Version 1.1
Load Shed Philosophy
Guideline
APPLICABILITY: Reliability Coordinator
I. Background
While time limits exist for mitigation of the exceedance of certain types of system operating
limits, events can occur quickly necessitating a responsive action as quickly as possible.
Load shedding is a mitigation tool that can be implemented quickly to prevent a Bulk Electric
System (BES) Emergency or widespread adverse events such as cascading outages or
voltage collapse. The North American Electric Reliability Corporation (NERC) defines a BES
Emergency as “Any abnormal system condition that requires automatic or immediate
manual action to prevent or limit the failure of transmission facilities or generation supply
that could adversely affect the reliability of the Bulk Electric System.” It is important to
understand that, while acceptable system performance is expected at all times, there may
be cases where system integrity is potentially in jeopardy.
Often, in these instances, drastic actions must be taken pre-contingent to prevent even
potential exceedances. However, in other cases, such as some thermal limitations, there
may be some ability to use post-contingent actions. The Reliability Coordinator System
Operator (RCSO) maintains the authority in all cases to use best judgment to determine
whether and when it is necessary to direct load shed; and this philosophy provides some
framework to help make that decision. In addition, Transmission Operators (TOP) and
Balancing Authorities (BA) maintain the authority to take action up to and including shedding
firm load at their own discretion.
II.
Regulatory Context
Many NERC standards require TOPs, BAs and RCs to have plans for and take action up to
and including shedding firm load. Some standards require automatic load shedding, which is
not addressed here, while others address manual load shedding. For a table of such
standards, see Appendix A. In addition, there are also standards that provide for clear
decision-making authority to act or to direct actions to preserve the integrity and reliability of
the Bulk Electric System. 1
III.
Basis for Philosophy
Current system conditions, available indicators from Real-Time Contingency Analysis
(RTCA) and other available tools, RCSO judgment and other considerations such as public
health, safety, and potential equipment damage must always be considered. Internal event
analysis following events will be conducted to develop lessons learned and for improving
processes and tools.
1
IRO-001-1.1 R3 and TOP-001-1a R1
Classification: Public
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Load Shed Philosophy
Guideline
IV.
Load Shed Philosophy
A directive for Load Shed should be considered to prevent issues that may be considered
an Emergency once all other viable mitigating actions have been taken. As system
conditions change, Real-time Assessments 2 of the system should be performed in
consultation with Operations Engineering staff, time and availability permitting, as well as
with affected Transmission Operator(s) to determine whether potential Emergencies may
exist. To the extent that there is insufficient time or information available to fully analyze
potential system events, it is expected that RCSOs will act conservatively. The risk to
reliability should be evaluated for preventing issues that are expected to be localized, and
should be discussed with the affected Transmission Operator(s). However, it is important to
understand that there is not a discrete point that separates Emergency from local. It is a
continuum with extremes and all points in between.
V.
Application of Load Shed Philosophy
Directing Load Shed should be considered an option in situations including, but not limited
to:
• The system is one contingency away from cascading outage
• An Interconnection Reliability Operating Limit (IROL) is exceeded
• A TOP-007-WECC-1 SOL exceedance will exceed 30 minutes
• Imbalance of generation and demand causes reliability concerns (large sustained
frequency excursions, Energy Emergency Alerts, large sustained ACE excursions,
unresolved Disturbance Control Standard (DCS) events, etc.)
• Following a Real-time Assessment, it is unclear whether the system can sustain the
next contingency
Directing Load Shed may not be necessary when:
• Other mitigating actions can resolve the issue in a timely manner
• Load shed would violate safety, equipment, or regulatory or statutory requirements
• System risks are expected to be contained and controlled to a defined area as
demonstrated by system studies
• Load at risk is not sufficiently more than load that would have to be shed
2
“An examination of existing and expected system conditions, conducted by collecting and
reviewing immediately available data.” Glossary of Terms Used in NERC Reliability
Standards (August 2013)
Classification: Public
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Load Shed Philosophy
Guideline
Version History
Rev.
Date
Action
By
Change Tracking
1.0
09/10/2013
Issued for Implementation
M Mizumori
Original WECC RC Guideline
1.1
12/19/2013
Revised and Reissued
M Granath
Updated to Peak Reliability template
Classification: Public
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Load Shed Philosophy
Guideline
Attachment A: NERC Standards associated with load shed
Standard Requirement
EOP-001-2.1b R2.3
Each Transmission Operator and Balancing Authority shall…Develop,
maintain, and implement a set of plans for load shedding.
EOP-002-3.1 R6.6
If the Balancing Authority cannot comply with the Control Performance and
Disturbance Control Standards, then it shall immediately implement remedies
to do so. These remedies include, but are not limited to… Reducing load,
through procedures such as public appeals, voltage reductions, curtailing
interruptible loads and firm loads.
EOP-002-3.1 R7.1
Once the Balancing Authority has exhausted the steps listed in Requirement
6, or if these steps cannot be completed in sufficient time to resolve the
emergency condition, the Balancing Authority shall…Manually shed firm
load without delay to return its ACE to zero…
EOP-003-1 R1
After taking all other remedial steps, a Transmission Operator or Balancing
Authority operating with insufficient generation or transmission capacity shall
shed customer load rather than risk an uncontrolled failure of
components or cascading outages of the Interconnection.
EOP-003-1 R3
Each Transmission Operator and Balancing Authority shall coordinate load
shedding plans among other interconnected Transmission Operators and
Balancing Authorities.
EOP-003-1 R6
After a Transmission Operator or Balancing Authority Area separates from
the Interconnection, if there is insufficient generating capacity to restore
system frequency following automatic underfrequency load shedding, the
Transmission Operator or Balancing Authority shall shed additional load.
EOP-003-1 R8
Each Transmission Operator or Balancing Authority shall have plans for
operator-controlled manual load shedding to respond to real-time
emergencies. The Transmission Operator or Balancing Authority shall be
capable of implementing the load shedding in a timeframe adequate for
responding to the emergency.
IRO-005-3.1a R5
Each Reliability Coordinator shall monitor system frequency and its
Balancing Authorities’ performance and direct any necessary rebalancing to
return to CPS and DCS compliance. The Transmission Operators and
Balancing Authorities shall utilize all resources, including firm load
shedding, as directed by its Reliability Coordinator to relieve the emergent
condition.
Classification: Public
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Load Shed Philosophy
Guideline
TOP-001-1a R2
Each Transmission Operator shall take immediate actions to alleviate
operating emergencies including curtailing transmission service or energy
schedules, operating equipment (e.g., generators, phase shifters, breakers),
shedding firm load, etc.
TOP-001-1a R8
During a system emergency, the Balancing Authority and Transmission
Operator shall immediately take action to restore the Real and Reactive
Power Balance. If the Balancing Authority or Transmission Operator is
unable to restore Real and Reactive Power Balance it shall request
emergency assistance from the Reliability Coordinator. If corrective action or
emergency assistance is not adequate to mitigate the Real and Reactive
Power Balance, then the Reliability Coordinator, Balancing Authority,
and Transmission Operator shall implement firm load shedding.
TOP-004-2 R4
If a Transmission Operator enters an unknown operating state (i.e. any state
for which valid operating limits have not been determined), it will be
considered to be in an emergency and shall restore operations to respect
proven reliable power system limits within 30 minutes.
VAR-001-2 R8
Each Transmission Operator shall operate or direct the operation of
capacitive and inductive reactive resources within its area – which may
include, but is not limited to, reactive generation scheduling; transmission line
and reactive resource switching; controllable load; and, if necessary, load
shedding – to maintain system and Interconnection voltages within
established limits.
VAR-001-2 R12
The Transmission Operator shall direct corrective action, including load
reduction, necessary to prevent voltage collapse when reactive resources
are insufficient.
Classification: Public
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