 A per km
for a 6 kV single-core cable with a cross-section of 150 mm2 , XLPE insulation and a
capacitance of 0.63 mF/km). Action does not need to be taken to clear this rst fault,
making this solution advantageous in terms of maintaining service continuity.
However, this entails the following consequences:
b the insulation must be continuously monitored and faults that are not yet cleared
must be indicated by an insulation monitoring device or by a neutral voltage
displacement protection unit (ANSI 59N) (g. 2),
b subsequent fault tracking requires complex automatic equipment for quick
identication of the faulty feeder and also maintenance personnel qualied
to operate the equipment,
b if the rst fault is not cleared, a second fault occurring on another phase will cause
a real two-phase-to-earth short circuit, which will be cleared by the phase protection
The basic advantage is service continuity since the very low fault current does not
cause automatic tripping for the rst fault; it is the second fault that requires tripping.
Fig. 2. Insulation monitoring device (IMD).
DE57204
b The failure to eliminate transient overvoltages through the earth can be a major
handicap if the overvoltage is high.
b Also, when one phase is earthed, the others reach a phase-to-phase voltage
at power frequency (U = 3 V) in relation to the earth, and this increases the
probability of a second fault. Insulation costs are higher since the phase-to-phase
voltage may remain between the phase and earth for a long time with no automatic
b Insulation monitoring is compulsory, with indication of the rst fault.
b A maintenance department with the equipment to quickly track the rst insulation
fault is required.
b It is difcult to implement protection discrimination for the rst fault.
b There are risks of overvoltages created by ferroresonance.
The faulty feeder may be detected by a directional earth fault protection unit
(ANSI 67N) (g. 3).
The diagram shows that discrimination is implemented by a comparison of the
phase displacement angle between the residual voltage and residual currents, for
the faulty feeder and for each fault-free feeder.
The current is measured by a core balance CT and the tripping threshold is set:
b to avoid nuisance tripping,
b lower than the sum of the capacitive currents of all the other feeders.
This makes it difcult for faults to be detected in power systems that are limited
in size, consisting of only a few hundreds of meters of cable.
This solution is often used for industrial power systems ( 15 kV) that require service
It is also used for the public distribution systems in Spain, Italy and Japan.
Fig. 3. Detection for directional earth fault protection.
DE57205
A resistor is intentionally connected between the neutral point and earth.
In this type of power system, the resistive impedance limits the earth fault current Ik1
and still allows satisfactory evacuation of overvoltages.
However, protection units must be used to automatically clear the rst fault. In power
systems that supply rotating machines, the resistance is calculated so as to obtain
a fault current Ik1 of 15 to 50 A. This low current must however be IRN 2 Ic (where
Ic is the total capacitive current in the power system) to reduce switching surges and
allow simple detection.
In distribution power systems, higher values are used (100 to 300 A) since they are
easier to detect and allow the evacuation of lightning overvoltages.
IRN Ik1
Fig. 1. Earthing with accessible neutral:
resistor between neutral and earth.
DE55200
b This system is a good compromise between low fault current and satisfactory
overvoltage evacuation.
b It does not require equipment with phase-to-earth insulation sized for the phaseto-phase voltage.
b The protection units are simple and selective and the current is limited.
b The service continuity of the faulty feeder is downgraded and earth faults must be
cleared as soon as they occur (rst fault tripping).
b The higher the voltage and the current limited, the higher the cost of the earthing
DE55206
Fig. 2. Earthing with accessible neutral:
resistor on single-phase transformer secondary circuit.
b If the neutral point is accessible (star-connected windings with an accessible
neutral), the earthing resistor may be connected between the neutral and earth (g. 1)
or via a single-phase transformer with an equivalent resistive load on the secondary
winding (g. 2).
b When the neutral is not accessible (delta-connected winding) or when the protection
system study shows that it is appropriate, an articial neutral point is created using a
zero sequence generator connected to the busbars; it consists of a special
transformer with a very low zero sequence reactance.
v star-delta transformer with solidly earthed primary neutral, and a delta connection
including a limiting resistor (LV insulation, therefore the most inexpensive solution)
(g. 3),
v star-delta transformer with limiting resistor (HV insulation) between the primary
neutral point and earth, and a closed delta connection (no resistor); this solution
is less often used (g. 4).
Earthing with inaccessible neutral:
Fig. 3. Limiting resistor
Fig. 4. Limiting resistor on
on secondary circuit.
To detect a fault current Ik1 that is low, protection functions other than phase
overcurrent are required (g. 5).
These earth fault protection functions detect fault current:
b directly in the neutral earthing connection 1,
b or in the power system by the vector sum of the 3 currents measured by:
v 3 current sensors supplying the protection units 2,
v or a core balance CT 3: preferred method since more accurate.
DE57208
The threshold is set according to the fault current Ik1 calculated without taking
into account the source and connection zero sequence impedance in relation to
the impedance RN, in compliance with two rules:
b setting > 1.3 times the capacitive current of the power system downstream
from the protection unit,
b setting in the range of 10 to 20% of the maximum earth fault current.
Fig. 5. Earth fault protection solutions.
In addition, if 3 CTs are used for detection, in view of current technologies, the
setting should be within 5 to 30% of the CT rating to account for the uncertainty
b transient current asymmetry,
b CT saturation,
b scattering of performance.
Public and industrial MV distribution systems.
DE57209
A reactor is intentionally connected between the neutral point and earth.
For power system voltages greater than 40 kV, it is preferable to use a reactor rather
than a resistor because of the difculties arising from heat emission in the event
of a fault (g. 1).
In this type of power system, an inductive impedance limits earth fault current Ik1 and
still allows satisfactory evacuation of overvoltages. However, protection units must
be used to automatically clear the rst fault.
To reduce switching surges and allow simple detection, the current IL must be much
higher than the total capacitive current of the power system Ic.
In distribution systems, higher values are used (300 to 1000 A) since they are easier
to detect and allow the evacuation of lightning overvoltages.
ILN Ik1
Fig. 1. Earthing with accessible neutral.
b This system limits the amplitude of fault currents.
b Protection discrimination is easy to implement if the limiting current is much greater
than the capacitive current in the power system.
b The coil has a low resistance and does not dissipate a large amount of thermal
energy; the coil can therefore be reduced in size.
b In high voltage systems, this solution is more cost-effective than resistance earthing.
b The continuity of service of the faulty feeder is downgraded; earth faults must be
b When earth faults are cleared, high overvoltages may occur due to resonance
between the power system capacitance and the reactance.
DE55210
neutral), the earthing reactance may be connected between the neutral and earth.
system study shows that it is appropriate, an articial neutral point is created by
a neutral point coil connected to the busbars; it consists of a zigzag coil with
an accessible neutral (g. 2).
The impedance between the two parts of the winding, essentially inductive and low,
limits the current to values that remain greater than 100 A.
A limiting resistor may be added between the coil neutral point and earth to reduce
the amplitude of the fault current (HV insulation).
Fig. 2. Earthing with inaccessible neutral.
b The protection setting is in the range of 10 to 20% of the maximum fault current.
b The protection function is less restrictive than in the case of resistance earthing,
especially considering the high value of ILN given that Ic is less than the limited
Public and industrial MV distribution systems (currents of several hundred amperes).
Compensation reactance
DE57211
A reactor tuned to the total phase-to-earth capacitance of the power system
is inserted between the neutral point and earth so that the fault current is close
to zero if an earth fault occurs (g. 1).
ILN + IR
Fig. 1. Earth fault in power system with compensation
reactance earthing.
DE55212EN
current in the reactor
Fig. 2. Vector diagram of currents during an earth fault.
This system is used to compensate for capacitive current in the power system.
The fault current is the sum of the currents owing through the following circuits:
b reactance earthing circuit,
b fault-free phase capacitances with respect to earth.
The currents compensate for each other since:
b one is inductive (in the earthing circuit),
b the other one is capacitive (in the fault-free phase capacitances).
They therefore add up in opposite phase.
In practice, due to the slight resistance of the coil, there is a low resistive current
of a few amperes (g. 2).
b The system reduces fault current, even if the phase-to-earth capacitance is high:
spontaneous extinction of non-permanent earth faults.
b The touch voltage is limited at the location of the fault.
b The installation remains in service even in the event of a permanent fault.
b The rst fault is indicated by detection of current owing through the coil.
b The cost of reactance earthing may be high since the reactance needs
to be modied to adapt compensation.
b It is necessary to make sure that the residual current in the power system during
the fault is not dangerous for people or equipment.
b There is a high risk of transient overvoltages on the power system.
b Personnel must be present to supervise.
Fault detection is based on the active component of the residual current.
The fault creates residual currents throughout the power system, but the faulty
is the only one through which resistive residual current ows.
In addition, the protection units must take into account repetitive self-extinguishing
faults (recurrent faults).
When the earthing reactance and power system capacitance are tuned
(3 LN C 2 = 1)
b fault current is minimal,
b it is a resistive current,
b the fault is self-extinguishing.
The compensation reactance is called an extinction coil, or Petersen coil.
Public and industrial MV distribution systems with high capacitive current.
DE57213
An electrical connection with zero impedance is intentionally set up between
the neutral point and earth.
Since the neutral is earthed without any limiting impedance, the phase-to-earth fault
current Ik1 is practically a phase-to-neutral short-circuit, and is therefore high (g. 1).
Tripping takes place when the rst insulation fault occurs.
Fig. 1. Earth fault in a solidly earthed neutral power system.
b This system is ideal for evacuating overvoltages.
b Equipment with insulation sized for phase-to-neutral voltage may be used.
b Specic protection units are not required: the normal phase overcurrent protection
units can be used to clear solid earth faults.
b This system involves all the drawbacks and hazards of high earth fault current:
maximum damage and disturbances.
b There is no service continuity on the faulty feeder.
b The danger for personnel is high during the fault since the touch voltages created
Impedant faults are detected by a delayed earth fault protection unit (ANSI 51N),
set in the range of the rated current.
b This type of system is not used in European overhead or underground MV power
systems, but is prevalent in North American distribution systems. In the North
American power systems (overhead systems), other features come into play
to justify the choice:
v distributed neutral conductor,
v 3-phase or 2-phase + neutral or phase + neutral distribution,
v use of the neutral conductor as a protective conductor with systematic earthing
at each transmission pole.
b This type of system may be used when the short-circuit power of the source is low.
A short-circuit is one of the major incidents
affecting power systems.
This chapter describes short-circuits and
their effects on power systems and their
interaction with equipment.
It also provides a method and the main
equations to calculate currents and voltages
when short-circuits occur.
b A short-circuit is an accidental connection between conductors by a zero
(solid short-circuit) or non-zero impedance (impedant short-circuit).
b A short-circuit is referred to as internal if it is located within equipment or external
if its occurs on links.
b The duration of a short-circuit is variable. A short-circuit is said to be
self-extinguishing if its duration is too short for tripping of the protection devices,
transient if cleared following tripping and reclosing of the protection devices
and continuous or sustained if it does not disappear following tripping.
b The causes of a short-circuit can be mechanical (a shovel, a branch, an animal),
electrical (damaged insulation, overvoltages) or human (operating error) (g. 1).
Effects of short-circuit currents
DE57355EN
The consequences are often serious, if not dramatic.
b A short-circuit disturbs the power system environment around the fault point by
causing a sudden drop in voltage.
b It requires disconnection, through the operation of the protection devices,
of a part (often large) of the installation.
b All equipment and connections (cables, lines) subjected to a short-circuit
are subjected to high mechanical stress (electrodynamic forces) that can cause
breaks and thermal stress that can melt conductors and destroy insulation.
b At the fault point, there is often a high-energy electrical arc, causing very heavy
damage that can quickly spread.
Although short-circuits are less and less likely to occur in modern, well-designed,
well-operated installations, the serious consequences they can cause are an incentive
to implement all possible means to swiftly detect and eliminate them.
The short-circuit current at different points in the power system must be calculated to
design the cables, busbars and all switching and protection devices and determine
DE55356EN
Ia = I sin( t + )
Ic = I sin e
Characterization of short-circuits
A number of types of short-circuits can occur in a power system.
b Three-phase short-circuit: a fault between the three phases.
This type generally provokes the highest currents (g. 2).
b Phase-to-earth short-circuit: a fault between a phase and earth.
This type is the most frequent (g. 3).
b Two-phase short-circuit clear of earth: a fault between two phases (phase-tophase voltage). The resulting current is lower than for a three-phase short-circuit,
except when the fault is in the immediate vicinity of a generator (g. 4).
b Two-phase-to-earth short-circuit: a fault between two phases and earth (g. 5).
Isc = Ia + Ic
Moment fault occurs
DE57216
DE57215
Fig. 1. Graphical representation of a short-circuit current based
on an equivalent diagram.
DE55229EN
2 2 Ik
Fig. 2. Three-phase short-circuit (5% of cases).
Fig. 4. Two-phase short-circuit clear of earth.
Fig. 3. Phase-to-earth short-circuit (80% of cases). Fig. 5. Two-phase-to-earth short-circuit.
Fig. 6. Typical short-circuit current curve.
Short-circuit current at a given point in the power system is expressed as the rms
value Ik (in kA) of its AC component (g. 6).
The maximum instantaneous value that short-circuit current can reach is the peak
value Ip of the rst half cycle. This peak value can be much higher than 2 Ik
because of the damped DC component IDC that can be superimposed on the AC
This DC component depends on the instantaneous value of the voltage at the start
of the short-circuit and on the power system characteristics. The power system
is dened by the short-circuit power, according to the equation:
Ssc = 3 Un Ik (in MVA).
This theoretical value has no physical reality; it is a practical conventional value
comparable to an apparent power rating.
During normal, balanced symmetrical operation, analysis of three-phase systems is
similar to that of an equivalent single-phase system, characterized by the phase-toneutral voltages, phase currents and power system impedances (called cyclical
impedances). As soon as a signicant dissymmetry appears in the conguration or
in power system operation, simplication is no longer possible. It is not possible to
establish simple electrical relations in the conductors, using the cyclical
In this case, the symmetrical-components method is used, which consists of
expressing the real system as a superposition of three independent, single-phase
power systems, called:
b positive sequence (designated by a subscript 1, e.g. V1),
b negative sequence (designated by a subscript 2, e.g. V2),
b zero-sequence (designated by a subscript 0, e.g. V0).
For each system (positive-, negative- and zero-sequence respectively), voltages
V1, V2, V0 and currents I1, I2, I0 are related by the impedances Z1, Z2, Z0 of the same
The symmetrical impedances are a function of the real impedances, notably
the mutual inductances.
The notion of symmetrical components is also applicable to power.
V1 = V 1 + V 2 + V 0
--- (V1 + a V2 + a2 V3 )
V2 = a2 V 1 + a V2 + V0
1- (V1 + a2 V2 + a V3 )
V 2 = -3
V3 = a V1 + a2 V2 + V0
--- (V1 + V2 + V3 )
DE55214EN
j -----3
Decomposition of a three-phase system into symmetrical components
Decomposition into symmetrical components is not simply a mathematical technique,
it corresponds to the physical reality of the phenomena. It is possible to directly
measure the symmetrical components (voltages, currents, impedances)
of an unbalanced system.
The positive-, negative- and zero-sequence impedances of an element in the power
system are the impedances of the element subjected to voltage systems that are,
respectively, positive three-phase, negative three-phase and phase-to-earth
on three parallel phases.
Generators produce the positive-sequence component and faults may produce
the negative and zero-sequence components.
In the case of motors, the positive-sequence component creates the useful rotating
eld, whereas the negative-sequence component creates a braking rotating eld.
For transformers, an earth fault creates a zero-sequence component that produces
a zero-sequence eld passing through the tank.
DE57217EN
Three-phase short-circuit between the phase
conductors (g. 1)
The value of the three-phase short-circuit current at a point F within the power system is:
Ik3 = ---------------------3 Zsc
where U refers to the phase-to-phase voltage at point F before the fault occurs and
Zsc is the equivalent upstream power system impedance as seen from the fault point.
In theory, this is a simple calculation; in practice, it is complicated due to the difculty
of calculating Zsc, an impedance equivalent to all the unitary impedances of series
and parallel-connected units located upstream from the fault. These impedances
are themselves the quadratic sum of reactances and resistances.
Zs c =
Calculations can be made much simpler by knowing the short-circuit power Ssc
at the connection point for utility power. It is possible to deduce the equivalent
impedance Za upstream of this point.
Fig. 1. Three-phase short-circuit.
U---Za = --Ssc
--I sc = ---------U
I 1 = --E
---Z1
DE55219EN
Similarly, there may not be a single source of voltage, but rather several sources
in parallel, in particular, synchronous and asynchronous motors which act
as generators when short-circuits occur.
The three-phase short-circuit current is generally the strongest current that can ow
in the power system.
I2 = I0 = 0
V 1 = V2 = V 0 = 0
Model of a three-phase short-circuit using the symmetrical components.
Single-phase short-circuit between a phase
conductor and earth (g. 2)
DE57218EN
The value of this current depends on the impedance ZN between the neutral
and earth. This impedance can be virtually nil if the neutral is solidly earthed
(in series with the earthing resistance) or, on the contrary, almost innite if the
neutral is isolated (in parallel with the power system phase-to-earth capacitance).
The value of the phase-to-earth fault current is:
Fig. 2. Phase-to-earth short-circuit.
3--------U
---------------------Ik1 = -----------------------( Z 1 + Z 2 + Z 0 + 3Z N)
This calculation is required for power systems in which the neutral is earthed by
an impedance ZN. It is used to determine the setting of the earth fault protection
devices which must break the earth-fault current.
If Z1, Z2 and Z0 are negligible with respect to ZN, then:
Ik1 = --------------------3 ZN
I 1 = I 2 = I 0 = ---------------------E
----------------------Z 1 + Z 2 + Z 0 + 3Z
E( Z + Z + 3Z )
V 1 = -------------2------------0------------------Z 1 + Z 2 + Z 0 + 3Z
V 2 = ---------------------2----------------------Z 1 + Z 2 + Z 0 + 3Z
V 0 = ---------------------0----------------------Z 1 + Z 2 + Z 0 + 3Z
DE55220EN
This is the case, for example, when Ik1 is limited to 20 A in an MV power system
supplied by a high-power transformer (10 MVA).
Model of a phase-to-earth short-circuit using the symmetrical components.
Two-phase short-circuit between phase conductors (g.1)
The value of the two-phase short-circuit current at a point within the power system is:
---------I k2 = --------U
In a power system supplied by a transformer (fault far from the sources), the value of
the two-phase short-circuit current at a point within the power system is:
----------Ik2 = -------U
The two-phase short-circuit current is weaker than three-phase short-circuit current,
by a ratio of 3/2, i.e. approximately 87%.
If the fault occurs close to a generator (Z2 Z1), the current can be higher
than in a three-phase fault.
I 1 = ---------------------------Z1 + Z2 + Z
Fig. 1. Two-phase short-circuit clear of earth.
I 2 = ---------------------------Z1 + Z2 + Z
DE55224EN
DE57221EN
E( Z + Z )
V 1 = --------------2-------------Z1 + Z2 + Z
V 2 = --------------------2-------Z1 + Z2 + Z
Model of a two-phase short-circuit using the symmetrical components.
Two-phase short-circuit between two phase
conductors and earth (g. 2)
For a solid short-circuit (fault far from the sources), the value of the two-phaseto-earth short-circuit is:
IkE2E = --------------------------(Z + 2Z )
E ( Z + Z + 3Z )
I 1 = -------------------------------2------------0----------------------------------Z 1 Z 2 + (3Z + Z0 ) ( Z 1 + Z 2 )
E (Z + 3 Z )
= ---------------------------------------0--------------------------------------Z 1 Z 2 + (3Z + Z0 ) ( Z 1 + Z 2 )
I 0 = -----------------------------------------------2------------------------------Z 1 Z 2 + ( 3Z + Z0 ) ( Z 1 + Z 2)
Fig. 2. Two-phase-to-earth short-circuit.
DE55225EN
DE57222EN
Model of a two-phase-to-earth short-circuit using the symmetrical components.
Short-circuit across generator
DE55223EN
DE55228EN
Fig. 1. Typical curves for short-circuit currents across generator
Subtransient component
Steady-state component
Total-current curve
Fig. 2. Decomposition of the short-circuit current.
It is more complicated to calculate short-circuit current across the terminals
of a synchronous generator than across the terminals of a transformer connected
to the power system.
This is because the internal impedance of the machine cannot be considered constant
after the start of the fault. It increases progressively and the current becomes weaker,
passing through three characteristic stages:
b subtransient (approximately 0.01 to 0.1 second), the short-circuit current
(rms value of the AC component) is high, 5 to 10 times the rated continuous current.
b transient (between 0.1 and 1 second), the short-circuit current drops to between
2 and 6 times the rated current.
b steady-state, the short-circuit current drops to between 0.5 and 2 times the rated
The given values depend on the power rating of the machine, its excitation mode and,
for the steady-state current, on the value of the exciting current, therefore on the
load on the machine at the time of the fault.
What is more, the zero-sequence impedance of the AC generators is generally
2 to 3 times lower than their positive-sequence impedance. The phase-to-earth
short-circuit current is therefore greater than the three-phase current.
By way of comparison, the steady-state three-phase short-circuit current across
the terminals of a transformer ranges between 6 and 20 times the rated current,
depending on the power rating.
It can be concluded that short-circuits across generator terminals are difcult
to assess, in particular their low, decreasing value makes protection setting difcult.
IEC method (standard 60909)
DE55226EN
2 2 I"k
2 2 Ib
Fig. 1. Graphic representation of short-circuit quantities
as per IEC 60909.
2-phase-to-earth
----------Un
------3 Z1
c--------Un
--------Z1 + Z2
c Un 3 Z2
-----------------------------------------------------------------Z1 Z2 + Z2 Z0 + Z1 Z0
----------3-Z1 + Z2 + Z0
Short-circuit currents as per IEC 60909 (general situation).
---------3 Z1
---------2 Z1
c---------Un
Z 1 + 2Z0
2Z1 + Z 0
Short-circuit currents as per IEC 60909 (distant faults).
The rules for calculating short-circuit currents in electrical installations are presented
in IEC standard 60909, issued in 2001.
The calculation of short-circuit currents at various points in a power system can
quickly turn into an arduous task when the installation is complicated.
The use of specialized software accelerates calculations.
This general standard, applicable for all radial and meshed power systems, 50 or 60 Hz
and up to 550 kV, is extremely accurate and conservative.
It may be used to handle the different types of solid short-circuit (symmetrical or
dissymmetrical) that can occur in an electrical installation:
b three-phase short-circuit (all three phases), generally the type producing
the highest currents,
b two-phase short-circuit (between two phases), currents lower than three-phase faults,
b two-phase-to-earth short-circuit (between two phases and earth),
b phase-to-earth short-circuit (between a phase and earth), the most frequent type
(80% of all cases).
When a fault occurs, the transient short-circuit current is a function of time
and comprises two components (g. 1):
b an AC component, decreasing to its steady-state value, caused by the various
rotating machines and a function of the combination of their time constants,
b a DC component, decreasing to zero, caused by the initiation of the current
and a function of the circuit impedances.
Practically speaking, one must dene the short-circuit values that are useful
in selecting system equipment and the protection system:
b Ik: rms value of the initial symmetrical current,
b Ib: rms value of the symmetrical current interrupted by the switching device when
the rst pole opens at t min (minimum delay),
b Ik: rms value of the steady-state symmetrical current,
b Ip: maximum instantaneous value of the current at the rst peak,
b IDC: DC value of the current.
These currents are identied by subscripts 3, 2, 2E, 1, depending on the type of shortcircuit, respectively three-phase, two-phase clear of earth, two-phase-to-earth,
phase-to-earth.
The method, based on the Thevenin superposition theorem and decomposition into
symmetrical components, consists in applying to the short-circuit point an equivalent
source of voltage in view of determining the current. The calculation takes place in
b Dene the equivalent source of voltage applied to the fault point. It represents the
voltage existing just before the fault and is the rated voltage multiplied by a factor
taking into account source variations, transformer on-load tap changers and the
subtransient behavior of the machines.
b Calculate the impedances, as seen from the fault point, of each branch arriving
at this point. For positive and negative-sequence systems, the calculation does not
take into account line capacitances and the admittances of parallel, non-rotating
b Once the voltage and impedance values are dened, calculate the characteristic
minimum and maximum values of the short-circuit currents.
The various current values at the fault point are calculated using:
b the equations provided,
b a summing law for the currents owing in the branches connected to the node:
v Ik, see the equations for Ik in the tables opposite, where voltage factor c is dened
by the standard; geometric or algebraic summing,
v ip = 2 Ik, where k is less than 2, depending on the R/X ratio of the positivesequence impedance for the given branch; peak summing,
v Ib = q Ik, where m and q are less than 1, depending on the generators
and motors, and the minimum current interruption delay; algebraic summing,
v Ik = Ik, when the fault is far from the generator,
v Ik = Ir, for a generator, where Ir is the rated generator current and is a factor
depending on its saturation inductance; algebraic summing.
Equipment behaviour
during short-circuits
DE55227EN
There are 2 types of system equipment, based on whether or not they react when
a fault occurs.
This category comprises all equipment which, due to its function, must have
the capacity to transport both normal current and short-circuit current.
This equipment includes cables, lines, busbars, disconnecting switches, switches,
transformers, series reactances and capacitors, instrument transformers.
For this equipment, the capacity to withstand a short-circuit without damage
is dened in terms of:
b electrodynamic withstand (expressed in kA peak), characterizing mechanical
resistance to electrodynamic stress,
b thermal withstand (expressed in rms kA for 1 to 5 seconds), characterizing
maximum permissible heat rise.
IAC: peak of the periodic component.
IDC: aperiodic component.
Fig. 1. Rated breaking current of a circuit breaker subjected to
a short-circuit as per IEC 60056.
This category comprises the equipment designed to clear short-circuit currents, i.e.
circuit breakers and fuses. This property is expressed by the breaking capacity and,
if required, the making capacity when a fault occurs.
Breaking capacity (g. 1)
This basic characteristic of a current interrupting device is the maximum current
(in rms kA) it is capable of breaking under the specic conditions dened by the
standards; it generally refers to the rms value of the AC component of the short-circuit
current. Sometimes, for certain switchgear, the rms value of the sum
of the 2 components (AC and DC) is specied, in which case, it is the asymmetrical
The breaking capacity depends on other factors such as:
v voltage,
v R/X ratio of the interrupted circuit,
v power system natural frequency,
v number of breaks at maximum current, for example the cycle: O - C/O - C/O
(O = opening, C = closing),
v device status after the test.
The breaking capacity is a relatively complicated characteristic to dene and it
therefore comes as no surprise that the same device can be assigned different
breaking capacities depending on the standard by which it is dened.
In general, this characteristic is implicitly dened by the breaking capacity because
a device should be able to close for a current that it can break.
Sometimes, the making capacity needs to be higher, for example for circuit breakers
protecting generators.
The making capacity is dened in terms of the kA peak because the rst asymmetric
peak is the most demanding from an electrodynamic point of view.
For example, according to standard IEC 60056, a circuit breaker used in a 50 Hz
power system must be able to handle a peak making current equal to 2.5 times the
rms breaking current.
Prospective short-circuit breaking current
Some devices have the capacity to limit the fault current to be interrupted.
Their breaking capacity is dened as the maximum prospective breaking current that
would develop during a solid short-circuit across the upstream terminals of the device.
Specic device characteristics
The functions provided by various interrupting devices and their main constraints
yes, if withdrawable
Longitudinal input/output isolation
Earthing switch: short-circuit making capacity
Making and breaking of normal load current
With a fuse: short-circuit breaking capacity in fuse no-blow zone
Rated making and breaking capacities
Maximum making and breaking capacities
Duty and endurance characteristics
Minimum short-circuit breaking capacity
Maximum short-circuit breaking capacity
Protection and measuring devices require
data on the electrical rating of the equipment
For technical, economic and safety reasons,
this data cannot be obtained directly
from the high-voltage power supply
of the equipment. The following intermediary
devices are needed:
b phase-current sensors,
b core balance CTs to measure earth fault
b voltage transformers (VT).
These devices fulll the following functions:
b reduction of the value to be measured
(e.g. 1500/5 A),
b galvanic isolation,
b provision of the power required for data
processing and for the protection function
The role of a phase-current sensor is to provide its secondary winding with a current
proportional to the measured primary current. They are used for both measurements
b current transformers (CT),
b current transformers with a voltage output (LPCT).
General characteristics (g. 1)
The current transformer is made up of two circuits, the primary and the secondary,
coupled by a magnetic circuit.
When there are a number of turns in the primary circuit, the transformer is of the woundprimary type.
When the primary is a single conductor running through a sensor, the transformer
may be of the bar-primary type (integrated primary made up of a copper bar), support
type (primary formed by an uninsulated conductor of the installation) or the toroidal
type (primary formed by an insulated cable of the installation).
The CTs are characterized by the following values (according to standard IEC 60044)(1).
CT rated insulation level
This is the highest voltage applied to the CT primary.
Note that the primary is at the HV voltage level and that one of the secondary
terminals is generally earthed.
Similar to other equipment, the following values are dened:
b maximum1 min. withstand voltage at power frequency,
b maximum impulse withstand voltage.
Example. For a 24 kV rated voltage, the CT must withstand 50 kV for 1 minute at 50 Hz
and an impulse voltage of 125 kV.
DE57330
Rated transformation ratio
It is usually given as the transformation ratio between primary and secondary current
Ip/Is.
The rated secondary current is generally 5 A or 1 A.
Ip: primary current
Is: secondary current (proportional to Ip and in phase)
Fig. 1. Current transformer.
It is dened by the composite error for the accuracy-limit current.
The accuracy-limit factor is the ratio between the accuracy-limit current and the
b For class P:
5P10 means 5% error for 10 In and 10P15 means 10% error for 15 In,
5P and 10P are the standard accuracy classes for protection CTs,
5 In, 10 In, 15 In, 20 In are the standard accuracy-limit currents.
b The PR class is dened by the remanence factor, the ratio between the remanent
ux and the saturation ux, which must be less than 10%.
5PR and 10PR are the standard accuracy classes for protection CTs.
b Class PX is another way of specifying CT characteristics based on the knee-point
voltage, the secondary resistance and the magnetizing current (see next page, g. 1,
CT response in saturated state).
This is the apparent power in VA that the CT is intended to supply to the secondary
circuit at the rated secondary current without causing the errors to exceed the values
specied.
It represents the power consumed by all the connected devices and cables.
If a CT is loaded at a power lower than its rated output, its actual accuracy level
is higher than the rated accuracy level. Likewise, a CT that is overloaded loses accuracy.
Expressed in kA rms, the maximum current permissible for 1 second (Ith)
(the secondary being short-circuited) represents the thermal withstand of the CT
to overcurrents. The CT must be able to withstand the short-circuit current for the time
required to clear it. If the clearing time t is other than 1 sec., the current the CT can
withstand is I th t
Electrodynamic withstand expressed in kA peak is at least equal to 2.5 Ith
Normal values of rated primary currents (in A):
10 - 12.5 - 15 - 20 - 25 - 30 - 40 - 50 - 60 - 75 and multiples or decimal submultiples.
(1) Also to be taken into account are elements related to the type of assembly,
characteristics of the site (e.g. temperature, etc.), power frequency, etc.
CT response in saturated state
When subjected to a very high primary current, the CT becomes saturated.
The secondary current is no longer proportional to the primary current. The current
error which corresponds to the magnetization current increases signicantly.
Imagnetizing
Fig. 1. Equivalent diagram of a CT secondary current...
Im at Vk
and CT magnetization curve.
The CT secondary satises the equation:
(RCT + Rload + Rwire) ALF Isn2 = constant
where Isn = rated secondary current
ALF = accuracy-limit factor
Isat = ALF Isn
CT for phase overcurrent protection
For denite-time overcurrent protection, if saturation is not reached at 1.5 times
the current setting, operation is ensured no matter how high the fault current (g. 2).
For IDMT overcurrent protection, saturation must not be reached at 1.5 times the
current value corresponding to the maximum in the useful part of the operation curve
(g. 3).
DE55332EN
CT for differential protection (g. 4)
The CTs should be specied for each application, according to the operating principle
of the protection unit and to the protected component. Refer to the instruction manual
of the protection unit.
DE57334EN
DE57331EN
Knee-point voltage (g. 1)
This is the point on the current transformer magnetization curve at which a 10%
increase in voltage E requires a 50% increase in magnetization current Im.
Low-power current transducers (LPCT) (g. 1)
DE57336
These are special voltage-output sensors of the Low-Power Current Transducer
(LPCT) type, compliant with standard IEC 60044-8.
LPCTs are used for measurement and protection functions.
They are dened by:
b the rated primary current,
b the rated extended primary current,
b the rated accuracy-limit primary current.
They have a linear output over a wide current range and begin to saturate at levels
above the currents to be interrupted.
Fig. 1. LPCT-type current sensors.
Example of measurement characteristics as per IEC 60044-8
b Rated primary current Ipn = 100 A
b Rated extended primary current Ipe = 1250 A
b Secondary voltage Vsn = 22.5 mV
b Class 0.5:
v accuracy 0.5% from 100 A to 1250 A,
v accuracy 0.75% at 20 A,
v accuracy 1.5% at 5 A.
DE55337EN
Example of protection characteristics as per IEC 60044-8
b Primary current Ipn = 100 A
b Class 5P from 1.25 kA to 40 kA (g. 2).
20 A 100 A
Fig. 2. LPCT accuracy characteristics.
Zero-sequence current - residual current
DE55338
The residual current characterizing the earth-fault current is equal to the vector sum
of the 3 phase currents (g. 1).
The residual current is equal to three times the zero-sequence current I0.
Ir sd = 3 I0 = I1 + I 2 + I3
Fig. 1. Denition of
residual current.
Detection of the fault current
Earth-fault current can be detected in a number of ways.
for earth-fault
A few amperes
DE57340EN
balance CT
DE57339
Direct measurement by special core
balance CT connected directly to the
protection relay. The CT is installed
around the live conductors and directly
creates the residual current.
It can also be installed on the accessible
neutral to earth link. The result is high
measurement accuracy; a very low
detection threshold (a few amperes)
10% of InCT (DT)
5% of InCT (IDMT)
1 or 5 A
DE57342EN
DE57341EN
Toroidal CT +
interposing ring
DE57343EN
Differential measurement using a classic
toroidal CT installed around the live
conductors and generating the residual
current, plus an interposing ring CT used
as an adapter for the protection relay.
3 phase CTs +
The toroidal CT can also be installed on
the accessible neutral to earth link with
an interposing ring CT.
This solution offers good accuracy and
exibility in CT selection.
Measurement of the currents in the three
phases with one CT per phase and
measurement of the residual current
by a special interposing ring CT.
3 phase CTs
(Irsd calculated
by relay)
No H2 restraint
30% InCT (DT)
10% InCT (IDMT)
With H2 restraint
10% InCT (DT)
5% InCT (IDMT)
DE57344
Practically speaking, the residual-current threshold must be:
b Is0 10% InCT (DT protection),
b Is0 5% InCT (IDMT protection).
Calculation based on measurement of the currents in the three phases with one CT
b The residual current is calculated by the protection relay.
b Measurement accuracy is not high (sum of CT errors and saturation characteristics,
calculated current).
b Installation is easier than in the previous case, but measurement accuracy is lower.
Practically speaking, the protection threshold settings must comply with the
b Is0 30% InCT for DT protection (10% InCT for a protection relay with H2 restraint),
b Is0 10% InCT for IDMT protection.
The role of a voltage transformer is to provide
its secondary winding with a voltage
proportional to that applied to the primary
circuit. Voltage transformers are used
for both measurements and protection.
Measurement of phase-to-phase voltages
The voltage transformer is made up of two windings, the primary and the secondary,
coupled by a magnetic circuit, and connections can be made between phases or
between a phase and earth.
DE57345
Voltage transformers are characterized by the following values:
(publications IEC 60186, IEC 60044-2 and NFC 42-501) (1)
b power frequency, generally 50 or 60 Hz,
b highest primary voltage in the power system,
b rated secondary voltage 100, 100/3, 110, 110/3 volts depending on the type
b rated voltage factor used to dene the heat-rise characteristics,
b apparent power, in VA, that the voltage transformer can supply to the secondary,
without causing errors exceeding its accuracy class, when connected to the rated
primary voltage and to its rated load. Note that a VT must never be short-circuited
on the secondary, because the power supplied increases and the transformer
can be damaged by the resulting heat rise,
b accuracy class dening the guaranteed error limits for the voltage ratio and phasedisplacement under the specied power and voltage conditions.
A number of measurement assemblies are possible:
b 3-transformer star assembly (g. 1)
(requires 1 insulated high-voltage terminal per transformer)
Transformation ratio: ------------------100 3
b 2-transformer V assembly, (g. 2)
(requires 2 insulated high-voltage terminals per transformer)
Transformation ratio: Un 100
DE57346
Fig. 1. Star-connected voltage transformers (VT).
In isolated neutral systems, all phase-neutral VTs sufciently loaded to avoid the risk
of ferromagnetic resonance.
(1) Elements related to the type of assembly, characteristics of the site (e.g. temperature), etc.
Measurement of residual voltage
Fig. 2. V-connected voltage transformers (VT).
The residual voltage characterizing the neutral-point voltage with respect to earth
is equal to the vector sum of the 3 phase-to-earth voltages.
The residual voltage is equal to three times the zero-sequence voltage V0:
DE55347
Vrsd = 3 V0 = V1 + V2 + V3 (g. 3)
DE57348
Fig. 3. Denition of residual voltage.
DE57349
The occurrence of this voltage signals the existence of an earth fault.
It can be measured or calculated:
b measurement using three voltage transformers whose primaries are star connected
and the secondaries, in an open delta arrangement, supply the residual voltage (g. 4),
b calculation by the relay, using three voltage transformers whose primaries and
secondaries are star connected (g. 5).
Fig. 4. Direct measurement of residual
Fig. 5. Calculation of residual voltage.
The protection relays that continuously
monitor power system variables include
combinations of basic functions to suit
the power system components being
The relay includes (g. 1):
b analog measurement input for the variable observed, received from the sensor,
b logic result of measurement processing (noted S),
b instantaneous logic output of the protection function, used for indication,
for example (noted Si),
b delayed logic output of the protection function, used to control circuit breaker
tripping (noted St).
DE57270
Characteristics (g. 2)
I > Is
The protection function work mode involves characteristic times (IEC 60255-3):
b operating time: this is the time between the application of the characteristic quantity
(at twice the threshold setting) and the switching of the output relay (instantaneous
output),
b overshoot time: this is the difference between operating time and the maximum
time during which the characteristic quantity can be applied with no tripping,
b reset time: this is the time between a sudden decrease in the characteristic quantity
and the switching of the output relay.
Fig. 1. Relay operating principle.
(example of ANSI 51 phase overcurrent protection relay)
Note: other non-standardized terms are commonly found as well, the denitions of which may
vary from one manufacturer to another: reclaim time, no response time, instantaneous tripping
time, memory time.
To improve stability, the functions have a drop out/pick up ratio d that is a %
of the threshold setting: in the example in gure 3, S goes from 1 to 0 when I = d Is
DE55272EN
Overshoot time
Maximum no trip time
Fig. 2. Protection function characteristic times.
DE55271
Fig. 3. Drop out/pick up ratio.
Some protection functions may be set by the user, in particular:
b tripping set point: it sets the limit of the observed quantity that actuates
the protection function.
b tripping time:
v denite time delay (DT)
The example in gure 1, applied to a current relay, shows that above the current
threshold Is, the protection tripping time is constant (time delay setting T).
DE55273EN
Fig. 1. Denite time tripping principle.
v IDMT delay (IDMT: Inverse Denite Minimum Time)
The example in gure 2, applied to a current relay, shows that above the current
threshold Is, the higher the current, the shorter the protection tripping time.
DE55274EN
Fig. 2. IDMT tripping principle.
There are several types of curves, determined by equations and dened by the
various standardization organizations: for example, the IEC denes the following
(g. 3):
- standard inverse time (SIT),
- very inverse time (VIT),
- extremely inverse time (EIT).
DE55275
Fig. 3. IDMT tripping curves.
b timer hold: adjustable reset time,
b restraint: inhibition of tripping according to percentage of second harmonic,
b time constants (e.g. thermal overload ANSI 49RMS),
b characteristic angle (e.g. directional overcurrent ANSI 67).
The main protection functions are listed with a brief denition in the table below.
They are listed in numerical order by ANSI C37.2 code.
Detection of rotating machine overspeed
Detection of rotating machine underspeed
Impedance measurement detection
Underimpedance
Back-up phase-to-phase short-circuit protection for generators
Overuxing check
Check before paralleling two parts of the power system
Protection for control of voltage sags
Positive sequence undervoltage
Protection of motors against operation with insufcient voltage
Remanent undervoltage
Third harmonic undervoltage
Check on the disappearance of voltage sustained by rotating machines
after the power supply is disconnected
Detection of stator winding insulation earth faults (impedant neutral)
Directional active overpower
Protection against active overpower transfer
Directional reactive overpower
Protection against reactive overpower transfer
Phase undercurrent
3-phase protection against undercurrent
Directional active underpower
Protection against active underpower transfer
Directional reactive underpower
Protection against reactive underpower transfer
Protection against overheating of rotating machine bearings
Protection of synchronous machines against faults or eld loss
Negative sequence / unbalance
Protection against unbalanced phase current
Negative sequence voltage protection and detection of reverse rotation of rotating machines
Excessive starting time and locked rotor
50N or 50G
Protection of motors against starting with overloads or reduced voltage, and for loads
that can block
Protection against overheating of machine windings
Instantaneous phase overcurrent
3-phase protection against short-circuits
Checking and protection if the circuit breaker fails to trip after a tripping order
Instantaneous voltage-restrained phase
Inadvertent generator energization
Protection against earth faults:
50N: residual current calculated or measured by 3 CTs
50G: residual current measured directly by a single sensor (CT or core balance CT)
3-phase protection against short-circuits with voltage-dependent threshold
51N or 51G
Detection of inadvertent generator energization
Delayed phase overcurrent
3-phase protection against overloads and short-circuits
Delayed earth fault
67N/67NC
Delayed voltage-restrained phase overcurrent
51N: residual current calculated or measured by 3 CTs
51G: residual current measured directly by a single sensor (CT or core balance CT)
Protection against excessive voltage or sufcient voltage detection
Insulation fault protection
Detection of transformer internal faults (gas, pressure)
Restricted earth fault differential
Earth fault protection for star-connected 3-phase windings with earthed neutral
100% generator stator earth fault
Detection of stator winding insulation earth faults (impedant neutral power systems)
Successive starts
Protection function that monitors the number of motor starts
3-phase short-circuit protection according to current ow direction
Earth fault protection depending on current ow direction
(NC: Neutral compensated)
Vector shift disconnection protection
Pole slip
Detection of loss of synchronization of synchronous machines
Automated device that recloses the circuit breaker after transient line fault tripping
Protection against abnormally high frequency
Protection against abnormally low frequency
Protection for fast disconnection of two parts of the power system
3-phase protection against busbar internal faults
3-phase protection against internal faults in AC generators
3-phase protection against line internal faults
3-phase protection against internal faults in motors
3-phase protection against internal faults in transformers
The protection functions are completed
b additional control functions,
b operation monitoring functions,
b operation functions,
b indication functions,
b metering functions,
b diagnosis functions,
b communication functions,
for enhanced operation of power systems.
All of these functions may be provided
by the same digital protection unit.
This function controls the different types of switchgear closing and tripping coils.
This function indicates switchgear trip circuit failures.
This function is used to implement logic discrimination by the sending and/or reception
of blocking signals by different protection units.
These functions perform logic equation operations to generate additional data
or orders used for the application.
These functions make operation more convenient for the user.
b Transformer on-load tap changers,
b Reactive energy regulation,
b Fault locator (ANSI 21FL),
b Capacitor bank control,
b Remaining operating time before thermal overload tripping.
These functions provide information required for a good understanding
of power system operation.
b Phase current,
b Tripping current,
b Residual current,
b Differential and through currents,
b Current THD (total harmonic distortion),
b Phase-to-neutral and phase-to-phase voltages,
b positive sequence, negative sequence and residual voltages,
b Voltage THD (total harmonic distortion),
b Frequency,
b Active, reactive and apparent power,
b Power factor (cos j),
b Active and reactive energy,
b Peak demand current, active and reactive power,
b Temperature,
b Motor starting time,
b Disturbance recording.
Switchgear diagnosis functions
b Switchgear closing and fault tripping operation counters,
b Operation time,
b Charging time,
b Sensor supervision (VT, CT); this function monitors the voltage or current
transformer measurement chain and acts on the related protection functions,
b Cumulative breaking current (kA2).
These functions are used for the exchange of available data by the different power
system components (measurements, states, control orders).
Protection functions form a consistent
system depending on the overall structure of
the power distribution system and the neutral
earthing arrangement. They should therefore
be viewed as a system based on the principle
of discrimination, which consists of isolating
the faulty part of the power system and only
that part as quickly as possible, leaving all
the fault-free parts of the power system
Various means can be used to implement
discrimination in power system protection:
b time-based discrimination,
b current-based discrimination,
b discrimination by data exchange, referred
to as logic discrimination,
b discrimination by the use of directional
protection functions,
b discrimination by the use of differential
b combined discrimination to ensure better
overall performance (technical and economic),
or back-up.
Time-based discrimination consists of assigning different time delays to the overcurrent
protection units distributed through the power system.
The closer the relay is to the source, the longer the time delay.
The fault shown in the diagram opposite (g. 1) is detected by all the protection units
(at A, B, C, and D). The contacts of delayed protection unit D close faster than those
of protection unit C, which themselves close faster than those of protection unit B
Once circuit breaker D tripped and the fault current has been cleared, protection units
A, B and C, which are no longer required, return to the stand-by position.
The difference in operation time DT between two successive protection units
is the discrimination interval. It takes into account (g. 2):
b breaking time Tc of the downstream circuit breaker, which includes the breaker
response time and the arcing time,
b time delay tolerances dT,
b upstream protection unit overshoot time: tr,
b a safety margin m.
T should therefore satisfy the relation:
T Tc + tr + 2dT + m
Considering present switchgear and relay performances, T is assigned a value
of 0.3 s.
Example: Tc = 95 ms, dT = 25 ms, t r = 55 ms; for a 300 ms discrimination interval,
the safety margin is 100 ms.
DE55242EN
DE57241EN
Discrimination interval T
Fig. 2. Breakdown of a discrimination interval.
TA = 1.1 s
TB = 0.8 s
This discrimination system has two advantages:
b it provides its own back-up; for example if protection unit D fails,
protection unit C is activated T later,
b it is simple.
TC = 0.5 s
TD = 0.2 s
Fig. 1. Time-based discrimination principle.
However, when there are a large number of cascading relays, since the protection
unit located the furthest upstream has the longest time delay, the fault clearing time
becomes prohibitive and incompatible with equipment short-circuit current withstand
and external operating necessities (e.g. constraint imposed by utility).
DE57243
This principle is used in radial power systems. (g. 1)
The time delays set for time-based discrimination are activated when the current
exceeds the relay settings. The settings must be consistent.
There are two cases, according to the type of time delay used.
IsA, TA
Denite time relays (g. 2)
The conditions to be fullled are: I sA > I sB > I sC et TA > TB > TC.
The discrimination interval T is conventionally in the range of 0.3 seconds.
IsB, TB
DE55244EN
IsC, TC
Fig. 1. Radial power system with time-based discrimination.
IscC IscB IscA
Fig. 2. Time-based discrimination with denite time relays.
IDMT relays (g. 3)
If the thresholds are set to the rated current In, overload protection is ensured
at the same time as short-circuit protection and setting consistency is guaranteed.
I nA > I nB > I nC
I sA = I nA, l sB = I nB, and I sC = I nC
The time delays are set to obtain the discrimination interval T for the maximum
current seen by the downstream protection relay. The same family of curves is used
to avoid overlapping in a portion of the domain.
DE55245
Fig. 3. Time-based discrimination with IDMT relays.
Current-based discrimination uses the principle that within a power system,
the further the fault is from the source, the weaker the fault current is.
A current protection unit is installed at the starting point of each section: the
threshold is set to a value lower than the minimum short-circuit current caused by a
the monitored section, and higher than the maximum current caused by a fault
downstream (outside the monitored area).
With these settings, each protection device is only activated by faults located
immediately downstream, within the monitored zone, and is not sensitive to faults
outside that zone.
For sections of lines separated by a transformer, it can be of benet to use this system
since it is simple, cost-effective and quick (tripping with no delay).
An example is given below (g.1):
IscBmax < IsA < IscAmin
IsA = current setting
IscB on the transformer primary is proportional to the maximum short-circuit current
on the secondary.
Time delays TA and TB are independent, and TA may be shorter than TB.
The upstream protection unit (A) does not provide back-up for the downstream
protection unit (B).
In practice, it is difcult to dene the settings for two cascading protection units, and
still ensure satisfactory discrimination, when there is no notable decrease in current
between two adjacent areas. This is the case in medium voltage power systems,
except for sections with transformers.
The following example concerns current protection of a transformer between
two cable sections.
The overcurrent protection setting I s satises the relation:
1.25 I scBmax < IsA < 0.8 I scAmin
Discrimination between the two protection units is ensured.
DE57246EN
IscBmax
Discrimination curves
IsA < IscAmin
IsA > IscBmax
Fig. 1. Current-based discrimination operation.
DE57247EN
This system was developed to solve the drawbacks of time-based discrimination.
This principle is used when short fault clearing time is required (g. 1).
The exchange of logic data between successive protection units eliminates the need
for discrimination intervals, and thereby considerably reduces the tripping time of the
circuit breakers closest to the source.
In radial power systems, the protection units located upstream from the fault are
activated; those downstream are not. The fault point and the circuit breaker to be
tripped can therefore be clearly located.
Each protection unit activated by a fault sends:
b a blocking signal to the upstream level (an order to increase the upstream relay
time delay),
b a tripping order to the related circuit breaker unless it has already received a
blocking signal from the downstream level.
Time-delayed tripping is provided as back-up.
The principle is illustrated in gure 2:
b when a fault appears downstream from B, the protection unit at B blocks
the protection unit at A,
b only the protection unit at B triggers tripping after the delay TB, provided it has not
received a blocking signal,
b the duration of the blocking signal for the protection unit at A is limited to TB + T3,
with T3 opening and arc extinction time of circuit breaker B (typically 200 ms),
b if circuit breaker B fails to trip, protection unit A gives a tripping order at TB + T3,
b when a fault appears between A and B, protection unit A trips after the delay TA.
DE57248EN
Fig. 1. Logic discrimination principle.
TB + T3
Tripping time is not related to the location of the fault within the discrimination chain
or to the number of protection units in the chain.
This means that discrimination is possible between an upstream protection unit
with a short time delay and a downstream unit with a long time delay. For example,
a shorter time delay may be used at the source than near the loads.
The system also has back-up designed into it.
Since logic signals must be transmitted between the different levels of protection
units, extra wiring must be installed. This can be a considerable constraint when
the protection units are far apart each other, in the case of long links, for example
(several hundreds of meters long).
This difculty may be bypassed by combining functions: logic discrimination
in the nearby switchboards and time-based discrimination between zones
that are far apart (refer to chapter on combined logic + time-based discrimination).
This principle is often used to protect medium voltage power systems that include
radial branches with several levels of discrimination.
Fig. 2. Logic discrimination operation.
In a looped power system, in which faults are fed from both ends, it is necessary
to use a protection unit that is sensitive to the direction of the ow of fault current
in order to locate and clear the fault selectively. This is the role of directional
overcurrent protection units.
DE57249EN
The protection actions differ according to the direction of the current (gs. 1 and 2),
i.e. according to the phase displacement of the current in relation to a reference given
by the voltage vector; the relay therefore needs both current and voltage data.
The operating conditions, namely the position of the tripping and no tripping zones,
are adapted to t the power system to be protected (g. 3).
Example of the use of directional protection units (g. 4):
Circuit breakers D1 and D2 are equipped with directional protection units
that are activated if the current ows from the busbars to the cable.
If a fault occurs at point 1, it is only detected by the protection unit at D1.
The protection unit at D2 does not detect it, because of the detected current direction.
The D1 circuit breaker trips.
If a fault occurs at point 2, it is not detected by these protection units
and the D1 and D2 circuit breakers remain closed.
Other protection units must be included to protect the busbars.
DE57250EN
Directional protection principle
Fig. 1. Protection unit active.
The solution is simple and may be used in a large number of cases.
Voltage transformers must be used to provide a phase reference to determine
the direction of the current.
This principle is used to protect parallel incomers and closed loop power systems
and also for certain cases of earth fault protection.
Fig. 2. Protection unit not active.
I busbars V cable
No tripping zone
DE57252EN
DE55251EN
Tripping zone
I cable V busbars
Fig. 3. Detection of current direction.
Fig. 4. Example of two parallel incomers.
DE57253EN
These protection units compare the current at the two ends of the monitored section
of the power system (g. 1).
Any amplitude or phase difference between the currents indicates the presence
of a fault: The protection units only react to faults within the area they cover and
are insensitive to any faults outside that area. This type of protection is therefore
selective by nature.
Instantaneous tripping takes place when IA-IB 0
In order for differential protection to work, it is necessary to use current transformers
specically sized to make the protection units insensitive to other phenomena.
What makes differential protection units stable is that they do not pick up as long as
there are no faults in the zone being protected, even if a differential current is detected:
b transformer magnetizing current,
b line capacitive current,
b error current due to saturation of the current sensors.
There are two main principles according to the stabilization mode:
b high impedance differential protection: the relay is series-connected to a stabilization
resistor Rs in the differential circuit (gs. 2 and 3),
b percentage-based differential protection: the relay is connected independently to
the circuits carrying the currents IA and IB. The difference between the currents IA
and IB is determined in the protection unit and the protection stability is obtained by
a restraint related to the through current (gs. 4 and 5).
Fig. 1. Differential protection principle.
DE55256EN
DE57254EN
Fig. 2. High impedance differential protection diagram.
Fig. 4. Percentage-based differential protection diagram.
DE55257EN
DE57255EN
Fig. 3. Stability by resistance.
Threshold % It
Fig. 5. Stability by restraint.
b Protection sensitive to fault current less than the rated current of the protected
b Zone protection that can trip instantaneously.
b The cost of the installation is high.
b It takes skill to implement the system.
b An overcurrent back-up function needs to be included.
Comparison of the two principles
b High impedance differential protection:
v the upstream and downstream CTs must have the same rated currents (primary
and secondary),
v the resistance of the stabilization resistor is chosen to avoid tripping by external
faults with a saturated CT and to allow the relay to be supplied by the CT,
v The relay is relatively simple, but requires the use of stabilization resistors.
b Percentage-based differential protection:
v can be adapted to t the equipment to be protected,
v the relay is relatively more complicated, but is easy to use.
Differential protection may concern all priority high power components:
motors, generators, transformers, busbars, cables and lines.
Combined discrimination is a combination of
basic discrimination functions that provides
additional advantages in comparison to
individual types of discrimination.
b total discrimination,
b redundancy or back-up.
Several practical examples of applications using combined
discrimination are given below:
b current-based + time-based,
b logic + time-based,
b time-based + directional,
b logic + directional,
b differential + time-based.
DE57258EN
Current-based + time-based discrimination
The example shows an arrangement with both of the following:
b current-based discrimination between A1 and B,
b time-based discrimination between A2 and B.
This provides total discrimination, and the protection unit at A provides back-up
for the protection unit at B.
IsA1, TA1
IsA2, TA2
DE55259EN
Fig. 1. Current-based + time-based discrimination.
DE57260
Logic + back-up time-based discrimination
IsA, TA1
b logic discrimination between A1 and B,
The A2 protection unit provides back-up for the A1 protection unit, if A1 fails to trip
due to a blocking signal fault (permanent blocking signal).
IsA, TA2
DE55261EN
Fig. 2. Logic + back-up time-based discrimination.
DE57262EN
Logic + time-based discrimination
Fig. 3. Comparison of combined (logic + time-based)
discrimination and time-based discrimination tripping times.
b logic discrimination inside a switchboard
(between A and B and between C and D).
b time-based discrimination between two switchboards B and D, with TB = TD + T.
It is not necessary to install a logic signal transmission link between two switchboards
that are far apart. The tripping delays are shorter than with time-based discrimination
alone (g. 3).
b back-up time-based discrimination needs to be included at points A and C
(refer to the paragraph above).
DE57263
Time-based + directional discrimination
D1 and D2 are equipped with short time-delayed directional protection units;
H1 and H2 are equipped with time-delayed overcurrent protection units.
If a fault occurs at point 1, it is only detected by the D1 (directional), H1 and H2
protection units. The protection unit at D2 does not detect it, because of the detected
current direction. D1 trips. The H2 protection unit drops out, H1 trips and the faulty
section H1-D1 is isolated.
TH1 = TH2
TD1 = TD2
TH = TD + T
DE57264EN
Fig. 1. Time-based + directional discrimination.
Logic + directional discrimination
BSIG 51
Fault at D2 end:
b tripping at D2 and B,
b D1 is blocked by B (BSIG: blocking signal).
51 BSIG
The example shows that the orientation of blocking signals depends on the direction
of the current ow.
This principle is used for busbar coupling and closed loops.
Fault at D1 end:
b tripping at D1 and B,
b D2 is blocked by B (BSIG: blocking signal).
Fig. 2. Logic + directional discrimination.
Differential + time-based discrimination
DE57265EN
b instantaneous differential protection,
b a phase overcurrent or earth fault protection unit at A as back-up for the differential
protection unit,
b a current protection unit at B to protect the downstream zone,
b time-based discrimination between the protection units at A and B, with
TA = TB + T.
This provides back-up for the differential protection function, but double-wound
current transformers are sometimes necessary.
Note: time-based discrimination may be replaced by logic discrimination.
Fig. 3. Differential + time-based discrimination.
Power-system protection should:
b detect faults,
b isolate the faulty parts of the power system,
keeping the fault-free parts in operation.
Protection units are chosen according to
the power-system conguration (parallel
operation of generators or transformers,
loop or radial power system, neutral
earthing arrangement).
b phase-to-phase fault protection,
b earth fault protection, linked to the neutral
earthing arrangement.
The following types of systems will be
examined: single-incomer, dual-incomer,
open loops and closed loops.
Phase-to-phase faults (g. 1)
The incomer and feeders are equipped with phase overcurrent protection units
(ANSI 51).
Time-based discrimination is used between the incomer protection unit (A)
and the feeder protection units (D).
The protection unit at D detects fault 1 on the feeder and trips circuit breaker D
after a delay TD.
The protection unit at A detects fault 2 on the busbars and trips after a delay TA.
It also acts as back-up should protection D fail.
Choose: IsA IsD and TA TD + T
T: discrimination interval (generally 0.3 s).
The protection unit at D must be selective in relation to the downstream protection units:
if the delay required for protection A is too long, logic or combined (logic + time-based)
discrimination should be used.
Phase-to-earth faults
Resistance earthing on the transformer (g.2)
Earth fault protection units (ANSI 51N) are installed on the feeders, incomer and
neutral earthing connection.
Time-based discrimination is used between the different protection units.
These units are necessarily different from phase fault protection units since the fault
currents are in a different range.
The feeder protection units are set selectively in relation to the incomer protection
unit, which is itself set selectively in relation to the neutral earthing protection unit
(in accordance with discrimination intervals).
The fault current ows through the capacitances of the fault-free feeders
and the earthing resistance.
All the fault-free feeder sensors detect capacitive current.
To prevent inadvertent tripping, the protection unit on each feeder is set higher than
the feeders capacitive current.
b fault at 3: the D1 circuit breaker is tripped by the protection unit linked to it,
b fault at 4: the A circuit breaker is tripped by the incomer protection unit,
b fault at 5: the protection unit on the neutral earthing connection trips circuit breaker H
on the transformer primary circuit. (g. 1).
DE57231EN
DE57230
if the delay required for protection A is too long, logic discrimination should be used.
The neutral earthing protection unit at H acts as back-up should the incomer
protection unit at A fail to trip.
The incomer protection unit at A acts as back-up should a feeder protection unit
at D fail to trip.
IsD, TD
IsD IsA
IsD IsA IsH
Fig. 1. Phase-to-phase fault protection.
Fig. 2. Phase-to-earth fault protection (resistance-earthed neutral at transformer).
Phase-to-earth faults (contd)
DE57232
51G IsA, TA
51G IsD, TD
Resistance-earthed neutral at busbars (g. 1)
A zero sequence generator is used for resistance-earthing.
Earth fault protection units (ANSI 51G) are installed on the feeders, incomer and zero
The feeder protection units and incomer protection unit are set selectively in relation
to the earthing impedance protection unit. As in the previous case, the protection
unit on each feeder is set higher than the feeders capacitive current.
In the event of a fault on feeder 1, only the D1 feeder circuit breaker trips.
In the event of fault on the busbars 2, only the protection unit on the earthing
connection detects the fault. It trips circuit breaker A.
In the event of fault on the transformer secondary circuit 3, the incomer protection
units detects the fault. It trips circuit breaker H.
Note: when circuit breaker A is open, the transformer secondary circuit neutral
is isolated. It may be necessary to protect it by a neutral voltage displacement
measurement (ANSI 59N).
The zero sequence generator protection unit acts as back-up should the incomer
protection unit at A or a feeder protection unit at D fail to trip.
If the condition IsD > 1.3 Ic cannot be satised for a feeder, a directional earth fault
protection unit may be used to discriminate between fault current and capacitive
The same procedure is used as for resistance-earthing at the transformer or busbars.
DE57233EN
Fig. 1. Phase-to-earth fault protection
(resistance-earthed neutral at busbars).
Isolated neutral (g. 2)
A fault, regardless of its location, produces current which ows through the capacitance
of the fault-free feeders.
In industrial power systems, this current is generally weak (a few amperes), allowing
operations to carry on while the fault is being tracked.
The fault is detected by an insulation monitoring device or a neutral voltage
displacement protection unit (ANSI 59N).
When the total capacitive current of a power system is high (in the range of ten
amperes), additional measures must be taken to quickly clear the fault.
Directional earth fault protection can be used to selectively trip the fault feeder.
This is similar to resistance-earthing at the transformer, but the capacitive currents
are negligible compared to the fault current, so the protection function is simpler
Compensated neutral
The power system is earthed at the transformer. Faults are detected by a specic
directional earth fault protection unit (ANSI 67NC), which monitors the active
residual current and recognizes faults during their initial transient phase.
Fig. 2. Phase-to-earth fault protection (isolated neutral).
DE57234
51 TD
Power system with two transformer incomers or two line incomers
The feeders are equipped with phase overcurrent protection units with delays set
to TD.
The two incomers A1 and A2 are equipped with phase overcurrent protection units
(ANSI 51) set selectively in relation to the feeders, i.e.
TA TD + T.
They are also equipped with directional protection units (ANSI 67) with delays set
at TR < TA T.
Time-based discrimination is used between the incomer A protection units
and feeder D protection units.
Current-based discrimination is used between the power supply H protection units
and incomer A protection units.
This means that a fault at 1 is cleared by the tripping of D2 after a delay TD.
A fault at 2 is cleared by the tripping of A1 and A2 with a delay of TA (the directional
protection units do not detect the fault).
A fault at 3 is detected by the A1 directional protection unit which trips at the time TR,
allowing continued operation of the fault-free part of the power system.
However, the fault at 3 is still fed by T1. At the time TH TA + T,
H1 is tripped by the phase overcurrent protection unit linked to it.
Phase-to-earth faults (g. 2)
DE57235
51G TN
51G TD
Fig. 2. Phase-to-earth fault protection (resistance-earthed
neutral at the transformer).
Resistance-earthed neutral at incomer transformers
Earth fault protection units (ANSI 51G) are installed on the feeders and set higher
than the corresponding capacitive currents, with delays of TD.
Directional earth fault protection units (ANSI 67N) are installed on incomers A1 and
A2, with time delays of TR.
Earth fault protection units (ANSI 51G) are installed on the earthing connections and
set higher than the incomer and feeder protection units, with time delays such that
TN TD + T.
This means that a fault at 4 is cleared by the tripping of D1.
A fault at 5 is cleared by the tripping of A1, A2, H1 and H2 by the protection units
located on the neutral earthing connections of the 2 transformers.
A fault at 6 is detected by the A1 directional protection unit which trips at the time TR,
However, the fault at 6 continues to be supplied up to the time TN at which
the protection unit on the corresponding transformer earthing connection trips
the H1 circuit breaker.
Resistance-earthed neutral at the busbars
Earth fault protection units are installed on the feeders, incomers and zero sequence
The system operates in the same way as in single-incomer power systems.
This is similar to resistance-earthing, but the phase-to-earth current is higher
and reaches the phase-to-phase current level.
Only one earthing coil is in service at a given time to ensure power system
capacitance matching; this is similar to single-incomer power systems.
Additional protection functions
Coupling (g. 1)
The synchro-check function (ANSI 25) is used to check that the circuits to be connected
have voltage amplitude, phase and frequency differences within acceptable limits
to allow closing of the coupling circuit breaker.
DE57236
When electrical installations are supplied by the utility and an independent power
source, interference between the two sources as a result of events such as a utility
failure or earth faults should be avoided. The consequences include voltage and
frequency uctuations and current and power exchanges between the different circuits.
Protection functions are often advocated or imposed in the distributors technical
There are several methods of decoupling two sources:
b monitoring of the active power direction and protection by a reverse power
protection relay (ANSI 32P),
b monitoring of voltage amplitude and under or overvoltage protection
(ANSI 27 or 59),
b monitoring of frequencies and underfrequency (ANSI 81L) or overfrequency
(ANSI 81H) protection,
b protection against phase shifts caused by faults (ANSI 78),
b monitoring of frequency variations and ROCOF (rate of change of frequency)
protection (ANSI 81R) with respect to a threshold. This protection function is faster
than the frequency protection functions and more stable than phase shift protection.
DE57237EN
Fig. 1. Power system coupling protection.
Fig. 2. Automatic source transfer.
Automatic source transfer (g. 2)
The system in gure 2 shows an installation with two busbars normally supplied
by two sources with the coupling open (2/3 conguration).
If source 1 is lost, the power system is recongured. Source 1 is opened and
the coupling is closed; this automatic source transfer takes place according
to a procedure:
b initialization of the transfer by the detection of undervoltage (ANSI 27) on source 1
resulting in opening of the source 1 circuit breaker: Us = 70% Un,
b inhibition of transfer if a fault is detected downstream from source 1 by an overcurrent
protection unit (ANSI 50 and 50N),
b enabling of transfer after the disappearance of voltage sustained by rotating machines
is checked by the remanent undervoltage protection unit (ANSI 27R):
Us = 25% Un,
b enabling of transfer after verication that there is sufcient voltage (ANSI 59)
on source 2 and closing of coupling circuit breaker: Us = 85% Un.
In distribution systems that include
substations supplied in open loops,
protection is provided at the head
The power system is operated as an open loop and protection is provided at the ends
of the loops, which are equipped with circuit breakers (g. 1).
The switching devices used on the substations are switches.
Faults cause power outages.
Phase overcurrent and earth fault protection units (ANSI 51 and 51N) are installed
on the circuit breakers at the head of each loop.
A fault occurring in a cable that connects 2 substations may trip either of these circuit
breakers depending on the position of the loop opening.
The protection is often completed by an automated device that:
b clears the fault (with the power off) by opening the devices located at the ends
of the faulty cable, after the faulty cable has been located by the fault detector,
b closes the circuit breaker that has tripped at the head of the loop,
b closes the device that ensured the normal opening of the loop in order to restore
power to the fault-free downstream half of the loop.
The power system can be put back into its initial operating state after the faulty circuit
The outage may last from a few seconds to a few minutes depending on whether
the loop is recongured automatically or manually.
DE57238EN
Fig. 1. Open loop protection principle.
substations supplied in closed loops,
protection is provided for different sections.
The power system may be operated in closed loops, with each section protected by
circuit breakers at the ends of the section. Most faults do not cause power outages.
Various protection solutions may be used.
DE57239EN
Differential protection (g. 1)
Each cable is equipped with a line differential protection unit (ANSI 87L) and each
substation is equipped with a busbar differential protection unit (ANSI 87B).
This type of protection is very quick.
If the neutral is resistance-earthed, the sensitivity of the differential protection units
must cover phase-to-earth faults.
Fig. 1. Closed loop differential protection.
DE57240
Overcurrent protection and directional logic discrimination (g. 2)
The circuit breakers in the loop are equipped with overcurrent and directional
protection units. Logic discrimination is used to clear faults as quickly as possible.
A fault in the loop activates:
b all the protection units if the loop is closed,
b all the protection units upstream from the fault when the loop is open.
Each protection unit sends a blocking signal to one of the adjacent units in the loop,
according to the data transmitted by the directional protection unit.
Protection units that do not receive a blocking signal trip with a minimum delay that
is not dependent on the faults position in the loop:
b the fault is cleared by two circuit breakers, one on either side of the fault if the loop
is closed, and all the switchboards remain energized,
b the fault is cleared by the upstream circuit breaker if the loop is open.
This solution is a comprehensive one since it protects cables and busbars.
It is fast, selective and includes back-up protection.
Fig. 2. Loop overcurrent protection and directional logic discrimination.
Types of faults and
DE57281EN
Busbars are electrical power dispatching
nodes that generally have more than
Specic busbar protection may be provided
in a variety of ways, using basic functions.
51N TB = 0.4 s
51N TC = 0.1 s
DE57282EN
Fig. 1. Time-based discrimination.
TA1 = 0.1 s
TA2 = 0.7 s
TB = 0.4 s
TC = 0.1 s
DE57283
Fig. 2. Logic discrimination.
Fig. 3. Differential protection.
Phase-to-phase and phase-to-earth faults
The use of time-based discrimination with the overcurrent (ANSI 51) and earth fault
(ANSI 51N) protection functions may quickly result in excessive fault clearing time
due to the number of levels of discrimination.
In the example (g. 1), protection unit B trips in 0.4 s when there is a busbar fault
at point 1; when a busbar fault occurs at point 2, protection unit A trips in 0.7 s,
since the discrimination interval is set to 0.3 s.
The use of logic discrimination (g. 2) with overcurrent protection provides a simple
solution for busbar protection.
A fault at point 3 is detected by protection unit B, which sends a blocking signal
to protection unit A.
Protection unit B trips after 0.4 s.
However, a fault at point 4 is only detected by protection unit A, which trips after 0.1 s;
with backup protection provided if necessary in 0.7 s.
51N TA = 0.7 s
Differential protection (ANSI 87B) is based on the vector sum of the current entering
and leaving the busbars for each phase. When the busbars are fault-free, the sum
is equal to zero, but when there is a fault on the busbars, the sum is not zero and
the busbar supply circuit breakers are tripped.
This type of protection is sensitive, fast and selective.
b With percentage-based, low impedance differential protection, the difference
is calculated directly in the relay. The threshold setting is proportional to the through
current and CTs with different ratios may be used. However, the system becomes
complicated when the number of inputs increases.
b With high impedance differential protection (g. 3), the difference is calculated
in the cables, and a stabilization resistor is installed in the differential circuit. The CTs
are sized to account for saturation according to a rule given by the protection relay
manufacturer. The threshold setting is approximately 0.5 CT In and it is necessary to
use CTs with the same ratings.
Load shedding function
The load shedding function is used when a shortage of available power in comparison
to the load demand causes an abnormal drop in voltage and frequency:
certain consumer loads are disconnected according to a preset scenario,
called a load shedding plan, in order to recover the required power balance.
Different load shedding criteria may be chosen:
b undervoltage (ANSI 27),
b underfrequency (ANSI 81L),
b rate of change of frequency (ANSI 81R).
The breaker failure function (ANSI 50BF) provides backup when a faulty breaker
fails to trip after it has been sent a trip order: the adjacent incoming circuit breakers
are tripped.
The example (g. 1) shows that when a fault occurs at point 1 and the breaker
that has been sent the trip order fails, the breaker failure protection function is faster
than action by upstream protection time-based discrimination: 0.6 s instead of 0.7 s.
DE57284EN
Fig. 1. Breaker failure.
Link (line and cable)
The term link refers to components designed
to convey electrical power between two points
that are several meters to several kilometers
apart: links are generally overhead lines with
bare conductors or cables with insulated
A specic type of protection is required
Protection against overheating due to overload currents in conductors under steady
state conditions is provided by the thermal overload protection function (ANSI 49RMS),
which estimates temperature buildup according to the current measurement.
Phase-to-phase short circuits
51N or 67N
DE57286
DE57285EN
b Phase overcurrent protection (ANSI 51) may be used to clear the fault,
the time delay being set to provide discrimination.
A distant 2-phase fault creates a low level of overcurrent and an unbalance;
a negative sequence / unbalance protection function (ANSI 46) is used to complete
the basic protection function (g. 1).
b To reduce fault clearance time, a percentage-based differential protection function
(ANSI 87L) may be used. It is activated when the differential current is equal to more
than a certain percentage of the through current. There is a relay at either end
of the link and information is exchanged by the relays via a pilot (g. 2).
Fig. 1. Link protection
by overcurrent relay.
Fig. 2. Link protection
by differential relays.
Phase-to-earth short circuits
Time-delayed overcurrent protection (ANSI 51N) may be used to clear faults with
a high degree of accuracy (g. 1).
For long feeders though, with high capacitive current, the directional earth fault
protection function (ANSI 67N) allows the current threshold to be set lower than the
capacitive current in the cable as long as system earthing is via a resistive neutral.
DE57279EN
Distance protection (ANSI 21) against faults affecting line or cable sections is used
in meshed power systems (parallel links, several sources).
It is selective and fast, without requiring time-based discrimination. Sensitivity
depends on the short-circuit power and the load. It is difcult to implement when the
type of link is not the same throughout (overhead line + cable).
It operates according to the following principle:
b measurement of an impedance proportional to the distance from the measurement
point to the fault,
b delimitation of impedance zones which represent line sections of different lengths
(g.1),
b tripping by zone with time delay.
The example in gure 2 shows the following for the protection unit at point A in line
b an impedance circle at 80% of the length of the line (zone 1), inside which tripping
is instantaneous,
b an impedance band between 80% and 120% of the length of the line (zone 2),
in which tripping is delayed (200 ms),
b an impedance circle at 120% of the length of the line (zone 3), outside which
there is long-time delayed backup tripping of protection unit B outside AB,
b an impedance circle at 120% downstream to provide backup for downstream
b When there is communication between the protection units at the ends,
tripping can take place instantaneously between 0 and 100%.
DE55280EN
Fig. 2. Impedance circles.
The recloser function (ANSI 79) is designed to clear transient and semi-permanent
faults on overhead lines and limit down time as much as possible. The recloser
function automatically generates circuit breaker reclosing orders to resupply
overhead lines after a fault. This is done in several steps:
b tripping when the fault appears to de-energize the circuit,
b time delay required for insulation recovery in the location of the fault,
b resupply of the circuit by reclosing.
Reclosing is activated by the link protection units.
The recloser may be single-phase and/or 3-phase, and may comprise one or more
consecutive reclosing cycles.
Fig. 1. Distance protection principle.
The transformer is a particularly important
power system component.
Transformers requires effective protection
against all faults liable to damage them,
whether of internal or external origin.
The choice of a protection unit is often
based on technical and cost considerations
related to the power rating.
The main faults that can affect transformers are:
b short-circuits,
b frame faults.
Overloads may be caused by an increase in the number of loads supplied
simultaneously or by an increase in the power drawn by one or more loads.
Overloads result in overcurrent of long duration, causing a rise in temperature that is
detrimental to the preservation of insulation and to the service life of the transformer.
Short-circuits can occur inside or outside the transformer.
Internal short-circuits: faults between different phase conductors or faults
between turns of the same winding. The fault arc damages the transformer winding
and can cause re. In oil transformers, the arc causes the emission of
decomposition gas.
If the fault is slight, a small amount of gas is emitted and the accumulation of gas
A violent short-circuit can cause major damage liable to destroy the winding and also
the tank frame by the spread of burning oil.
DE55288EN
External short-circuits: phase-to-phase faults in the downstream connections.
The downstream short-circuit current creates electrodynamic stress in the transformer
that is liable to have a mechanical effect on the windings and lead to an internal fault.
Information on transformer operation
Fault current according to the position of the fault in the
Transformer energizing (g. 3)
Transformer energizing creates a transient peak inrush current that may reach 20
times the rated current with time constants of 0.1 to 0.7 seconds. This phenomenon
is due to saturation of the magnetic circuit which produces a high magnetizing
current. The peak current is at its highest when energizing takes place as the voltage
goes through zero and there is maximum remanent induction on the same phase.
The waveform contains a substantial amount of 2nd harmonics.
This phenomenon is part of normal power system operation and should not
be detected as a fault by the protection units, which should let the peak energizing
current through.
DE55289
( t) = Ie
Fig. 3. Transformer energizing.
Ie: inrush current envelope
e: time constant
Frame faults are internal faults. They may occur between the winding and the tank
frame or between the winding and the magnetic core.
They cause gas emission in oil transformers. Like internal short-circuits, they can cause
transformer damage and re. The amplitude of the fault current depends on the
upstream and downstream neutral earthing arrangements, and also on the position
of the fault in the winding:
b in star connected arrangements (g.1), the frame fault current varies between 0
and the maximum value depending on whether the fault is at the neutral or phase
end of the winding.
b in delta connected arrangements (g. 2), the frame current varies between 50 and
100% of the maximum value depending on whether the fault is in the middle or at the
Overuxing
Transformer operation at a voltage or frequency that is too low creates excessive
magnetizing current and leads to deformation of the current by a substantial amount
of 5th harmonics.
DE57290
Overcurrent of long duration may be detected by a denite time or IDMT delayed
overcurrent protection unit (ANSI 51) that provides discrimination with respect
to the secondary protection units.
The dielectric temperature is monitored (ANSI 26) for transformers with liquid
insulation and the winding temperature is monitored (ANSI 49T) for dry type
Thermal overload protection (ANSI 49RMS) is used for more sensitive monitoring
of temperature rise: heat rise is determined by simulation of the release of heat
according to the current and thermal inertia of the transformer.
For MV/LV transformers, overloads may be detected on the low voltage side
by the long time trip function of the main LV circuit breaker.
Fig. 1. Transformer differential protection.
DE57291EN
Max. LV Max. HV
DE57292
Fig. 2. Transformer overcurrent protection.
Several protection functions may be implemented.
b For oil transformers, devices that are sensitive to gas emission or oil movement
(ANSI 63) caused by short-circuits between turns of the same phase or phase-to-phase
v Buchholz relays for free breathing HV/HV transformers,
v gas and pressure detectors for hermetically sealed HV/LV transformers.
b Transformer differential protection (ANSI 87T) (g.1) which provides fast protection
against phase-to-phase faults. It is sensitive and used for vital high power transformers.
To avoid nuisance tripping, the 2nd harmonic of the differential current is measured
to detect transformer energizing (H2 restraint) and the 5th harmonic is measured
to detect overuxing (H5 restraint).
The use of this protection function with neural network technology provides
the advantages of simple setting and stability.
b An instantaneous overcurrent protection unit (ANSI 50) (g. 2) linked to the circuit
breaker located on the transformer primary circuit provides protection against violent
short-circuits. The current threshold is set higher than the current due to short-circuits
on the secondary winding, thereby ensuring current-based discrimination.
b HV fuses can be used to protect transformers with low kVA ratings.
DE57293
Fig. 3. Transformer tank frame fault protection.
DE57294
Fig. 4. Earth fault protection. Fig. 5. Restricted earth fault
b Tank frame fault (g. 3)
This slightly delayed overcurrent protection unit (ANSI 51G), installed on the
transformer frame earthing connection (if the setting is compatible with the neutral
earthing arrangement), is a simple, effective solution for internal winding-to-frame
faults. In order for it to be used, the transformer must be isolated from the earth.
This protection function is selective: it is only sensitive to transformer frame faults
on the primary and secondary sides.
Another solution consists of using earth fault protection:
b earth fault protection (ANSI 51N) located on the upstream power system for frame
faults that affect the transformer primary circuit.
b earth fault protection (ANSI 51N) located on the incomer of the switchboard being
supplied, if the neutral of the downstream power system is earthed on the busbars
(g. 4).
These protection functions are selective: they are only sensitive to phase-to-earth
faults situated in the transformer or on the upstream and downstream connections.
b restricted earth fault protection (ANSI 64REF) if the downstream power system
neutral is earthed at the transformer (g. 5). This is a differential protection function
that detects the difference between residual currents measured at the neutral
earthing point and at the three-phase output of the transformer.
b neutral point earth protection (ANSI 51G) if the downstream power system is
earthed at the transformer (g. 6).
b neutral voltage displacement protection (ANSI 59N) may be used if the downstream
power system neutral is isolated from the earth (g. 7).
Fig. 6. Neutral point earth
Fig. 7. Neutral voltage
displacement protection.
Appropriate protection function
Dielectric temperature monitoring
(transformers with liquid insulation)
(dry type transformers)
Alarm at 95C; tripping at 100C
Alarm at 150C; tripping at 160C
49 RMS
Alarm threshold = 100% of thermal capacity used
Tripping threshold = 120% of thermal capacity used
Time constant in the 10 to 30 minute range
Denite time overcurrent
Choice of rating according to appropriate method for switchgear concerned
Percentage-based differential
Buchholz or gas and pressure detection
Tank frame overcurrent
51N/51G
IDMT overcurrent
High threshold > downstream Isc
Low threshold < 5 In
Delay downstream T + 0.3 seconds
IDMT low threshold, selective with downstream, approximately 3 In
Slope = 15% + setting range
Min. threshold 30%
Neutral point earth fault
Threshold > 20 A, delay 0.1 seconds
Threshold 20 % of maximum earth fault current and > 10% of CT rating
(with 3CTs and H2 restraint)
Delay 0.1 seconds if earthing is on the power system
Time delay according to discrimination if earthing is on the transformer
Threshold 10% of In, no delay
Threshold < permanent limitation resistance current
Threshold approximately 10% of residual overvoltage
Threshold > 1.05 Un/fn
Delay: constant time, 1 hour
DE57295
DE57296
Low rated HV/LV transformer
High-rated HV/LV transformer
51G (2 x)
DE57298
DE57297
Low-rated HV/HV transformer
High-rated HV/HV transformer
Motors are the interface between electrical
and mechanical equipment. They are
connected to the machines they drive
and are therefore exposed to the same
Motors may be subjected to internal
mechanical stress due to their moving parts.
A single faulty motor can disrupt an entire
production process. Modern motors have
optimized characteristics which make them
unsuitable for operation other than according
to their rated characteristics. This means
that they are relatively fragile electrical loads
that need to be carefully protected.
There are asynchronous motors (mainly
squirrel-cage motors or wound-rotor motors)
and synchronous motors (motors with DC
rotor excitation).
Questions concerning synchronous motors
are the same as those that concern
asynchronous motors plus those that
concern generators.
Motors are affected by:
b faults related to the driven loads,
b power supply faults,
b motor internal faults.
Faults related to the driven loads
If the power drawn is greater than the rated power, there is overcurrent in the motor
and an increase in losses, causing a rise in temperature.
Excessive starting time and frequency of starts
Motor starting creates substantial overcurrents which are only admissible for short
durations. If a motor starts too frequently or if starting takes too long due to insufcient
motor torque compared to load torque, overheating is inevitable and must be avoided.
Rotation suddenly stops due to blocking of the driven mechanism. The motor draws
the starting current and stays blocked at zero speed. There is no more ventilation and
overheating occurs very quickly.
Loss of pump priming or a break in load coupling causes no-load operation of the motor,
which does not directly harm the motor. However, the pump itself is quickly damaged.
This causes motors to operate as generators when the inertia of the driven load
This reduces motor torque and speed: the slow-down causes increased current
and losses. Abnormal overheating therefore occurs.
3-phase power supply may be unbalanced for the following reasons:
b the power source (transformer or AC generator) does not supply symmetrical
3-phase voltage,
b all the other consumers together do not constitute a symmetrical load and
this unbalances the power supply system,
b the motor is powered by two phases after a fuse has blown on one phase,
b The phase order is reversed, changing the direction of motor rotation.
Power supply unbalance creates negative sequence current which causes very high
losses and quick rotor overheating.
When the voltage is re-supplied after a motor power failure, the motor sustains
remanent voltage that may lead to overcurrent when the motor starts again or even
a mechanical break in transmission.
Motor internal faults
Phase-to-phase short-circuits
These faults vary in strength according to where they occur in the coil and they cause
Stator frame fault
The amplitude of the fault current depends on the power system neutral earthing
arrangement and the position of the fault within the coil.
Phase-to-phase short-circuits and stator frame faults require motor rewinding,
and frame faults can also irreparably damage the magnetic circuit.
Rotor frame faults (for wound-rotor motors)
Rotor insulation breakdown can cause a short-circuit between turns and produce
a current that creates local overheating.
Overheating of bearings due to wear or faulty lubrication.
This fault affects synchronous motors; motor operation is asynchronous and the rotor
undergoes considerable overheating since it is not designed accordingly.
This fault also affects synchronous motors, which may lose synchronism for different
b mechanical: sudden load variation,
b electrical: power supply system fault or eld loss.
Overloads may be monitored the following:
b IDMT overcurrent protection (ANSI 51),
b thermal overload protection (ANSI 49RMS), which involves overheating
due to current,
b RTD temperature monitoring (ANSI 49T).
The same function provides both types of protection (ANSI 48-51LR).
For excessive starting time protection, an instantaneous current threshold is set
below the value of the starting current and activated after a delay that begins when
the motor is energized; the delay is set longer than the normal starting time.
Locked rotor protection is activated outside starting periods by current above
a threshold, after a delay.
The successive starts protection function (ANSI 66) is based on the number of starts
within a given interval of time or on the time between starts.
Loss of pump priming
This is detected by a denite time undercurrent protection unit (ANSI 37)
which is reset when the current is nil (when the motor stops).
Additional protection may be provided by the direct measurement of rotation speed
by mechanical detection on the machine shaft.
The underspeed protection function (ANSI 14) detects slow-downs or zero speed
resulting from mechanical overloads or locked rotors.
The overspeed protection function (ANSI 12) detects racing when the motor is
driven by the load, or a loss of synchronization for synchronous motors.
Loss of supply is detected by a directional active power protection unit (ANSI 32P).
This is monitored by a delayed positive sequence undervoltage protection unit
(ANSI 27D).
The voltage threshold and delay are set to allow discrimination with the power
systems short-circuit protection units and to tolerate normal voltage sags such as
those that occur during motor starting. The same protection function may be shared
by several motors in the switchboard.
Protection is provided by the detection of negative sequence current by an IDMT
or denite time protection unit (ANSI 46).
The phase rotation direction is detected by the measurement of negative sequence
overvoltage (ANSI 47).
Motor remanence is detected by a remanent undervoltage protection unit (ANSI 27R)
which enables resupply when the voltage drops below a certain voltage threshold.
They are detected by a delayed overcurrent protection unit (ANSI 50 and 51).
The current threshold is set higher than the starting current and a very short delay
is applied to prevent the protection unit from tripping on transient inrush currents.
When the corresponding breaking device is a contactor, it is associated with fuses
which ensure short-circuit protection.
For large motors, a high impedance or percentage-based differential protection
system (ANSI 87M) is used (g.1).
As an alternative, by appropriate adaptation of the connections on the neutral side
and by the use of 3 summing current transformers, a simple overcurrent protection
unit (ANSI 51) can be used to provide sensitive, stable detection of internal faults (g.2).
The type of protection depends on the neutral earthing arrangement. High sensitivity
is required to limit damage to the magnetic circuit.
If the neutral is solidly earthed or impedance-earthed, a delayed residual overcurrent
protection unit (ANSI 51N/51G) may be used to protect the main windings.
In isolated neutral arrangements, a neutral voltage displacement protection unit
(ANSI 59N) may be used to detect neutral voltage displacement. If the motor feeder
is capacitive (long cable), a directional earth fault protection unit (ANSI 67N) is used.
Fig. 1. Phase-to-phase short-circuit.
Differential protection (ANSI 87M)
DE57301
Rotor frame fault
An insulation monitoring device with AC or DC current injection detects winding
Overheating of bearings
The bearing temperature is measured by RTDs (ANSI 38).
For synchronous motors: refer to the chapter on generators.
Fig. 2. Phase-to-phase short-circuit.
Autodifferential overcurrent protection (ANSI 51)
Setting that enables starting
Delayed current threshold
Counting of number of starts
Mechanical detection of overspeed,
According to motor operating characteristics
(time constant in the range of 10 to 20 minutes)
Depends on the thermal class of the motor
Threshold in the 2.5 In range
Delay: starting time + a few seconds
Threshold: 2.5 In
Delay: 0.5 to 1 second
According to motor manufacturer
Threshold in the range of 70% of drawn current
Threshold 5% of rated speed
Delay of a few seconds
Phase rotation direction
Threshold 5% of Sn
Threshold from 0.75 to 0.80 Un
Delay in the 1 second range
b Denite time
Is1 = 20% In, delay = starting time + a few seconds
Is2 = 40% In, delay 0.5 seconds
b IDMT
Is = 10% In, tripping time at 0.3 In > starting time
Negative sequence voltage threshold at 40% of Un
Threshold < 20 to 25% of Un
Delay in the 0.1 second range
Internal motor faults
Rating that allows consecutive starts
Threshold > 1.2 starting I, delay in the 0.1 second range (DT)
Slope 50%, threshold 5 to 15% of In, no delay
Power system with
10% of maximum earth fault current
Delay in the 0.1 second range (DT)
Threshold = 30% of Vn
Minimum threshold according to sensor
Threshold 30% of Sn
Same as for generator
Specic synchronous motor faults
DE57302
DE57303
Motor-transformer unit: asynchronous
motor/transformer
Example: 1 MW crusher
Asynchronous motor controlled by circuit
Example: 250 kW fan
DE57305
Asynchronous motor controlled by fuse
and contactor
Example: 100 kW pump
Priority synchronous motor
Example: 2 MW compressor
Generator operation can be altered by both
faults within the machine and disturbances
occurring in the power system to which
A generator protection system therefore
has a dual objective: to protect the machine
and protect the power system.
The generators referred to here are
synchronous machines (AC generators).
Faults such as overloads, unbalance and internal phase-to-phase faults are the same
type for generators and motors.
Only faults specically related to generators are described below.
External phase-to-phase short-circuits
When a short circuit occurs in a power system close to a generator, the fault current
looks like the current shown in gure 1.
The maximum short-circuit current should be calculated taking into account
the machines substransient impedance Xd.
The short-circuit current detected by a protection unit with a very short time delay
(about 100 ms) should be calculated taking into account the machines transient
impedance Xd.
The short-circuit current in steady state conditions should be calculated taking into
account the synchronous impedance X.
It is low, generally less than the generators rated current.
Voltage regulators can often keep it higher than the rated current (2 or 3 times higher)
DE55306EN
Internal phase-to-frame faults
This is the same type of fault as for motors and the effects depend on the neutral
earthing arrangement used. There is a difference however in comparison to motors
in that generators can be decoupled from the power system during start-up and
shutdown and also in test or stand-by mode. The neutral earthing arrangement
may differ according to whether the generator is connected or disconnected
and the protection functions should be suitable for both cases.
When a generator coupled with a power system loses its eld, it becomes
desynchronized with respect to the power system. It then operates asynchronously,
at a slight overspeed, and it draws reactive power.
This causes stator overheating since the reactive current may be high and rotor
overheating since the rotor is not sized for the induced currents.
Fig. 1. Short circuit currents across generator terminals.
The loss of generator synchronization occurs when balanced steady state operation
is disrupted by strong disturbances: for example, when a short-circuit in the power
system causes a drop in the electrical power supplied by the generator and
the generator accelerates, still driven by the prime mover.
Operation as a motor
When a generator is driven like a motor by the power system (to which it is connected),
it applies mechanical energy to the shaft and this can cause wear and damage
to the prime mover.
Voltage and frequency variations under steady state conditions are due to regulator
malfunctions and cause the following problems:
b frequencies that are too high cause motor overheating,
b frequencies that are too low cause motor power loss,
b frequency variations cause motor speed variations, that may cause mechanical
damage and malfunctioning of electronic devices,
b voltage that is too high puts stress on the insulation of all parts of the power system,
causes magnetic circuit overheating and damages sensitive loads,
b voltages that are too low cause torque loss and an increase in current and motor
b voltage uctuations cause motor torque variations resulting in icker (ickering of
light sources).
Normal generator management may be disturbed:
b inadvertent energization when the normal starting sequence is not complied with:
the generator, shut down but coupled to the power system, runs like a motor and may
damage the prime mover,
b power management: when there are several parallel sources, the number of
sources must be adapted to suit the power drawn by the loads; there is also the case
of islanded operation of an installation with its own power generation.
The overload protection functions for generators are the same as those for motors:
b IDMT overcurrent (ANSI 51),
b thermal overload (ANSI 49RMS),
Protection is ensured, the same as for motors, by IDMT or denite time negative
sequence current detection (ANSI 46).
(in the power system)
DE55307EN
0.2 Is
0.3 Un
Fig. 1. Voltage restrained overcurrent protection threshold.
b As the value of short-circuit current decreases over time to approximately the rated
current, if not lower, in steady state conditions, simple current detection may be
insufcient.
This type of fault can be detected effectively by a voltage-restrained overcurrent
protection device (ANSI 51V), the threshold of which increases with the voltage (g.1).
Operation is delayed.
b When the machine is equipped with a system that maintains the short-circuit at
about 3 In, the use of a phase overcurrent protection unit (ANSI 51) is recommended.
b Another solution consists of using a delayed underimpedance protection unit (ANSI
21G), which may also provide back-up (ANSI 21B) for the overcurrent protection unit.
DE57308
Internal phase-to-phase short-circuits (in the stator)
b High impedance or percentage-based differential protection (ANSI 87G) provides
a sensitive, quick solution.
b If the generator is operating in parallel with another source, a directional phase
overcurrent protection unit (ANSI 67) can detect internal faults.
b In certain cases, particularly for generators with low power ratings compared to the
power system to which they are connected , internal phase-to-phase short-circuit
protection may be provided as follows (g. 2):
v instantaneous overcurrent protection (A), validated when the generator circuit
breaker is open, with current sensors on the neutral point side, set lower than
the rated current,
v instantaneous overcurrent protection (B), with current sensors on the circuit breaker
side, set higher than the generator short-circuit current.
Fig. 2. AC generator coupled with other sources.
b If the neutral is earthed at the generator neutral point, earth fault protection
(ANSI 51G) or restricted earth fault protection (ANSI 64REF) is used.
b If the neutral is earthed within the power system rather than at the generator neutral
point, a stator frame fault is detected by:
v an earth fault protection unit on the generator circuit breaker when the generator is
coupled to the power system,
v by an insulation monitoring device for isolated neutral arrangements when
the generator is decoupled from the power system.
b If the neutral is impedant at the generator neutral point, 100% stator frame fault
protection (ANSI 64G) is used. This protection combines two functions:
v neutral voltage displacement, which protects 80% of the windings (ANSI 59N)
v third harmonic (H3) neutral point undervoltage, which protects the 20%
of the windings on the neutral side (ANSI 27TN).
b If the neutral is isolated, frame fault protection is provided by an insulation
monitoring device. This device operates either by detecting residual voltage
(ANSI 59N) or by injecting DC current between the neutral and earth. If this device
exists on the power system, it monitors the generator when it is coupled; a special
generator device, validated by the open position of the generator circuit breaker
being in the open position, is needed to monitor insulation when the generator
is uncoupled.
When the excitation current circuit is accessible, frame faults are monitored
by an insulation monitoring device.
Field loss is detected either by a delayed reactive overpower protection unit
(ANSI 32Q) for high power rating systems or by an underimpedance protection unit
(ANSI 40) for islanded power systems with generators, or by direct monitoring
of the excitation circuit if it is accessible (ANSI 40DC).
Protection against the loss of synchronization is provided by a specic pole slip
protection function (ANSI 78PS) ; the pole slip measurement principle is based on
either an estimate of machine instability according to the equal-area criterion,
or by the detection of active power swings (g. 1); an overspeed protection unit
(ANSI 12) may be used as back-up.
DE55310EN
Without loss of synchronization
2 appearance of fault
4 clearing of fault
11 power swings
With loss of synchronization
Fig. 1. Active power ows in a generator following a short-circuit.
This is detected by a relay that detects reverse active power (ANSI 32P) drawn
by the generator.
Voltage variations are monitored by an overvoltage-undervoltage protection unit
(ANSI 59 and 27) and frequency variations by an overfrequency-underfrequency
protection unit (ANSI 81H and 81L).
The protection units are delayed since the phenomena do not require instantaneous
action and because the power system protection units and voltage and speed
controllers must be allowed time to react.
The ux control function (ANSI 24) can detect overuxing.
DE57309
The starting of generators according to a normal sequence is monitored by the
inadvertent energization protection function (ANSI 50/27). This protection involves
the simultaneous use of:
b an instantaneous overcurrent function and an undervoltage protection function,
b the undervoltage protection function is delayed to avoid unwanted 3-phase fault
tripping, and there is another delay to allow generator starting without the presence
of current before coupling.
Fig. 2. Independent operation of an installation with its own
The distribution of active power ows can be managed appropriately by the use of
directional active underpower protection units (ANSI 37P), which provide adequate
control of source and load circuit breaker tripping (example in g. 2).
Prime mover related faults
as a motor
In threshold, IDMT curve
According to the generator operating characteristics:
maximum thermal capacity used 115 to 120%
Depends on the thermal class of the generator
Threshold 5% of Sn (turbine) to 20% of Sn (diesel)
Power supply system faults
maintained at 3 In
Voltage-restrained
Threshold 2 In
Delay for discrimination with downstream protection
Threshold 1.2 In
About 0.3 Zn
Current threshold = 10% of generator In
Voltage threshold = 80% of Un
Inhibit time after voltage sag = 5 seconds
Minimum current appearance time after voltage appearance = 250 ms
Generator internal faults and generator control
If neutral
is earthed at
100% stator frame fault
is impedant at
is earthed within
Earth fault on generator
circuit breaker side
64G/59N
64G/27TN
Threshold 5 to 15% of In
Slope 50%, threshold 5 to 15% of In
Delay according to discrimination with the other sources
Threshold 15% of In
Threshold = 10% of maximum earth fault current
Threshold 10% of In
Vrsd threshold = 30% of Vn
Delay of 5 seconds
Adaptive threshold = 15% of 3rd harmonic Vrsd
Threshold 10 to 20% of maximum earth fault current
displacement if the
generator is decoupled
Xa = 0.15 Zn, Xb = 1.15 Zn, Xc = 2.35 Zn
Zn circle delay: 0.1 second
Xd circle delay: discrimination with downstream protection
Equal-area criterion: delay of 0.3 seconds
Power-swing criterion: 2 revolutions, 10 seconds between 2 power swings
Threshold 110% of Un
Threshold 80% of Un
Threshold + 2 Hz of rated frequency
Threshold - 2 Hz of rated frequency
According to manufacturers specications
DE57312
DE57311
DE572314
DE57313
Low power generator-transformer
Medium power generator-transformer
Capacitor banks are used to compensate for
reactive energy drawn by power system
loads and occasionally in lters to reduce
harmonic voltage. Their role is to improve
the quality of the power system.
They may be connected in star, delta and
double star arrangements, depending on
the level of voltage and the total rated power
A capacitor comes in the form of a case
with insulating terminals on top. It comprises
individual capacitors (g. 1) which have
limited maximum permissible voltages
(e.g. 2250 V) and are mounted in groups:
b in series to obtain the required voltage
b in parallel to obtain the desired power
There are 2 types of capacitor banks:
b without internal protection,
b with internal protection where a fuse
is added for each individual capacitor.
b frame faults,
b short-circuit of an individual capacitor.
An overload is due to continuous or temporary overcurrent:
b continuous overcurrent due to:
v an increase in the supply voltage,
v the ow of harmonic current due to the presence of non-linear loads such as static
converters (rectiers, variable speed drives), arc furnaces, etc.,
b temporary overcurrent due to energizing of a capacitor bank step.
Overloads result in overheating which has an adverse effect on dielectric withstand
and leads to premature capacitor aging.
A short-circuit is an internal or external fault between live conductors, phase-to-phase
(delta connection of capacitors) or phase-to-neutral (star connection).
The appearance of gas in the gas-tight case of the capacitor creates overpressure
which may lead to the opening of the case and leakage of the dielectric.
A frame fault is an internal fault between a live capacitor component and the frame
made up of the metal case that is earthed for safety purposes.
The fault current amplitude depends on the neutral earthing arrangement and
on the type of connection (star or delta).
Similar to an internal short-circuit, the appearance of gas in the gas-tight case
of the capacitor creates overpressure which may lead to the opening of the case
and leakage of the dielectric.
Short-circuit of an individual capacitor
DE55315
Dielectric breakdown of an individual capacitor results in a short-circuit.
Without internal protection, the parallel-wired individual capacitors are shunted
by the faulty unit:
b capacitor impedance is modied,
b the applied voltage is distributed to one less group in the series,
b each group is subjected to greater stress, which may result in further, cascading
breakdowns, until a full short-circuit.
Figure 2 shows the situation where group 2 is shunted following breakdown
of an individual capacitor.
With internal protection, blowing of the related internal fuse clears the faulty
individual capacitor:
b the capacitor remains fault-free,
b its impedance is modied accordingly.
Figure 3 shows the situation where the individual capacitor in group 2 is cleared
by its internal fuse and group 2 remains in service.
DE57316EN
Fig. 1. Capacitor bank.
Fig. 2. Capacitor bank without internal fuses.
Fig. 3. Capacitor bank with internal
Capacitors should not be energized unless they have been discharged.
Re-energizing must be time-delayed in order to avoid transient overvoltages.
A 10-minute time delay allows for sufcient natural discharging.
Fast discharge inductors may be used to reduce discharging time.
b Extended overcurrents due to increases in the supply voltage can be avoided
by overvoltage protection (ANSI 59) that monitors the power-system voltage.
This protection may cover the capacitor itself or a larger part of the power system.
Given that the capacitor can generally accommodate a voltage of 110% of its rated
voltage for 12 hours a day, this type of protection is not always necessary.
b Extended overcurrents due to the ow of harmonic current are detected by an overload
protection of one the following types:
v thermal overload (ANSI 49RMS),
v time-delayed overcurrent (ANSI 51), provided it takes harmonic frequencies
b The amplitude of short overcurrents due to the energizing of a capacitor bank step
is limited by mounting impulse inductors in series with each step.
Short-circuits are detected by time-delayed overcurrent protection (ANSI 51).
Current and time-delay settings make it possible to operate with the maximum
permissible load current as well as close and switch capacitor bank steps.
This type of protection depends on the neutral earthing arrangement.
If the neutral is earthed, time-delayed earth fault protection (ANSI 51G) is used.
Fault detection is based on the modication of the impedance created:
b by short-circuiting the component for capacitors with no internal protection,
b by clearing the faulty individual capacitor for capacitors with internal fuses.
When the capacitor bank is double star-connected, the unbalance created by
the change in impedance in one of the stars causes current to ow in the connection
between the neutral points. This unbalance is detected by a time-delayed sensitive
overcurrent protection device (ANSI 51).
and examples of applications
Suitable protection functions
Time-delayed earth fault
Threshold 110% Un
Threshold 1.3 In
Time constant in the 10-minute range
Threshold 1.3 In, IDMT curve
Threshold approximately 10 In
Time delay approximately 0.1 s (DT)
Threshold 20% I maximum earth fault
Threshold 10% CT rating is supplied by 3 CTs, with H2 restraint
Threshold approx. 1 A, depending on the application
Time delay approximately 1 s (DT)
DE57320
DE57322
DE57321
Delta compensation
51, 51G
Double-star compensation
Active power in MW
Apparent power in MVA
Compensation coil (Petersen coil)
Denite-time delay
IDMT delay
Impedant neutral
Insulation monitoring device (IMD)
Key words and denitions
The part of the apparent power that can be converted into mechanical or thermal
Average value (that drops to zero) of the upper and lower envelopes of a current
during energization or the initiation of a short-circuit.
Power in MVA drawn by the loads in a power system.
Order sent to an upstream protection device by a device that has detected a fault.
Maximum current that a breaking device is capable of interrupting under prescribed
The power system is earthed via a reactor tuned to the phase-to-earth capacitances.
Neutral earthing reactor tuned to the phase-to-earth capacitances.
Current sensor used to measure the residual current by summing the magnetic
Cosine of the angle between the fundamental components of the current and voltage.
Operation whereby a source or part of a power system is connected to a power
system already in operation when the necessary conditions are fullled.
Device used to obtain a value related to the current.
Discrimination system based on the fact that the closer the fault is located to
the source, the stronger the fault current.
Operation whereby a source or part of a power system is disconnected from a power
Time delay before device tripping that does not depend on the measured current.
Capacity of a set of protection devices to distinguish between conditions
where a given protection device must operate and those where it must not.
Capacity of a power system to return to normal operation following a sudden
Cables arriving from a set of busbars and supplying one or more loads or substations.
Series of sinusoidal signals whose frequencies are multiples of the fundamental
Variable time delay before device tripping that is inversely dependent upon
the measured current.
International standard dealing with the calculation of short-circuit currents
in three-phase power systems.
The power system is earthed via a resistance or a low reactance.
A line supplying energy from a source to the busbars of a substation.
Transient current that occurs when a load is connected to a power system.
For inductive loads, it comprises an aperiodic component.
In an isolated neutral system, device that veries the absence of a fault.
The power-system neutral is not earthed except for high-impedance connections
to protection or measurement devices.
Restoration of supply to loads that have been shed, when normal power system
operating conditions have been re-established.
Disconnection of non-priority loads from the power system when normal power
system operating conditions no longer exist.
Discrimination system in which any protection device detecting a fault sends
a no-trip order (blocking signal) to the upstream protection device.
The upstream protection trips a circuit breaker only if it did not receive a blocking
signal from the downstream device.
Maximum current that a breaking device is capable of making under prescribed
conditions. It is at least equal to the breaking capacity.
Method by which the power system neutral is connected to earth.
Load drawing a current with a waveform that is not identical to that of the voltage.
Current variations are not proportional to the voltage variations.
Overcurrent lasting a long time and affecting one of the elements in the power
Protection-system study
Reactive power in Mvar
System reconguration
Zero-sequence generator
In a directional phase protection function, the phase-to-phase voltage value
in quadrature with the current for cos = 1. In a directional earth-fault protection
function, it is the residual voltage.
Ratio between the active power and the apparent power. For sinusoidal signals,
the power factor is equal to cos .
Set of electrical-power production and consumption centres interconnected
by various types of conductors.
Protection function settings determined by the protection-system study.
Set of devices and their settings used to protect power systems and their components
against the main faults.
Rational selection of all the protection devices for a power system, taking into account
its structure and neutral earthing system.
Protection used for rapid decoupling of a source supplying a power system
in the event of a fault.
The part of the apparent power that supplies the magnetic circuits of electrical
machines or that is generated by capacitors or the stray capacitance of the links.
Automatic device that recloses a circuit breaker that has tripped on a fault.
Sum of the instantaneous line currents in a polyphase power system.
Sum of the instantaneous phase-to-earth voltages in a polyphase power system.
Protection of a three-phase winding with earthed neutral against phase-to-earth faults.
Accidental contact between conductors or between a conductor and earth.
Theoretical power in MVA that a power system can supply. It is calculated
on the basis of the rated power system voltage and the short-circuit current.
The power-system neutral is earthed via a connection with zero impedance.
Operation whereby a power system is disconnected from one source and connected
to another. The sources may or may not be parallel connected.
Period lasting between 0 and 100 ms following the appearance of a fault.
Three independent single-phase systems (positive sequence, negative sequence
and zero sequence) superimposed to describe any real system.
Operation, following an incident, involving switching of circuit breakers and switches
to resupply power system loads.
Intentional delay in the operation of a protection device.
Discrimination system in which protection devices detecting a fault are organized
to operate one after the other. The protection device closest to the source has
the longest time delay.
Ratio of the rms value of the harmonics to that of the fundamental.
Period lasting between 100 ms and 1 second following the appearance of a fault.
Value of the monitored parameter that trips operation of the protection device.
Device used to obtain a value related to the voltage.
Three-phase transformer used to create a neutral point in a power system
for neutral earthing.
b IEC 60050 international electrotechnical vocabulary
b IEC 60044 current transformers
b IEC 60186 voltage transformers
b IEC 60255 electrical relays
b IEC 60909 calculation of short-circuit currents in three-phase AC systems
b IEEE C37.2 standard electrical power system device function numbers
and contact designations
b MV design guide
b Protection of power systems (Published by Herms)
b MV partner
b Cahier technique publications
v N 2 Protection of electrical distribution networks by the logic-selectivity system
v N 18 Analysis of three-phase networks under transient conditions using
v N 62 Neutral earthing in an industrial HV network
v N 113 Protection of machines and industrial HV networks
v N 158 Calculation of short-circuit currents
v N 169 HV industrial network design
v N 174 Protection of industrial and tertiary MV networks
v N 181 Directional protection equipment
v N 189 Switching and protecting MV capacitor banks
v N 192 Protection of MV/LV substation transformers
v N 194 Current transformers: how to specify them
v N 195 Current transformers: specication errors and solutions
b Schneider Electric site: http://www.schneider-electric.com
b Sepam protection-relay site: http://www.sepamrelay.com
b Sepam catalogues
b Les techniques de lingnieur (Engineering techniques)
b Guide de lingnierie lectrique (Electrical engineering handbook) (Lavoisier)
accuracy-limit factor
capacitance of a phase with respect to earth
difference between the operating times of two protection
tolerance of time delays
phase-to-neutral voltage of the equivalent single-phase
initial symmetrical short-circuit current
zero-sequence component of current
positive-sequence component of current
negative-sequence component of current
neutral point coil
winding resistance in a current transformer
neutral-point earthing resistance
phase-to-phase voltage threshold
phase 2 current
phase 3 current
symmetrical short-circuit current interrupted when the rst
pole separates
decreasing aperiodic component of the short-circuit
continuous short-circuit current
continuous phase-to-earth short-circuit current
two-phase short-circuit current
three-phase short-circuit current
current owing in the neutral earthing reactor
rated current of an electrical component
current owing in the solidly earthed neutral-point circuit
rated current of a current transformer
peak value of short-circuit current
primary current in a current transformer
circuit owing in the neutral earthing resistor
current threshold setting
saturation current in a current transformer
the highest short-circuit current
secondary current in a current transformer
maximum permissible current for 1 s
stabilization resistance in a differential circuit
tripping time delay
circuit breaker breaking time
(minimum time before separation of 1st pole)
protection overshoot time
rated phase-to-phase voltage
neutral-point earthing reactance
zero-sequence component of voltage
positive-sequence component of voltage
negative-sequence component of voltage
phase 1 phase-to-neutral voltage
phase 2 phase-to-neutral voltage
phase 3 phase-to-neutral voltage
knee-point voltage
rated phase-to-neutral voltage
phase-to-neutral voltage threshold
zero-sequence impedance
positive-sequence impedance
negative-sequence impedance
apparent rated impedance (transformer, capacitor, motor,
impedance between the neutral point and earth
aperiodic component 18
IEC 60909 17
blocking signal 27, 31, 34, 35, 41, 42
breaking capacity 18
busbars 4, 5, 33
line 18, 33, 44, 45
load shedding 43
LPCT 19, 21
cable 18, 33, 41, 44, 45
capacitor 18, 27, 60, 61, 62
capacitor bank 27
characteristic angle 25
circuit breaker 17, 18, 27, 3643, 45
circuit-breaker failure 43
neutral point 9
Petersen 10
contactor 2, 18, 52, 54
core balance CT 7, 8, 22, 26
coupling 35, 39, 46, 57
residual 10, 22
short-circuit 1219, 28, 30
current sensors 19-22, 33
making capacity 18
asynchronous 14, 50, 54, 55
synchronous 14, 50, 53, 54
decoupling 19, 26, 39
busbars 26
high impedance 33, 58
percentage-based 48, 58
restricted earth fault 26, 47, 48, 56, 58
transformer 26
combined 34, 36
current-based 30, 34, 47
differential 35
logic 34, 35, 36
time-based 28, 29, 31, 34, 35, 38
earthing 611
fault, characterization 12, 18
fuse 18, 47, 50, 52, 60
generator 1417, 33, 5559
harmonics 46, 47, 56, 58, 60
compensated 6, 26, 37, 38
impedant 26, 56, 58
isolated 6, 7, 23
solidly earthed 11, 37, 38
neutral earthing 6-11
neutral point 611, 37, 47, 48, 52, 56
overuxing 47
overload 44, 47, 51, 56, 61
overvoltage 612, 61
active 27, 39, 51, 53, 57, 58
apparent 19, 23
rated output 19
reactive 53, 55, 57, 58
short-circuit 11, 12, 45
architecture 3, 4, 5
loop 4, 5, 32, 35, 40, 41
radial 4, 5, 29, 31, 36
power factor 27
100% generator stator 26
busbars 42, 43
capacitor 6062
circuit breaker failure 26
differential 20, 26, 33, 35, 41, 42, 44, 47, 52, 53, 56
directional active overpower 26
directional active underpower 26, 58
directional reactive overpower 26, 53, 58
directional reactive underpower 26
distance 26, 45
excessive starting time and locked rotor 26, 51
eld loss 26, 50, 52, 53, 55, 57, 58
generator 5559
inadvertent generator energization 26
links 44, 45
motor 5054
negative sequence / unbalance protection 26, 44, 53, 58
negative sequence overvoltage 26
neutral voltage displacement 26, 48, 53, 58
delayed earth fault 11, 26, 44, 61, 62
delayed phase 26, 47, 52, 62
delayed voltage-restrained phase 26, 56
directional earth fault 7, 26, 37, 44, 52, 53
directional phase 26, 56, 58
earth fault 36, 37, 38, 40, 42, 48, 53, 56, 58
instantaneous earth-fault 26
instantaneous phase 26, 47, 48
instantaneous voltage-restrained phase 26
phase 20, 36, 38, 40, 44, 56
overuxing 26, 48, 57
overfrequency 26, 58
overspeed 26, 53, 58
overvoltage 26, 37, 47, 58, 62
phase undercurrent 26, 53
pole slip 26, 50-58
positive sequence undervoltage 26, 51, 53
power system 3641
pressure 26, 47, 48
rate of change of frequency (rocof) 26, 39, 43
recloser 26, 45
remanent undervoltage 26, 51, 53
residual undervoltage (third harmonic) 26, 56, 58
RTD 26, 51, 53, 56, 58
successive starts 26
synchro-check 26, 39
temperature monitoring 26
thermal image 26, 44, 47, 51, 53, 56, 58, 61, 62
thermostat 26
transformer 4649
underfrequency 26, 58
underimpedance 26, 53, 56, 57, 58
underspeed 26, 53, 58
undervoltage 26, 57, 58
vector shift 26
protection coordination 2
protection relays 22, 24, 42
protection settings 14
protection system study 2, 3, 8, 9
of a CT 8, 19, 20, 22, 33, 42
of a transformer 46
phase-to-earth 12, 14, 17
phase-to-phase 12, 14, 17, 44, 47, 52, 56
three-phase 12, 14, 17
two-phase 15, 17
two-phase clear of earth 12
two-phase-to-earth 7, 12, 15, 17
source transfer 39
subtransient 16, 17, 55
switch 2, 18, 40
symmetrical components 13, 14, 15, 17
temperature 27, 47, 51, 52
operation 24, 28
overshoot 24, 28
reset 24, 25
timer hold 25
tripping 24, 25, 31, 34, 53
denite 25
IDMT 25
total harmonic distortion 27
transformation ratio 23
current 19, 21, 27, 33, 35, 52
voltage 19, 23, 27, 32
transformer energization 46
transient 6, 7, 10, 16, 46, 55
tripping threshold 7, 25, 48
zero-sequence generator 8, 37, 38
rate of change of frequency 26, 39, 43
residual voltage 7, 23, 37, 47, 52, 56
H2 (second harmonic) 22, 25, 47, 48, 62
H5 (fth harmonic) 47
voltage 26, 56, 58
ART.065193 2008 Schneider Electric - All rights reserved
89. Boulevard Franklin Roosevelt
Tel.: +33 (0)1 41 29 85 00
As standards, specications and designs change from time to time, please ask for conrmation
http://www.sepamrelay.com
Design: Graphme
Publication: Schneider Electric
Printed: Imprimerie du Pont-de-Claix / JPF - Made in France
CG0021EN
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