IEEE Transactions on Power Delivery, Vol. 8. No. 3, July 1993

FEASIBILITY OF ADAPTIVE PROTECTION AND CONTROL

975

Prepared by the Feasibility of Adaptive Protection and Control Working Group of the Substation Protection Subcommittee

of the IEEE Power System Relaying Committee

J. S . Thorp, Chairman

M. Adamiak W. C. Kotheimer G. D. Rockefeller H. N. Banerjee L. L. Mankoff R. Ryan J. A. Bright A. Munandar M. S . Sachdev T. W. Cease S. L. Nilsson H. S . Smith D. M. Clark A. G. Phadke E. A. Udren E. M. Gulachenski R. Ramaswami C. L. Wagner S . H. Horowitz

ABsTRA(JT

A summary of 90 responses to a survey on the use of adaptive functions in protective relaying systems is given. The survey was developed by Working Group K8 of the Substation Protection Subcommittee of the IEEE Power System Relaying Committee. This paper presents some background on the concept of adaptive protection, summarizes the questionnaire and gives an analysis of the questionnaire responses. Section A of the questionnaire which concerns satisfaction with existing relaying schemes is summarized in two figures. Section B of the survey which outlines 16 possible adaptive features is repeated in its entirety. A number of adaptive features suggested by the respondents are also analyzed

The concept of adaptive as stated in the questionnaire is: A function within a protective relay or system that automatically adjusts the operating characteristics (setting or state change) of the relay system in response to changing power system conditions can be said to be adaptive. Many existing relays are adaptive to a limited extent. An inverse-time over current relay, for example, could be considered to adapt its trip time to the current level. With the advent of microprocessor based relaying systems and the possibility of improved communication, more complicated adaptation can be conceived. More extensive adaptation requires additional input signals to the protective relay or control device. These inputs may be from within the substation, from adjacent substations, or even from remote locations in the power system.

In an attempt to determine industry acceptance of adaptive relaying and control a questionnaire was prepared by Working Group K-8 of the Power System Relaying Committee. The first part of the questionnaire provided the respondents with the opportunity to express their satisfaction with existing relaying schemes. The results are summarized here with two tables, one for solid state relays and one for electromechanical relays. The second part of the questionnaire was a list of 16 possible adaptive functions whose value the respondents were asked to rate. In addition, the respondents were asked whether these functions were available in existing relays. Each of the questions along with the mean and standard deviation of the responses are given. Finally, the respondents were asked to provide suggestions for possible adaptive features. The paper concludes with an analysis of the questionnaire results and the new suggested adaptive schemes.

92 SM 384-8 PWRD by the IEEE Power System Relaying committee of the IEEE Power Engineering Society for presentation at the IEEE/PES 1992 Summer Meeting, Seattle, WA, July 12-16, 1992. Manuscript submitted February 18, 1992; made available for printing April 6, 1992.

A paper recommended and approved

SUMMARY OF RESULTS

SURVEY ON THE USE OF ADAPTIVE FUNCTIONS IN PROTECTIVE RELAYING SYSTEMS

se of the Ou- Recent advances in technology including the advent of microprocessor based protective relays and improved communication systems have made it possible to consider added adaptive protection and control features. A questionnaire was developed by Working Group K-8 of the Power System Relaying Committee for the purpose of identifying adaptive features in use as a part of protective relaying systems and to assess the need for new adaptive features in such systems to mprove system performance. The questionnaire was mailed to approximately 260 protective relaying engineers working for electric utilities selected from the list of relay engineers in North America that is maintained by the Power System Relaying Committee. A total of 90 responses were received. The average respondent had 17.74 years of experience with protective relaying systems and worked for a utility with a 4115 MW peak load.

. .

RESPONDENT: (Circle the appropriate category and fill in the desired information in the spaces provided)

E n g i s 9 ) ; Staff Engineer( 11); Engineer(8); Other(29):

Systems: 17.74 (from 3 to 38)

dent's Job Title; Chief Engineer(l3); Supervising

er of Years Ex- with Protective Relaying

Orearuzatlon: Utility(82); Cogenerator(1); Other(7); . .

UTILITY DATA

peak LQ& 41 15(MW)

i ' Circuit of (a) 345 kV and above 575 (miles) (b) 230 kV 564 (miles) (cj 11 5 through 169 kV (d) 4 1 5 kV

1249 (miles) 2544 (miles)

Definitim A function within a protective relay or system that automatically adjusts the operating characteristics (setting or state change) of the relay system in response to changing power system conditions can be said to be an adaptive function. The adaptive function may require additional measurements or signals or it may use the normal protective relaying input signals combined with enhanced signal processing techniques for its operation. For example, a relay that changes its characteristics in response to a change in the source impedance ratio could be said to have an adaptive feature.

0885-8977/93$03.00 0 1992 IEEE

976

(A) On a scale of 1 to 7, how would you rate your satisfaction with the functional performance of the different relaying systems that you are f+iliar with or F e using: B-.II

Note that 7 is excellent performance and 1 is not satisfactory.

The relaying schemes listed in Section A and the responses are summarized in Figures 1 and Figure 2 for solid state and electro-mechanical relays respectively. The top bar next to each relaying scheme indicates how many respondents were using that scheme. The second bar indicates the mean of the respondent’s satisfaction with the scheme and the third bar is the standard deviation (0) in the satisfaction response. For example, 70 respondents use solid-state multiphase distance relays, the mean satisfaction was 5.8 (out of 7.0) and the standard deviation was less than one ( the responses are concentrated near 5.8). It should be noted that the statistics are less meaningful when only a few respondents are using that scheme.

Figure 1 Summary of Survey Responses to Section (A) for Solid-state Relays. The number using each scheme along with the mean and standard deviation of the satisfaction

WalltV of “f1C x l u m h a w r s rel&!ss this aueShon but only

index for that scheme. Transmission Line

Multiphase Distance Relays

Ground Distance

Overcurrent Ground Relays

Blocking

Unblocldng

Permissive Ovareaching Transfer Trip

Pilot -Directional Comparison

Figure 2 Summary of Survey Responses to Section (A) for Electromechical Relays. The number using each scheme along with the mean and standard deviation of the satisfaction index for that scheme.

Transmission Line . . . . . . Multiphase Distance Relays

Ground Distance . . . . . . I . . . . ’ I * . . . .

Overcurrent Ground Relays Pilot - Directional Comparison

Blocking . . . . . . . Unblocking

P e m k i v e Overreaching Transfer Tnp Permissive Undanaching Transfer Tnp Direct Undemaching Transfer Trip

Phase Comparison - Segregated . . . . . . . . . . . - Non-Segregated . . ’ . . . . . . . . . . . . . . . . . - . . . . .

. . . .

. I * . . . . . . . . . . . . . . .

Current Differential

Pilot wine

Reclosing Relays - High Speed

Time Delay

. . Diffmntial Without . . Harmonic Restraint Differential With Harmonic Restraint

. . Transformer

. . . . . . * I * . . . . . . . - I . . . . . . . . . . a . a . . . . . - . . . . . . . * . - I . . . . - .

Permissive Underreaching Transfer Trip

Transfer Trip Direct Underreaching * .

Phase Comparison - Segregrated

Non-Segregated

Current DSeEntiaI

Pilot wire

Reclosing Relays - High Speed . . . . . a . . . . . - . . . . . - . .

Time Delay . . Transformer Differential Without Harmom Restramt DSemntial With Harmom Restraint

Sudden F’ressum Relays

Volts-per-Hertz

Overcurrent Relays -Primary Relays

Backup to Differential

Tertiary Protection

Neueal Protection

High Impedance Relays Bus and Breaker

Linear Couplers

Current DSemntial

Pole. Disagreement

Breaker Flashover

Breaker Failure

-1 Standard Deviation

Sudden PESSUR. Relays

Volts-per-Hcrtz

Overcunent Relays -Primary Relays

Backup to Differential

Tertiary Protection

Neutral Protection

High Impedance Relays Bus and Breaker

Linear couplers

Current Differential

Pole Disagreement

Breaker Flashover

Breaker Fail-

I Number UsingJ20

IJ Standard Deviation Mean

Next Section B of the survey is repeated in its entirety. The mean and standard deviation of the responses to part a) of each question have been inserted before part b) of that question. The number of respondents who felt that the feature was not available or available or that they were using the feature are inserted in the spaces in part b. The results are summarized in Figure 3

(B) In your opinion, on a scale of 1 to 7 (with 7 being very valuable and 1 being not needed), how valuable is it or would it be to have relays adapt to the following situations. Note that the question has two parts. In part (b) please indicate if in your view the adaptive feature is available or not available to you in your relays or if you are using the feature. If so. would you please indicate the type of relay which provides the feature.

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1. a) Operat h g Time as a F u n c t h of the Distance to Fault (settable or not)

A distance relay having this feature determines, before making a trip decision, how far inside the zone the fault appears. If it seems to be near the boundary, the relay integrates fault values for a long time to overcome noise and locate the fault precisely. If the fault appears well inside, the relay can operate quickly without danger of overreaching. Such a relay is sometimes described as having an inverse time-distance Characteristic. The mathematical curve describing the fault location and trip time may arise implicitly in the design. In some relays, the user can choose from a family of curves depending on whether trip speed or reach accuracy is more important. The adaptive feature is to automatically select a specified curve shape which gives the best combination of security and dependability depending on the system state, i.e., normal, alert, emergency or restorative.

mean = 4.3 (r= 1.6 b) Not Available (..66..) Available (..5..) Using (..2..)

. . 2. a) Mutual Coupling Cwensat ion in Ground Impedance I?"

Mutual coupling due to transmission lines on the same tower or paralleled along the same right-of-way produces a distance measurement error in ground impedance relays. This error is a function of the system configuration and the resulting zero sequence mutual impedance. Conventional techniques can compensate only if the ground current from the coupled line is available in the same station. An adaptive scheme that knows the system impedance matrix can precalculate the effect on the relaying assuming, for example, a fault at the end of the zone 1 reach, even if the coupled line doesn't terminate in the same station. The impedance matrix would reflect the changed effect occurring when the coupled line is taken out of service and grounded. Adaptive protection can coqemate the ground impedance measurement by cakdating the f l i t of the mutually coupled line.

mean = 4.9 Q = 1.7 b) Not Available (..68..) Available (..O..) Using (..O..)

3. a) n g High Source i A high ratio of positive sequence source impedance to

positive sequence line impedance (SIR) results in a low voltage and current and, more importantly, a small change in voltage for a fault at either end of a line. Unusually large changes in the SIR which might accompany a wide-area disturbance, can result in voltages that are below the range of the relay. An adaptive relay, for example, can be made to adapt its voltage and current semitivity to accommodate such excursions.

mean = 4.8 Q = 1.3 b) Not Available (..73..) Available (..O..) Using (..O..)

er Detection for ~ - S L = e d

Reliably sense the opening of the far end, for far-end faults on the protected line, to achieve instantaneous sequential clearing for those applications where conventional instantaneous overcurrent units will not provide this response. This will provide fast clearing backup of a conventional pilot relaying system, possibly obviating the need for two pilot systems. Only information available within the substation would be used at the instant of the fault. Information about the external system would be transmitted to the substation prior to the disturbance. This information would allow the identification of distinctive current change patterns at the time of far-end opening, measuring current in various lines within the substation. The adoptive feature is the abirity to respond to changing external system c d i t w n s .

Available (..20..) Using (..4..) mean = 5.1 Q = 1.3

b) Not Available (..57..)

5. a) Load Flo w C- Re-fault load current influences several relay settings and in

general makes a relay less sensitive. Using fault detectors. it would be possible to compensate for the pre-fault load data and

to improve the sensitivity of overcurrent, differential, or impedance relays. The latter could be made significantly less sensitive to the effect of fault resistance. The adaptive feoture is the improvanent in relay sensitivity by eliminating the effects of prelfavlt load flow on fault-related memuements.

mean= 5.5 Q = 1.1 b) Not Available (..72..) Available (..6..) Using (..2..)

6. a) Fault Tvpe (multiuhase vs. sinele phase) Changing Speed of - . Operation

Using only normal relaying input signals, the relay would trip multiphase faults more quickly than single phase faults. The benefits -would be in t&ns of i m ~ o v e d security and dependability of the protection system. The adaptive feature is farter clearing of multiphase faults.

mean = 4.7 Q = 1.4 b) Not Available (.,64..) Available (..lo..) Using (..1..)

7. a) & J M e v a Zone 1 reach can be increased and Zone 2 overreach can be

reduced by inputting the approximate current infeed ratios. These ratios can be precalculated and changed only when the condition of the remote breakers (open vs closed) are changed, or the ratios could be changed based on rough impedance calculations made by the substation computer based on information about system changes. The adaptive feature is improved multi-teminal protection in response to external system ir$ormation.

mean = 5.0 CT= 1.5 b) Not Available (..70..) Available (..2..) Using (..1..)

8. a) Variable Br eaker Failure Measurements of system quantities (for example, positive

sequence voltage) during a fault indicates the severity of the fault (the lower the voltage the more severe the fault). By increasing the breaker failure timing for the less severe faults, back-up breaker tripping can be minimized. The particular time vs. positive-sequence voltage characteristic could be preset or could vary with conditions external to the substation based on input from a central computer. The adaptive feature is a reduction in back-up breoker tripping bawd on measurement of system quantities during the fault.

mean = 4.0 Q = 1.6 b) Not Available (..67..) Available (..4..) Using (..3..)

9. a) &missive Reclosing Reclosing time can be controlled to maximize success and

minimize the delay by initiating the reclose after extinguishing the secondary arc. Until this arc is extinguished, the voltage to ground on the faulted phase(s) will be depressed. When the arc goes out, the voltage will suddenly increase because of the coupling to adjacent phases of the protected line and to parallel lines. With three phase tripping, the sound phases of the protected line are left with a trapped charge or are experiencing oscillations with line-side shunt reactors. This energy will cause a '2ump" in the faulted phase voltage once the secondary arc is extinguished. With single phase tripping, the sound phase conductors remain energized by the system. By measuring line side voltages, reclose time can be controlled to maximize success and minimize delay.

mean = 4.3 Q = 1.5 b) Not Available (..64..) Available (..4..) Using (..O..)

10. a) Maptive Reclosing If the poles of a reclosing circuit breaker could be

individually controlled, it would be possible to close one pole first, and then after examining the coupled voltages on the un-energized phases, determine if a fault remains on the transmission line. The first pole to be closed could be selected on the basis of having the least likelihood of having a fault. The voltages on the three phases, with only one phase energized can

978

be used to detect phase-to-phase faults involving either the energized phase and one of the un-energized phases (the two voltages are equal), or both the un-energized phases (again these two voltages are equal. although smaller than when one of the faulted phases is energized). This procedure would minimize the possibility of ever reclosing all three poles into a multi-phase fault. The adaptive feature is the elimination of reclosing into a multi-phase farclt by controlled individuul pole reclosing.

mean = 3.9 B = 1.5 b) Not Available (..63..) Available (..2..) Using (..O..)

11. a) * Respom High-speed reclosing is a very effective means of

responding to sympathy trips (incorrect tripping during a fault). Some users will reclose following a bus differential trip, but not for a transformer differential operation. Also, some users will not high-speed reclose on a transmission line near a power plant because of shaft-fatigue considerations. For applications where high-speed reclosing is not conventionally implemented, such reclosing could be utilized when the computer system detects that the trip was sympathetic. These trips could, for example, be detected by noting the appearance of near normal voltage on the protected circuit immediately after breaker clearing if one end stays closed or by the detection of power flow out of the circuit for a supposedly intemal fault. The high-speed reclose could be contingent upon power system conditions; for example, additional shaft fatiguing would be tolerated only when system conditions are critical. Thus tire reclosing strategy would be adaptable to system conditions.

mean = 4.3 Q = 1.6 b) Not Available (..65..) Available (..O..) Using (..O..)

12. a) m v e S r The synchronism check angle for reclosing a circuit breaker

is generally selected to: i) control the inrush current and synchronizing torque for cases involving generation, and ii) control closing current in a circuit breaker to limit breaker and power system wear when reconnecting segments of a power system where an angular difference exists. In either case, the application engineer will preselect the synch check angle to provide reasonable margin to avoid generator or breaker wear or damage. In the event of a system-wide disturbance, with the danger of system collapse, it should be possible to increase the synch check angle by means of secure, remote communication. The predetermined change (or calculated required change) would increase the probability of reclosing one or more key transmission lines to enhance the chances of system survival. The amount of temporary increase in synch check angle at any location would be such that equipment capability is not exceeded (generator, breaker) but the margin is reduced, resulting in the possibility of limited but acceptable higher duty for the infrequent incident. By temporarily changing the synchronism check angle for reclosing in the event of a severe system disturbance, the possibility of system survival is increared.

b) Not Available (..66..) Available (..5..) Using (..2..) mean = 4.8 Q = 1.4

13. a) Proactive Load Sheddine A major disturbance in an integrated power system can

cause cascading which can result in the formation of islands due to the imbalance between generation and load. In existing power systems, "reactive" load shedding relays are employed to balance load generation mismatch during a disturbance. Since the load shedding relays are reacting to the event, large amounts of load can be tripped for major disturbances. However, if a "proactive" load shedding scheme is employed on a system basis, the total amount of load shed during an event may be reduced by the faster acting scheme. This type of scheme would use a centrally located computer to compare the current system conditions with study scenarios to decide the best course of action to avoid or reduce the impact of the disturbance. Thus the proactive frrnction of recognizing the synptm of a disturbance and applying the proper load shedding wouId be the adaptive function.

mean = 4.2 B = 1.6 b) Not Available (..72..) Available (..2..) Using (..O..)

must accommodate dissimilar ct accuracy classification and performance due to different primary voltage levels, mismatch of current ratios due to the use of standard cts and fixed relay taps, and power transformer ratio changes due to tap changers, both fixed and Tap Changing Under Load. Large slopes make the relay insensitive to partial winding faults. An adaptive transformer differential relay could reduce the slope by "learning" the ct mismatch and responding to measurements of the tap positions. The +five feature is the improvement in semlivify by leaming the ct mismatch and using measurements of the tap posirions.

mean = 5.2 B = 1.4 b) Not Available (..71..) Available (..1..) Using (..1..)

. . 15. a) Voltape C h a n g e w f m , & d J h & Inrushcurrents appear in response to step changes in the

applied voltage of the transformer and in response to energization of a transformer parallel to the one protected. Thus, if a large differential current appears suddenly without any preceding change in voltage, this would be an indication of an internal fault and allow for fast tripping of the breakers. This function would be disabled after about the fist half cycle of the fault before dc saturation of the cts can cause incorrect operation during an external fault. The adrqDtive f d w e i~ f&er tripping on large differential currents not preceded by a voltage change.

mean = 4.9 B = 1.4 b) Not Available (..66..) Available (..O..) Using (..O..)

Bus Protective Re-

16. a) Bus Protection Restr aint for Arrester Applications. Bus protection applied to systems with lighming arresters

connected to the bus can be affected by arrester operations and must p" slowed down or made less sensitive to avoid false operation. The arrester current can be used to temporarily restrain the operation of the relay to permit fast operation of the relay for bus faults. Altemately, voltage supervision might be feasible to use for high speed operating elements of the bus protection because there is some bus voltage even when the arrester operates. Protection for the case when the lightning arrester fails will be provided but may be somewhat slowed down for these cases. The adaptive feature is fmtm response for systems with arrester qp l i ca t im .

. .

mean = 4.1 B = 1.4 b) Not Available (..70..) Available (..O..) Using (..O..)

The responses to the questions in Section A are summarized in Figure 1 (for solid state relays) and Figure 2 (for Electromechanical relays). An analysis of the responses to Section A provides a good introduction into a discussion of the purposes of the questionnaire, namely to identify adaptive features presently in use and to assess the need and acceptability for new or improved adaptive features.

The responses involve subjective reactions to the level of satisfaction with present day relaying. The "mean" of the response shown in the two Figures is, in effect, a happiness index indicating a uniformly high degree of satisfaction. It is not surprising that relay engineers are satisfied with the equipment that they use. Protective relaying has been an evolving technology of which the respondees have been a part and the performance has been highly successful. The results are interesting however, compared to Section B; an indication of adaptive features that would be desirable. The summary of

979

Section B responses is shown in Figure 3 which has the same format as the first two figures. Figures 1 and 2 also give an indication of the comparative use of the various relaying schemes.

In transmission line relaying, the most commonly used relays are the phase distance and directional overcurrent relays. This is as would be expected since their versatility as primary and backup independent protection and use in pilot schemes presents so many application opportunities. The interesting result in Section B, however, is the high mean of such adaptive features as load compensation, remote-end open-breaker detection, multi-terminal coverage, and adaptive speed vs fault type. This suggests that despite the success of and satisfaction with existing relays, there is still a strong desire and possibly a need for additional functions.

The many variations of pilot schemes result in the individual schemes appearing to be used less than the distance relays. This is misleading since the number of respondents using pilot schemes exceeds those using phase dismce relays. Here again, both the usage and satisfaction indices are not surprising and the desire for the adaptive features mentioned above would apply. Those schemes employing distance relays are more popular in both sections than the less versatile phase comparison and pilot wire schemes and the responses show this characteristic.

Reclosing and breaker failure responses have been listed under the category of transmission line protection. Both, as expected, are widely used and are candidates for adaptive treatment. The rationale here would seem to rest on the complex logic involved in developing and implementing the circuitry. Clearly, there are many factors that are involved in selecting how and when to trip backup breakers: factors that are intimately related to the system configuration as it exists at the moment of operation. This is an obvious adaptive problem.

The level of satisfaction with the transformer and busbeaker failure schemes listed in Section A is high, which would seem to imply that there would be a reluctance to apply adaptive features. When the impact on system security of failures of this type of protection is considered, there must be very significant advantage before adding any complexity to an already complex circuit. There is, however, one notable exception. The harmonic restraint transformer differential is widely used and has a high satisfaction level, a not-unexpected result, but also received a high mean for adaptive features. This parallels the discussion on the transmission line relays. There is a coupling of familiarity. satisfaction and versatility that leads the relay engineer to consider additional features such as adaptability.

It is clear that, despite the extensive use of and satisfaction with existing equipment, relay engineers will seriously consider the improvements that adaptive technology can provide.

In Section B the respondents were given the opportunity to comment on the availability of the proposed adaptive functions. A number of respondents perceived that certain features were available and in use, in situations where the respondents may not have focused on the bold adaptive feature or where the existing capabilities may have been stretched somewhat. For some of the questions in Section B specific relays were mentioned by the respondents. Members of the Working Group contacted manufacturers of such relays who concurred with the following summaries: B-4. Dete- for &gl&gd

B-5.

B-9.

. . . digital distance relays cited meet the

intent of this survey item, in that conditions extemal to the protected line are not directly known to these relays. However, these products do provide sequential high-speed tripping of unbalanced faults in response to the drop in the sound-phase current. . The respondents cited products which do meet the intent of this item. Analog products were identified which do utilize filtering which rejects the signals' fundamental component. Digital designs are available which can

ation to remove pre-fault current.

The products cited do not meet the intent of this adaptive

Flow C-

concept, which detects the extinguishing of the secondary arc. The Working Group is not aware of any such available device. . #?synchknism-check relay mentioned will double or triple its angular-range setting in response to an external contact closure.

Figure 3 Summary of responses to Section (B), the sugges- ted adaptive features. The number of responses along with the mean and standard deviation of the desirablility rating.

B-12 tive S r

Operating Time . . . . .

. . . a . ' * . . . . . * .

. . . . . * Mutual Coupling Compensation . . High SIR

U : : : : : . . . . - - . . . I . ' . * . ' . ' . - Remote-End Open Breaker Detection

U ; ; ; ; ; ' . . . _ . . . . . . . Fault Type Changing Speed

Multi-Terminal Distance Relay Coverage

. . . .

. . . a

. . . . . * . . . . . * I . ' . . . . . -

. . . . . a

Variable Breaker Failure Timing . . . . . . . . . . . .

I . ' . . ' Permissive Reclosing I......

. . . . . I . . * . . e I . ' . . .

. .

Adaptive Reclosing U! I ! : : : . . . . . . . . . . . . . .

I . . . . ' e . *

Sympathy Trip Response

U , . . . . ' . . ' . . . . _ . . _ . .

~ ~~

AdaptiveSynchCheck * i . . I * . U : : : : : . . . . . . . . . . . '

I . . . . . . . . Proactive Load Shedding

u...... . . . . . - . . . -

._. . . . . : : : . . e Adaptive Transformer

Differential U ; ; ; ; ; . . . . . . . . . . -

. * . . . . . * Voltage Supervision . . of Differential Unit

1 ~ 1 1 , i I i l i l i l i 0 1 2 3 4 5 6 7 - Number of ResponseQO

Mean Standard Deviation

W e s t e d aptive F u n c m .

The final question in Section B asked for other suggested adaptive functions not included in the list of 16. This subsection includes a brief discussion of some of these suggested functions along with Working Group analysis of the feature.

A number of respondents expressed dissatisfaction with the tendency of sudden pressure relays to operate on highcurrent extemal faults where winding movements can create a significant

980

pressure wave. A technique is in use to provide security against such faults which involves blocking tripping for faults exceeding a set level of through current. For high current intemal faults where the sudden pressure is blocked, we depend on the electrical relay and overcurrent backup. The Working Group considers this an elementary form of adaptive relaying.

The ability to change settings to achieve greater sensitivity to faults was suggested. One such case is included under 2(a) "Mutual Coupling Compensation in Ground Impedance Protection". Also 3(a), "High Source Impedance Ratio Changing" relates to the suggestion. However, this suggestion is to increase the sensitivity of the relay to get higher sensitivity to faults when e.g. "..another line outage creates fault currents below emergency load values". As indicated in questions 2(a) and 3(a), there could be many valid reasons for the proposed feature to solve relay performance problems under unusual conditions. The recent earthquake in the San Francisco region $ointed to a need to change settings of many relays to achieve protection of the transmission system as it was restored. The restoration involved bypassing normal breaker positions and tapping lines without breakers, which the protection system was not designed to handle. Manual setting of the relays was needed to get some measure of protection. Had this been possible to do via communication lines this process could have been much easier. Thus, there may be merit in having the capability for other reasons.

overcurrent relay protection and the change in pickup as a function of present load level or protracted lack of load (cold load condition). Ground overcurrent pickup must also be set in relation to loading on 4-wire distribution, since the distribution transformers are connected phase to multigrounded neutral, accordingly, techniques similar to those described are also applicable to the ground protection. There is a problem with this approach in coordinating with downstream non-adaptive reclosers. Adaptive microprocessor reclosers may be the solution. Adaptive pickup is also applicable to the distribution substation transformer overcurrent relaying - this must be set above emergency loading occasioned by loss of another transformer in the same or another substation.

The possibility of changing generator loss of field relay setting with changes in system impedance was suggested. In a few cases this may be a viable solution. However, generators are not run close to the limit in normal operation. Once or twice a year when peaks occur this solution could be useful. The question is whether the few times a year when the extra power or MVAR could be obtained would justify a more complex and possibly unreliable relaying system for the rest of the year.

Several other possible adaptive features received brief mention. These included the detection of downed distribution - line conductors, the capability to switch from one set of input devices to another as configuration changes or as equipment experiences failure, increased reliability in an integrated substation, overload detection for transmission lines, and capacitor bank and transformer protection in the event of a solar magnetic disturbance. To one extent or another, all of these topics are part of a number of on-going research projects.

Adaptive Reclosing

Several respondents mentioned the desire for flexible reclosing relays. A number of recent offerings go in this direction: also, some users are programming PLC's to customize their designs. One respondent using a custom PLC program mentioned breaker selection based on what breakers are in service and which line relays operated. The objective of the reclosing function is to re-energize previously tripped power equipment (i.e. a bus, a transformer, or a transmission line) with the minimum possible delay, consistent with the need to minimize the disturbance created by the act of reclosing. The reclosing relays presently in use meet these twin objectives through a multiplicity of contact sensing logic circuits, timers, and checks on various voltages and currents. To a large extent, the reclosing relays are already adaptive in nature. However, by following the general principles of adaptive relaying considered in this paper, possible additional adaptive features are revealed, and the technical means of achieving these features need to be

One respondent addresses

evaluated.

New Aditgti ve reclo

Section B of the survey reveals that several new concepts of reclosing would be desirable candidates for adaptive treatment. Sections 9, 10, 11, and 12 describe in detail these reclosing concepts and the results of the survey. A summary of the sense of each section is as follows:

(1) Avoid reclosing until fault arc is extinguished: By measuring line side voltages, reclose time can be controlled to maximize success and minimize delay.

(2) Avoid reclosing into a multi-phase fault: By controlling individual pole reclosing, reclosing into a multi-phase fault is eliminated.

(3) Immediate reclose on sy-athy trips: By interpreting the protective mne involved, the type of fault, and other parameters at the the fault, reclosing strategy would be adaptable to system conditions.

(4) Adaptive synchronism check angle for reclosing: By temporarily changing the synchronism check angle for reclosing in the event of a severe system disturbance, the possibility of system survival is increased.

In addition, the concepts that the questionnaire revealed which were directed toward single transmission line reclosing. could be extended to encompass all the terminals at a given substation.

(5) Reclosing logic for an entire substation: It may be desirable to have a single reclosing relay for an

entire substation. Such a relay would determine which breaker is to reclose, depending on the status of various buses and disconnect switches in the substation. It would take into account any changes in the bus configuration (such as a split bus condition), and check the appropriate bus and line side voltages for status and synchronism checks. The relay would also count the number of relay zone incursions in a pre-selected interval, to detect low magnitude intermittent faults which may not last long enough to operate back up relays. This feature, in conjunction with a fault locator, would help in finding intermittent faults on the system. In principle, this would be the relay that has all the features of adaptive reclosing discussed in items (1)-(4) above. The questions of reliability of such a device may dictate a duplicate system - or even a voting scheme.

Technical feasibilitv; The functions described above in items (1),(2),(3), and (5)

require certain input data within specific time periods, after the initiation of a svstem disturbance. The inDut data consists of ac system voltages and currents, status o f circuit breakers and disconnect switches, and measurement of time intervals between different events taking place on the power system. All of these inputs, whether generated locally within the substation, or at a remote substation, are technically feasible with computer based relay input systems. The voltage and current signals are generally rich in non-fundamental frequency components, and careful analog and digital filtering is needed to obtain the desired estimate of the input, upon which relaying decision could be based. Techniques for achieving a secure estimation of the required parameters have been developed over the past several years, and present no insurmountable difficulties at this time. The ability to track the changing status of circuit breakers and disconnect switches also exist, and could be made less susceptible to error by applying logical checks on appropriate voltages and currents to confirm the detected status of a switch or breaker.

Certain aspects of item (4). which describe the desirable reclosing angle from the point of view of the breaker or generator health, are also capable of being implemented with currently known computer relaying techniques. However, the subject of determining the correct reclosing angle limits so that the reclosed system will remain stable and in a viable state, requires an

entirely new type of capability. It requires the ability to predict the outcome of a dynamic process to be initiated by the act of reclosing. This subject requires considerable analytical ability in a central location where a dynamic state model of the power system is maintained and used'for the prediction task. The safe synch check angle limit is a complex function of various state variables of the power system, and considerable analytical and computational effort may be needed to achieve that goal. It may be possible to find altemative procedures that are not quite optimum. but nevertheless provide a good, practical, solution to the problem.

Communication Needs . .

It may be well to think of an adaptive reclosing relay in the functional block representation shown in Figure 4. Each of the relays is connected to the substation host computer and through it, would acquire the information needed for the adaptive reclosing functions. The substation host computer would also communicate with other substation hosts and a remote central host as needed Data may also be obtained over these links to accomplish the adaptive reclosing task. The station host may be included within one of the relays. Similarly, the station reclosing relay may reside within the substation host computer.

Adaptive reclosing functions 1 and 2 require voltage and current inputs which exist within the relay, and therefore require no externpl communications. Function 3 above - reclosing on sympathy trips - requires local signals. In addition to the inputs normally seen by the reclosing relay, it would also benefit from inputs from the other relays in the substation, which may have initiated the original trip. Also needed may be the breaker and disconnect switch status within the station. All of these inputs are available within the substation host computer, and would be available over the local communication link for the adaptive reclosing function.

Adaptive reclosing task (5 ) requires data from all the breakers and disconnect switches in the substation, and local communication links with the substation host acting as a data concentrator would meet those needs.

Task (4) requires access to data at the system control center for some of its functions. Data about the present state of the power system and its vulnerability is needed. Safe reclosing angle limits may be sent to the relay over the remote communication channel.

. I

Substation A Sub.

. ..._ I B I .

Relay n

Sub. A

Figure 4 Adaptive Reclosing Relay

ClONCLUSIONS

The responses of 90 protective relaying engineers to a questionnaire developed by Working Group K-8 of the Power System Relaying Committee have been analyzed for the purpose of identifying adaptive features in use as a part of protective relaying systems and assessing the need for new adaptive features in such systems. The responses to the first part of the questionnaire involve subjective reactions to satisfaction with present day relaying and provide a background for evaluating acceptability of new or improved adaptive features. It is not surprising that, in general, relay engineers are satisfied with the equipment that they use. The two figures summarizing the

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results give an indication of the comparative use of various relaying schemes. The second part of the questionnaire was an attempt to determine the acceptability of new adaptive features.

As might be expected, the most commonly used relays in transmission line relaying, are phase distance and directional overcurrent relays. It is interesting, however that such adaptive features as load compensation, remote-end open-breaker detection, multi-terminal coverage and adaptive speed vs fault type score well as adaptive features.

Both reclosing and breaker failure responses are widely used and are candidates for adaptive treatment. The harmonic restraint transformer differential is widely used and received a high mean for adaptive features. This parallels the discussion on the transmission line relays. It is clear that there is a great deal of satisfaction with existing relays. Nevertheless, adaptive functions would still be considered as additions to phase distance and directional-overcurrent relays as well as the less commonly used phase comparison and pilot wire relays. In effect, there is a coupling of the satisfaction and familiarity with existing equipment and the inherent versatility of the newer digital devices. This situation strongly suggests that relay engineers will welcome the adaptive technology and the improvements that it can provide,

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Discussion

G. D. Rockefeller (Rockefeller Associates, Inc., Morris Plains, NJ): Fig. 3 seems to suggest that response was luke warm and that there was a lack of strong preference amongst the various concepts. Perhaps this indicates that the respondees saw few benefits. While this may be the case, we should recognize that many probably had little prior exposure to the concepts and had only a brief description to react to. Moreover, I suspect that many did not have a high level of relaying experience.

Accordingly, I think the jury is still out on how fast adaptive concepts are going to be accepted and implemented. Judged by the number of recent papers purporting to apply adaptive techniques, much interest has been aroused, although I sense that "adaptive" is more of a buzzword than an indication of highly effective implementa- tions. However, one shouldn't castigate such usage, since the Relay- ing Committee has not blessed a definition for the term.

I think that relatively little progress can be achieved using just the information conventionally available. Rather, wider knowledge of the condition of the power system, through on-line communication links opens the door to much more potential for innovation. This can first be more information about local substation conditions, but eventually down loading of data from area or dispatch computer complexes.

Manuscript received May 22, 1992.

ROBERT E. WILSON, Western Area Power Adminis- tration, Golden, CO; The committee has pro- duced an informative and readable report. It presents not only current practices in relay- ing but also innovation for the future. The report is particularly valuable because it reflects the judgement and experience of practicing relay engineers. The statistics calculated from the question-

naire were helpful in showing the Ithappiness indext1 and the clustering of the responses. An important result is that, on the one hand, respondents were generally satisfied with existing relays. At the same time, the engi- neers would welcome many adaptive features. This report should be read by all relay manu- facturers. In my opinion, some of the adaptive features

could easily be implemented. For example, by using distance relays with multiple setting groups a modern SCADA system could modify line relay settings when one terminal was removed for maintenance. In modern microprocessor- based relays measuring principle and control logic changes could be made by replacing one or a mall number of electronic chips. Expen- sive relay replacement should not be neces- sary. The report mentions that Ifthe mathematical

curve describing the fault location and trip time may arise implicitly in the design." This result has been observed for one model of a digital distance relay in a study which I did of relay modeling within EMTP. A relay that uses the mho measuring principle and a discrete Fourier detector to recover phasor information was simulated. The response time of the digital relay grew shorter as the simulated fault was moved closer to the relay location (1) . An adaptive feature I would like to see

involves reclosing close to generating plants or series capacitor banks. Utilities may decide never to reclose any transmission line coming from a generating plant because the fault may be permanent and close to the plant. If the protection system could quickly esti- mate the relative distance to the fault and

the fault type, then more informed reclosing decisions could be made. A utility may decide to reclose if the fault involves only one phase and close to the opposite line terminal.

If the permanent fault is closer to the oppo- site terminal, there may be enough Vubbertl in the system to lessen the effects of an unsuc- cessful reclose on system stability. The report summarized many sound ideas that

are no longer viewed as radical or as tech- niques to be avoided. In several years we may be reading reports about the successful and wide-spread application of adaptive relaying. I enjoyed reading the report.

References

[l] R.E. Wilson and J.M. Nordstrom, "EMTP Transient Modeling of a Distance Relay and a Comparison with EMTP Laboratory Testing, IEEE POWER Summer Power Meeting, Paper 92 SM 378-0 PWRD, July 12-16, Seattle, WA.

Manuscript received August 3 , 1992 .

R. Rodrigues (Electricidade de Portugal), Pinto de Sa (Instituto Superior Tkcnico) Lisbon, Portugal: This survey is very valuable for helping researchers and manufacturers to drive their work with adap- tive relaying to real life problems.

However, we feel that the most desired adaptive feature for utility engineers is the capability for remote setting, if not only because it will avoid the travelling time and staff needed for local setting. This may also be the most feasible adaptive feature, because it only re- quires from the relays a remote communication capability as well as a Computer Aid for Protection Coordination, both of which are already appearing in the market. The links between such Coordination Aids and the relays are a matter of SCADA and EMS openness, as well as of Substation LANs. Standards for all these aspects are being actively addressed namely in the IEEE, so that a medium term remote adaptive setting feature appears as quite feasible as a result of several parallel developments.

The fact that such an adaptive feature would be a very welcome event is recognized in the paper in the "Suggested Adaptive Features" sec- tion. Theory for automatic adaptive Coordination and Setting has also been a matter of research [Cl]. We feel that the feature of adaptive remote setting from a Coordination Center would be a very interest- ing question that should have been included in the survey.

References: [Cl] - A.K. Jampala, S.S. Venkata, M.J. Damborg, "Adaptive Transmission Protection - Concepts and Computational Issues", IEEE Trans. on PWRD, January 1989, Vol. 4, n'. 1, pp. 177-185.

Manuscript received August 5, 1992 .

THE WORKINGGROUP The Working Group would like to thank the Discussers for their comments. In general, the Working Group agrees that there are many additional attractive features that could be envisioned. The Working Group was only limited by the of the survey that respondents would be willing to answer. There are a large number of additional adaptive features in [l] and [2] that were not included in the questionnaire.

Messrs Rodrigues and Pinto de Sa suggest automatic remote Coordination and Setting. The capability for changing a relay setting from a remote location is attractive. Manually making such a change is an acceptable feature of digital relays. Changing the setting adaptively from a central location, however, is an area for additional research. At a minimum, it requires that virtually all of the relays in a given area be digital, that a secure and dependable communication system be in place and a comprehensive coordination program be available. As the

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discussers note, all of these areas are under study and this feature may be widely implemented.

Mr Rockefeller comments on the lack of strong preferences in the responses. The Working Group was encouraged that there were few strong negative responses and that the relay community (with little experience with adaptive features) saw some value in most of the suggested features. Perhaps the Working Group should have included some clearly undesirable features as a control. We concur that adaptive is a "buzz word" but suspect that ""buzz words" are part of the process of technical innovation.

We agree with the general tone of hkRockefeller's discussion. The survey may have been the first introduction of the adaptive relaying concept to many of the respondees. As a result, the responses must be viewed more as a "wish list" than a specific requirement. The important conclusions, however, are that engineers are becoming more comfortable with the concept and are beginning to generate features in which relay performance is more intimately tied to real-time system conditions.

Substantial progress will certainly increase with more data

and/or more data transmission. As the survey indicates, however, there are many desirable features that are immediately obtainable with existing local and remote information. This is a necessary first stage in an evolving technology.

Mr. Wilson has suggested adaptive reclosing functions and multiple settings in distance relays. Certainly such adaptive features are immediately obtainable. The phased approach mentioned above is emphasized by his discussion.

[l] G.D. Rockefeller, C.L. Wagner, J.R. Linders, K.L. Hicks, and D.T. Rizy, "Adaptive Transmission Relaying Concepts for Improved Performance", IEEE Trans. Power Delivery, vo1.3, no.4, pp. 1446-1458, Oct 1988

[2] S.H. Horowitz, A.G. Phad!ce and J.S. Thorp, "Adaptive Transmission System Relaying , IEEE Trans. Power Delivery, vo1.3, no.4, pp. 1436-1445, Oct 1988

Manuscript received September 2 4 , 1992