Load Shed Philosophy

Transcription

1 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. III. 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 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 R3 and TOP-001-1a R1 Classification: Public Page 1 of 5 Effective Date: January 1, 2014

2 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 Page 2 of 5 Effective Date: January 1, 2014

3 Version History Rev. Date Action By Change Tracking /10/2013 Issued for Implementation M Mizumori Original WECC RC /19/2013 Revised and Reissued M Granath Updated to Peak Reliability template Classification: Public Page 3 of 5 Effective Date: January 1, 2014

4 Attachment A: NERC Standards associated with load shed Standard Requirement EOP b R2.3 Each Transmission Operator and Balancing Authority shall Develop, maintain, and implement a set of plans for load shedding. EOP 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 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 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 R3 Each Transmission Operator and Balancing Authority shall coordinate load shedding plans among other interconnected Transmission Operators and Balancing Authorities. EOP 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 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 a 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. BA TOP RC Classification: Public Page 4 of 5 Effective Date: January 1, 2014

5 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 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 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 R12 The Transmission Operator shall direct corrective action, including load reduction, necessary to prevent voltage collapse when reactive resources are insufficient. Classification: Public Page 5 of 5 Effective Date: January 1, 2014