Distance relay using a polarizing voltage

A distance relay for protection of power transmission lines, having a plurality of mho elements, each with a predetermined mho impedance characteristic. The relay has the capability of calculating a polarized reference voltage which remains constant through fluctuations in measured transmission line voltage. The difference voltage between the measured line voltage and the product of the measured line current and the mho element characteristic impedance is then compared against the polarized reference voltage in a product-type phase comparator to determine under-impedance conditions. If an under-impedance condition is present, the mho element produces a set output and a calculated "torque" value. The set outputs, if any, are then compared against a look-up table for fault type determination. If more than one set output is present, the magnitudes of the respective torque values are compared for fault type determination.

TECHNICAL FIELD 
This invention relates generally to relay apparatus for the protection of 
power transmission lines and more particularly concerns such a relay 
apparatus for determining faults in a transmission line based on the 
detection of under-impedance (less than normal) conditions along the line. 
BACKGROUND OF THE INVENTION 
Among a wide variety of protective relay devices used with power 
transmission lines, distance relays respond to faults which cause a less 
than normal impedance condition on the line, referred to as an 
under-impedance condition. A distance relay capable of accomplishing such 
protection includes one or more mho elements, each mho element having a 
circular impedance plane characteristic, such as shown in FIG. 1. FIG. 1 
shows resistance R along one axis and reactance Z along the other axis. 
Z.sub.R at angle .theta. is the impedance setting of the particular mho 
element and defines the boundary or "reach" of the circular impedance 
plane characteristic shown in FIG. 1. 
In FIG. 2, which is in the voltage plane, a voltage Z.sub.R.I, which is the 
product of the measured value of current on the transmission line at the 
relay and the relay impedance Z.sub.R, is shown, as is the measured value 
of voltage V on the transmission line at the relay. I and V are the 
instantaneous values of current and voltage on the transmission line at 
the relay, taken at the same time. The difference voltage dV between the 
measured voltage V and the calculated voltage Z.sub.R.I is also shown in 
FIG. 2. The three specific phasor quantities V, Z.sub.R.I and dV form a 
triangle, as shown. Angle "a", which is the angle between the voltage 
phasors V and dV at point P on the mho circle is 90.degree.. A distance 
relay is designed to discriminate between transmission line impedances 
which are larger or smaller than its characteristic impedance Z.sub.R, on 
the basis that a smaller impedance will result in a mho circle of smaller 
diameter, which results in an angle greater than 90.degree. between the V 
and dV phasors, while a larger impedance will cause the angle between the 
V and dV phasors to be less than 90.degree.. As shown in FIG. 3, this 
discrimination is accomplished by a 90.degree. phase comparator 10, which 
compares the measured instantaneous voltage V from the transmission line 
on input line 12 with the calculated value dV on input line 14. The output 
of the phase comparator 10 on line 16 provides an indication of whether 
the impedance condition of the transmission line is normal or less than 
normal, which is indicative of a possible transmission line fault. A great 
enough under-impedance condition will result in the relay producing a 
control signal to trip a circuit breaker for the particular transmission 
line in question, thus protecting the line. 
Different types of phase comparators have been used in the manner described 
above. These include induction cylinder units, Hall Effect devices, and 
thermal and solid state networks. More recently, a system involving 
coincidence timing of sinusoidal inputs, involving the phase relationship 
between the input signals, has been used. Also, computers have been used 
to provide calculated simulations of mechanical and electro-mechanical 
devices. For instance, for an induction cylinder type of phase comparator, 
the difference between the input signals produces a mechanical torque 
which increases in magnitude in accordance with such difference, resulting 
in a contact closing when a predetermined under-impedance condition exists 
on the transmission line. In a computer, the mechanical action of the 
cylinder unit can be simulated through the multiplication of dV and a 
polarizing voltage. The resulting product can still be referred to 
conveniently as a "torque." The sign of the product indicates an under or 
over-impedance, i.e. whether the impedance is inside or outside the mho 
circle characteristic. The actual magnitudes of the torque values, 
however, were heretofore not considered to be useful. 
In the protection of a typical transmission line which carries three phase 
AC power, a distance relay includes a plurality of individual mho 
elements, each having a specified impedance characteristic, to cover the 
various distance line fault possibilities. For instance, with three power 
phases A, B and C, a total of six mho elements will cover three 
phase-to-ground faults, i.e. A-G, B-G and C-G, as well as three 
phase-to-phase faults, i.e. A-B, B-C and C-A. 
Although the above-described systems for determining distance faults, 
including those utilizing computer calculations instead of 
electrical-mechanical devices, provide to a significant extent accurate 
fault information, there are a number of fault situations which may not be 
accurately detected by the abovedescribed systems. For instance, when 
there is a fault close to the relay itself, the measured voltage V may 
approach zero, leading to inaccuracies in the resulting calculations. This 
is true in particular when the mho elements are self-polarized, i.e. the 
same voltage which is used as a component in determining the difference 
voltage dV is also used as the polarizing, i.e reference, voltage V for 
phase comparison. 
To overcome this problem, polarizing voltages can be used which are based 
on memory, or which are based on a phase or phases free from faults FIG. 4 
shows a mho element voltage plane characteristic using a polarizing 
voltage VP. The polarizing voltage increases the accuracy and 
fault-determining capacity of distance relays. However, such relays are 
still characterized by other significant problems. One problem concerns 
the response of more than one mho element to a particular fault, with 
resulting confusion as to the fault type. Providing reliable fault-type 
identification in single-pole tripping systems, for targeting and other 
applications, is also a significant issue in many systems. Attempting to 
satisfy all of the criteria leads to compromises in relay sensitivity, as 
well as increased cost when additional external logic must be utilized. 
The present invention uses a particular form of polarizing voltage and a 
system for analyzing the calculated torque measurements from each mho 
element to provide a significant improvement in reliable fault 
information, including fault-type determination. 
DISCLOSURE OF THE INVENTION 
Accordingly, the present invention is a relay apparatus for protection of 
power transmission lines. The relay apparatus includes at least one relay 
element, such as a mho element, having a selected impedance 
characteristic. The voltage and current values on a given section of 
transmission line are measured and used to produce a polarized reference 
voltage. The difference voltage between the measured value of line voltage 
and the product of the measured value of line current and the impedance 
characteristic of the relay element is then determined. The difference 
voltage and the polarized reference voltage are then compared to produce 
an output which includes both a signal indication of a possible fault on 
the transmission line and a magnitude value associated therewith.

BEST MODE FOR CARRYING OUT THE INVENTION 
In a power distribution system involving a plurality of individual 
transmission lines, there will typically be protective equipment, in the 
form of a plurality of protective relays, at both ends of each section of 
transmission line. This is shown in FIG. 5, which includes one section of 
transmission line 28 between two distribution points, i.e. substations, 30 
and 32. The protective equipment shown generally at 34 in the vicinity of 
distribution point 30 looks forward along the line 28 toward distribution 
point 32 and beyond, while protective equipment 36 in the vicinity of 
distribution point 32 will look back along the line 28 toward distribution 
point 30 and beyond. Typically, there will be protective equipment or 
elements, along with associated timers, designed to cover three zones, 
i.e. zone 38 (the first zone), zone 40 (the second zone) and zone 42 (the 
third zone) for forward-looking equipment 34, to provide in-depth and 
reliable coverage for the section of transmission line 28 between 
substation points 30 and 32. There are also three zones of coverage 39, 41 
and 43 looking back along the line 28 from protective equipment 36. This 
multiple zone approach has a number of advantages, including the ability 
to avoid tripping the breaker for line 28 when the fault exists on a 
section of the transmission line downstream from point 32 or upstream from 
point 30. 
The array of protective equipment, i.e. 34, includes, among others, 
elements known as distance relays, discussed above in some detail, which 
look for under-impedance conditions on the line. In a three phase system, 
a distance relay at 34 will include six mho elements, each with its own 
specific impedance characteristic, such as shown in FIG. 1. A separate mho 
element covers each of the following fault types on a typical three-phase 
transmission line: the A phase line-to-ground, the B phase line-to-ground, 
the C phase line-to-ground, the A phase line to the B phase line, the B 
phase line to the C phase line, and the C phase line to the A phase line. 
Three phase faults and double line-to-ground faults also are covered by 
one or more of the above-described elements. 
In the present invention, a positive sequence memory voltage is used to 
develop the polarizing voltage for the mho element, from no-fault voltage 
measurements of VA, VB and VC. Positive sequence voltage is well-known in 
the art and refers to the combined phasor line voltages VA, VB and VC, 
rotating in a positive sequence. The resulting polarizing voltage, 
referred to as VP, is a memory voltage, because it is calculated from 
no-fault values of VA, VB and VC and remains constant and thus will not 
vary when there is a line fault which would otherwise produce a change in 
one of the phasor voltages. 
FIG. 6 shows the circuit in the distance relay for calculating the 
polarizing voltage. The three phase voltages VA, VB and VC are applied on 
lines 42, 44 and 46 to circuitry 40 which samples the signals at 
established intervals, and filters the sampled signals through a positive 
sequence filter to produce VA1 in accordance with the following equation: 
VA1=1/3 [VA+(a-1)VB+(a.sup.2 -1)VC], where "a" has a magnitude of one and 
an angle of 120.degree.. The result of this is that the VB and the VC 
voltages are shifted by .+-.60.degree. and inverted, which provides a good 
transient response. The output of the positive sequence filter, referred 
to a VA1, appears on line 48. 
The next step in obtaining the polarization voltage VP is to apply VA1 on 
line 48 to a memory filter 50 which operates according to the following 
equation: VA1M=1/16 (VA1.sub.k)-15/16 (VA1M.sub.k-2), where VA1M is the 
output on line 52 of the memory filter, and the index k is counted in one 
quarter cycle steps. The memory filter 50 in effect provides an output 
from a point earlier in time by one half cycle or 180.degree., inverts it 
and then scales it by a factor of 15/16. The inversion removes the 
180.degree. phase shift introduced by the half cycle delay. The filter 
then adds 1/16 of the most recent measurement of the positive sequence 
voltage. The filter output (VA1M) has a time constant of approximately 
four cycles and provides a polarization signal for a period covering 
twenty cycles. This time coverage provides good results, as documented in 
oscillogram tests. 
The output of the memory filter, VA1M, on line 52, is used directly as a 
polarizing voltage VP for VA phase lines, and shifted in phase by 
.+-.60.degree. and inverted by elements 53 and 55 to produce polarizing 
voltage VB1M on line 54 and VC1M on line 56. The VB1M and VC1M polarizing 
voltages are used by the VB and VC mho elements, respectively. The 
calculation of the polarizing voltage VP occurs four times per power 
cycle, i.e. every 90.degree., in the embodiment shown. 
The voltage and current on the transmission line are applied to each mho 
element at a sampling rate of approximately four times per cycle in the 
embodiment shown. The values of dV are calculated and then a phase 
comparison is made between dV and VP for each mho element by forming the 
product dV.multidot.VP. If an underimpedance condition is determined, 
which is indicated by the sign of the product, an output signal is 
produced, referred to hereinafter for explanation purposes as a set 
output, and illustrated as a "one" in FIG. 8. The magnitude of the product 
is also significant, as discussed below, and referred to hereinafter as a 
torque magnitude. Thus, each mho element produces, at specified intervals, 
a set output (or not) based on the sign of the product dV.multidot.VP and 
a torque magnitude output. 
FIG. 7 is a block diagram showing the structure of the present invention 
which identifies particular line faults based on the set and torque 
magnitude outputs provided by the individual mho elements. Block 60 shows 
the six mho elements as a group, with each of the mho elements, as 
explained above, capable of producing a set output (or not) indicating an 
under-impedance condition determined by that mho element, as well as the 
magnitude of the "torque" for each mho element, i.e. the magnitude of the 
product dV.multidot.VP. Normally in the embodiment shown, the magnitude of 
any torque from a mho element indicating an under-impedance condition will 
be positive, but it would also be possible to utilize negative torques as 
well. The outputs of block 60 for the mho elements are shown collectively 
for both the set (binary one) outputs and the torque (magnitude) outputs, 
although it should be understood that in each case separate lines for each 
mho element are provided. The set-not set output information on line 62 is 
applied to a software look-up table processor 66 and processed relative to 
one or more of the individual mho elements having a set output. 
One embodiment of such a table is shown in FIG. 8. On the left hand side of 
the table are listed the six mho elements covering zone three in the 
embodiment shown (see FIG. 5), with an indication of whether or not they 
produce a set-not set (one-zero) output. On the other side of the table is 
the action which is to result for the mho elements for zones 1 and 2. 
Table 8 lists virtually all of the possible set-not set combinations of 
the six mho elements in the relay. By way of example, in rows 2, 3 and 5, 
where only one mho element has a set output, then that element and that 
element alone is run for zones 1 and 2, shown as block 71, on the 
transmission line, to identify the fault. An appropriate output is 
provided from the look-up table processor 66 on line 73. The zone three 
protective equipment is on a timer and will not trip the breaker for the 
line unless the fault remains for the preestablished time. When only one 
mho element is run for zones 1 and 2, a substantial amount of processing 
time is saved. If the fault is within the particular line section, i.e. 
line 28, then the single elements for zones 1 and 2 will identify it and 
the breaker will be tripped. 
The present invention, however, is capable of identifying which one 
particular mho element should be run in zones 1 and 2 when there are two 
or more mho elements having set outputs for zone 3. Typically, a 
comparison of the torque values is made by the comparator 68. For 
instance, in row 10, where the elements AB and A both have set outputs, 
there will be a comparison made in block 68 of the magnitudes of the 
respective torques for those mho elements, obtained on line 64. The set 
output indications will be obtained from the look-up table processor 66 on 
line 70. The maximum torque element typically will be the element that is 
run in zones 1 and 2. It is typical that torque comparisons are made for 
any condition in which two or more mho elements have set outputs, and 
where the fault type cannot be simply obtained from the look-up table 
(FIG. 8) for multiple mho element set outputs. 
It has been found that this particular look-up table and the torque process 
described above provides reliable fault-identification information in 
identifying which mho elements to run in zones 1 and 2. This processing of 
zone 3 mho element information, using both set outputs and torque 
magnitudes, is very helpful, since it reduces the overall processing time 
of the protective system significantly, i.e. only one mho element need be 
run in zones 1 and 2 to provide substantially complete distance fault 
information concerning whether or not to trip the breaker for the line. A 
flow chart shown in FIG. 9 sets forth the particular sequence of steps in 
the analysis process leading to the determination of the particular action 
steps shown in FIG. 8. 
In conclusion, the combination of set outputs and torque values are useful 
in fault-type discrimination for identification of a particular fault 
location, saving substantial processing time for zones 1 and 2 elements. A 
positive sequence memory voltage is used for the mho element polarization 
voltage. The present invention may be used for a broad range of power 
system configurations. Performance in a particular system configuration 
can be optimized, adjusting selectivity versus sensitivity as required. In 
one example, as the resistance of an AG (phase line A-to-ground) fault 
increase, the torque of the CG element can exceed the AG element torque. 
In three-pole systems, the maximum torque element can be used for maximum 
sensitivity, while with single-pole systems, the use of a look-up table 
which relates the AG-CG combined pickup to the actual AG fault provides 
satisfactory results. In another example, where the resistance of a 
close-in AG fault increases, the AB element torque might in some cases 
exceed the AG element torque. Again, the system sensitivity can be 
optimized for a three-pole trip system, while selectivity can be 
emphasized in a single-pole trip scheme, through the use of a look-up 
table. 
In addition, the system of the present invention can provide accurate fault 
information for systems in which the source-to-line impedance is strong, 
in which systems it is well known that fault-type identification is 
typically more difficult. 
In a variation of the system of FIG. 5, one or more of the torque values 
from the mho elements, such as phase-to-phase mho elements, may be 
weighted by a scale factor from circuit 80 in order to make an adjustment 
for a particular system situation. For instance, certain torque inputs may 
be scaled depending upon the direction or the level of the current flow 
load through the line. A phase-to-phase weighting step is shown in FIG. 9. 
A typical scaling factor might be 1.25. 
It has been found that the present invention also provides a capability of 
fault discrimination in many complex or difficult system fault situations, 
including close-in phase and ground faults, double line-to-ground faults 
and may also be used to prevent uninvolved mho elements from operating 
during single-pole open conditions. The individual mho elements in the 
relay of the present invention are typically very stable in operation 
during open-pole intervals due to the constant phase relationship 
maintained by the polarizing voltages for all elements and during all 
disturbances. 
Although a preferred embodiment of the invention has been disclosed herein 
for illustration, it should be understood that various changes, 
modifications and substitutions may be incorporated in such embodiment 
without departing from the spirit of the invention as defined by the 
claims which follow. For instance, the present invention has been 
described using as an example the well-known mho relay element. However, 
other relay elements, such as offset mho and various combinations of 
directional elements could also be used.