Through its Reliability Risk Analysis team, NERC regularly examines events with significant risks to bulk power system reliability and issues lessons learned reports to ensure industry is well informed of system impacts, emerging trends, risk analysis and expected corrective actions. NERC’s lessons learned identify areas in which new or enhanced compliance monitoring and enforcement initiatives are warranted and provides guidance to help the utility industry avoid repeating errors that impact reliability.
In its latest guidance, issued in December 2018, NERC addresses recently identified risk situations related to:
Protection System Misoperations
NERC has been focusing attention on protection system misoperation for a number of years. A utility with a relatively high rate of Bulk Electric System misoperations over the last several years desired to improve its system performance. The utility determined that
NERC provided this lessons learned report to help other utilities in a similar situation to take proactive steps. A multi-pronged effort was initiated to address the company’s misoperation performance involving identifying the worst performing types of protection and communications schemes on its system and other process and procedure changes.
Avoiding Interconnection Reliability Operating Limit (IROL) Exceedances
IROL exceedances are a significant risk to reliability and are also a potential NERC compliance violation. For one utility, during commissioning of a new substation, one of three transformers tripped via differential protection because of a mis-wiring. The remaining two transformers picked up the load and the system operator immediately received critical alarms from the energy management system (EMS). These alarms indicated a potential IROL exceedance if there were to be an additional transformer trip because the last remaining transformer would then become overloaded and also trip.
The utility implemented an incident review effort. The results of the effort lead to improving guidance for operators, improving training related to operator handling of IROL and SOL (System Operating Limit) alarms, and performing field commissioning tests prior to energization, among other changes to avoid a similar event in the future.
NERC provided this lessons learned report to help other utilities in a similar situation to take proactive steps including conducting field commissioning tests to detect miswirings on pre-fabricated protection panels.
Avoiding Cascading Events
A 138 kV line tree contact followed by the misoperation of a 345/138 kV transformer resulted in two contingency overloads. Upon performing cascading analysis, the utility realized that one of the contingencies could initiate a cascade if left unmitigated. The operator took action to shed load on a pre-contingent basis to prevent the possible cascade.
The utility implemented an incident review consisting of several efforts. The results lead to identifying additional procedures and process controls, improving information presentation on the EMS system, a review of auto calculated relay settings, among other changes in order to avoid a similar event in the future.
NERC’s specific guidance for utilities facing similar situations is to consider developing procedures for running additional studies (beyond N-1 criteria) during a hot weather alert before taking non-emergency outages. In this particular case, an N-1-1 study would not have revealed any overloads (as Outages #4 and #5 did not result in any base-case violations); however, the RC is investigating software approaches to run additional studies during a hot weather alert. Also, utilities should identify additional controls to help ensure that EMS models accurately align with the system configurations, especially during multi-phased reconfiguration and construction projects.
Utilities should review each of these lessons learned reports to better understand the facts and circumstances and to determine whether they are applicable to their own systems. TRC understands that you may have limited technical resources in certain areas of planning and design. Our experts can supplement your existing staff and programs to develop internal initiatives that help you proactively respond to the reliability risks identified in the lessons learned reports. We provide assessment, corrective action and program implementation solutions for commissioning procedures, protection system design and cascading event avoidance.
This regulatory update is a service to TRC’s utility clients, helping keep you informed of issues that impact your company’s electric system reliability and resilience risks along with related topics regarding future regulatory developments to help you achieve your company’s business goals.