C.E Henville *

rotective relays continuously monitor the power system to detect abnormal conditions, such as short circuits, that could be damaging to equip-

ment or to the integrity of the system as a whole. They initiate corrective actions (often tripping circuit break- ers) to remove the smallest possible portion of the elec- tric system to isolate faulted equipment and allow the remaining part of the system to continue to generate and deliver power. The relay engineer uses “fault studies,” or models of the power system operat- ing under normal and short-circuit conditions, to determine what parameters should be mea- sured and what relay settings should be used to discriminate reliably between acceptable and unacceptable system conditions. Like all mod- els, t he usefulness of fault study models depends on their validity and accuracy.

This article encourages digital relay users to look more closely at their records of disturbances. In addition to the normal use of checking relay performance, these records can also be used to check the accuracy of the power system model.

Fault Studies and Model Validation Under normal operating conditions, the validity of power system models can be checked by comparing actual measured parameters (cur- rents, voltages, and power flows) with those pre- dicted by the models. Steady-state operating conditions simplify the process of taking measurements and readily allow comparison of the model with the real power sys- tem. However, power system measurements under steady-state conditions provide only information about the performance of the system under three-phase bal- anced conditions. They do not provide information about the performance under unbalanced, short-circuit condi- tions, particularly when currents through the ground arc involved. In the past, special recording instruments (fault recorders) have been used. Fault recorders are either installed for a long term, to capture fault data under natu- rally occurring fault conditions, or temporarily installed to capture data from specially staged short circuits. The fault recorders are costly to purchase and install, and staged

* BC Hydro

short circuits are difficult to arrange, because they are both costly and stressful on the power system.

Modern digital protective relays provide a low-cost window allowing engineers to look at the power system under transient fault conditions. These devices constant- ly measure parameters such as currents and voltages. When a short circuit occurs, the parameters measured iust before and durine the fault are recorded and stored

as event reports for later retrieval. These “snapshots” of faults allow regular observation of system parameters during short circuits. The event reports are only a byproduct of a device that has the primary function of disconnecting faulty equipment from the power system. Therefore, there are some limitations in the use of the reports for transient analysis of a power system. On the other hand, the format of the reports simplify the study of transient conditions under which the protective relay has to perform its prime function.

System models used for fault studies normally consid- er fundamental frequency (usually 60 or 50 Hz) parame- ters only. The impedances of the various components of the power system (generators, transformers, transmis- sion and distribution lines) arc best k:nown at the funda- mental frequency a t which they were designed to operate. Fault studies, therefore, usually ignore tran-

sients at higher and lower frequencies (which are usually present in currents and voltages during the transition from prefault to fault conditions). Therefore, convention- al protective relays are designed to respond mainly to the fundamental frequency component of the voltages and currents that they measure.

Digital relays often employ filters to reject transient signals at frequencies other than fundamental. These fil- ters must stabilize before their output can be considered to be accurate. The stabilization period forms a signifi- cant part of the duration of a event fault report. Plotting phasor magnitude and angle simplifies the process of checking that filters have stabilized. The filters are usual- ly only accurate at fundamental frequency, and this method of plotting output helps determine the adequacy of filter output at off-nominal frequencies.

When the fault location and resistance are known, the event reports can be used to calculate the impedance between the measuring (relay) location and the fault point. The measured impedance can then be compared with the impedance assumed in the fault study model for the same circuit component. This type of model valida- tion is frequently used to check the validity of transmis- sion line models, since transmission lines are the most frequently faulted component of a power system. Howev- er, the majority of transmission line faults are temporary and disappear when the transmission line is de-ener- gized; so the exact fault location is often unknown.

It is often possible to extend the use of relay event reports to check the power system model as a whole (includinl: transformer and generator impedances) and independent of fault location and resistance. The records are widely available, and in a form that makes it easy to produce the same parameters that are used for develop ing relay settings.

Fault Recording Fault recorders have been the traditional device used to monitor the power system. However, fault recorders are too expensive to monitor every voltage and current that

for fundamental frequency analy- sis. Unfiltered data is sometimes

Relays are primarily designed and built for specific functions. They monitor with only as much bandwidth and accuracy as they need. Thus, they provide records that are useful for monitoring their performance and for validating the models upon which settings are based. Limitations are in trying to use the records to monitor other aspects of the power system and instrument trans- former performance.

lsl As monitors for low frequency events such as dynamic swings, they may not be useful. Record length is limited, and accuracy at off-nominal fre- quency may be unknown.

~BI Low sampling rates may limit their usefulness for high frequency events related to switching and lightning surges, and circuit breaker transient recovery voltages.

P Fundamental frequency filtering may limit their use- fulness for small deviations from nominal frequency.

Given these limitations, it would appear that the best use of digital relay event records is for analysis of distur- bances against which the relays are supposed to protect. Since the relay is designed for the application, the record will be useful for power system model validation for that application. The record will be of less use for general system monitoring. Fault recorders and other monitors continue to have a place as general system monitors.

Plotting Measured Quantities Oscillographic Plots The conventional method of plotting sampled voltages and currents is to graph the instantaneous sampled val- ues versus time. This type of plot is known as an oscillo- gruphic plot, Graphical software packages are available for many digital relays, which allow the measured ana- log quantities to be plotted for easier visualization of the progress of the event. Figure 1 shows the plot of the instantaneous values of the samples from one type of digital relay. The higli-frequency resolution of the plot is limited by the sampling rate (960 Hz) to less than about 480 Hz.

l ~ , u s w U. - 1Y --I *,_' -,I- r-: -:*Pm.'= !,, p , ~ . . . . .. -I : I - _n- i d . . L ".e._ 1- *.e,_: ..- Sl. _,,-.*: . W _ L JC .i-..-. ..'\,-i _' ' -___fiii4 i ,;' =.=I *"*. **I *IS Y".

is measured by protective relays.

be processed to isolate the funda- Further, fault recorder data must

mental frequency signals. Data from fault recorders usually cov- ers only t h e most important equipment and provides only a narrow view of the power system.

On the other hand, relay event reports are available from almost eve^ digital relay. Filtered data is ... often available and is used directly

ness due to low sampling rate. iustantaneous values mental fvequeucy comporrent

January 2000 27

Figure 1 shows the fundamental frequency component of the prefault load quantities and the fault quantities. The higher and lower frequency components of the fault and postfault transients can also be seen.

In some cases, only the filtered samples are recorded by the relay, and made available in an event report. In such cases, it is possible to connect the data points by artificially constructed fundamental frequency sinusoidal waveforms of the correct amplitude and phase to show

Time (114 Cycle Increments)

Figure 3. Mugnitudes ofphusors

I 2500, ,160 8 I 140 & - 2000 m

8 1500

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40 > 20

500

0 0 1 5 9 13 17 21 25 29 33 37 41

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I -c 12 (A) I V2 (kV) + Angl2 - AngV2 (Deg)

Figure 4. Negative sequence phasors

- 2 2 U 80

1000 60 $ 40 > 20

5c

0 0 1 5 9 13 17 21 25 29 33 37 41

Time (114 Cycle Increments)

I -c 12 (A) I V2 (kV) + Angl2 - AngV2 (Deg)

Figure 4. Negative sequence phasors

oscillographic-like plots of the fundamental frequency component of the measured signals. Figure 2 shows one such example, with only the fundamental frequency com- ponents of the fault quantities visible.

The plot of the fundamental frequency component is called oscillogruphic like because the real signals will always contain other frequency components, which are not shown in plots like those of Figure 2.

Plots such as those of Figures 1 and 2 attempt to repli- cate the output of an oscillographic chart recorder. These display types are useful for observing the phase and magnitude of currents and voltages in transient events, and the operation of various functions within the protection system during the fault. If their bandwidth is adequate, they may also be useful for observation of var- ious power system transient phenomena, such as har- monics, dc offset, and low and high frequency transients.

Phasor Magnitude and Angle Plots A new type of plot, which is now introduced, is a plot of the magnitude and angle of the phasors of fundamental

28 IEEE Cum~iiter A ~ ~ l i c a t i o n s in Pouuer

frequency components during transients. For instance, Figure 3 shows a plot of the magnitudes of the voltages from the same event record plotted in oscillographic- like fashion in Figure 2. Figure 3 shows, more clearly than Figure 2, the time at which the filter output stabi- lizes. A plot in the form of magnitudes allows filter sta- bilization to be readily observed.

Figure 3 shows that the recorded fault was unbalanced. The two phases with low voltages and high currents were short circuited to each other. The data recorded is, there fore, useful for analyzing the performance of the power system under unbalanced short-circuit conditions. The analysis is usually performed using symmetrical compo- nents to simplify the mathematics. Symmetrical positive, negative, and zero sequence quantities can readily be cal- culated from the recorded three-phase quantities. Plots of their magnitudes will also show when filter stabilization occurs. The angles of the phasors can also be plotted. The phasors are constantly rotating at fundamental frequency; so it is better to plot relative angles of one phasor with respect to some other phasor than its absolute angle. Fig- ure 4 shows the plots of the negative sequence current and voltage magnitude, and the angle between them for the same fault plotted in Figures 2 and 3.

Figure 4 shows that, before and after the fault, the neg- ative sequence quantities are negligible. During the fault, after the filter stabilizes, the negative sequence voltage lags the negative sequence current by about 97 degrees.

Figures 3 and 4 have shown the ease by which filter stabilization may be checked, when plotting magnitudes and angles of recorded phasors (instead of oscillograph- ic plots of instantaneous values). Consider the type of information that may be derived from the new plots.

Calculations for Model Verification Equipment in Front of the Relay An intuitive use of a relay event record is to validate the model of the protected equipment in front of the relay. The simplest concept is that of a balanced, three-phase short circuit at a known location on a transmission line with negligible impedance in the short circuit. Since the short circuit is symmetrical, the equivalent circuit can be represented as a singlephase circuit as shown in Figure 5. All impedances, voltages, and currents in this circuit are shown as their values in the positive sequence network. The point marked R is the relaying point at which the volt- ages and currents can be determined from fault records. All currents and voltages in this figure are phasors, and impedances are complex numbers.

In Figure 5 , the transmission line impedance is modeled as ZL1, and the short circuit is at a known per unit distance m, along the line. Given that the impedance of the short circuit is negligible, the impedance of the line can be calcu- lated from the formula

ZLl=Vrl/(m Vrl)

Thus, using the measured values of Vrl, Irl, and the known value of m, the actual impedance of the line can be calculated and compared with the value used in the model. If the model is valid, and ZL1 is known, the distance to the fault can be calculated in a process known as fault location.

In real life, fault location is complicated by the fact that the fault is usually unbalanced, and the fault imped- ance may not be negligible. Numerous techniques have been developed to overcome the fault location problems caused by unbalanced faults and to take into account the fact that the fault impedance is not negligible. The description of these techniques is beyond the scope of this article. To verify the transmission line impedance by measured data, the fault location must always be known. Unfortunately for the verification process, the location of temporary faults on a transmission line is often unknown. Further, faults in equipment other than trans- mission lines (such as cables, generators, and transform- ers) are very rare, and after they become faulted, they are often replaced with different equipment that must be put into the model. Other means of verifying the power system model should be considered.

Power System Behind the Relay It is often possible to use an event record to calculate the Thevenin equivalent source impedance behind a relay. In that case, the location of the fault and the mag- nitude of the fault resistance lose their importance. This section reviews the ways in which this calculation may be done, including the effect of unbalanced faults.

Consider the decomposition of the threephase power system impedances into three singlephase systems using the principle of symmetrical components. The positive sequence system has been shown in Figure 5. The nega- tive and zero sequence equivalent circuits are as shown in Figure 6. Given the directions of currents and voltages shown in Figure 6, the calculation of the negative and zero sequence source impedances is simple:

ZsZ=-Vr2/1?2 and ZsO=-Vdl/rO.

Calculation of the positive sequence source impedance is a little more difficult, because the equivalent voltage Es behind the source impedance in Figure 5 needs to be known. This is sometimes difficult to estimate accurately from a relay event record, because of the effect of prefault load currents. If these are large, and not all measured by the relay, it may not be possible to calculate 0 from the event record. However, if all the prefault load current is measured by the relay, the effect of the unknown Es can be eliminated as follows. Under prefault conditions,

Es=/rl'*Zsl+ Vrl',

where the prime mark indicates prefault currents and voltages. Under fault conditions,

Es=/rl*Zsl+Vrl.

Es can be eliminated from the two equations (fault and prefault) to yield the positive sequence source impedance from the measured currents and voltages:

Zsl =(Vrl-Vrl')/(/rl'-Irl),

From these equations, it can be seen that the negative and zero sequence impedances behind the relay can be calculated from the measured fault quantities, as long as all the fault current through the source impedance also flows through the relay. The positive sequence source impedance behind the relay can also be calculated from the fault and prefault quantities, as long as all the load and fault current that flows through the source impedance also flows through the relay.

The requirement that the relay measure all the cur- rent that flows through the source impedance limits the applicability of the measurement to simple radial sys- tems, or two machine systems. If there are connections behind the relay to the remote terminal of the protected equipment, through which significant fault current flows, the source impedance cannot normally be calculated from a single relay event record

S.

Positive Sequence Network

Figure 5. Three-phase fault ut known locution

Examples Some examples of the application of the plotting tech- nique, and use of the calculation to verify the source impedances behind relay terminals in system models, fol- low. The technique is usecl to measure

Zero Sequence Network- 1 ?cure 6. Theveuin euuivulent

source impedances behind the relaying

I

source impedances in symmetri- cal components

points. The measured source impedances are compared with the impedances calculated for the same points using a conventional fault study program. The comparisons reveal the validity (or lack of validity) of the power sys- tem model at specific locations.

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Figure 7. Cuwents and voltages during fault impedance change

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Figure 8. Negative sequence source impedance

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Figure 9. Zero sequence current and voltage

Negative Sequence Source Impedance The Thevenin equivalent negative sequence source impedance behind a relay at one terminal of a 60 kV line was determined for a fault with unknown impedance and at unknown location. The line terminal was nearly the equivalent of one terminal of a two-machine system. There was, in fact, a very weak connection to the remote terminal. The fault used in this example was a phaseto phase fault cleared after a zone 2 time delay. At one point during the fault, the apparent impedance of the fault changed, and the fault currents increased substan- tially, as shown in Figure 7.

The calculated negative sequence source impedance during this event was plotted as shown in Figure 8. Figure 8 shows that the magnitude and angle of the negative sequence source impedance stays constant even as the

fault current increases with the decrease of fault imped- ance. The impedance must be measured before and after the transient change in fault quantities. During the change, and for a short while after it, the filter must be allowed to stabilize before meaningful measurements can be taken,

The positive, negative, and zero sequence source impedances behind this terminal calculated from the observation of nine fault records were within t10 per- cent of the source impedances given by the system model on which the fault study for this terminal was based. In two cases, the measured source impedances were significantly higher than the values given by the model. In those two cases, the system disturbance reports (prepared by the system dispatcher) stated that the system was normal at the time of the disturbance. It is probable that, for these two incidents, there was some system element that was out of service but not recorded by the dispatcher. These two discrepancies indicate that source impedances can be higher than expected, even under normal conditions.

Zero Sequence Source Impedance Figure 9 shows the zero sequence current and voltage and the angle between them during a single-line-to- ground fault on a 138 kV circuit connecting a generating station to an integrated system. The relay producing this record was on one terminal of a true two-machine sys- tem, since there were no other parallel paths connecting the generator to the integrated system.

The rise in zero sequence current and voltage at the start of the fault is clearly seen. The zero sequence source impedance measured from the above current and voltage is plotted in Figure 10.

It would appear from Figure 10 that the zero sequence source impedance starts out at about 1,000 to 630 ohms, before dropping to a value less than 100 ohms during the fault. However, study of Figure 9 reveals that the zero sequence current and voltage magnitudes before the fault starts are too small for a meaningful measurement of zero sequence source impedance. This observation empha- sizes the importance of ensuring that zero and negative sequence currents and voltages due to an unbalanced fault are much larger than steady-state measuring errors and unbalances due to load flow through an unbalanced system. Small errors i n phase quantities that are processed to derive negative and zero sequence camp@ nents can result in large errors when the values of those components are small. Viewing the phasor magnitudes has made it easy to see the difficulty in trying to verify the response of the power system to unbalanced short-circuit currents by observation of the performance during bal- anced nonfault load conditions. The relay event record has provided the lowcost window to the system during unbalanced fault conditions.

The measured zero sequence source impedance at this station during this fault and other faults was 66 ohms, at

30 IEEE Computer Applications in Power

an angle of 87 degrees. This was significantly lower than the value of 84.5 ohms at an angle of 90 degrees, given by the system model. At this station, the zero sequence source impedance is all made up from the unit stepup transformer zero sequence impedance. The value assumed by the system model is taken from a drawing of the transformer nameplate, which shows only the positive sequence impedance. The transformer is about 40 years old, and factory test records of the zero sequence imped- ance measurements are not available. In this case, the relay record has provided a more realistic value of the zero sequence impedance than the assumed value. The record has shown up a deficiency in the model.

Positive Sequence Source Impedance Figure 11 shows the current and voltages at the low volt- age side of a generator stepup transformer during a sin- gle line to ground fault in the circuit connecting the high voltage side of the transformer to the integrated system. The transformer is connected delta wye, with the delta connected winding on the low voltage side, therefore, no zero sequence current flows in the transformer during the high voltage single-line-toyround fault. The circuit connecting the high voltage side of the transformer to the system is a single circuit; so this terminal is one terminal of a twemachine system, similar to the case described for the zero sequence source impedance example.

Figure 11 also shows the change in currents and volt- ages when the remote terminal of the circuit opens due to tripping by instantaneous protection at that end. The protection at this terminal was time delayed; so fault cur- rent was continued to be supplied by the generator for a total time of approximately 2.5 seconds.

Figure 12 shows the positive sequence source imped- ance behind the relay. Because this impedance is a gener- ator impedance, it varies over time as it changes from subtransient to transient timeframes. The positive sequence impedance was not measured until the six- teenth sample. The fourth to seventh samples (inclusive) were used to establish the prefault conditions, which were used in combination with the fault conditions to eliminate the unknown positive sequence source voltage.

The initial value of the measured positive sequence impedance (at the sixteenth sample) is about twice as large as the subtransient impedance that had been assumed in the system model for the 50-year old genera- tors at the station. There are two possible reasons for the large discrepancy between the measured value and the value assumed in the model.

In the case of a generator, the prefault source imped- ance is the synchronous reactance of the generator, which is different from the impedance during the fault. This means that the equation for calculating the positive sequence source impedance is not valid, because that equation assumed the same prefault and fault source impedance. Note however, that the equation is based on

the same assumption of constant source impedance that the majority of fault simulation studies use. Only electrc- magnetic transients programs model the time varying characteristics of generator impedances.

The generator source impedance changes during the fault from the subtransient reactance to the transient reactance. The delay before stabilization in the relay fil- ter means that the initial subtransient reactance cannot be measured.

In spite of the difficulty of measuring the actual posi- tive sequence source impedance, it was clear that the assumed value was unnecessarily small.

Benefits and Conclusions Widespread availability of digital relay event reports p r e vides more information than ever before about the power system in transient conditions. The information is useful to verify power system models. These models are used universally to study the system to ensure proper protective relay applications and settings. Verification of the source impedance behind a relay terminal can often be done regardless of whether or not the fault imped- ance and location on the protected equipment is known.

Studies of the type described were done with hundreds of fault records from dozens of stations on the BC Hydro system. It was generally observed that when the positive sequence source impedance behind the relay contained a significant amount of rotating machine impedance, the measured source impedance was usually significantly higher than the source impedance used in the model. The model normally uses the direct axis subtransient reac- tance of the generator to give maximum calculated fault currents. The dynamic nature of the generator reactance in transition from prefault to fault conditions, and during the fault, makes it difficult to measure the source imped- ance accurately and to model it accurately.

When rotating machine impedances were not signif- icant, positive sequence impedances used in t h e model were usually validated by the relay reports. Modeled negative sequence impedances were usually validated, whether or not rotating machine imped- ances were significant.

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Figure 12. Positive sequence source impedance

Discrepancies in zero sequence impedances were more frequently observed, and it was not always possible to discover where the model was wrong. Where a single piece of equipment makes up the zero sequence imped- ance (as in the case of the example of the generator step- up transformer described above), the impedance used in the model can simply be corrected to the value deter- mined from the relay event report. Usually however, the cause of the discrepancy will be more obscure and may not be able to be discovered. In such cases, wider than normal margins in relay settings may be required to accommodate the uncertainty in the system model.

The measured angles of source impedances are often several degrees different from those given by the system model. Part of the reason is that resistances of transform- ers and generators in the BC Hydro system model are normally neglected. Fortunately, accurate knowledge of this angle is usually not vely important for relay applica- tion and setting.

Measured source impedances were sometimes higher than a t other times, even though the system was believed to have been normal at the time of the event. This would indicate that large margins for sensitivity of relay settings are desirable. Settings must usually be sen- sitive enough to cover for contingency conditions that result in higher than normal source impedances; so small increases in source impedance are not usually a concern. However, with digital relays that have the capa- bility of having different settings under different system conditions, there could be a tendency to try to reduce sensitivity margins, and rely on setting changes to cover

32 IEEE Coni~uter A~~i icut ions in Power

contingency conditions. This tendency could result in dangerous situations if margins are reduced so much that the sensitivity under system normal conditions is not adequate for increased source impedance caused by a change in system configuration in a remote location.

The filter delay in event reports can complicate the val- idation of the system model under transient conditions. It is important to ensure that the measured quantities have stabilized sufficiently to produce valid measurements.

Plotting the magnitudes and angles of the phasors will help in the identification of suitable time spans when measurements may be made. However, the speed of change in the transient condition is an important factor in determining the validity of the filtered signal. When the system conditions are gradually changing, useful information can still be derived.

Relay event records cannot replace fault recorders as a means of monitoring power system performance dur- ing transients. The wider bandwidth and longer record- ing time of fault recorders will continue to justify their application in important parts of the power system. How- ever, one traditional value of fault recorders, that of monitoring fundamental frequency parameters during short circuits, is gradually being lessened by the relay event reports. The possibility of using filtered informa- tion from the relay reports simplifies their use to mea- sure fundamental frequency components.

It is widely recognized that relay event records may be used to check the impedances of the protected equip- ment in front of the relay. This article points out the addi- tional benefits of using the record to check the system impedances behind the relay. Since these event records usually come "free" with the relay, it would be wise to take advantage of the availability of the records to check the validity of the system model as much as possible.

Acknowledgment This articie is hased on a PES Prize Paper by C.F. Henville, "Digital relay reports verify power system models," IEEE Trmrocfioris on Power Deiiuey, April 1998, pages 36.393.

For Further Reading C.O. Schweitzer 111, D.C. Rogers, "Practical benefits ai micniprnccssor- based relaying," I'loceedings of the We.$tem Frotectioe Rdoy Conference. Spokane, Washington, 1988.

C.F. Henviile, A.F. Eineweihi, "Spreadsheet help for the protection engineer," Proceedings of the Western Pmtectiue Reluy Conference. Spokane, Washington, 1989.

Biography Charles E Henville was born in Bcsseterre St. Kitts. He was awarded a BA and an MA liy Cambridge University, England. anti an M.Eng by the University of British Columbia, Canada. His initial engineering career was as a commissioning engineer fur a British engineering company anti lor RC Hydro. He has been involved in power system protection since 1977, anti is now a specialist protection engineer with BC Hydro. He is senior member of the IEEE, a registered professional engineer in British Colum- bia. a member of the IEEE Power System Relaying Committee, and trea- surer of the Vaiicaiiver Section of the IEEB.