POWER LOSS RIDE-THROUGH IN A VARIABLE SPEED DRIVE SYSTEM
Copyright Material PCIC Europe
Paper No. PCIC Europe AM-07

Tino Wymann
ABB MV Drives
Austrasse, 5300 Turgi
Switzerland
Abstract - Voltage dips or power interruptions in the gridcause huge problems for the users. The ride-throughbehavior of frequency converters can be a very goodsolution to bridge the gap. But how does the whole drive
system react in case of power loss? The kinetic energy
recovery is only successful when all the importantelements in the chain fit together. This paper reconsiders
the topic on a power drive system level and gives
practical information to the users, based on theexperience of a drive manufacturer.
Index Terms — Grid disturbance, voltage sag, variable
speed drive control, power-loss ride-through, industrial
power systems, sub-synchronous torsional interaction.
I.

INTRODUCTION

Plant operators depend on reliability of their processcontrols and of the respective drivers. A failure in thesystem can be very expensive. The general practice is tointroduce redundancy for critical parts of the system andchoose safety factors during the design phase. Thisadditional effort makes the whole system robust tocomponent failures. But yet a short voltage dip in the
power supply can bring it down within a second. The
variable speed drives as one of the largest controlled
electrical devices in a plant are of special concern, when it
comes to power loss and how to behave under such a
condition.

Fig. 1 Variable speed drive system and its interfaces
The whole installation can be split into three parts:
feeding line, power drive system (PDS) and driven
equipment, which are all supported by electricallypowered auxiliaries (Fig. 1). Important for the system
integrator are the two interfaces, the electrical and themechanical. The performance at these points is crucial for
the whole design, under regular, but also under
exceptional boundary conditions. Interesting enough,hardly any performance requirements are stipulated
during ride-through at the interfaces themselves. On the
one hand, because the borderline between what canrealistically be claimed and what is technically unfeasible
is hard to tell during planning phase, on the other handbecause it is application dependent, possibly drive-type
dependent and interdisciplinary. Writing performance

Pieder Jörg
ABB MV Drives
Austrasse, 5300 Turgi
Switzerland
requirements around the electric interface touches theareas of power systems engineering, automation andcontrol, as well as power electronics; defining
requirements around the mechanical interface involves
motor control, torsional dynamics and process control
issues.
Nevertheless, project requirements in processindustries normally talk about a minimum power loss
period that has to be “survived” after the network has
dropped under a certain level in RMS voltage. A typicalrequirement would be to stay operational for 4s at lessthan 85% of nominal voltage. Coming from thesemiconductor manufacturing industry, it’s also common
to define the required time as function of undervoltage
level [11]). As will be shown in the paragraph II of thispaper, these requirements leae the supplier of large PDS
in a grey-zone with respect to what “operational” means,since there is no commercially viable energy storage
technology to bridge even outages of few cycles. The
implicit minimum expectation in industry is:••

no trip signal raised
drive reapplies referenced torque without delayThis definition has the advantage of being simple andgenerally valid, at least at first glance. Conditions can be
defined quantitatively or even contractually between thevariable speed drive (VSD) manufacturer and the process
designer of the end-user. It is generally interpreted assuch, that the VSD is providing full torque and following
speed reference above the defined grid RMS voltage level
and that it is producing no torque while riding-through thepower dip. The minimum expectation versus a large PDS
is based on experience with the behavior of large electricmotors, connected directly on the supply line (DOL
motors). As however will be shown in paragraph III, the
DOL motor’s behavior during a power outage is quite
complex, e.g. for small asymmetric undervoltage, it is still
providing torque, but with untypical torque harmonics.
In reality, the criteria under which network disturbancelevel the VSD would physically need to stop following
torque and – if applicable – speed reference is much
more complex. Depending on VSD technology (thyristor
based LCI, voltage source inverter type) there aredifferent electrical limits reached at different stages, inside
the power electronic circuit as well as with respect to theinteraction of the VSD with the power grid. As aconsequence, transition into ride-through (with torque off
and frozen speed controller) could very often be deferred
as compared to the simple definition. This will be
discussed in paragraph IV.
In process industries, the expected behavior of the VSD
during the grid disturbance is hardly ever described.
“Riding through” usually implies zero-torque at the motor
shaft during that time interval. Advanced control strategies
with modern power electronics may offer some torque oreven full torque under grid-disturbance, however typicallytraded against another “evil” like untypical grid or torqueharmonics. Relaxing some criteria and limits duringtransients opens new possibilities. In the following, we will
use these definitions:•••

Zero torque ride-through: no or negligible torqueapplied to load, VSD remains connected to line,with no or negligible current flow (“classic” ridethrough).
Full torque ride-through: full torque applied to load,
but VSD is deliberately operated outside spec fordefined criteria or limits.
Reduced torque ride-through: reduced torqueapplied to load, VSD may deliberately be operated
outside spec for defined criteria or limits.

The engineers designing variable speed wind-power
generation (essentially VSDs with opposite power flow)
have walked that road to some extent, under the gridowner’s expectation to keep generators generating when
the grid needs support most. However in this case, theVSDs must not make things worse in the grid, and as aconsequence a variety of national grid codes has
appeared over the last decade, defining how operation
under a degraded grid has to be, depending on the actualor on the statistically most likely root-cause, and on thenature of the disturbance at the point of common coupling
PCC [5]. We will discuss operation and the potential
consequences under degraded conditions at the electricalinterface (i.e. at the VSD’s PCC) with examples. The
paragraph IV and the summary in Fig. 9, may be seen asa catalogue of things to be checked for full-torque ridethrough.
If during a grid disturbance, partial power is of any use
for the process, compared to zero in classic ride-through,then this could be implemented for certain classes of
network disturbances in form of a reduced-torque ridethrough strategy. This possible reduced-torque ridethrough, very similar to the behavior of a DOL motor, isdiscussed in paragraph V-B.
A further point, which is neglected by the common
definition of the ride-through criteria is the transition fromride-through back to normal operation, especially for zerotorque ride-through. Many drive trains are elastic and fast
torque steps are not allowed from torsional perspective,
hence anything but a slow transition requires at least theknowledge about the mechanical properties of the driven
equipment. Since this information is not always available,a softly ramped transition is implemented by default andprecious time is lost before full power is again released tothe process. Unfortunately, coordination between the VSD
and the overriding controller is very often not studied forsuch a slow transient, leading to oscillations and possibly
tripping of the process during that ramp-up phase.
Paragraph V-C describes possible ways how to improve
performance and reduce time lost during the transition
phase.
Ultimately, the PDS, the process control system withsensors and actuators and the auxiliary systems (cooling,synchronous motor field exciter, etc.) are all supplied byelectricity, usually from the same grid. Whatever options
for full-torque or partial-torque ride-through shall be
exploited, the auxiliary systems need to support that aswell, as will be outlined in paragraph VI.

II.

ENERGY IN THE DRIVE SYSTEM

During a network disturbance the energy flow to theprocess is interrupted or at least limited. There is energy
stored in the installation and it will be consumed by theprocess’ load if the energy from the grid doesn’t comeback. One can spot the energy storage at two places; theVSD and the rotating masses of the process.
It might be helpful to calculate the energy-to-power ratio(E/P). Where E is the total energy and P the process
power. E/P is a time, namely the time during which P canbe consumed until the energy is zero. In reality P changes
with speed so this is not an accurate calculation but anindication.
A VSD stores its energy in the dc-link. This energy canbe considered as very small compared to the power
required by the process. Its only purpose is to decouple
the input and output side of the VSD. A practical example:
For a driven power of 5 MW a motor of 6 MVA is used. In
this case the stored energy in the VSD is around 30 kWs.
The resulting energy-to-power ratio is 6ms.This gives arough feeling of the order of magnitude of the stored
energy in the dc-link.
The rotating masses of the process have a certain
kinetic energy. If no power is provided from the grid therotational speed will slow down. How much depends on
the total inertia of the whole drive system and the load
characteristic of the process. The kinetic energy stored in
the system is equal to
1
(1)
2
where
Total inertia of the rotating masses
Angular velocity of the shaft
During a total power loss the required process energy
will be taken from. E.g.: When during a voltage dip
10% of the energy has been consumed from the rotating
masses, the new speed is √0.9
95%.
Calculating the energy-to-power ratio for a real motor
2(14MW, 1494rpm, 915kgm ) gives, applying (1) and E/P,
800ms. If the process of the end user has a lot of inertia
this can be a figure with the magnitude of seconds.
Compared with the dc-link, which has energy stored in themagnitude of milliseconds, one can see a factor ofthousand between. From the process side the rotating
masses have a big influence on the ride-through behaviorof the whole PDS.
III.

BEHAVIOR OF DOL MOTORSBefore analyzing the behavior of VSDs, the study of thedirect online (DOL) driven motors gives us some basic
ideas of the problematic. The active element in this setup
is only the motor starter breaker. Usually a motor
protection relay is used which opens the breaker underfaulty grid conditions. Also a ride-through is possible
where the breaker stays closed during a voltage dip. Dips
down to zero are frequently called blackout, whereas
partial dips (e.g. to 60% on one phase only) are called
brownout.
A common practice is the automated re-acceleration of
the critical motors in a plant as described in [1] and [2].
The drawback is the high reactive power consumption
which prolongs the dip additionally and avoids the fastrecovery of the voltage (see [3]). Synchronous generators
can support the network with introduction of reactive
power. Intelligent excitation systems will take care of that.
A. Brownout Ride-ThroughAn electrical motor which is connected directly to gridalways follows its frequency. The flux in the motor isrotating with network frequency (or a fraction of it when
having several poles). This speed is followed even during
a brownout. In order to keep the speed of the process, thesame shaft power is needed. This implies that the current
has to increase to a level that the power is the same as
before, as shown in table I.

C. Phase Loss Ride-ThroughExactly the same simulation has been used but only
one phase was lost for 100ms (Fig. 4). During thisasymmetric distortion the torque oscillates at double
network frequency but its average is non-zero (Fig. 5).
This means that there is still energy flowing to the process
and the speed decreases less than before. After clearing
the distortion the torque oscillation changes back to
network frequency.
ia [p.u]

Voltage
Current
Speed
Motor flux
Torqueic [p.u]

8
6

TABLE I
DOL MOTOR VALUES DURING BROWNOUT

Initial State
100 %
100 %
100 %
100 %
100 %

ib [p.u]

10

4
2
0

Brownout
80 %125 % (1/0.8)
100 %
80 %
100 %

- 2
- 4
- 6
- 8
- 10

0

0.2

0.4

0.6

0.8

1

Time [sec]Fig. 4 Motor phase currents for a 100ms phase loss

The increased current leads to a temperature rise in thecomponents. How long and if the drive system cansurvive a certain brownout depends mainly on the motor
design. The protection relay has to trip the breaker if the
motor is operated outside the specified range, at least as
long as lifecycle cost (due to accelerated ageing) is more
important than continuing the process.

tem [p.u]n [p.u]

6

4

2

0

B. Blackout Ride-Through- 2

The influence on the torque and current of a short
power loss (100ms on a 2 MW induction motor) was
simulated. Figure 2 and 3 show the results at theinterfaces. Even if the speed just drops a few percent themechanical and electrical stress can be tremendous.
Depending on the system this is not acceptable and thebreaker has to trip before the voltage comes back.
ia [p.u]ib [p.u]

8
6
4
2
0
- 2
- 4
- 6
- 8
0

0.2

0.4

0.6

0.8

1

Time [sec]Fig. 2 Motor phase currents for a 100ms total power loss
tem [p.u]n [p.u]

6

4

2

0

- 2
0

0.2

0.4

0.2

0.4

0.6

0.8

1

Time [sec]Fig. 5 Torque and speed for a 100ms phase loss
IV.
A.

ELECTRICAL INTERFACE OF A PDS

When to give up on normal operation

ic [p.u]

10

- 10

0

0.6

0.8

Time [sec]Fig. 3 Torque and speed for a 100ms total power loss

1

The control system of the PDS needs quantitative(measurable and programmable) criteria when to give up
on normal operation and to go in a special ride-throughprocedure. Let’s recall two things: hard electricallimitations (overcurrent, overvoltage) have to berespected at any time. Performance limitations (e.g. toprevent increased torque pulsations) have normally to berespected, but might be temporarily relaxed for shorttransients.
The common way of defining when to give up on
normal operation and to initiate a classic zero-torque ridethrough with a VSD is specifying an RMS voltage level,
under which operation can be interrupted with a signal tothe overriding control system, however without tripping
the VSD. This definition makes intuitively sense, when
thinking of full load operation under a symmetrical
undervoltage of the grid: in order to keep full power flow,load current will have to rise inverse proportional to thevoltage dip and at some point, it will reach the design limitof the electronic circuit of the VSD (see Fig. 6). Every
VSD is designed for full operation with certain
undervoltage, compensated by elevated input current.
This designed undervoltage operation has to bedifferentiated against full-torque (Fig. 6, curve 4) or
reduced-torque ride-through (Fig.6, curve 3), where some
“soft” performance factors are neglected, compared tounrestricted performance in designed undervoltage
operation. The trade-off in performancess is illustrated inFig. 6 with torque, resp. current ripple, bu
ut there could beother trade-offs.

Fig. 6 Drive reaction on voltage dip withw
the different
strategies: 1) designed undervoltage op
peration with no
impact on performance, 2) classic zero-torque ridew performancethrough, 3) partial-torque ride-through withcompromise, 4) full-torque ride-through withw performancecompromise
ems less intuitive
Upon a closer look, this definition see
for several reasons:1.) When a light power dip occurs under partial or
low load conditions, it is equally intuitivve to wonder, if
deeper dips could not be survived, as thee distance to thedesign limits for the current must be large
er. This is in fact
the case to some extent, however other limits have to bewatched, often not directly measurable asa the maximum
design current.
2.) The voltage at the point of connect is notindependent of the load of the VSD itself. This becomesV
is going into
most obvious at the moment when the VSD
zero-torque ride-through and voltage is increasing backe the load is off
to acceptable values immediately, since
now. In cases where the VSD comprisess filter banks andcapacitors for reactive power compensation, the voltage
may even rise above nominal level due to local
ult that originallyovercompensation, even though the fau
led to the undervoltage is not cleared yet. For symmetric
undervoltage dips, the maximum converrter input current
would actually be a much better criterionn to determine, if
a zero-torque ride-through should to be innitiated.
3.) In practice, most power dips will be
es are phase toasymmetric, as their typical root cause
neutral shorts (tree falling on overheadd line, insulation
breakdown resulting in earth fault), phasepto phaseshorts (overhead lines touching under strrong side winds)
or inrush events (transformer magnetization, DOL motor
start, etc.). Again intuitively, one would expectethe samedesign restriction on maximu
um current as for symmetric
dips to limit operation. However when looking into thegrid behavior under such a fault, depending on fault
location and transformer coonfigurations between fault
and VSD, other design limiits may be reached earlier
than the maximum design current.
cWhich limits will bereached, depends on the choosen control strategies. Fig.
9 in the following section listss the most important limits.
4.) Another possible mootivation to go into a classic
zero-torque ride-through is not the dip itself but thehe voltage comes back fast,
recovery of the voltage. If thboth the voltage source inve
erter, as well as the current
source inverter topology ma
ay have problems. The DC
capacitor bank of a voltage
e source inverter forms aresonance together with grid- and transformer
impedance, which is excitedd by the returning voltage;
the reactor current of a line commutated current source
e voltage recovery happens
inverter may overshoot if thejust after one thyristor has fired. Then it takes relatively
long before the next commu
utation can take place, andreduce the current again. Thee overshooting of the dc-link
can partly be handled by overdesigning VSDs.
Alternatively special control strategies are necessary to
he voltage recovery, one of
prepare the converter for ththem is to reduce power too gain current-margin on athyristor controlled LCI, i.e. pa
artial-torque ride-through.
5.) Another limitation is linked with the electricmotor: when the input voltag
ge is lower than the terminal
voltage of the motor. For all drive topologies withectifiers at the input (i.e. notcontrolled or un-controlled reactive front-ends AFE), it beccomes challenging to rotate
flux in the motor, as they create the motor stator voltage
e. Thus the flux in the motor
directly out of the line voltage
has to be reduced, which
h for synchronous motors
happens only slowly over th
he rotor’s field exciter or byloading the motor with inductive reactive power with theconverter. Depending on inve
erter topology, the VSD canreduce the stator flux directly and very fast before therotor flux is able to react, thus enabling full torque during
short dips.
Taking the simple relationnship (2) between flux andvoltage (stator resistance ca
an be neglected at nominal
speed) one can see why the fluxf
has to be reduced during
a voltage dip.
Ψ

(2)

where
Ψ

Stator flux vector
v
Stator volta
age vector

n of (2) is given in Fig. 7 and
The graphical interpretationshows that for a circular moveement of the flux, the voltage
is equal to the tangential veloccity.

Fig. 7 The tangential velocityy of the flux is given by theavailable voltage. For undervo
oltage uuv, also flux needs tobe reduced to continue rotatting the rotor (and flux) at agiven motor speed.

The angular velocity is given by the motor speed, and if
motor speed shall be kept constant during a voltage dip;
but if the necessary tangential speed can’t be reachedwith the available voltage, then the flux needs to be
reduced. Reducing flux on the other hand will again
reduce torque and speed might drop anyway, if the torqueproducing part of the current is not increased at the sametime. And increasing current limited by the current that
already flows to reduce the flux.
Understanding these limits and linking them to the grid
topology and to potential grid disturbances is of major
importance when trying to continue operating under low
load condition. It may even be necessary to quantify the
expected limits for each load point a priori, so that output
power could be reduced actively in those cases wherepartial operation is beneficial for a process to mitigate theimpact of a power dip, i.e. when thanks to advanced
process control, advantage can be taken from reduced
power, rather than having to accept a period of zero
power (see section V-B).
B. Full Torque Ride-Through1)
Drawing Parallels with Power Systems
Experience: As mentioned in the introduction, variable
speed wind power generators are required by mostnational grid codes to continue operation also under
severe grid disturbances, very much like every fixed
speed synchronous generator (e.g. hydro generator)
would do. Thanks to the IEC norm 61400-21, which
describes how to measure performance under griddisturbance, the various grid codes have a quite
common way of defining, which grid disturbances haveto be tolerated. These represent at the same time also
the most frequent disturbances. According our
experience, they also correspond to the most likely
disturbances at industrial plants. The focus is on both
symmetrical and asymmetrical voltage drops. The
disturbances are described in terms of depth andduration. The same profiles in depth over time areassumed to happen both on all the three phases as wellas on a single or two phases. The phase angle, at which
the event is starting, is usually not discussed. Under
asymmetric conditions, the phases of the 3-phase
supply-grid will change phase relationship and not be
shifted by 120deg against each other anymore. This is
usually also not discussed, but is self-understood tosome extent, as it depends on the specific installation
with its transformer configurations.
Fig. 8 shows an overview over what representative grid
codes require to be handled. The x-axis shows theduration of the dip and the y-axis indicates thecorresponding worst-case depth. In other words, long dips
down to zero voltage are so unlikely, that they are
excluded. The longer the dip, the less deep it is expected
to be.
The grid-codes do even go further and describe what
the generators, especially the VSD based systems, haveto do during the dip. The focus is clearly on supporting thegrid. In view of the other process loads and the VSD
auxiliaries connected on the same grid, this philosophy
also would make sense for industrial drives, only that
VSDs of voltage source inverter type usually have passive
front-ends (diode rectifiers), which much restricts
possibilities. The classic no-torque ride-through known inprocess industry is also foreseen in power generation,

however needs agreement with the system operator andusuallyislimitedtotimes<1.5s.

Fig. 8 Typical dip depth and duration requirements forwind power generators, above the curve, the generators
have to support the grid with capacitive reactive power!
Restoration of pre-dip power is usually also required
within a given time. The UK grid code e.g. specifies
restoration of active power to 90% of pre-conditions within
500ms for short dips (<140ms), or within 1s for longer
dips after the grid returned to 90%.
It has to be noted, that these requirements come from agrid operator perspective. They are characterized by two
guiding ideas: first, the classes of grid disturbances ofconcern shall be well described and cover the most likely
events; second, the stability of the grid has high priority
and must be supported. Both ideas should also be
followed in process industries, latest when powerelectronic (VSD) loads start to become predominant,compared to other loads. Only describing the classes ofdisturbances gives electric equipment designers arealistic chance to evaluate bottlenecks and to develop
strategies for full-load or partial-load ride-through. And
with all the other electric equipment important for theprocess (auxiliaries, controls), keeping the grid withintolerances must be prioritized in order to prevent
secondary faults, which may also bring down the process.
2)
Challenges and Opportunities for Full TorqueRide-through: Short asymmetric voltage dips of durationand depth as shown in Fig. 8 are the most common griddisturbances also in industrial plants. When only one or
two phases are dipping, then still enough energy can be
drawn from the grid to provide the desired torque withthe electric motor, however the deeper the dip, the more
the system will resemble a single-phase supplied load,
where power flow is pulsing with 100Hz. Despite therelatively small DC-link storage, this is a pulsation thatstill can be smoothened with the DC-link and with therotational inertia of the load. When trying to implement
full torque ride-through strategies for this class of
disturbances, several criteria have to be watched, whichstill may require a zero-torque ride-through at somepoint. Beyond the obvious maximum continuous
designed current of each phase, we foresee at least thecategories and issues listed in Fig. 9, that need to bechecked. Several issues are even site-dependent andwill require at least some routine check-ups before full-torque ride-through can be applied to a specific project.
Others are more general and may be resolved by
overdesigning or adapting the VSD for the specificapplication. The list only includes issues with the VSD
main-circuit and neglects auxiliaries, which will be
discussed in paragraph VI.

Fig. 9 Categories of challenges that may need to beaddressed before riding through with full torque
In order to determine which compromises have to be
expected in performance, the two basic classes of gridside power electronics circuits need to be distinguished:• controlled or uncontrolled rectifier bridge (dioderectifier front-end of voltage source inverters,
thyristor-controlled rectifier of an LCI)
• voltage source inverter with an active front-enda) Limited Control Dynamics: The first challenge
lies in the fact that both voltage as well as phase of theline can rapidly change during asymmetric dips. Thisputs the PDS control designer into a dilemma: thereference tracking behavior for current (resp. torque)under nominal operation, constrained by a cost effective
size of DC-reactor or DC-capacitor, is optimal with timeconstants in the range of a cycle (15-20ms), while therecovery of the grid happens faster than this. Adapting
the controller to follow the grid voltage disturbances
makes the reference tracking “nervous”, i.e. the torque
will tend to overshoot, when operating points arechanged. It would also become more susceptible toother noise coming from grid. For all that noise, theconverter will look like a negative resistance from thegrid side, i.e. current will rise when voltage drops.
Experience with the large LNG plant reported in [9]show, that increasing current controller gains by a factorof 3 (thus including roughly another half decade of
harmonics into the bandwidth of the current controller of
the PDS) increases grid voltage THD by roughly one
percentage point; i.e. harmonics between 5Hz (normal
bandwidth) and 35Hz (increased bandwidth) are also
actively rejected, but the PDS looks like a negative
resistor to the grid.
When tuned normally (i.e. slower), the current controller
will show transient errors at the beginning and at the endof a grid disturbance. Transient control errors resulting
from grid voltage changes usually lead to errors in current
control, and for sure in one of the two transients of thegrid-disturbance in overcurrent. Under high loads, such aovercurrent together with the high fundamental current
may reach trip limits. In addition, for LCIs with their current
controlled rectifiers, torque errors as function of transientcurrent errors may be applied to the driven load. Thesetransients may be cyclic (at least few cycles) and need to
be studied if critical to the load.
Transient control errors resulting from phase changes
also lead to errors in current, but in LCIs also to untypicalharmonics in the current and thus in the load torque.
Single and double grid frequency torque pulsations haveto be expected during transients. The uncontrolled dioderectifier is the most robust solution in terms of transient
errors due to control.
b) Non-Uniform Power Flow: In the extreme caseof an asymmetric voltage sag with one phase dropping
to zero, power cannot be taken continuously from thatgrid anymore, but will pulse at double grid frequency.
Also in less severe dips, the topology of the powerelectronics and the drive transformers may prevent
uniform, continuous power flow. Power flow will besmoothened out of the intermediate storage (capacitor
bank or DC-reactor), as long as the load is continuing tobe driven with uniform, continuous power, which willlead to oscillations in voltage resp. current. The
oscillations may lead to complete depletion anddiscontinuity on one hand, or to overload (e.g.
overvoltage across DC-capacitor) on the other hand.
c) Untypical Harmonics: Network disturbances willcontain a rich spectrum of harmonics. The frequency
converters of VSDs are non-linear loads and especiallythe integer harmonics in the grid voltage will betransformed into other integer harmonics inside theinverter, which again will result in different current
harmonic spectra in the line-side current. During regular
operation, a frequency converter will only have integer
harmonics of the order 6k ±1. This fact is used in design
to place parasitic resonances of electric filters on the
non-occurring even and triplen harmonics. During griddisturbances, any other integer harmonic can occur, andparticularly even harmonics cannot be excluded. Someof them may thus excite parasitic resonances of filters
during grid disturbances and their impact needs to bechecked. Special attention needs to be paid to filter
protection settings, as protection must not trip on griddisturbances ear-marked for full-torque ride-through.
1

2

1

2

th

Fig. 10 The parasitic resonance of a 5 harmonic filter (2)this seen near 4 harmonic from grid (1) and may beexcited during full-torque ride-throughTo illustrate a case, which would be sensitive to evenharmonics, Fig. 10 shows as an example the fifth
harmonic filter of a LCI drive, curve 2 as seen by the VSD
at 265Hz and curve 1 as seen from the bus by another
VSD, with parasitic resonance near 200Hz.
The diode rectifier and the controlled thyristor rectifierare, as a special case of transformation of harmonics intondother harmonics, transforming positive sequence 2
harmonics into DC-components on the converter side ofthe drive transformer. This will after some time-constantlead to the well-known core-saturation of the transformer,
with a detrimental impact on the adjacent loads on thesame bus. As shown in [6] by Stemmler, the controlled
rectifier may be instable for firing angles in inverter modend
between 0..45 degrees, if a positive sequence 2
harmonic occurs over certain time under grid imbalance.
Depending on reactor size, the stable range will be
extended from 45 degrees downwards. As shown in thesame publication, the effective reactor size as function of
the physical reactor size, together with active control“virtual” inductance matters for stability.
Contrary to the reported HVDC transmission
application, drive transformers have smaller saturation
time constants (approximated by Lm/Rm) and may
consequently already saturate during a long full-torqueride-through. On a plant electric system level, powerelectronics controls play an eminent role in two ways:• For the VSD under investigation itself, whether its
control scheme is capable of preventing core
saturation also at small inverter firing angles.
• For other VSDs in the plant, if their complexndadmittance, exhibited to the grid at 2 harmonic is
amplifying positive sequence voltage disturbancend
as function of 2 harmonic current, thus pushingtransformer core saturation.
The important role of the complex admittance, resp.
impedance of controlled active loads is further elaborated
in the next section or in [7,8 and 9].
The untypical harmonics generated by a network
disturbance into the VSD may also appear in the currentson the load side and finally as untypical torque harmonics
in the electric motor air-gap. This is specially the case for
LCI-type VSDs. The most likely harmonics to occur under
network disturbance and full-torque ride-through however
are the 1X and 2X torque pulsation, which should notpose problems to the driven load, as these frequencies
may occur for mechanical reasons as well and should be
accounted for, at least to some extent.
d) Grid-Unfriendly Load Behavior: The VSDs,
especially the ones with controlled front-ends (VSI withactive front-end and LCI with controlled thyristor rectifier)have to be understood as complex, time-variant
impedances or admittances on the grid. Other
industries, such as the electric railway traction, put very
conservative criteria on this complex admittance. They
typically require passivity for the VSD admittance over
the whole frequency domain. Such criteria have to be
fulfilled also during grid disturbances.
The VSD control strategy for full-torque ride-throughcould also pose a problem to the grid and its recovery
from the disturbance. Simple strategies tend to load
phases with undervoltage more than healthy phases,
which may lead to a knock-on effect on the grid and on

auxiliaries. Having a look at grid-codes for wind power
generators again, grid-operators have started to requirecapacitive reactive power support to the unhealthyphases.
V.
A.

MECHANICAL INTERFACE OF A PDS

General Behavior

The VSD can provide regular operation for dips ofabout 20-30% depth (remaining 70-80% of voltage) then it
switches usually to zero-torque ride-through. The torque
will be reduced to zero or even a small negative values inorder to support the dc-link with energy (for voltage
source inverters). This prevents large inrush currents andvoltage overshoots when the voltage comes back andalso keeps the induction motors magnetized. The
technique is also known as kinetic recovery and was
discussed here [4]. The advantage is that the VSD
doesn’t have to trip and is able to re-accelerate the motor
immediately after the distortions is cleared. The crucial
point is that the rotational mass does not drop under themechanical system’s minimum speed or even come to astop during the dip. This criterion is process depended
and cannot be generalized.
B. Reduced Torque Ride-ThroughDepending on the nature of the power dips, different
supervision rules could be applied in parallel, each ofwhich can initiate a classic ride-through reaction. The
items to supervise correspond to the issues identified in
the previous paragraph. For cases, where the process
could profit from getting partial power, a reduced torqueride-through could be initiated in a similar fashion. The
rules however would have to be carefully selected and will
largely depend on the application and the plant.
The integration of partial-torque ride-through into aplant-wide control design needs to be thought through.
Obviously, a compressor will approach the surge-line
slower as compared to a classical zero-torque ridethrough and so will buy time for the compressor
performance control and the process control to adjust
valve positions. Wherever the outer loop may end-up
being faster than the speed controller of the VSD underpartial-torque ride-through, the cascade of controls mayneed to be extended with dynamic resets (dynamic antiwindup).
Fig. 11 shows the full decision tree when considering all
three options for riding through grid disturbances in a
concrete project. The decision tree is guiding the work
during project engineering, but needs to be implemented
in the PDS controller for later operation as well. The topics
in the rectangular blocks are defining the studies to bedone during project engineering: the disturbance types
need to be addressed, an impact study for trying to run
full torque under those disturbances needs to be carriedout, then VSD internal “ultimate limits”, i.e. effects wherezero-torque ride-through becomes mandatory, need to be
defined and last but not least, the dependency of thosecriteria (“ultimate limits”) on load needs to be studied.

The torque control loop should ideally be put to theactual load torque. Load might have changed over thetime the grid was disturbed and needs to be identified.
Special attention needs to be given to the build-up of
torque at this stage. The torque increase could be madevery sharp from electric perspective, but then might excite
torsional resonances. Torque can be reapplied
immediately, if the disturbance had lasted less than ¼
period of the dominant torsional natural frequency
between motor and load. For longer disturbances, load
torque has to be ramped back with slow ramps or withsteeper ramps, where the reference was filtered at thecritical frequencies. If the controls are tuned fast for strong
disturbance rejection, then there is also the risk that thosefast controllers un-damp the lower torsional modes,
because the torsional resonance interacts with the control
circuit, which may even result in continued torsional
excitation, as reported in [10].
The speed controller needs to be re-initialized at actual
speed, which may have dropped over the time of the griddisturbance. It must only be released from anti-windup by
the time the inner loop is again following the reference.
The same is true for any overriding integrating controller.
VI.

Fig. 11 Design tree for utilization of all advanced ridethrough features. The quadratic sub-process blocks
indicated preparatory studies to be carried out.
Already the classic zero-torque ride-through could be
improved by going from “cause” based initialization criteria
to “effect” based initialization criteria. An example could
be to wait for an over-current rather than to go into zerotorque based on grid-voltage levels, which don’t pose a
problem to operations per se yet. Criteria for full-torqueride-through are defined in an impact study, which
essentially checks the impact of dropping performance in
certain points, in order to continue full torque under
disturbance. The list of criteria gives the ultimate pain
threshold. For those thresholds, load dependency has tobe evaluated in a second study, in order to know thederating levels to allow partial load ride-through.
C. Transition back to normal operation
At some point, the grid disturbance will disappear and
the PDS has to recover from ride-through and go back tonormal operation. The topic is hardly ever addressed in
specifications.
There are two control loops that have to return working
normally: The inner torque control loop and the outer
speed control loop (if used). Both loops should be reinitialized to their present state of operation.
After full-torque ride-through, there will be no transition
back to normal, as both loops have kept on working. Bothafter reduced-torque as well as after zero-torque ridethrough, initialization is necessary.

AUXILIARY AND CONTROL POWER

During the different grid disturbances that could
happen, also the auxiliary and control power is affected
when not fed from a different source. This has a nonnegligible effect on the ride-through behavior of the whole
drive chain. The auxiliary voltage powers small 3-phaseinduction motors for cooling. The effect of different failures
is analog to those on the DOL motors in section III. During
a brownout, the coolant flow is still given as long thefrequency stays stable. But the auxiliary motors will
overheat and their protection relays may trip at a certain
point.
In contrast, a total auxiliary power loss implies animmediate stop of the coolant flow (usually water or air).
The heat cannot be transported any longer and the heatsinks of the semiconductors keep heating up. As thecoolant temperature at the power-part outlet is no longer
meaningful for the self-protection, the time without
auxiliary power is limited to some seconds by the drive
software. The used delay before the trip depends on thethermal capacity of the heat sinks and on the actual load.
If the main power fails together with the auxiliary power ofcourse the classic zero-torque ride-through time can bequite large due to reduced losses. A redundant line from adifferent sub-station or bus bar can be used to increase
reliability of the cooling systems. Especially in the casewhere auxiliary power distortions are more common thanmain power distortions.
The control power is not allowed to fail at any time. The
control boards need the energy in order to control the
VSD and also provide the protection. That’s why thecontrol line should always be backed-up by an external(from the plant) or an internal UPS.
The cooling of transformer and motor is for shortinterruptions of several seconds less critical than the oneof the VSD. The thermal time constant could be minutes
whereas the VSD has seconds. A brownout instead could
also be critical to those components because the coolingfans might overheat.
Also to consider are the switchgears and main circuit
breakers. They should not open during a network
distortion. This might be due to control power loss or the

protection might trigger. If the PDS shall survive griddisturbances, the whole factory protection and auxiliary
supply concept has to be designed accordingly.
For synchronous motors also an excitation unit isneeded, which is fed from the auxiliary power or with anadditional transformer from the main power. As theexcitation unit is an active device it reacts to network
distortions and needs to do a ride-through itself ifnecessary. During an excitation unit ride-through nocurrent will be driven in the rotor windings and the flux
decreases with rotor time constant. With reduced flux
limited torque can be produced in the motor. The rotor
time constant limits the ride-through time which itself
depends on the motor size.
VII.

CONCLUSION

Short voltage dips down to about 70-80% remaining
voltage can usually be handled without losing
performance. For deeper voltage sags, it may still bepossible to provide full power, but only by compromisingon “soft” performance criteria, e.g. at the cost of increased
harmonic pollution to the network. This temporary
operation with less performance, but providing full torqueto the load, is defined as “full torque ride-through”. A
special design of the PDS might be required to handle
overshooting when the voltage comes back and in case ofsymmetric dips provide the additional current that isneeded in order to compensate for the voltage (1/0.7 =
143% current). Dips below this threshold will be handled
as zero or reduced torque ride-through. State of the art isthe zero torque ride-through. The drive tries to survive thedistortion without tripping and continues the operation
after the voltage comes back.
Reduced-torque ride-through is very complex andrequires an interdisciplinary approach. But there we see
also a major opportunity for improvement in the future.
Especially with compressors with their small rotational
inertia, which are quickly running towards surge, alreadyfor short dips of some 200ms, a reduced torque ridethrough could help to stay away from surge longer and to
gain time for valves to adjust load. In order to keep on
operating the PDS with partial torque, the exact limitations
have to be determined in studies, e.g. in simulation, andthese limits need to be defined as function of load.
Crucial is also the interaction between process controls
and the PDS controls. When the PDS runs into a torquelimit (e.g. due to an undervoltage) it has to communicatethat in a proper way to the process control. The processcontrol can react and prevent integrator wind-up. In a
scenario with partial-torque, advanced anti-windup
schemes might be necessary such as dynamic reset.
These schemes will have to ensure, that the overriding
controller doesn’t wind-up or overshoot, when the PDS
controllers are working at reduced performance.
Special attention needs also to be given to auxiliaries
that need to be supplied during grid disturbances as well.
Mostly UPS will be used to buffer the control circuits,
whereas cooling circuits are designed in a robust way forthe majority of grid disturbances.
VIII.
[1]

REFERENCES

Rekha T. Jagaduri et al., “Field Evaluation ofAutomatic Restart of Essential Motors Using
Microprocessor-Based Protective Relays”, IEEE
PCIC Conference Record, PCIC-2011-82[2]

Robert A. Hanne et al., “Minimizing Refinery Upset
During Power Interruption Using PLC Control”, IEEE
PCIC Conference Record, PCIC-92-15[3] Math H. J. Bollen, ”The Influence of Motor
Reacceleration on Voltage Sags”, IEEE Transaction
on Industry Applications, Vol. 3, No. 4, Jul/Aug 1995
[4] Kai Pietiläinen, Voltage Sag Ride-Through of AC
Drives, Stockholm, KTH, Royal Institute ofTechnology, 2005
[5] T. Bublat, T. Gehlhaar, “Comparison of hightechnical demands on grid connected wind turbines
defined in international Grid Codes”, EWEC
European Wind Energy Conference & Exhibition,
2008, Brussels[6] H. Stemmler, “HVDC back-to-back interties on weakAC systems second harmonic problems, analysis
and solutions”, CIGRE Symposium, 1987, Boston[7] L. Harnefors, L. Zhang, and M. Bongiorno,“Frequency-domain
passivity-based
current
controller design,” IET Power Electron., vol. 1,no. 4,
pp. 455–465, Dec. 2008
[8] L. Harnefors, M. Bongiorno, and S. Lundberg,“Input-admittance calculation and shaping forcontrolled voltage-source converters,” IEEE Trans.
Ind. Electron., vol. 54, no. 6, pp. 3323–3334, Dec.
2007
[9] P. Jörg, A. Tresch, M. Bruha, “A model based
approach to optimize controls of a large LCI VSD for
minimal grid-side sub-synchronous torsional
interaction”, IEEE PCIC-Europe 2013, Istanbul
Turkey[10] P. Jörg, A. Lenzi, V. Depau, „Optimization ofTransient Behavior of Complex Turbocompressor
Shaft Lines”, in IEEE IAS Annual Meeting Record,
2011, Orlando[11] PLS Standard SEMI F47: Required voltage sag
tolerance for semiconductor fab equipment, PowerStandards Lab, Alameda CA, U.S.A.
IX.

VITA

Tino Wymann graduated from the Swiss Federal
Institute of Technology Lausanne in 2009 with a
M.Sc. in Power Conversion and Systems. The same
year he joined the R&D department of ABB Medium
Voltage Drives. Since 2011 he’s working in thesystem design team which supports sales with thetechnical aspects of variable speed drive solutions indifferent industries and applications.
tino.wymann@ch.abb.com
Pieder Jörg received his M.Sc. degree 1995 fromthe Swiss Federal Institute of Technology, Zurich. He
joined ABB at Corporate Research in the area of
power electronics. In 2002 he joined the business
unit Medium Voltage Drives as head of productdevelopment. Since 2010 he is focusing on businessand technology development for demanding drives
applications. He has been involved in various studies
and improvement projects involving large VSD
systemswithdemandingrotor
dynamics.
pieder.joerg@ch.abb.com