western systems coordinating council · synchronous condensers, cogenerators and induction motors)...

Western Systems Coordinating Council

AUGUST 1994

SUPPORTING DOCUMENT

FOR

RELIABILITY CRITERIA FOR

TRANSMISSION SYSTEM PLANNING

J. Kondragunta, SCEand

WSCC Reliability Subcommittee

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TABLE OF CONTENTS

PAGE

1. INTRODUCTION ............................................................................................................. 1

2. BACKGROUND ............................................................................................................... 1

3. TRANSIENT VOLTAGE CRITERIA .............................................................................. 1

3.1 INTRODUCTION ................................................................................................. 1

3.2 SWING VOLTAGES ............................................................................................ 2

3.3 OVERVOLTAGE CRITERIA .............................................................................. 3

3.4 UNDERVOLTAGE CRITERIA ........................................................................... 3

4. TRANSIENT FREQUENCY CRITERIA ........................................................................ 7

4.1 OVERFREQUENCY CRITERIA ......................................................................... 7

4.2 UNDERFREQUENCY CRITERIA ...................................................................... 7

5. POST TRANSIENT VOLTAGE DEVIATION .............................................................. 9

LIST OF TABLES

Table 1 Disturbance-Performance Table........................................................................... 10

Table 2 Performance Levels.............................................................................................. 11

Table 3 Disturbances Ordered by Frequency of Occurrence............................................. 12

Table 4 Sinusoidal Real System Voltage Dip <100%....................................................... 13

Table 5 Sinusoidal Real System Voltage Dip <90%......................................................... 14

Table 6 Sinusoidal Real System Voltage Dip <85%......................................................... 15

Table 7 Average Peak Voltage Dip <100% ...................................................................... 16

Table 8 Average Peak Voltage Dip <90% ........................................................................ 17

Table 9 Average Peak Voltage Dip <85% ........................................................................ 18

Table 10 Load Loss with Voltage Swing of 0.7 Hz .......................................................... 19

Table 11 Load Loss with Voltage Swing of 0.28 Hz ........................................................ 20

Table 12 WSCC Load Shedding ....................................................................................... 21

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LIST OF FIGURES

Figure 1 Schematic Diagram for Swing Voltage Measurements ...................................... 22

Figure 2 Load Sensitivity to Voltage Dips ........................................................................ 23

Figure 3 Voltage Dip Limit - Level A Disturbance - 0.7 Hz Swing ................................. 24

Figure 4 Voltage Dip Limit - Level B,C,& D Disturbances - 0.7 Hz Swing .................... 25

Figure 5 Voltage Dip Limit - Level A Disturbance - 0.28 Hz Swing ............................... 26

Figure 6 Voltage Dip Limit - Level B,C,& D Disturbances - 0.28 Hz Swing .................. 27

Figure 7 Voltage Performance Parameters........................................................................ 28

APPENDIX I: Examples of Disturbance Classifications............................................................. 29

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1. INTRODUCTION:

The purpose of this document is to provide the rationale for the parameters in the PerformanceTable 1 in the Reliability Criteria for Transmission System Planning, Part I of the WSCCReliability Criteria dated March, 1993.

The parameters in the performance table, Table 1, were selected in such a way that theapplication of new criteria, dated March 1993, do not result in a higher or lower level ofreliability for the WSCC system when compared to the level of reliability from the applicationof the old reliability criteria, dated March 1990.

The reliability criteria in its final form has been developed by lengthy discussions among themembers of the Reliability Subcommittee, comments received from the PCC members as wellas extensive work performed by the Ad-hoc Reliability Criteria Parameters Evaluation Group(ARCPEG) of TSS.

2. BACKGROUND:

Table 2, from the old criteria, shows the allowable actions or conditions on systems other thanthe one in which the disturbance occurred. The performance table shown in Table 1 wasdeveloped in such a way that the actions resulting from application of Table 1 for variouslevel of disturbances will be essentially the same as Table 2.

The Subcommittee classified various disturbances into level A, B, C and D. The necessaryprobability data shown in Table 3, provided by Bonneville Power Administration, was used toestablish relative outage rates for various elements of the system. Also the table shows therelative rate of outage of each element with respect to a single line outage.

3. TRANSIENT VOLTAGE CRITERIA:

3.1 Introduction:

Historically, some WSCC members have used a minimum swing voltagemagnitude of 0.8 p.u. as the performance criteria in assessing acceptable bulktransmission transient stability results. Consequently, the corresponding percentvoltage dip could vary widely as long as the magnitude of the dip did not go below0.8 p.u.. The 0.8 p.u. voltage criteria provided margin for nuclear unit auxiliaryundervoltage trip protection that is usually set at a magnitude of 0.65 p.u. to 0.70p.u. based on the rated generator terminal voltage. In most instances it would takeat least a N-2 outage to produce such critical voltage deviations under the newcriteria. Nuclear plant owners should be especially careful in setting power flowconditions for nuclear units, because under the right set of conditions a N-2 outagecould cause an allowed voltage deviation of 25% that could lead to a nuclear unittrip if the initial voltage was below 1.0 p.u..

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The auxiliary protection on fossil-fired units has not been investigated at this time.However, unit owners are encouraged to determine what the undervoltage tripsettings are and their relationship to the allowable voltage dips in the PerformanceTable to assess stability performance.

3.2 Swing Voltages:

•Measurement:

Figure 1 and the following examples illustrate why using a maximum percentagevoltage swing deviation is more appropriate than using a fixed minimum voltagemagnitude when examining study results.

In the WSCC data bank, the 500 kV and 230 kV systems are fairly well represented,but the lower voltage systems (below 230 kV down to the distribution level) aresparsely represented, if they are represented at all. There is also additional voltageregulation at the distribution level (either regulating transformers, or regulators onindividual feeders), which is generally not represented in the WSCC data. Thisleads to the following problems (assuming that a given percent voltage change on ahigh voltage bus causes an equal percent swing on the load bus):

1) If the load is represented at a 500 kV or 230 kV bus, the initial voltage may bewell above 1.00 p.u.. In the attached example, the 230 kV bus voltage is at 1.052p.u. and the 500 kV bus voltage is at 1.1 p.u.. If a criteria of 0.8 p.u. is used forvoltage swings, the voltage on the low voltage bus (where the load actually isconnected) could deviate between 0.76 p.u. (0.8/1.052) and 0.73 p.u. (0.8/1.1).

2) If the load is represented at a 115 kV or other low voltage bus, the initial voltagemay be below 1 p.u.. In the attached example, the 115 kV voltage is 0.974 p.u.. Therefore, the allowable swing may be only 0.82 p.u. (0.8/0.974) on the 13.8 kV lowvoltage load bus.

From the example, it is clear that using a criteria of 0.8 p.u. does not appropriatelyrepresent effects on the load itself. It is logical to use a percentage voltage diprather than the absolute minimum valuation for voltage swings.

Note that the actual voltage realized at the load bus may differ from this radialexample due to the network configuration and active devices in the system. Forsystems having subtransmission networks containing active elements (for example,synchronous condensers, cogenerators and induction motors) the load bus voltage dipis more complex than the simple radial example. For some 500 kV or 230kV disturbances, the subtransmission buses could have a higher or lower dip thanthe 500 kV or 230 kV voltage dips.

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3.3 Overvoltage Criteria:

The ARCPEG recommendation was that an overvoltage criteria is not needed forWSCC member systems, based on the survey, and is not recommended herebecause it is usually a local problem.

3.4 Undervoltage Criteria:

The Reliability Subcommittee has established voltage dip criteria designed toavoid uncontrolled loss of load. The values selected were based on the estimatedresponse of electronic equipment such as computers to voltage dips. These loads arebelieved to be most sensitive. Load sensitivity data was obtained from severalsources that were somewhat imprecise and inconsistent. During the one-year trialperiod Reliability Subcommittee conducted load sensitivity and evaluation of theWSCC voltage dip criteria.

The results of the study indicate that the WSCC voltage dip criteria now in placeare appropriate and that no modification is warranted at this time.

The Reliability Subcommittee is in general agreement that the following table willapply for voltage dip criteria to meet its objectives.

Instantaneous Maximum DurationLevel Minimum dip of V Dip exceeding 20%

A 25% 20 cyclesB 30% 20 cyclesC 30% 40 cyclesD 30% 60 cycles

Figure 7 shows the interpretation of the voltage performance parameters.

As indicated in Table 3 the disturbances that occur frequently need to meet level Aperformance. As the frequency of occurrence of disturbances decreases, the levelof performance changes from level A to levels B, C and D. The allowable transientvoltage dips for these levels were selected based on the frequency of occurrence ofevents. In other words, maximum transient voltage dip for level A disturbancesshould be less than that of level B disturbances and so on.

BACKGROUND — At the time the criteria were established, several sources ofinformation were available relating equipment response to voltage dips. The Ad-hoc Reliability Criteria Parameters Evaluation Group (ARCPEG) suggested diplevels and duration for the Disturbance-Performance Table considering SDG&E,CBEMA (Computer and Business Equipment Manufacturers Association)/IEEE,and ASEA data. The SDG&E curve was believed to be the most conservative andwas used as the basis for their recommendations. The values adopted by the

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RS for the table, referred to the SDG&E curve, were believed to provide marginagainst load loss for levels A, B, and C, and to be at the threshold for level D.

Duane Braunagel of Platte River Power Authority also reviewed these and otherdata sources and noted a discrepancy in what was purported to be the CBEMAcurve. This curve was shown about the same in the SDG&E, U.S. Dept. ofCommerce publication FIPS PUB 94, and an IEEE Transactions on PowerDelivery paper from 1990. However, ANSI/IEEE Standard 446-1987 shows a"Typical Design Goals of Power-Conscious Computer Manufacturers" curve thatresembles the so-called CBEMA curve but is noticeably more conservative. Usingthe ANSI/IEEE curve as a reference, it appeared that levels A and B as now shownin the table would be at the load loss threshold and levels C and D would result inuncontrolled load loss. (Any load loss, firm or interruptible, by transient voltagedip would be considered uncontrolled and therefore unacceptable.) TheANSI/IEEE source seemed the most authoritative and appropriate data to use forWSCC criteria development.

Duane also provided information to the Subcommittee indicating that voltage diptests were performed by imposing a step change dip in voltage on electronicequipment for various time durations. He pointed out that, on the real system,voltage dips are sinusoidal in shape which would cause less time exposure to lowvoltage for equipment than a step change, thereby providing some margin if theANSI/IEEE data is directly applied to the system. The question of concern was, howgreat is this margin and is it sufficient to generally offset the higher load sensitivityand resultant loss of load indicated by the ANSI/IEEE curve?

Refer to Brent Vossler's October 7, 1992 letter and final report to the ARCPEG,Duane Braunagel's February 9 and 22, 1993 letters to Mike Raezer, and his June 3,1993 letter to the RS for details about the above information.

CBEMA was contacted to obtain better information on electronic load sensitivity tovoltage dips. Bill Hanrahan of CBEMA's standards department said that CBEMAhas not developed any curves showing computer voltage tolerance, although he wasaware that various such curves have been represented as CBEMA curves. Hereferred to the curves in ANSI/IEEE Std. 446-1987 and in FIPS PUB 94 as the bestsources for guidance (neither document represents the curves as being fromCBEMA), but did not know the origin of the curves. He was not aware that thecurves were different and on being informed, recommended using the moreconservative ANSI/IEEE curve. He also suggested contacting John Roberts of IBM,Chair of CBEMA's Power Interfaces Committee for further information.

Mr. Roberts was contacted and was able to provide some historical background andinformation on some new standards work. He said that the so-called CBEMA curvewas developed several years ago by a gentleman (didn't get the name) inthe Navy Department for their computers. The curve was adopted by CBEMAfor its use. The latest information that Mr. Roberts had was a standard developed

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by IBM and proposed for adoption by the International Electrotechnical Commission(IEC). The IEC was apparently dragging its feet on adopting the standard but Mr.Roberts expected that the European equivalent of CBEMA was very likely to adoptit. He said that this standard best represents the sensitivity of modern computers tovoltage dip. Mr. Roberts confirmed that the sensitivity tests are conducted bysubjecting the loads to a step reduction in voltage followed by a step increase back tothe original level.

SELECTION OF A LOAD SENSITIVITY CURVE — Figure 2 shows loadsensitivity to voltage dips according to the ANSI/IEEE Std. 446-1987, IBM, SanDiego, and "CBEMA" (as represented in the San Diego study) standards. The IBMcurve is based on 3 discrete points from the standard at 0, 75, and 80 percentvoltage. The ANSI/IEEE data is shown in two ways. The data was given in the446-1987 standard in the form of a curve that was rather more like a sketch than anaccurate plot. It was apparently intended more for guidance than to serve as aprecise standard. Duane Braunagel enlarged the curve on a copy machine andtransferred the data as plotted to semi-log graph paper to make it easier to read. This is shown as the ANSI/IEEE curve "ANSI FIG 4-SCALED." However, discretevalues for three points on the curve were indicated at 0, 30, and 87 percent voltage,so a curve was also constructed using these three data points. This is shown as theANSI/IEEE curve "ANSI FIG 4 - 3PTS." The two reproduced curves do notcoincide. The original was obviously not drawn to the scale indicated by the datapoints.

The IBM standard is in close agreement with the San Diego curve in the region ofinterest, that is, between 70 and 80 percent voltage and between 20 and 80 cycles.These curves are based on measured equipment response. The "CBEMA" is leastconservative, the ANSI/IEEE most conservative, of all the curves. The sources forthe ANSI/IEEE curves are not known. In summary, the IBM standard is most up-to-date and from a known source, is probably more indicative of today's equipmentcapability, and is somewhat mid-range among the various options along with the SanDiego curve. It seems the best choice for evaluating the WSCC voltage dipstandard.

COMPARING THE SINUSOIDAL REAL SYSTEM VOLTAGE DIP WITH THESQUARE WAVE TEST DIP — If we assume that the energy delivered to aresistive load is an approximate indicator of load reaction to a voltage dip we canconvert the system sinusoidal dip to an equivalent square wave dip of equal energy(Tables 4-6). We can then compare this square wave dip to the allowable squarewave dip on the curve. Alternatively, we can assume that the load responds directlyto peak voltage rather than energy delivered, and convert the sinusoidal voltageduring the dip to an average peak voltage (Tables 7-9). We can then compare asquare wave dip of the same value to the allowable dip on the curve. The twoapproaches produced about the same result with the energy based equivalent squarewave dip generally being slightly greater.

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The effect was also examined by varying the portion of the dip that was assumed toaffect the load, assuming that regulated power supplies in electronic equipment arenot affected by low voltage until the voltage drops below a critical level. Thevarious curves suggest that equipment can continuously tolerate voltage that doesnot drop below about 85% to 90%. Tables 4-9 show that the difference between asinusoidal dip and a square wave dip diminishes as the critical voltage threshold islowered. Thresholds of 100%, 90% and 85% were evaluated. The moreconservative 90% threshold, rather than 100%, was used for the primary evaluationof load loss at the limiting conditions specified in the WSCC criteria.

WILL APPLICATION OF THE WSCC CRITERIA RESULT IN LOSS OF LOAD?— The limiting condition was determined for voltage swings of 0.7 and 0.28 Hz atperformance levels A, B, C, and D of the criteria. Examination of the time voltageremained below 80% at the maximum allowed voltage dip for each level indicatedwhether the maximum dip or the time less than 80% would be limiting (Figures 3through 6).

The limiting conditions are summarized in Tables 10 and 11, as are the equivalentsquare wave voltage dips, and time allowed by the IBM standard for the dip at thelimiting condition. Load was assumed lost if the dip time exceeded the time allowedby the IBM standard. Time margin is also indicated.

The analysis indicated that no load would be lost for a 0.7 Hz voltage swing (a"typical" swing) at any performance level. Level A has about a one second marginand level B 12 cycles. Levels C and D have no margin and would appear to be atthe threshold of load dropping. This is consistent with the rationale used to developthe voltage dip standard.

For a 0.28 Hz voltage swing (a "slow" swing), no load would be lost for levels Aand B with more than a half second margin. Level C has virtually no margin andwould be at the threshold of load dropping. There would appear to be loss of loadat level D with about a negative one second margin. For a sensitivity toassumptions comparison, data is also shown on the basis that the voltage dip affectsload at all levels below initial voltage, a less severe condition for a sinusoidal dip. For this condition no load loss would occur for level D.

CONCLUSIONS — The voltage dip performance standards in the WSCCPlanning Criteria Disturbance-Performance Table will likely produce the intendedsystem response during credible disturbances. It was intended that no load be lostdue to voltage dips for level A through D disturbances, with some margin at thehigher levels and little or no margin at the lower levels. There appears to besome risk of load loss at level D for a very low frequency voltage swing using themore conservative load response assumptions, but the risk does not seem greatenough to warrant changes in the criteria at this time.

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4. TRANSIENT FREQUENCY CRITERIA:

4.1 Overfrequency Criteria:

WSCC Reliability Subcommittee did not find a need to define a newoverfrequency criteria. This recommendation was presented to PCC.

The ARCPEG's recommendation is that an overfrequency criteria is not neededbecause:

— the member survey did not indicate a need and— most overfrequency problems are associated with generators which have

local protection that may vary widely by generator type, frequency setpoints, and time delays.

This recommendation from ARCPEG also supports the Subcommittee'srecommendation regarding the overfrequency criteria.

4.2 Underfrequency Criteria:

The frequencies chosen are intended to coordinate with underfrequency loadshedding (UFLS) programs. UFLS is expected to arrest frequency decline andassure continued operation of the system within any islands that may be formed as aresult of a disturbance. In each island, underfrequency load shedding relay settingsare coordinated with underfrequency protection of generating units and any othermanual or automatic actions which can be expected to occur under conditions offrequency decline. The coordinated automatic load shedding program is based onstudies of system dynamic performance, under conditions which would cause thegreatest potential imbalance between load and generation.

In the long run, the frequency specifications are expected to remain relativelyconstant. If a system sets load shedding above a particular threshold the actionwill be interpreted as accepting loss of that load for the specified class ofdisturbance.

Table 12 summarizes the load preservation program data from the"COORDINATED BULK POWER SUPPLY PROGRAM 1992-2002"(OE-411) report. The numbers shown in Table 12 are estimated additional MWrelief at each frequency level.

The rationale for each performance level is as follows:

Level A (59.6 Hz)

Based on the old criteria, no load dropping is allowed and this concept iscarried out in selecting the frequency.

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The intent is to have no firm or interruptible load shedding. 59.6 Hz wasselected and this is above nearly all known settings. Some interruptibleloads are now set at 59.75 Hz and tripping of these is acceptable.

Level B (59.4 Hz)

Based on the old criteria, dropping of interruptible load is allowed and thisconcept is carried out in selecting the frequency.

A minimum frequency of 59.4 Hz was selected and this allows dropping ofinterruptible load only and will not allow dropping of firm load.

Level C (59.0 Hz)

Based on the old criteria, controlled opening of interconnections, systemislanding and automatic under frequency load dropping are allowed and thisconcept is carried out in selecting the frequency.

In order to satisfy the above requirements a frequency of 59.0 Hz was selectedand this allows dropping of only the first two blocks of load which includesfirm load. Also this setting leaves additional load to be dropped for level D.

Level D (58.1 Hz)

Based on the old criteria controlled dropping of firm load and generationseparation are allowed and this concept is carried out in selecting thefrequency.

A minimum frequency of 58.1 Hz was selected to allow shedding of most,but not all, load under UFLS control. Maintaining some load shedding setbelow the minimum criteria frequency is important insurance against totalcollapse of a major island.

Also, at present interconnections among California Power Pool (CPP)members open at 58.2 Hz. A minimum frequency of 58.1 Hz allowscontrolled opening of CPP interconnections during disturbances.

The Reliability Subcommittee is in agreement that the following table will applyfor frequency criteria to meet its objectives.

Level Minimum Transient Frequency

A 59.6 HzB 59.4 HzC 59.0 HzD 58.1 Hz

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5. POST TRANSIENT VOLTAGE DEVIATION:

In developing a criteria for post transient voltage deviation, it was the intent of theReliability Subcommittee to provide some measure of the ability of the system torecover to acceptable operating conditions following an outage. The Subcommitteewas concerned both with load protection and system integrity. While post transientvoltage can provide some insight into the incipient voltage collapse problems, initself it is not adequate as a voltage stability criteria. The Subcommittee recognizesthat a voltage collapse criteria may include reactive power margin, minimumvoltage and the ability to consider local constraints.

The simple voltage criteria as proposed does provide some measure of systemrecovery following an outage and may be an initial step toward developing a morecomplete criteria in the future. The language was placed into the current reliabilitycriteria:

For the purpose of these criteria, the post transient time frame is one to threeminutes after a system disturbance occurs. This allows available automaticvoltage support measures to take place, but does not allow the effects ofoperator manual actions or Area Generation Control response. Therecommended simulation is a post transient power flow that simulatesall automatic action but not manual actions and not area interchange control.

These criteria are not intended to fully address potential voltage collapseproblems; to do so would require consideration of local constraints that are noteasily generalized.

The Subcommittee is still concerned that voltage collapse should be addressed inthe planning process. Footnote 7 of the Disturbance-Performance Table is intendedto override the reliability criteria table in cases where voltage collapse is known tooccur at voltage levels higher than those in the table. Planning and operation mustbe constrained to avoid voltage collapse.

At this time, voltage collapse avoidance requires more sophisticated analysis thancan be reflected in a simple voltage deviation criterion. The Subcommittee has notrecommended any type of var/watt/voltage margin requirements in the criteria, dueto the likelihood that these criteria are sensitive to the robustness of the effectedarea and the type of disturbance.

Until more is known about voltage collapse throughout the WSCC system, eachmember system should evaluate its criteria to avoid voltage collapse for knowndisturbances. It is the expectation of the Subcommittee that these voltage criteria beaddressed through the peer review process. Within the procedures for ratingtransmission paths, it is the responsibility of the affected parties to identify potentialvoltage collapse problems and bring them to the attention of the sponsors of the pathbeing rated.

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TABLE 1WSCC DISTURBANCE-PERFORMANCE TABLE

OF ALLOWABLE EFFECT ON OTHER SYSTEMS(1)

PerformanceLevel

Disturbance(2)Initiated By:No Fault3 Ø Fault With Normal ClearingSLG Fault With Delayed ClearingDC Disturbance (3)

TransientVoltageDipCriteria

(4)(5)(6)

MinimumTransientFrequency

(4)(5)

PostTransientVoltageDeviation(4)(5)(6)(7)

LoadingWithinEmergencyRatings

Damping

A GeneratorOne CircuitOne TransformerDC Monopole (8)

Max V Dip - 25%

Max Duration of VDip Exceeding 20%

- 20 cycles

59.6 hz 5% Yes >0

B Bus Section Max V Dip - 30%


- 20 cycles

59.4 hz 5% Yes>0

C Two GeneratorsTwo CircuitsDC Bipole (8)

Max V Dip - 30%


- 40 cycles

59.0 hz 10% Yes >0

D Three or More circuits on ROWEntire SubstationEntire Plant Including Switchyard

Max V Dip - 30%


- 60 cycles

58.1 hz 10% No >0

(1) This table applies equally to the system with all elements in service and the system with one element removedand the system adjusted.

(2) The examples of disturbances in this table provide a basis for estimating a performance level to which adisturbance not listed in this table would apply.

(3) Includes disturbances which can initiate a permanent single or double pole DC outage.

(4) Maximum transient voltage dips and duration, minimum transient frequency, and post transient voltagedeviations in excess of the values in this table can be considered acceptable if they are acceptable to theaffected system or fall within the affected system's internal design criteria.

(5) Transient voltage and frequency performance parameters are measured at load buses (including generating unitauxiliary loads); however, the transient voltage performance parameters for level D apply to all buses. Allowable post transient voltage deviations apply to all buses.

(6) Refer to Figure 1.

(7) If it can be demonstrated that post transient voltage deviations that are less than these will result in voltageinstability, the system in which the disturbance originated and the affected system(s) should cooperate inmutually resolving the problem. Simulation of post transient conditions will limit actions to automatic devicesonly and no manual action is to be assumed.

(8) Refer to section 7.0 - Application to DC Lines, paragraph 7.2.

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TABLE 2. PERFORMANCE LEVELS

ALLOWABLE ACTIONS OR CONDITIONS ON SYSTEMSOTHER THAN THE ONE ON WHICH THE DISTURBANCEOCCURRED

PERFORMANCE LEVEL *

A B C D

1. ACTIONS AS PERMITTED BELOW

a. DROPPING OF INTERRUPTIBLE LOAD NO YES YES YES

b. CONTROLLED GEN. DROPPING OREQUIVALENT REDUCTION OF ENERGYINPUT TO THE SYSTEM NO YES YES YES

c. CONTROLLED OPENING OF INTER-CONNECTIONS INCLUDING SYSTEMISLANDING AND AUTOMATIC UNDER-FREQUENCY LOAD DROPPING NO NO YES YES

d. CONTROLLED DIRECT DROPPING OF FIRMLOAD NO NO NO YES

e. CONTROLLED SUB ISLANDING ANDGENERATION SEPARATION NO NO NO YES

2. POST DISTURBANCE LOADINGS ANDVOLTAGES OUTSIDE OF EMERGENCY LIMITSPRIOR TO ADJUSTMENT NO NO NO YES

* A "YES" INDICATES THE ACTION OR CONDITION IS PERMITTED IN SIMULATION TESTING TO MEETTHE PERFORMANCE LEVEL IF REQUIRED TO PREVENT CASCADING. A "NO" INDICATES THAT THEACTION OR CONDITION IS NOT ALLOWED.

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TABLE 3. DISTURBANCES ORDERED BY FREQUENCY OFOCCURRENCE

Outage RateOutage Relative To

Element Lost Rate/Year Line NOTE

Generator 4.0 2 1One Line 1.8 1 1Two Generators 0.33 0.2 2Two Line Dependent 0.088 0.05 3Line + Generator 0.082 0.045 3Bus Section 0.072 0.04 3Transformer 0.037 0.018 4Generator + Transformer 0.012 0.007 5Line + Transformer 0.0051 0.003 5Two Line Independent 0.0030 0.002 5

NOTE:

1. Level A — Comparable to single line outages.2. Not used — Seen as non-simultaneous outage e.g., one unit trips while

another is down on maintenance.3. Level B — Dependent outages seen as variations of bus section outages.4. Level A — The Reliability Subcommittee is in general agreement that outage

of one transformer should be deemed Level A in spite of frequency data tomake the performance requirement consistent for loss of single element and tomaintain about the same level of performance provided by the previouscriteria.

5. Level C — (N-2) outages two orders of magnitude less likely than N-1.

TABLE 4

• SINUSOIDAL DIP IN 60 HZ, 1 VOLT PEAK SIGNAL• ENERGY INTO 1 OHM LOAD DURING PORTION OF DIP WHERE V < 100%

FREQUENCY FREQUENCY V DIP START TIME END TIME DURATION ENERGYHz radians % seconds seconds seconds watt-sec

____________ ____________ _____ ___________ __________ __________ _________0.70 4.398 25 0 0.7143 0.7143 0.25460.70 4.398 30 0 0.7143 0.7143 0.23680.28 1.759 25 0 1.7857 1.7857 0.63650.28 1.759 30 0 1.7857 1.7857 0.5919

• EQUIVALENT SQUARE WAVE DIP IN 60 HZ, 1 VOLT PEAK SIGNAL FOR SAME ENERGY INTO 1 OHM LOAD

V DIP DURATION ENERGY% seconds watt-sec

_______ ___________ __________15.6 0.7143 0.254418.6 0.7143 0.236615.6 1.7857 0.636018.6 1.7857 0.5916

Assumes energy delivered to resistive load is relevant indicator of voltage dip effect on electronic equipment.Assumes voltage dip affects load at all levels below initial voltage.

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TABLE 5



____________ ____________ _____ ___________ __________ __________ _________0.70 4.398 25 0.0936 0.6207 0.5271 0.16760.70 4.398 30 0.0773 0.6370 0.5597 0.16380.28 1.759 25 0.2339 1.5518 1.3178 0.41900.28 1.759 30 0.1932 1.5925 1.3993 0.40960.70 4.398 27 0.0863 0.6280 0.5417 0.16670.28 1.759 21 0.2822 1.5036 1.2214 0.41420.28 1.759 24 0.2443 1.5414 1.2971 0.4192



_______ ___________ __________20.3 0.5271 0.167423.5 0.5597 0.163820.3 1.3178 0.418523.5 1.3993 0.409521.6 0.5417 0.166517.6 1.2214 0.414619.6 1.2971 0.4192

Assumes energy delivered to resistive load is relevant indicator of voltage dip effect on electronic equipment.Assumes no effect of voltage dip on load until dip exceeds 10%.

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TABLE 6



____________ ____________ _____ ___________ __________ __________ _________0.70 4.398 25 0.1463 0.5680 0.4217 0.12650.70 4.398 30 0.1191 0.5952 0.4762 0.13070.28 1.759 25 0.3658 1.4199 1.0540 0.31610.28 1.759 30 0.2977 1.4880 1.1904 0.3266



_______ ___________ __________22.5 0.4217 0.126625.9 0.4762 0.130722.6 1.0540 0.315725.9 1.1904 0.3268

Assumes energy delivered to resistive load is relevant indicator of voltage dip effect on electronic equipment.Assumes no effect of voltage dip on load until dip exceeds 15%.

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TABLE 7

• SINUSOIDAL DIP IN 60 HZ, 1 VOLT PEAK SIGNAL• AVERAGE PEAK VOLTAGE DURING PORTION OF DIP WHERE V < 100%

FREQUENCY FREQUENCY V DIP START TIME END TIME DURATION V-AVG.Hz radians % seconds seconds seconds volts

____________ ____________ _____ ___________ __________ __________ _________0.70 4.398 25 0 0.7143 0.7143 0.84080.70 4.398 30 0 0.7143 0.7143 0.80900.28 1.759 25 0 1.7857 1.7857 0.84080.28 1.759 30 0 1.7857 1.7857 0.8090

• EQUIVALENT SQUARE WAVE DIP IN 60 HZ, 1 VOLT PEAK SIGNAL FOR SAME AVERAGE PEAK VOLTAGE

V DIP DURATION V-AVG.% seconds volts

_______ ___________ __________15.9 0.7143 0.840819.1 0.7143 0.809015.9 1.7857 0.840819.1 1.7857 0.8090

Assumes average peak voltage applied to load over given time is relevant indicator of voltage dip effect on electronic equipment.Assumes voltage dip affects load at all levels below initial voltage.

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TABLE 8



____________ ____________ _____ ___________ __________ __________ _________0.70 4.398 25 0.0936 0.6207 0.5271 0.80230.70 4.398 30 0.0773 0.6370 0.5597 0.77020.28 1.759 25 0.2339 1.5518 1.3178 0.80230.28 1.759 30 0.1932 1.5925 1.3993 0.7702



_______ ___________ __________19.8 0.5271 0.802323.0 0.5597 0.770219.8 1.3178 0.802323.0 1.3993 0.7702

Assumes average peak voltage applied to load over given time is relevant indicator of voltage dip effect on electronic equipment.Assumes no effect of voltage dip on load until dip exceeds 10%.

17

TABLE 9



____________ ____________ _____ ___________ __________ __________ _________0.70 4.398 25 0.1463 0.5680 0.4217 0.78430.70 4.398 30 0.1191 0.5952 0.4762 0.75190.28 1.759 25 0.3658 1.4199 1.0540 0.78430.28 1.759 30 0.2977 1.4880 1.1904 0.7519



_______ ___________ __________21.6 0.4217 0.784324.8 0.4762 0.751921.6 1.0540 0.784324.8 1.1904 0.7519

Assumes average peak voltage applied to load over given time is relevant indicator of voltage dip effect on electronic equipment.Assumes no effect of voltage dip on load until dip exceeds 15%.

18

TABLE 10

APPLICATION OF THE WSCC VOLTAGE DIP CRITERIAFOR VOLTAGE SWING OF TYPICAL FREQUENCY

CHECK FOR LOAD LOSS AT LIMITING CONDITION ALLOWED BY CRITERIA

0.7 HZ VOLTAGE SWINGWSCC CRITERIA

LIMITSWSCCPERF.LEVEL

V DIP(%)

T < 80%(CYCLES)

LIMITING AND CORRESPONDINGNON-LIMITING - ( )

CONDITION

EQUIV. SQUAREWAVE DIP & TIME(% V DIP) (CYCLES)

TIME ALLOWED BYIBM STANDARD

(CYCLES)

LOADLOSS?

(YES/NO)MARGIN

(CYCLES)

A 25 20 25% V DIP(<80% for 18~)25% V DIP***

20

16

32

43

100

> 100

NO

NO

69

> 57

B 30 20 <80% for 20~(27% V DIP)

22 33 45 NO 12

C 30 40 30% V DIP(<80% for 23~)30% V DIP***

24

19

34

43

34

> 100

NO

NO

0

> 57

D 30 60 30% V DIP(<80% for 23~)30% V DIP***

24

19

34

43

34

> 100

NO

NO

0

> 57

Based on energy delivered to resistive load during dip, and no effect on load until dip exceeds 10%.

***For comparison, assumes voltage dip affects load at all levels below initial voltage.

19

TABLE 11

APPLICATION OF THE WSCC VOLTAGE DIP CRITERIAFOR VOLTAGE SWING OF TYPICAL FREQUENCY

CHECK FOR LOAD LOSS AT LIMITING CONDITION ALLOWED BY CRITERIA

0.28 HZ VOLTAGE SWING

WSCC CRITERIALIMITSWSCC

PERF.LEVEL

V DIP(%)

T < 80%(CYCLES)

LIMITING AND CORRESPONDINGNON-LIMITING - ( )

CONDITION

EQUIV. SQUAREWAVE DIP & TIME(% V DIP) (CYCLES)

TIME ALLOWED BYIBM STANDARD

(CYCLES)

LOADLOSS?

(YES/NO)MARGIN

(CYCLES)

A 25 20 <80% for 20~(21% V DIP)

18 73 >100 NO > 27

B 30 20 <80% for 20~(21% V DIP)

18 73 > 100 NO > 27

C 30 40 <80% for 40~(24% V DIP)

20 78 80 NO 2

D 30 60 30% V DIP(<80% for 57~)30% V DIP***

24

19

84

107

32

> 100

YES

NO

-52

?

Based on energy delivered to resistive load during dip, and no effect on load until dip exceeds 10%.

***For comparison, assumes voltage dip affects load at all levels below initial voltage.

20

21

TABLE 12

WSCC LOAD SHEDDING (PEAK MW)(from the 1993 OE-411 Report)

FrequencyHz NWPP RMPA AZNM CASN TOTAL

By 59.3 3449 94 35 1542 5120By 59.1 4610 9 269 5757 10645By 58.9 7277 977 1217 5415 14886By 58.7 4464 86 1192 5747 11489By 58.5 1498 1164 1410 6300 10372By 58.2 985 91 1392 3201 5669By 58.0 1206 1180 1362 4858 8606By 57.8 208 1413 1810 3431By 57.6 720 802 460 1982By 57.4 455 74 614 1143

Below 57.4 693 795 1488

TOTALS 25565 3601 9166 36499 74831

29

Appendix I: Examples of Disturbance Classifications

This appendix is intended to assist WSCC members in determining the properperformance level for disturbances that involve substation elements.

30

Example Substation #1(Breaker and One Half Bus Arrangement)

NorthBus #1 #2 #3

South #4 #5 #6 Bus

CASE ONE

Disturbance: Fault in center breaker between Lines #2 and #5 resulting in the outage of those twolines.

Specified Performance Level: B (Bus Section: A bus section is considered to be a commonpoint in a substation for two or more system elements)

CASE TWO

Disturbance: Loss of Line #1 with delayed clearing (center breaker does not operate) resultingin the additional loss of Line #4.

Specified Performance Level: B (Bus Section)

CASE THREE

Disturbance: Loss of Line #1 with Normal Clearing. Additional loss of Line #6.

Specified Performance Level: Initially not on D-P Table, assuming no prior knowledge ofcommon mode failure and that the lines are not on a common right-of-way. Level C (TwoCircuits) would be required if a common mode for the outage has been identified and until thecommon mode has been eliminated.

31

CASE FOUR

Disturbance: Breaker on the bus side of Line #1 is initially out of service. Fault on Line #4with normal clearing that results in the outage of Line #4 and the isolation of Line #1.

Specified Performance Level: A (One Circuit)

CASE FIVE

Disturbance: Fault on Line #5 with delayed clearing resulting in clearing the south bus whichtakes out the transformer.


CASE SIX

Disturbance: Center breaker between Line #1 and Line #4 initially out of service. Fault onLine #5 with delayed clearing resulting in clearing the south bus which takes out the transformerand isolates Line #4.


32

Example Substation #2(Main and Transfer Bus Arrangement)

West EastBus #1 #2 #3 #4 #5 #6 Bus

CASE ONE

Disturbance: Loss of Line #1 with delayed clearing resulting in the loss of the entire west busof the substation.


Note: If there had not been a bus sectionalizing breaker, this disturbance would have resulted inthe loss of the entire substation. In this case, performance level B would still be required.

CASE TWO

Disturbance: Fault in the bus sectionalizing breaker resulting in the outage of the entiresubstation.

Specified Performance Level: D (Entire Substation)

Note: Refer to bus section discussion under the “Terms Used in the Disturbance-PerformanceTable” for special considerations provided for bus tie and bus sectionalizing circuit breakers.

33

Example Substation #3(Ring Bus)

#1 #2

#3 #4

CASE ONE

Disturbance: Line #1 is out for maintenance so the ring bus is open. A fault occurs on Line #4which splits the bus, separating Lines #2 and Line #3.

Specified Performance Level: A (One Circuit)

Note: The performance requirement is not reduced with a facility initially out of service.

CASE TWO

Disturbance: Fault in the breaker between Lines #1 and #2 resulting in the outage of those twolines.


34

Example Substation #4(Double Breaker Double Bus)

#1 #2 #3 #4 #5 #6 #7

CASE ONE

Disturbance: Fault on Line #1. Backup relaying is incorrectly set for both buses to trip beforeprimary relaying. All breakers operate clearing the entire substation.

Specified Performance Level: D (Entire Substation)

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