WECC-0100 Rationale for Requirement R1.3 and R1.4 1
WECC-0100 TPL-001-WECC-CRT-3 (CRT)
Transmission System Planning Performance
Proposed Transient Voltage Response
Rationale for CRT Requirements
R1.3 and R1.4
WECC-0100 CRT Drafting Team
July 17, 2015
155 North 400 West, Suite 200
Salt Lake City, Utah 84103-1114
Western Electricity Coordinating Council
WECC-0100 Rationale for Requirement R1.3 and R1.4 1
Executive Summary
The new NERC TPL-001-4 Transmission Planning standard [1] (effective 1/1/2016) 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. In addition to requirement R5, NERC has defined performance requirements in response to Planning Events in Table 1 Steady State & Stability Performance Planning Events.
The approach to the transient voltage response criterion is twofold. First, considering the impact of the FIDVR phenomena, specify a recovery voltage that allows enough time to recover during a FIDVR event. Second, for faults that don’t result in a FIDVR event, specify a voltage dip criterion to give a reasonable expectation of minimal loss of load subsequent to the initial recovery above 80% after fault clearingof the first swing. This accounts for events not located in areas with a high concentration of air conditioning type loads.
Based on this approach, the SDT is proposing the following transient voltage response criteria:
- Transient stability voltage response at applicable BES buses serving load (having no intermediate connection) shall recover to at least 80 percent of pre-contingency voltage within 10 seconds of the initiating event for all P1-P7 category events.
- For voltage swings subsequent to fault clearing and the first voltage recovery above 80%swing (after the initial recovery), voltage dips at each applicable BES bus serving load (having no intermediate buses) shall not dip below 70% of pre-contingency voltage for more than 30 cycles or remain below 80% of pre-contingency voltage for more than 2 seconds for all P1-P7 category events.
Table of Contents
Introduction
Proposed Criterion
Technical Discussion
BiobliographyNEEDED FORMATTING
Western Electricity Coordinating Council
WECC-0100 Rationale for Requirement R1.3 and R1.4 1
Introduction
The new NERC TPL-001-4 Transmission Planning standard [1] (effective 1/1/2016) 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. R5 states:
“Each Transmission Planning and Planning Coordinator shall have criteria for acceptable System steady state voltage limits, post-Contingency voltage deviations, and the transient voltage response for its System. For transient voltage response, the criteria shall at a minimum, specify a low voltage level and a maximum length of time that transient voltages may remain below that level. [Violation Risk Factor: Medium] [Time Horizon: Long-term Planning]”
In addition to requirement R5, NERC has defined performance requirements in response to Planning Events in Table 1 Steady State & Stability Performance Planning Events. These standards are intended, among other things, to prevent Adverse Reliability Impact[1] to the Bulk Electric System. Related to transient voltage performance, Table 1 of NERC Standard TPL-001-4 spells out the fault type, either three-phase or single-line-to-ground, and whether or not Non-Consequential Load loss is allowed for the different Planning Events. NERC’s definition of Non-Consequential Load includes loads other than (does not include) 1) loads lost consequential to the outage, 2) loads losst due to response of voltage sensitive loadsity response, and 3) loads disconnected by end-user equipment. The purpose of this White Paper is to propose a WECC Regional Criterion for transient voltage response as discussed in requirement R5 of the NERC Standard above.
Historically the Western Electricity Coordinating Council (WECC) voltage dip criterion as specified in Table W-1 of the WECC Regional Criteria for System Performance TPL-001-CRT-2.1 was based on a proxy for loss of voltage sensitive load in the absence of dynamic load modeling. The basis for this criterion is discussed in the August 1994 white paper “Supporting Document for Reliability Criteria for Transmission Planning” [2]. The Western Electricity Coordinating Council (WECC) is now using dynamic load models that more explicitly model the dynamic behavior of loads in simulations. These more complex ‘composite load models’ (CLM) make the applicability of the present WECC regional performance criteria obsolete. Using these new models has resulted in study performance closer to actual system performance as found through validation studies conducted through the MVWG.
Fault–induced delayed voltage recovery (FIDVR) is the phenomena where, after a fault is cleared, the initial voltage recovery is delayed due to stalling of single-phase compressor motors (such as in residential air-conditioners and refrigeration). This phenomenon can occur during a normally cleared fault, and cannot be prevented. Actual events of FIDVR have been observed in parts of the Western Interconnection.
With approval of the NERC TPL-001-4 Standard, the general philosophy of the System Performance criteria has changed from no loss of load due to voltage dips for planning contingencies to maintaining the integrity of the Bulk Electric System recognizing that loss of voltage sensitive loads or loads tripped by end-user equipment cannot be prevented. Based ona recognition that a) loss of voltage sensitive loads or loads tripped by end-user equipment cannot be prevented, b) more accurate load modeling, and c) observations of the FIDVR phenomena, the voltage dip criterion in the WECC System Performance Criterion TPL-001-WECC-CRT-2.1 is outdated and no longer applicable.
Proposed Criterion
The intended purpose of the transient voltage response criterion is to ensure stability of the Bulk Electric System (BES). It is also intended to prevent or minimize Non-Consequential Load Loss. However, NERC clearly defines the response of voltage sensitive load or load that is disconnected by end-user not to be non-consequential. The existing WECC transient voltage dip criteria is specifically based upon voltage sensitive load loss and is not appropriate for use in meeting the intent of the NERC TPL-001-4 Standard. Therefore, the SDT is proposing the following transient voltage response criteria:
- Transient stability voltage response at applicable BES buses serving load (having no intermediate connection) shall recover to at least 80 percent of pre-contingency voltage within 10 seconds of the initiating event for all P1-P7 category events.
- For voltage swings subsequent to fault clearing and the first voltage recovery above 80%swing (after the initial recovery), voltage dips at each applicable BES bus serving load (having no intermediate buses) shall not dip below 70% of pre-contingency voltage for more than 30 cycles or remain below 80% of pre-contingency voltage for more than 2 seconds for all P1-P7 category events.
Refer to Figure 1 for transient voltage response parameters for delayed voltage recovery. Refer to Figure 2 for transient voltage response parameters for normal voltage recovery.
Figure 1: Delayed Response Voltage Parameters
Figure 2: Normal Response Voltage Parameters[BPA_USER1]
Technical Discussion
The approach to the transient voltage response criterion is twofold. First, considering the impact of the FIDVR phenomena, specify a recovery voltage that allows enough time to recover during a FIDVR event. Second, for faults that don’t result in a FIDVR event, specify a voltage dip criterion to give a reasonable expectation of minimal loss of load subsequent to the initial recovery above 80% after fault clearingrecovery of the first swing. This accounts for events not located in areas with a high concentration of air conditioning type loads.
The performance measure in Table 1 of the NERC TPL-001-4 standard related to transient voltage response is whether or not Non-Consequential load loss is allowed for P1 through P7 planning events. As defined in the NERC reliability standards the definition of Non-Consequential Load does not include; 1) loads lost consequential to the outage, 2) loads losst due to response of voltage sensitive loadsity response, and 3) loads disconnected by end-user equipment.Presumably any load lost during a fault and during the first swing after a fault, and prior to voltage recovery to at least 80% isn not non-consequential load loss due to the response of load to a fault.
It is obvious that for a system to not cascade there must be a voltage recovery. Based upon past experience and sound engineering judgment it is proposed that the voltage must recover to at least 80% of the pre-disturbance voltage. Even though there is no hard technical justification for 80%, it is widely understood that if the voltage did not recover to at least 80%, there could be unintended consequences such as protection system Misoperation which could result in cascading. In addition, recovering in a maximum of 10 seconds seems like a reasonable time to recover during a FIDVR event based on experience and engineering judgment. Since the NERC TPL-001-4 Standard is new, and there remains to be much research to be done regarding the FIDVR phenomena and related dynamic load modeling practices, these parameters should be revisited as better information becomes available.
After the initial voltage recovery, subsequent voltage dips due to power swings could cause load loss that may occur on neighboring systems. To minimize this impact to other loads not lost due to response of the load to the fault, voltage dips should be limited to give a reasonable expectation of minimum loss of load subsequent to recovery of the first swing. The IEEE Standard 1668 “IEEE Trial Use Recommended Practice for Voltage Sag and Short Interruption Ride-Through Testing for End-Use Electrical Equipment Rated Less than 1000V” [3] defines a recommended practice for voltage-sag ride-through performance testing for electrical and electronic equipment connected to low-voltage power systems. This includes minimum voltage-sag immunity requirements based on actual voltage-sag data. The ride-through voltage and times specified in this Standard give a reasonable expectation for voltage ride-through of loads connected to distribution systems. The Standard classifies voltage-sag into three types. Type I and Type II cover For single-phase and two-phase faults, and Type III covers three-phase faults. For Type I and Type II connected loadstests, recommended ride-through parameters are:
- 50% for 12 cycles
- 70% for 0.5 seconds
- 80% for 2 seconds
For Type III tests, recommended three-phase connected loads, ride-through parameters are:
- 50% for 3 cycles
- 70% for 6 cycles
- 80% for 2 seconds
Since the majority of faults that occur on the BES are single-phase faults, Based on this Standard and given the proximity of the distribution system to the BES, it is reasonable to assume a voltage dip defined by Type I and II parameters above is appropriatewith a minimum voltage of 70% and not below 80% for 2 seconds.
These parameters also seem reasonable to not cause excessive tripping of distributed resources (DR). The IEEE Standard 1547 “IEEE Standard for Interconnecting Distributed Resources with Electrical Power Systems” [4], provides technical specifications and requirements for interconnection of distributed resources within an area electric power system. In this Standard there is a section that specifies the response of DR to abnormal conditions. Response to abnormal voltages specifies maximum clearing times for distributed resources to cease energizing the area for reasons of safety and protection of equipment. The response to abnormal low voltages listed in the Standard are:
In addition, referencing IEEE Standard 1547 “IEEE Standard for Interconnecting Distributed Resources with Electrical Power Systems” [4], this also seems reasonable with regard to clearing times associated with distributed resources.
- V < 50%, clearing time 0.16 seconds
- 50% < V < 88%, clearing time 2.00 seconds.
The range of clearing times specified for distributed generation is 2.0 seconds within a voltage range of 50% to 88%. The 80% for 2.0 seconds parameter at BES busses serving load for voltage sag immunity of loads falls within the upper end of the voltage range for distributed resource clearing time of 2.00 seconds.
Bibliography
[1] NERC Reliability Standards for the Bulk Electric Systems of North America, Standard TPL-001-4 – Transmission System Planning Performance Requirements.
[2] WECC White Paper, Supporting Document for Reliability Criteria for Transmission Planning, August, 1994.
[3] IEEE Standard 1668™-2014, IEEE Trial-Use Recommended Practice for Voltage Sag and Short Interruption Ride-Through Testing for End-Use Electrical Equipment Rated Less than 1000 V.
[4] IEEE Standard 1557™-2003, IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems.
Western Electricity Coordinating Council
[1]Defined in the NERC Glossary as “The impact of an event that results in Bulk Electric System instability or Cascading”
[BPA_USER1]Need to replace with new diagram.