1. NERC Published Lessons Learned Summary
August 2016

2. NERC Lessons Learned - August 2016
One (1) NERC Lessons Learned (LL) was published on August 30, 2016
LL “Tie Line Relay Coordination”

3. Details
Pre-event, equipment failure left the POTT relay communication system out of service at Sub 1
Phase to ground fault on Line 1 between Sub 1 and 2 caused it to trip
Circuit breaker opening times in response to this fault resulted in a current reversal at Sub 2 on Line 2
Permissive overreaching transfer trip (POTT)

4. Details
Line 2 tripped at Sub 2 due to the current reversal and receiver noise and the POTT communication system only being disabled at Sub 1
DTT then opened line 2 breakers at Sub 1
Line 3 open-ended after lines 1 and 2 opened
Three-element tripped for the fault that normally required one element trip
Permissive overreaching transfer trip (POTT)
Direct transfer trip (DTT)

5. Details
During this event, the Line 2 POTT equipment at Sub 2 was still in service when it should have been taken out of service at the same time as the POTT equipment at Sub 1
This resulted in a false Line 2 POTT receive signal because of the noise in the receiver at Substation 2
When the POTT equipment was removed from service at Sub 1, Entity 1 notified the primary control center (PCC) at Entity 2; however, the Entity 2 PCC did not notify their own relay department
Entity 1’s policy did not require both ends of a POTT communication scheme to be disabled when one end was removed from service

6. Corrective Actions
Both entities’ outage coordination procedures (forced and scheduled) have been revised to incorporate internal and external review of work associated on tie lines
Both entities’ primary control centers (PCC1 and PCC2) will provide relay departments periodic updates on the status of relay equipment at remote terminals of tie lines
Both PCCs have reviewed this event with their system operators and outage coordinators, stressing need for clear, accurate, and verified communications
Entity 1 revised its policy on POTT schemes to remove the relay system at both terminals when work is done

7. Lessons Learned
Do not rely on the relay department initiating the work (planned and unplanned) to provide the PCCs with the appropriate information regarding the position of relay switches (i.e., on/off)
All tie line relay work should be reviewed by both relay departments, regardless of which entity initiates
Special care and review should be taken to make certain that the policies and maintenance practices of both entities are the same when working on tie-line facilities

8. Lessons Learned Survey Link
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. A link is provided in the PDF version of each Lesson Learned.