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 Impact[1] 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 criterion[2]; 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 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

WECC 2013 HS Power Flow Case Buses Represented by Voltage
Voltage (kV) / Number of Buses
kV <= 35 / 5381
35< kV < 100 / 4017
100 / 227
115 / 4400
120 / 136
132 / 123
138 / 1471
161 / 108
200 / 1
230 / 1688
240 / 160
287 / 11
300 / 2
345 / 144
360 / 10
500 / 257
Sum = / 18136

buses because they are not, for the most part, represented in WECC cases. Further, since the high voltage buses represented in WECC cases 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 alternative, use of a percent voltage dip criteria in order to better represent the performance impact at lower voltage buses. Regardless of the relative merit of a voltage dip versus a hard limit performance criterion, the representation of lower voltage buses in recent WECC base cases is far superior than was the practice in 1994. For example, in a 2013 case, just over one-half the modeled buses are below 100 kV. Further, the NERC and WECC criteria only apply to the BES, so the question arises as to whether or not the WECC can or should promulgate a criterion that attempts to estimate impact on non-BES facilities (i.e., facilities below 100 kV, radial lines, distribution networks, etc.). Particularly since the goal of the criteria is to eliminate potential loss of consequential load following system contingencies.

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 definition[3] 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 equipment[4]. In other words, load tripped by customer equipment is not prohibited and the main premise for the voltage dip criteria is unfounded.

Second, the term ‘uncontrolled’ is defined in the WECC Glossary in an appendix for non-approved, non-official 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 ride-through. 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 step-up transformer) remains within the PRC-024 Voltage Ride-Through Time Duration Curve, the transient response would be deemed acceptable.

[1] 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

[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 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.