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Recommendations on How to Proceed with
Transient Voltage Performance Criteria
Dmitry Kosterev and John Undrill
June 20, 2013
I. SUMMARY
Members of WECC Modeling and Validation Work Group, because of their involvement
in the development and use of the composite load model, have been asked to provide a
technical opinion on the transient voltage dip criteria . This review covers how the
evaluation of simulations can be impacted by the evolution of system modeling as
typified by the new composite load model, and recognizes the actual phenomenon of
parts of the system where behaviors such as Fault Induced Delayed Voltage Recovery
(FIDVR) are observed.
Landscape:
- Table I of NERC TPL-001-4 Standard specifies the system performance
requirements in terms of firm transmission service interruption and loss of
consequential load for various categories of events
- NERC TPL-001-4 Standard requirement R5 states that each Transmission Planner
and Planning Coordinator shall have criteria for acceptable System steady state
voltage limits, post-Contingency voltage deviations, and transient voltage
response for its System. For transient voltage response, the criteria shall at
minimum specify a low voltage level and maximum length of time that transient
voltages may remain below that level
- WECC RC is considering transient stability performance requirements in the
operational planning horizon. It is desirable to have consistent performance
requirements among planning and operational engineers
- Planning entities in WECC has been using transient voltage dip criteria as a
measure of power system dynamic performance since early 1990s
- Actual events of Fault-Induced Delayed Voltage Recovery have been observed in
parts of WECC. The phenomenon is due to stalling of single-phase compressor
motors (such as in residential air-conditioners and refrigeration), that can occur
during a normally cleared fault, and cannot be prevented. System performance
during a FIDVR event can violate WECC transient voltage dip criterion.
- Actual events can include loss of load during a transmission fault because of enduse control and protection settings. WECC MVWG performed analysis of voltage
sensitivities of various end-uses. Loss of such load must be treated as
consequential to a fault.
Modeling expectations:
- Power system performance is evaluated using system models.
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Models can be tuned after the fact to reproduce an actual known disturbance event
and to explain what happened during the event, as it was done for September 8
2011 Arizona – Southern California outage, or August 10 1996 outage
The goal, however, is to predict the system performance during “planned for”
events in the future.
The power system model is expected to predict quantitatively system performance
during events initiated by small disturbances of voltages (e.g. a single-phase fault,
or no-fault) or frequency (e.g. a generator outage)
o Unexpected control action is possible, e.g. La Rosita unit trip following
Hassayampa – North Gila 500-k line tripping during September 8, 2011
outage
The power system model is expected to predict the general phenomenon during
the events initiated by large disturbances of voltages (e.g. a three-phase fault) or
frequency (e.g. islanding)
Modeling limitations need to be recognized when simulating large disturbances:
o WECC has completed the development and in process of phased
implementation of the composite load model. The new composite load
model is a major improvement over the interim load model used in WECC
since 2001. The model is capable of representing FIDVR phenomenon,
but the details of the voltage recovery will vary depending on the
assumptions on how much load tripped during a fault and how much of
air-conditioning load stalls due to low voltages
o NERC TPL-001-4 requires evaluation of power plant disturbance ridethrough capabilities, and NERC PRC-024 provides voltage and frequency
ride-through requirements for control and protection setting. There is no
guarantee that a power plant will ride through a disturbance even within
voltage or frequency envelope. Per NERC statistics, protection and
controls of power generators and its auxiliaries can come into play during
large disturbances resulting into unexpected plant trips and runbacks. It is
not reasonable to expect to develop models that accurately predict such
control responses for the “planned for” events.
 In April 2007, a three-phase fault on a 500-kV line caused tripping
of a wind power plant during a fault, a coal-fired generator a few
seconds later, a gas-turbine plant and hydro generators
o It is not practical to develop models that can predict whether a power plant
trips during a disturbance. A judgment on whether the plant trips during a
disturbance can be made based on proximity to NERC PRC-024
boundaries and by monitoring generator voltage, frequency, step-up
transformer V/Hz, active and reactive power, generator field current
relative to expected limits.
System Studies
- Initial studies with a composite load model show that the N-1 3-phase fault with
normal clearing performs significantly worse that N-2 1-phase fault with delayed
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clearing. This is opposite to the existing criteria. The severity of the initiating
disturbance matters
System Performance Expectation
Progress in system modeling over the last 4 years has made it clear that the present
system performance criterion, which is stated as an allowable amount and duration of
voltage dip, does not reasonably recognize the behavior of many parts of the power
system in the wake of severe disturbances. Specifically, both satisfactory and
unsatisfactory responses ‘fail’ when tested against the present criterion; this criterion thus
fails to discriminate between satisfactory and unsatisfactory conditions.
We argue, in the light of the current system performance simulation capabilities, that
performance should be evaluated against criteria that recognize the following:
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High probability disturbances, such as 1-phase L-G faults, simple generator trips
should be evaluated against criteria whose emphasis is continuity of supply to
customers. The essence of the evaluation is that the power system must withstand
these events without disadvantage to customers.

Low probability but severe events, such as three phase faults, should be evaluated
against criteria whose emphasis is the immediate and sustained security of bulk
transmission grid and generation. The essence of the evaluation is that the
elements of the grid that are essential to the re-establishment of supply should
take precedence over short term continuity of supply to all customers.
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Sensitivity studies with respect to modeling uncertainty are required for events
that are initiated by severe faults. Modeling uncertainty includes air-conditioner
voltage stall thresholds, motor tripping, and power plant ride-through capabilities.
The nature of the criteria should be open to comprehensive review. A significant volume
of impact studies is the essential next step in approaching this subject. We recommend
assigning MVWG, including experts in load modeling, to produce such system impact
studies for various regions with WECC.
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