WECC-0100 TPL-001-WECC-RBP-3 System Performance
Proposed Transient Voltage Response Criterion
Need
The pending NERC TPL-001-4 Transmission Planning standard requires, in R5, that each Transmission
planner and Planning Coordinator have criteria for acceptable transient voltage response including a low
voltage level and a maximum length of time that transient voltages may remain below that level.
The existing WECC criterion provided in the WECC TPL-001-WECC-RBP-2 Regional Business Practice can
be found in Table W1 and limits the provides that the transient voltage dip on other systems shall not
exceed:
 Category B: 25% at load buses or 30 percent at non-load buses, or 20% for more than 20 cycles
at load buses;
 Category C: 30% at any bus or 20% for more than 40 cycles at load buses; and
 Category D: Nothing in addition to NERC.
Background
The Western Electricity Coordinating Council (WECC) is updating load models used in dynamic
simulations. Based on early studies, these more complex ‘composite load’ models may impact the
applicability of the present WECC regional performance criteria which are used to evaluate the results of
both planning and operating studies. In addition, since these regional criteria were developed, the
planning world has changed considerably due to the issuance of multiple reliability standards by the
North American Electric Reliability Corporation (NERC) which provide mandatory performance
requirements for planning and operating the North American bulk power system. These standards are
intended, among other things, to prevent Adverse Reliability Impact1 to the Bulk Electric System. Since
these new standards (arguably) duplicate the function Table W1 of the WECC TPL-001-WECC-RBP-2
System Performance Criterion, this provides an opportunity to review the need for separate regional
criteria and, if the criteria are still needed, what changes, if any, may be required in order to
accommodate the use of new and more complex load models.
The WECC is one of eight Regional Entities with similar reliability responsibilities. One way to evaluate
the WECC transient voltage dip performance criterion is to compare it to the criteria used in the other
regions. To that end, a request was made to the Florida Reliability Coordinating Council (FRCC), the
Midwest Reliability Organization (MRO), the Northeast Power Coordinating Council (NPCC), the
ReliabilityFirst Corporation (RFC), the SERC Reliability Corporation (SERC), the Southwest Power Pool, RE
(SPP), and the Texas Reliability Entity (TRE) for information about their respective regional performance
criteria, if any. As it turns out, only NPCC has a regional performance criterion2; however it provides
general design guidance and is less prescriptive than the WECC criteria. For example, the NPCC criteria
notes that “…the limiting condition for voltages should recognize that voltages should not drop below
Defined in the NERC Glossary as “The impact of an event that results in Bulk Electric System instability or
Cascading”
2
NPCC - Regional Reliability Reference Directory # 1 - Design and Operation of the Bulk Power System
1
that required for suitable system stability performance, and should not adversely affect the operation of
the bulk power system.” Unacceptable system dynamic response is defined as “…an oscillatory
response to a contingency that is not demonstrated to be clearly positively damped within 30 seconds of
the initiating event.” Based on the response to these requests, it seems fair to suggest that the other
Reliability Entities defer to the NERC and each member system’s internal performance standards, and do
not rely on separate regional performance criteria.
The basis for the WECC regional performance criterion is discussed in an August 1994 white paper
Supporting Document for Reliability Criteria for Transmission Planning. Among other things, the paper
suggests that WECC base cases have only sparse representation of load buses below 230 kV and, as
such, simulations cannot adequately represent the impact of system contingencies on lower voltage
buses because they are not, for the most part, represented in WECC
WECC 2013 HS Power
Flow Case Buses
cases. Further, since the high voltage buses represented in WECC cases
Represented by Voltage
are so remote from load, the paper argues that a voltage limit criteria (0.8
per unit, for example) would be inadequate and suggests, as an
Voltage (kV)
Number
kV <= 35 of Buses
5381 alternative, use of a percent voltage dip criteria in order to better
35< kV < 100
4017 represent the performance impact at lower voltage buses. Regardless of
100
227 the relative merit of a voltage dip versus a hard limit performance
115
4400 criterion, the representation of lower voltage buses in recent WECC base
120
136 cases is far superior than was the practice in 1994. For example, in a 2013
132
123 case, just over one-half the modeled buses are below 100 kV. Further,
138
1471 the NERC and WECC criteria only apply to the BES, so the question arises
161
108 as to whether or not the WECC can or should promulgate a criterion that
200
1
attempts to estimate impact on non-BES facilities (i.e., facilities below 100
230
1688
kV, radial lines, distribution networks, etc.). Particularly since the goal of
240
160
the criteria is to eliminate potential loss of consequential load following
287
11
system contingencies.
300
345
360
500
Sum =
2
144
10
257
18136
The foundation for the WECC criterion is the assumption that “Any load
loss, firm or interruptible, by transient voltage dip would be considered
uncontrolled and therefore unacceptable”. This assumption is
problematic for several reasons. First, load lost due to planned protective
system action is consequential load loss, a NERC defined term which is by definition3 acceptable for all
contingencies, and load that is tripped by automatic or manual switching of system elements for reasons
other than protective actions to isolate a fault is defined as non-consequential load (which is highly
discouraged for Category B contingencies). The definition of non-consequential load loss specifically
excludes the response of voltage sensitive customer load which is disconnected by end-user
equipment4. In other words, load tripped by customer equipment is not prohibited and the main
premise for the voltage dip criteria is unfounded.
3
Consequential Load Loss: All Load that is no longer served by the Transmission system as a result of Transmission
Facilities being removed from service by a Protection System operation designed to isolate the fault.
4
Non-Consequential Load Loss: Non-Interruptible Load loss that does not include: (1) Consequential Load Loss,
(2) the response of voltage sensitive Load, or (3) Load that is disconnected from the System by end-user
Second, the term ‘uncontrolled’ is defined in the WECC Glossary in an appendix for non-approved, nonofficial but commonly-used terms as “The unanticipated switching of system elements at locations and
in a sequence which have not been planned.” Therefore, if load is lost during a transient voltage dip by
the action of customer equipment and not by switching of system elements, it does not fit the definition
of uncontrolled. So, once again, the foundational argument for the criterion does not apply.
The conclusion of the supporting documentation states that “…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”. While that may have been the intention of the criteria, preventing
consequential loss of load is not a requirement in the NERC standards, nor is it required in any of the
other seven reliability regions. Further, since the WECC criterion only applies to ‘Other Systems’, in
reality it simply promotes an unequal application of criteria. For example, at an area boundary or a bus
with multiple owners, the criteria by rule apply unless waived by the impacted entities. However, if the
same event were to occur internal to an area in a location remote from other systems, the criteria
effectively do not apply. So if the
original intent was to avoid loss of
load due to voltage dips, it only
protects loads in other systems close
to a system boundary.
Recommendation
The NERC PRC-024 generator
performance standard identifies a
minimum time versus voltage curve
that generators are required to ridethrough. Since keeping generators
on-line during disturbances is clearly
desirable, one option that WECC
could take would be to substitute
this curve for the voltage dip criteria
in Table W1; as long the transient
voltage at the generator Point of
Interconnection bus (presumed to be
the high-side of the generator stepup transformer) remains within the
PRC-024 Voltage Ride-Through Time
Duration Curve, the transient
response would be deemed
acceptable.
equipment.” As can be seen, load lost due to voltage sensitive customer equipment is treated in the same way that
consequential load is treated; i.e., loss of these loads is not prohibited for contingencies.