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
WECC-0100 Rationale for Requirement R1.3 and R1.4
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Executive Summary
NERC Standard TPL-001-4 Transmission System Planning Performance Requirements (TPL) [1] (effective
1/1/2016), Requirement R5 requires 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 Fault-Induced Delayed Voltage Recovery (FIDVR) phenomena, specify a recovery voltage that
allows enough time to recover during a FIDVR event. Second, for faults not resulting in a FIDVR event,
specify a voltage dip criteria to give a reasonable expectation of minimal loss of load subsequent to the
initial recovery above 80% after fault clearing. This accounts for events not located in areas with a high
concentration of air conditioning type loads.
Based on this approach, the WECC-0100 drafting team (DT) is proposing the following transient voltage
response criteria:

Transient stability voltage response at applicable Bulk-Electric System (BES) buses serving load
(having no intermediate connection) shall recover to at least 80% 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%,
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.
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Introduction
The TPL in Requirement R5 requires each Transmission Planner and Planning Coordinator to 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 Impact1 to the BES. Related to transient voltage
performance, Table 1 of the TPL 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) load loss due to response of voltage sensitive loads, 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 criteria as specified in TPL001-CRT-2.1, System Performance, WECC Disturbance-Performance Table of Allowable Effects on
Other Systems (AKA: Table W-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]. 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 Modeling
Validation Work Group (MVWG).
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 those found 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 TPL, 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 BES
recognizing that loss of voltage sensitive loads or loads tripped by end-user equipment cannot be
prevented. Based on a recognition that: 1) loss of voltage sensitive loads or loads tripped by end-user
equipment cannot be prevented, 2) more accurate load modeling, and 3) observations of the FIDVR
1
Defined in the NERC Glossary as “The impact of an event that results in Bulk Electric System instability or Cascading”
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phenomena, the voltage dip criteria 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 BES. It is
also intended to prevent or minimize Non-Consequential Load Loss. However, NERC defines the
response of voltage sensitive load or load that is disconnected by end-user as not to be nonconsequential. The existing WECC transient voltage dip criteria are specifically based upon voltage
sensitive load loss and are not appropriate for use in meeting the intent of the TPL. Therefore, the DT
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% 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%,
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 precontingency 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.
MODIFY DIAGRAMS ACCORDINGLY
Refer to Figure 2 for transient voltage response parameters for normal voltage recovery.
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Figure 1: Delayed Response Voltage Parameters
FIGURE TWO NEEDS TO BE REPLACED WITH NEW DIAGRAM
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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 not resulting 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
clearing. 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 TPL 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) load loss due to response of voltage sensitive loads, and 3) loads
disconnected by end-user equipment. Presumably any load lost during a fault and prior to voltage
recovery to at least 80% is not non-consequential load loss.
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 TPL is new, and there remains to be much
research to be done regarding the FIDVR phenomena and related dynamic load modeling practices,
these parameters could 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 ridethrough 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 single-phase and two-phase faults, and Type III covers three-phase
faults. For Type I and Type II tests, recommended parameters are:



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50% for 12 cycles
70% for 0.5 seconds
80% for 2 seconds
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For Type III tests, recommended 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, it is reasonable to assume a
voltage dip defined by Type I and II parameters above is appropriate.
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:

V < 50%, clearing time 0.16 seconds

50% < V < 88%, clearing time 2.00 seconds.
The 80% for 2.0 seconds parameter at BES busses serving load 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.
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