Western Systems Coordinating Council
SUPPORTING DOCUMENT
FOR
RELIABILITY CRITERIA FOR
TRANSMISSION SYSTEM PLANNING
J. Kondragunta, SCE
and
WSCC Reliability Subcommittee
AUGUST 1994
TABLE OF CONTENTS
PAGE
1.
INTRODUCTION ............................................................................................................. 1
2.
BACKGROUND ............................................................................................................... 1
3.
TRANSIENT VOLTAGE CRITERIA .............................................................................. 1
4.
5.
3.1
INTRODUCTION ................................................................................................. 1
3.2
SWING VOLTAGES ............................................................................................ 2
3.3
OVERVOLTAGE CRITERIA .............................................................................. 3
3.4
UNDERVOLTAGE CRITERIA ........................................................................... 3
TRANSIENT FREQUENCY CRITERIA ........................................................................ 7
4.1
OVERFREQUENCY CRITERIA ......................................................................... 7
4.2
UNDERFREQUENCY CRITERIA ...................................................................... 7
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
i
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
ii
1.
INTRODUCTION:
The purpose of this document is to provide the rationale for the parameters in the Performance
Table 1 in the Reliability Criteria for Transmission System Planning, Part I of the WSCC
Reliability Criteria dated March, 1993.
The parameters in the performance table, Table 1, were selected in such a way that the
application of new criteria, dated March 1993, do not result in a higher or lower level of
reliability for the WSCC system when compared to the level of reliability from the application
of the old reliability criteria, dated March 1990.
The reliability criteria in its final form has been developed by lengthy discussions among the
members of the Reliability Subcommittee, comments received from the PCC members as well
as 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 than
the one in which the disturbance occurred. The performance table shown in Table 1 was
developed in such a way that the actions resulting from application of Table 1 for various
level of disturbances will be essentially the same as Table 2.
The Subcommittee classified various disturbances into level A, B, C and D. The necessary
probability data shown in Table 3, provided by Bonneville Power Administration, was used to
establish relative outage rates for various elements of the system. Also the table shows the
relative 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 voltage
magnitude of 0.8 p.u. as the performance criteria in assessing acceptable bulk
transmission transient stability results. Consequently, the corresponding percent
voltage dip could vary widely as long as the magnitude of the dip did not go below
0.8 p.u.. The 0.8 p.u. voltage criteria provided margin for nuclear unit auxiliary
undervoltage trip protection that is usually set at a magnitude of 0.65 p.u. to 0.70
p.u. based on the rated generator terminal voltage. In most instances it would take
at least a N-2 outage to produce such critical voltage deviations under the new
criteria. Nuclear plant owners should be especially careful in setting power flow
conditions for nuclear units, because under the right set of conditions a N-2 outage
could cause an allowed voltage deviation of 25% that could lead to a nuclear unit
trip if the initial voltage was below 1.0 p.u..
1
The auxiliary protection on fossil-fired units has not been investigated at this time.
However, unit owners are encouraged to determine what the undervoltage trip
settings are and their relationship to the allowable voltage dips in the Performance
Table to assess stability performance.
3.2
Swing Voltages:
•Measurement:
Figure 1 and the following examples illustrate why using a maximum percentage
voltage swing deviation is more appropriate than using a fixed minimum voltage
magnitude 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) are
sparsely represented, if they are represented at all. There is also additional voltage
regulation at the distribution level (either regulating transformers, or regulators on
individual feeders), which is generally not represented in the WSCC data. This
leads to the following problems (assuming that a given percent voltage change on a
high 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 be
well above 1.00 p.u.. In the attached example, the 230 kV bus voltage is at 1.052
p.u. and the 500 kV bus voltage is at 1.1 p.u.. If a criteria of 0.8 p.u. is used for
voltage swings, the voltage on the low voltage bus (where the load actually is
connected) 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 voltage
may 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 low
voltage load bus.
From the example, it is clear that using a criteria of 0.8 p.u. does not appropriately
represent effects on the load itself. It is logical to use a percentage voltage dip
rather than the absolute minimum valuation for voltage swings.
Note that the actual voltage realized at the load bus may differ from this radial
example due to the network configuration and active devices in the system. For
systems having subtransmission networks containing active elements (for example,
synchronous condensers, cogenerators and induction motors) the load bus voltage dip
is more complex than the simple radial example. For some 500 kV or 230
kV disturbances, the subtransmission buses could have a higher or lower dip than
the 500 kV or 230 kV voltage dips.
2
3.3
Overvoltage Criteria:
The ARCPEG recommendation was that an overvoltage criteria is not needed for
WSCC member systems, based on the survey, and is not recommended here
because it is usually a local problem.
3.4
Undervoltage Criteria:
The Reliability Subcommittee has established voltage dip criteria designed to
avoid uncontrolled loss of load. The values selected were based on the estimated
response of electronic equipment such as computers to voltage dips. These loads are
believed to be most sensitive. Load sensitivity data was obtained from several
sources that were somewhat imprecise and inconsistent. During the one-year trial
period Reliability Subcommittee conducted load sensitivity and evaluation of the
WSCC voltage dip criteria.
The results of the study indicate that the WSCC voltage dip criteria now in place
are appropriate and that no modification is warranted at this time.
The Reliability Subcommittee is in general agreement that the following table will
apply for voltage dip criteria to meet its objectives.
Level
Instantaneous
Minimum dip
A
B
C
D
25%
30%
30%
30%
Maximum Duration
of V Dip exceeding 20%
20 cycles
20 cycles
40 cycles
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 A
performance. As the frequency of occurrence of disturbances decreases, the level
of performance changes from level A to levels B, C and D. The allowable transient
voltage dips for these levels were selected based on the frequency of occurrence of
events. In other words, maximum transient voltage dip for level A disturbances
should be less than that of level B disturbances and so on.
BACKGROUND — At the time the criteria were established, several sources of
information were available relating equipment response to voltage dips. The Adhoc Reliability Criteria Parameters Evaluation Group (ARCPEG) suggested dip
levels 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 and
was used as the basis for their recommendations. The values adopted by the
3
RS for the table, referred to the SDG&E curve, were believed to provide margin
against 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 other
data sources and noted a discrepancy in what was purported to be the CBEMA
curve. This curve was shown about the same in the SDG&E, U.S. Dept. of
Commerce publication FIPS PUB 94, and an IEEE Transactions on Power
Delivery paper from 1990. However, ANSI/IEEE Standard 446-1987 shows a
"Typical Design Goals of Power-Conscious Computer Manufacturers" curve that
resembles the so-called CBEMA curve but is noticeably more conservative. Using
the ANSI/IEEE curve as a reference, it appeared that levels A and B as now shown
in the table would be at the load loss threshold and levels C and D would result in
uncontrolled load loss. (Any load loss, firm or interruptible, by transient voltage
dip would be considered uncontrolled and therefore unacceptable.) The
ANSI/IEEE source seemed the most authoritative and appropriate data to use for
WSCC criteria development.
Duane also provided information to the Subcommittee indicating that voltage dip
tests were performed by imposing a step change dip in voltage on electronic
equipment 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 low
voltage for equipment than a step change, thereby providing some margin if the
ANSI/IEEE data is directly applied to the system. The question of concern was, how
great is this margin and is it sufficient to generally offset the higher load sensitivity
and 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 to
voltage dips. Bill Hanrahan of CBEMA's standards department said that CBEMA
has not developed any curves showing computer voltage tolerance, although he was
aware that various such curves have been represented as CBEMA curves. He
referred to the curves in ANSI/IEEE Std. 446-1987 and in FIPS PUB 94 as the best
sources for guidance (neither document represents the curves as being from
CBEMA), but did not know the origin of the curves. He was not aware that the
curves were different and on being informed, recommended using the more
conservative 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 and
information on some new standards work. He said that the so-called CBEMA curve
was developed several years ago by a gentleman (didn't get the name) in
the Navy Department for their computers. The curve was adopted by CBEMA
for its use. The latest information that Mr. Roberts had was a standard developed
4
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 adopt
it. He said that this standard best represents the sensitivity of modern computers to
voltage dip. Mr. Roberts confirmed that the sensitivity tests are conducted by
subjecting the loads to a step reduction in voltage followed by a step increase back to
the original level.
SELECTION OF A LOAD SENSITIVITY CURVE — Figure 2 shows load
sensitivity to voltage dips according to the ANSI/IEEE Std. 446-1987, IBM, San
Diego, and "CBEMA" (as represented in the San Diego study) standards. The IBM
curve is based on 3 discrete points from the standard at 0, 75, and 80 percent
voltage. The ANSI/IEEE data is shown in two ways. The data was given in the
446-1987 standard in the form of a curve that was rather more like a sketch than an
accurate plot. It was apparently intended more for guidance than to serve as a
precise standard. Duane Braunagel enlarged the curve on a copy machine and
transferred 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, discrete
values 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 the
ANSI/IEEE curve "ANSI FIG 4 - 3PTS." The two reproduced curves do not
coincide. The original was obviously not drawn to the scale indicated by the data
points.
The IBM standard is in close agreement with the San Diego curve in the region of
interest, 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 least
conservative, the ANSI/IEEE most conservative, of all the curves. The sources for
the ANSI/IEEE curves are not known. In summary, the IBM standard is most up-todate and from a known source, is probably more indicative of today's equipment
capability, and is somewhat mid-range among the various options along with the San
Diego curve. It seems the best choice for evaluating the WSCC voltage dip
standard.
COMPARING THE SINUSOIDAL REAL SYSTEM VOLTAGE DIP WITH THE
SQUARE WAVE TEST DIP — If we assume that the energy delivered to a
resistive load is an approximate indicator of load reaction to a voltage dip we can
convert 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 square
wave dip on the curve. Alternatively, we can assume that the load responds directly
to peak voltage rather than energy delivered, and convert the sinusoidal voltage
during the dip to an average peak voltage (Tables 7-9). We can then compare a
square wave dip of the same value to the allowable dip on the curve. The two
approaches produced about the same result with the energy based equivalent square
wave dip generally being slightly greater.
5
The effect was also examined by varying the portion of the dip that was assumed to
affect the load, assuming that regulated power supplies in electronic equipment are
not affected by low voltage until the voltage drops below a critical level. The
various curves suggest that equipment can continuously tolerate voltage that does
not drop below about 85% to 90%. Tables 4-9 show that the difference between a
sinusoidal dip and a square wave dip diminishes as the critical voltage threshold is
lowered. Thresholds of 100%, 90% and 85% were evaluated. The more
conservative 90% threshold, rather than 100%, was used for the primary evaluation
of 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 at
performance levels A, B, C, and D of the criteria. Examination of the time voltage
remained below 80% at the maximum allowed voltage dip for each level indicated
whether the maximum dip or the time less than 80% would be limiting (Figures 3
through 6).
The limiting conditions are summarized in Tables 10 and 11, as are the equivalent
square wave voltage dips, and time allowed by the IBM standard for the dip at the
limiting condition. Load was assumed lost if the dip time exceeded the time allowed
by 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 margin
and level B 12 cycles. Levels C and D have no margin and would appear to be at
the threshold of load dropping. This is consistent with the rationale used to develop
the voltage dip standard.
For a 0.28 Hz voltage swing (a "slow" swing), no load would be lost for levels A
and B with more than a half second margin. Level C has virtually no margin and
would be at the threshold of load dropping. There would appear to be loss of load
at level D with about a negative one second margin. For a sensitivity to
assumptions comparison, data is also shown on the basis that the voltage dip affects
load 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 WSCC
Planning Criteria Disturbance-Performance Table will likely produce the intended
system response during credible disturbances. It was intended that no load be lost
due to voltage dips for level A through D disturbances, with some margin at the
higher levels and little or no margin at the lower levels. There appears to be
some risk of load loss at level D for a very low frequency voltage swing using the
more conservative load response assumptions, but the risk does not seem great
enough to warrant changes in the criteria at this time.
6
4.
TRANSIENT FREQUENCY CRITERIA:
4.1
Overfrequency Criteria:
WSCC Reliability Subcommittee did not find a need to define a new
overfrequency criteria. This recommendation was presented to PCC.
The ARCPEG's recommendation is that an overfrequency criteria is not needed
because:
—
—
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 set
points, and time delays.
This recommendation from ARCPEG also supports the Subcommittee's
recommendation regarding the overfrequency criteria.
4.2
Underfrequency Criteria:
The frequencies chosen are intended to coordinate with underfrequency load
shedding (UFLS) programs. UFLS is expected to arrest frequency decline and
assure continued operation of the system within any islands that may be formed as a
result of a disturbance. In each island, underfrequency load shedding relay settings
are coordinated with underfrequency protection of generating units and any other
manual or automatic actions which can be expected to occur under conditions of
frequency decline. The coordinated automatic load shedding program is based on
studies of system dynamic performance, under conditions which would cause the
greatest potential imbalance between load and generation.
In the long run, the frequency specifications are expected to remain relatively
constant. If a system sets load shedding above a particular threshold the action
will be interpreted as accepting loss of that load for the specified class of
disturbance.
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 MW
relief 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 is
carried out in selecting the frequency.
7
The intent is to have no firm or interruptible load shedding. 59.6 Hz was
selected and this is above nearly all known settings. Some interruptible
loads 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 this
concept is carried out in selecting the frequency.
A minimum frequency of 59.4 Hz was selected and this allows dropping of
interruptible load only and will not allow dropping of firm load.
Level C (59.0 Hz)
Based on the old criteria, controlled opening of interconnections, system
islanding and automatic under frequency load dropping are allowed and this
concept is carried out in selecting the frequency.
In order to satisfy the above requirements a frequency of 59.0 Hz was selected
and this allows dropping of only the first two blocks of load which includes
firm 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 generation
separation are allowed and this concept is carried out in selecting the
frequency.
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 set
below the minimum criteria frequency is important insurance against total
collapse 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 allows
controlled opening of CPP interconnections during disturbances.
The Reliability Subcommittee is in agreement that the following table will apply
for frequency criteria to meet its objectives.
Level
Minimum Transient Frequency
A
B
C
D
59.6 Hz
59.4 Hz
59.0 Hz
58.1 Hz
8
5.
POST TRANSIENT VOLTAGE DEVIATION:
In developing a criteria for post transient voltage deviation, it was the intent of the
Reliability Subcommittee to provide some measure of the ability of the system to
recover to acceptable operating conditions following an outage. The Subcommittee
was concerned both with load protection and system integrity. While post transient
voltage can provide some insight into the incipient voltage collapse problems, in
itself it is not adequate as a voltage stability criteria. The Subcommittee recognizes
that a voltage collapse criteria may include reactive power margin, minimum
voltage and the ability to consider local constraints.
The simple voltage criteria as proposed does provide some measure of system
recovery following an outage and may be an initial step toward developing a more
complete criteria in the future. The language was placed into the current reliability
criteria:
For the purpose of these criteria, the post transient time frame is one to three
minutes after a system disturbance occurs. This allows available automatic
voltage support measures to take place, but does not allow the effects of
operator manual actions or Area Generation Control response. The
recommended simulation is a post transient power flow that simulates
all automatic action but not manual actions and not area interchange control.
These criteria are not intended to fully address potential voltage collapse
problems; to do so would require consideration of local constraints that are not
easily generalized.
The Subcommittee is still concerned that voltage collapse should be addressed in
the planning process. Footnote 7 of the Disturbance-Performance Table is intended
to override the reliability criteria table in cases where voltage collapse is known to
occur at voltage levels higher than those in the table. Planning and operation must
be constrained to avoid voltage collapse.
At this time, voltage collapse avoidance requires more sophisticated analysis than
can be reflected in a simple voltage deviation criterion. The Subcommittee has not
recommended any type of var/watt/voltage margin requirements in the criteria, due
to the likelihood that these criteria are sensitive to the robustness of the effected
area and the type of disturbance.
Until more is known about voltage collapse throughout the WSCC system, each
member system should evaluate its criteria to avoid voltage collapse for known
disturbances. It is the expectation of the Subcommittee that these voltage criteria be
addressed through the peer review process. Within the procedures for rating
transmission paths, it is the responsibility of the affected parties to identify potential
voltage collapse problems and bring them to the attention of the sponsors of the path
being rated.
9
TABLE 1
WSCC DISTURBANCE-PERFORMANCE TABLE
(1)
OF ALLOWABLE EFFECT ON OTHER SYSTEMS
Performance
Level
A
B
Disturbance(2)
Initiated By:
No Fault
3 Ø Fault With Normal Clearing
SLG Fault With Delayed Clearing
DC Disturbance (3)
Generator
One Circuit
One Transformer
DC Monopole (8)
Bus Section
Transient
Voltage
Dip
Criteria
Minimum
Transient
Frequency
(4)(5)
Post
Transient
Voltage
Deviation
(4)(5)(6)(7)
Loading
Within
Emergency
Ratings
Damping
(4)(5)(6)
Max V Dip - 25%
59.6 hz
5%
Yes
>0
Max Duration of V
Dip Exceeding 20%
- 20 cycles
Max V Dip - 30%
59.4 hz
5%
Yes
>0
Max Duration of V
Dip Exceeding 20%
- 20 cycles
C
D
Two Generators
Two Circuits
DC Bipole (8)
Three or More circuits on ROW
Entire Substation
Entire Plant Including Switchyard
Max V Dip - 30%
59.0 hz
10%
Yes
>0
Max Duration of V
Dip Exceeding 20%
- 40 cycles
Max V Dip - 30%
58.1 hz
10%
No
>0
Max Duration of V
Dip Exceeding 20%
- 60 cycles
(1)
This table applies equally to the system with all elements in service and the system with one element removed
and the system adjusted.
(2)
The examples of disturbances in this table provide a basis for estimating a performance level to which a
disturbance 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 voltage
deviations in excess of the values in this table can be considered acceptable if they are acceptable to the
affected 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 unit
auxiliary 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 voltage
instability, the system in which the disturbance originated and the affected system(s) should cooperate in
mutually resolving the problem. Simulation of post transient conditions will limit actions to automatic devices
only and no manual action is to be assumed.
(8)
Refer to section 7.0 - Application to DC Lines, paragraph 7.2.
10
TABLE 2. PERFORMANCE LEVELS
ALLOWABLE ACTIONS OR CONDITIONS ON SYSTEMS
OTHER THAN THE ONE ON WHICH THE DISTURBANCE
OCCURRED
PERFORMANCE LEVEL *
A
1.
2.
*
ACTIONS
B
C
D
AS PERMITTED BELOW
a. DROPPING OF INTERRUPTIBLE LOAD
NO
YES
YES
YES
b. CONTROLLED GEN. DROPPING OR
EQUIVALENT REDUCTION OF ENERGY
INPUT TO THE SYSTEM
NO
YES
YES
YES
c. CONTROLLED OPENING OF INTERCONNECTIONS INCLUDING SYSTEM
ISLANDING AND AUTOMATIC UNDERFREQUENCY LOAD DROPPING
NO
NO
YES
YES
d. CONTROLLED DIRECT DROPPING OF FIRM
LOAD
NO
NO
NO
YES
e. CONTROLLED SUB ISLANDING AND
GENERATION SEPARATION
NO
NO
NO
YES
POST DISTURBANCE LOADINGS AND
VOLTAGES OUTSIDE OF EMERGENCY LIMITS
PRIOR TO ADJUSTMENT
NO
NO
NO
YES
A "YES" INDICATES THE ACTION OR CONDITION IS PERMITTED IN SIMULATION TESTING TO MEET
THE PERFORMANCE LEVEL IF REQUIRED TO PREVENT CASCADING. A "NO" INDICATES THAT THE
ACTION OR CONDITION IS NOT ALLOWED.
11
TABLE 3. DISTURBANCES ORDERED BY FREQUENCY OF
OCCURRENCE
Element Lost
Generator
One Line
Two Generators
Two Line Dependent
Line + Generator
Bus Section
Transformer
Generator + Transformer
Line + Transformer
Two Line Independent
Outage
Rate/Year
4.0
1.8
0.33
0.088
0.082
0.072
0.037
0.012
0.0051
0.0030
Outage Rate
Relative To
Line
2
1
0.2
0.05
0.045
0.04
0.018
0.007
0.003
0.002
NOTE
1
1
2
3
3
3
4
5
5
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.
Level B — Dependent outages seen as variations of bus section outages.
3.
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 to
make the performance requirement consistent for loss of single element and to
maintain about the same level of performance provided by the previous
criteria.
5.
Level C — (N-2) outages two orders of magnitude less likely than N-1.
12
TABLE 4
•
•
SINUSOIDAL DIP IN 60 HZ, 1 VOLT PEAK SIGNAL
ENERGY INTO 1 OHM LOAD DURING PORTION OF DIP WHERE V < 100%
FREQUENCY
Hz
____________
0.70
0.70
0.28
0.28
•
FREQUENCY
radians
____________
4.398
4.398
1.759
1.759
V DIP
%
_____
25
30
25
30
START TIME
seconds
___________
0
0
0
0
END TIME
seconds
__________
0.7143
0.7143
1.7857
1.7857
DURATION
seconds
__________
0.7143
0.7143
1.7857
1.7857
ENERGY
watt-sec
_________
0.2546
0.2368
0.6365
0.5919
EQUIVALENT SQUARE WAVE DIP IN 60 HZ, 1 VOLT PEAK SIGNAL FOR SAME ENERGY INTO 1 OHM LOAD
13
V DIP
%
_______
15.6
18.6
15.6
18.6
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.
DURATION
seconds
___________
0.7143
0.7143
1.7857
1.7857
ENERGY
watt-sec
__________
0.2544
0.2366
0.6360
0.5916
TABLE 5
•
•
SINUSOIDAL DIP IN 60 HZ, 1 VOLT PEAK SIGNAL
ENERGY INTO 1 OHM LOAD DURING PORTION OF DIP WHERE V < 90%
FREQUENCY
Hz
____________
0.70
0.70
0.28
0.28
0.70
0.28
0.28
•
FREQUENCY
radians
____________
4.398
4.398
1.759
1.759
4.398
1.759
1.759
V DIP
%
_____
25
30
25
30
27
21
24
START TIME
seconds
___________
0.0936
0.0773
0.2339
0.1932
0.0863
0.2822
0.2443
END TIME
seconds
__________
0.6207
0.6370
1.5518
1.5925
0.6280
1.5036
1.5414
DURATION
seconds
__________
0.5271
0.5597
1.3178
1.3993
0.5417
1.2214
1.2971
ENERGY
watt-sec
_________
0.1676
0.1638
0.4190
0.4096
0.1667
0.4142
0.4192
EQUIVALENT SQUARE WAVE DIP IN 60 HZ, 1 VOLT PEAK SIGNAL FOR SAME ENERGY INTO 1 OHM LOAD
14
V DIP
%
_______
20.3
23.5
20.3
23.5
21.6
17.6
19.6
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%.
DURATION
seconds
___________
0.5271
0.5597
1.3178
1.3993
0.5417
1.2214
1.2971
ENERGY
watt-sec
__________
0.1674
0.1638
0.4185
0.4095
0.1665
0.4146
0.4192
TABLE 6
•
•
SINUSOIDAL DIP IN 60 HZ, 1 VOLT PEAK SIGNAL
ENERGY INTO 1 OHM LOAD DURING PORTION OF DIP WHERE V < 85%
FREQUENCY
Hz
____________
0.70
0.70
0.28
0.28
•
FREQUENCY
radians
____________
4.398
4.398
1.759
1.759
V DIP
%
_____
25
30
25
30
START TIME
seconds
___________
0.1463
0.1191
0.3658
0.2977
END TIME
seconds
__________
0.5680
0.5952
1.4199
1.4880
DURATION
seconds
__________
0.4217
0.4762
1.0540
1.1904
ENERGY
watt-sec
_________
0.1265
0.1307
0.3161
0.3266
EQUIVALENT SQUARE WAVE DIP IN 60 HZ, 1 VOLT PEAK SIGNAL FOR SAME ENERGY INTO 1 OHM LOAD
15
V DIP
%
_______
22.5
25.9
22.6
25.9
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%.
DURATION
seconds
___________
0.4217
0.4762
1.0540
1.1904
ENERGY
watt-sec
__________
0.1266
0.1307
0.3157
0.3268
TABLE 7
•
•
SINUSOIDAL DIP IN 60 HZ, 1 VOLT PEAK SIGNAL
AVERAGE PEAK VOLTAGE DURING PORTION OF DIP WHERE V < 100%
FREQUENCY
Hz
____________
0.70
0.70
0.28
0.28
•
FREQUENCY
radians
____________
4.398
4.398
1.759
1.759
V DIP
%
_____
25
30
25
30
START TIME
seconds
___________
0
0
0
0
END TIME
seconds
__________
0.7143
0.7143
1.7857
1.7857
DURATION
seconds
__________
0.7143
0.7143
1.7857
1.7857
V-AVG.
volts
_________
0.8408
0.8090
0.8408
0.8090
EQUIVALENT SQUARE WAVE DIP IN 60 HZ, 1 VOLT PEAK SIGNAL FOR SAME AVERAGE PEAK VOLTAGE
16
V DIP
%
_______
15.9
19.1
15.9
19.1
DURATION
seconds
___________
0.7143
0.7143
1.7857
1.7857
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.
V-AVG.
volts
__________
0.8408
0.8090
0.8408
0.8090
TABLE 8
•
•
SINUSOIDAL DIP IN 60 HZ, 1 VOLT PEAK SIGNAL
AVERAGE PEAK VOLTAGE DURING PORTION OF DIP WHERE V < 90%
FREQUENCY
Hz
____________
0.70
0.70
0.28
0.28
•
FREQUENCY
radians
____________
4.398
4.398
1.759
1.759
V DIP
%
_____
25
30
25
30
START TIME
seconds
___________
0.0936
0.0773
0.2339
0.1932
END TIME
seconds
__________
0.6207
0.6370
1.5518
1.5925
DURATION
seconds
__________
0.5271
0.5597
1.3178
1.3993
V-AVG.
volts
_________
0.8023
0.7702
0.8023
0.7702
EQUIVALENT SQUARE WAVE DIP IN 60 HZ, 1 VOLT PEAK SIGNAL FOR SAME AVERAGE PEAK VOLTAGE
17
V DIP
%
_______
19.8
23.0
19.8
23.0
DURATION
seconds
___________
0.5271
0.5597
1.3178
1.3993
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%.
V-AVG.
volts
__________
0.8023
0.7702
0.8023
0.7702
TABLE 9
•
•
SINUSOIDAL DIP IN 60 HZ, 1 VOLT PEAK SIGNAL
AVERAGE PEAK VOLTAGE DURING PORTION OF DIP WHERE V < 85%
FREQUENCY
Hz
____________
0.70
0.70
0.28
0.28
•
FREQUENCY
radians
____________
4.398
4.398
1.759
1.759
V DIP
%
_____
25
30
25
30
START TIME
seconds
___________
0.1463
0.1191
0.3658
0.2977
END TIME
seconds
__________
0.5680
0.5952
1.4199
1.4880
DURATION
seconds
__________
0.4217
0.4762
1.0540
1.1904
V-AVG.
volts
_________
0.7843
0.7519
0.7843
0.7519
EQUIVALENT SQUARE WAVE DIP IN 60 HZ, 1 VOLT PEAK SIGNAL FOR SAME AVERAGE PEAK VOLTAGE
18
V DIP
%
_______
21.6
24.8
21.6
24.8
DURATION
seconds
___________
0.4217
0.4762
1.0540
1.1904
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%.
V-AVG.
volts
__________
0.7843
0.7519
0.7843
0.7519
TABLE 10
APPLICATION OF THE WSCC VOLTAGE DIP CRITERIA
FOR VOLTAGE SWING OF TYPICAL FREQUENCY
CHECK FOR LOAD LOSS AT LIMITING CONDITION ALLOWED BY CRITERIA
0.7 HZ VOLTAGE SWING
WSCC
PERF.
LEVEL
A
WSCC CRITERIA
LIMITS
V DIP
T < 80%
(%)
(CYCLES)
25
20
19
B
30
20
C
30
40
D
30
60
LIMITING AND
CORRESPONDING
NON-LIMITING - ( )
CONDITION
EQUIV. SQUARE
WAVE DIP & TIME
(% V DIP) (CYCLES)
TIME ALLOWED BY
IBM STANDARD
(CYCLES)
LOAD
LOSS?
(YES/NO)
MARGIN
(CYCLES)
25% V DIP
(<80% for 18~)
25% V DIP***
20
32
100
NO
69
16
43
> 100
NO
> 57
<80% for 20~
(27% V DIP)
22
33
45
NO
12
24
34
34
NO
19
43
> 100
NO
24
34
34
NO
0
19
43
> 100
NO
> 57
30% V DIP
(<80% for 23~)
30% V DIP***
30% V DIP
(<80% for 23~)
30% V DIP***
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.
0
> 57
TABLE 11
APPLICATION OF THE WSCC VOLTAGE DIP CRITERIA
FOR VOLTAGE SWING OF TYPICAL FREQUENCY
CHECK FOR LOAD LOSS AT LIMITING CONDITION ALLOWED BY CRITERIA
0.28 HZ VOLTAGE SWING
WSCC
PERF.
LEVEL
WSCC CRITERIA
LIMITS
V DIP
T < 80%
(%)
(CYCLES)
LIMITING AND
CORRESPONDING
NON-LIMITING - ( )
CONDITION
EQUIV. SQUARE
WAVE DIP & TIME
(% V DIP) (CYCLES)
TIME ALLOWED BY
IBM STANDARD
(CYCLES)
LOAD
LOSS?
(YES/NO)
MARGIN
(CYCLES)
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
84
32
YES
-52
> 100
NO
?
20
A
19
107
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.
TABLE 12
WSCC LOAD SHEDDING (PEAK MW)
(from the 1993 OE-411 Report)
Frequency
Hz
NWPP
RMPA
AZNM
CASN
TOTAL
By 59.3
By 59.1
By 58.9
By 58.7
By 58.5
By 58.2
By 58.0
By 57.8
By 57.6
By 57.4
Below 57.4
3449
4610
7277
4464
1498
985
1206
208
720
455
693
94
9
977
86
1164
91
1180
35
269
1217
1192
1410
1392
1362
1413
802
74
1542
5757
5415
5747
6300
3201
4858
1810
460
614
795
5120
10645
14886
11489
10372
5669
8606
3431
1982
1143
1488
TOTALS
25565
3601
9166
36499
74831
21
22
23
24
25
26
27
28
Appendix I: Examples of Disturbance Classifications
This appendix is intended to assist WSCC members in determining the proper
performance level for disturbances that involve substation elements.
29
Example Substation #1
(Breaker and One Half Bus Arrangement)
North
Bus
South
Bus
#1
#2
#3
#4
#5
#6
CASE ONE
Disturbance: Fault in center breaker between Lines #2 and #5 resulting in the outage of those two
lines.
Specified Performance Level: B (Bus Section: A bus section is considered to be a common
point in a substation for two or more system elements)
CASE TWO
Disturbance: Loss of Line #1 with delayed clearing (center breaker does not operate) resulting
in 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 of
common mode failure and that the lines are not on a common right-of-way. Level C (Two
Circuits) would be required if a common mode for the outage has been identified and until the
common mode has been eliminated.
30
CASE FOUR
Disturbance: Breaker on the bus side of Line #1 is initially out of service. Fault on Line #4
with 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 which
takes out the transformer.
Specified Performance Level: B (Bus Section)
CASE SIX
Disturbance: Center breaker between Line #1 and Line #4 initially out of service. Fault on
Line #5 with delayed clearing resulting in clearing the south bus which takes out the transformer
and isolates Line #4.
Specified Performance Level: B (Bus Section)
31
Example Substation #2
(Main and Transfer Bus Arrangement)
West
Bus #1
#2
#3
#4
#5
#6
East
Bus
CASE ONE
Disturbance: Loss of Line #1 with delayed clearing resulting in the loss of the entire west bus
of the substation.
Specified Performance Level: B (Bus Section)
Note: If there had not been a bus sectionalizing breaker, this disturbance would have resulted in
the 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 entire
substation.
Specified Performance Level: D (Entire Substation)
Note: Refer to bus section discussion under the “Terms Used in the Disturbance-Performance
Table” for special considerations provided for bus tie and bus sectionalizing circuit breakers.
32
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 #4
which 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 two
lines.
Specified Performance Level: B (Bus Section)
33
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 before
primary relaying. All breakers operate clearing the entire substation.
Specified Performance Level: D (Entire Substation)
34