NERC Lessons Learned Summary LLs Published in June 2015.

NERC Lessons Learned Summary

LLs Published in June 2015

NERC lessons learned published in 2015

Two NERC lessons learned (LL) published in June 2015• LL20150601 - Real-Time Contingency Analysis Failure due to a Modeling Error

• LL20150602 - Generator Distributed Control System Impact on Automatic Voltage Control

• Real-Time Contingency Analysis (RTCA) failed to converge for 46 minutes• During this time the State estimator converged intermittently• Both power flow and contingency analysis were available in study

application, and System Operators (SO) had full visibility and control of the system via SCADA

• SO did not recognize that the RTCA result had been non-convergent for 29 minutes because the alert status was not in an obvious location on the screen, it was not a prominent color, and it lacked an indication that it was critical

Real-Time Contingency Analysis Failure due to a Modeling Error


Corrective Actions

• Corrected model topology change The error was attributed to a change made during testing that was not reflected in the file that

was used to update the model on the live system

• Modified the process for making similar changes The new process now includes an additional step to recreate the file if any changes are made

directly to the development during testing. The changes will be compared and verified to the original file, and the newly created file will be used to populate the live system

• Display was adjusted with the status information moved to a more visible area, color of the status alert for “non-converged” was changed from brown to red, and unique audible alarm was also added to the “non-converged” status alert

• Event was communicated to all SOs, detailing the importance of understanding what the different solution status terms mean



Lessons Learned

• Additional change-control procedure needs to be in place to avoid the transfer of bad data to production after it has been successfully tested on the development system

• For verification of changes, automated review, peer review, or both must be implemented as part of any change process

• Real-time tools, state estimators, and RTCA solutions need to be validated on the production system after a model change

• Visual tools must prominently display critical alerts to the SO

• SOs must be able to recognize when their tools are abnormal and react according to their severity.

• Placement of status indicators, color of the text based upon the level of the alert, and type of audible alert must be considered for ease of identification by the SO



• RC and TOP observed unusual generator reactive output following the switching of a shunt reactor near the generators’ point of interconnection

• Generators at the plant were operating with their Automatic Voltage Regulators (AVR) regulating to maintain a constant voltage schedule

• The expected generator’s change in reactive output was initially observed when the shunt reactor was switched out of service

• Following switching the shunt reactor out of service, the generators’ reactive output immediately reduced by approximately the output of the reactor

• The resulting transmission voltage was slightly higher than it was prior to the switching of the reactor This response initially supported the assumption that the generators AVR were functioning properly

and were automatically regulating to maintain a constant voltage schedule

• However, after a few seconds the reactive output of the generators automatically slowly began to increase.

Generator control system impact on Voltage RegulatorsGenerator control system impact on Voltage Regulators

Corrective Actions• The coordinated controls review with the GOP resulted in modification of the plant DCS to

prevent this behavior The DCS was modified to have the ability to set the reactive objective to be the transmission system

voltage schedule that the plant was directed to control instead of a fixed MVAR output This control change to the DCS system resulted in the desired behavior from the plant following a

contingency or transmission element switching

• After a review of other facilities within the RC footprint, there were multiple plants found within with the same DCS issue These have been corrected with the same or similar modifications in coordination with the GOP


Lesson Learned• While a generator AVR may be properly set to control voltage schedule, other

plant control systems may override or counteract the appropriate AVR response Periodic review of plant AVR response to system events must be conducted to determine if

this occurs If proper AVR response is inhibited, the RC, TOP and GOP should review the interaction of

all related plant control systems to develop a corrective action plan for coordinating the plant control systems to ensure the AVR will have the uninhibited ability to control to the desired voltage set point.


Directions to Lessons Learned:Go to www.NERC.com > “Program Areas & Departments” tab > “Reliability Risk Management” (left side menu) > “Event Analysis” (left side menu) > “Lessons Learned” (left side menu)

NERC’s goal with publishing lessons learned is to provide industry with technical and understandable information that assists them with maintaining the reliability of the bulk power system. NERC requests that industry provide input on lessons learned by taking the short survey. The survey link is provided on each Lesson Learned.

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NERC Lessons Learned Summary LLs Published in June 2015.

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