1. NERC Published Lessons Learned Summary
October 2016

2. NERC Lessons Learned - October 2016
One (1) NERC Lessons Learned (LL) was published on October 04, 2016
LL “DC Grounds and AC Tied to DC Cause Multiple Relay Misoperations”

3. Details
Breaker status in a line relay momentarily changed state, indicating an open breaker
Line relay then keyed a breaker-open transfer trip to the remote end due to this false breaker-open status
At approx. the same time six high-speed aux relays operated
The relays are configured to trip associated generator breakers
Each breaker had a 352 breaker failure relay installed
Each breaker had two trip coils
Event records show all six breakers tripped by high-speed aux relay
If a protective trip is issued to a breaker, an input to the breaker failure relay is energized as a breaker fails to initiate

4. Details
Initial investigation indicated a strong negative ground on the 125 VDC system - Later disproved
An inadvertent connection between a 24 VDC battery circuit and the station annunciator 125 VDC common was identified
Once corrected the ground disappeared
It is believed this DC ground, along with a momentary AC voltage (see next slide) tied to the 125 VDC battery system caused the line relay breaker status to change resulting in a inadvertent “breaker-open” transfer trip to the remote station

5. Details
The aux relays in question were a high-threshold high-speed aux relay that requires a minimum of 70 V
This is intended to prevent it from operating on system grounds
Event records show indications of an AC signal on the breaker trip bus and breaker status input
If 120 VAC is momentarily tied to a 125 VDC battery system, the peak voltage on the DC system could rise to 305 V
120(√2)+135 VDC = 305 V peak
The normally open contacts had an extremely small gap (0.018 to in.)
Multiple event records taken from the breaker failure relays show indications of an ac signal on the breaker trip bus and breaker status input

6. Details
If the AC voltage was momentarily tied to the 125 VDC battery system, the aux relay contacts could have arced due to the small gap and the large peak voltage applied
Thus, causing DC tripping current to flow, tripping the breaker and picking up the aux relay’s target/seal-in unit
Target/seal-in units of the six breakers were found targeted, showing that tripping current went through the aux relays even though none of the relays that pick up these tripping auxiliaries operated
Despite thorough investigation, it is not currently known how the AC voltage was momentarily tied to the 125 VDC battery system
Gap about as thick as a business card.

7. Corrective Actions
The entity is considering a switch to fiber-optic contact-transfer- tripping auxiliaries and running fiber between the unit tripping relays and the control house
The entity is applying a modification to all similiar relays to add a two-cycle delay to the “breaker open” transfer trip in order to override a transient false input to the relay
Unit Tripping relays are located in the unit control rooms

8. Lessons Learned
A DC ground can cause a line relay to have a false-breaker-status indication, resulting in a relay misoperation
This lesson learned is an aggregate and similar to published Lessons Learned LL and LL

9. 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.