Tripping of Customer 69kV Line ( July 5, 2012 at 1934H )

Below is my root cause analysis (RCA) for the tripping of our 69kV Line. Since I could not speak for the Test and maintenance section-Relay Group, I presented the root cause using simple calculation and analysis. Please leave comments below for the improvement of this RCA.


BACKGROUND:

On July 5, 2012 at 193H, CUSTOMER 69 KV Line auto tripped but autoreclosed. Informed concerned officials about the tripping. Transmission Line personnel were dispatched to conduct line inspection.

PROFILE OF TRANSMISSION LINE:

 CUSTOMER FEEDER is a radial 69kV line connected at 69kV bus at our substation.

Commissioning date of the line:	1996
Line/Cable Specification:	336 MCM ACSR
Line Hardwares Involve:	N/A
Right-Of-Way Issue:	N/A
Date of last Inspection/Maintenance/ Row Clearing;	May 2012

CHRONOLOGY OF EVENTS:

Date/Time Out	Line/Equipment	Date/Time Energized	Relay Target/Fault Locator
July 5, 2012 / 1934H	CUSTOMER            69kV Line	July 5, 2012 / 1934H	SPAA 341C: I (time), Phase A B, C SEG: I (time), Phase A, B, C (Ia=157A, Ib=130A, Ic=143A)

Weather Condition: Strong winds with heavy rains and lightning prevailed in the area.

INITIAL ACTION/S TAKEN:

Substation Engineer on duty coordinated with Supervisors, TL Engineer and Transmission Line Crew about the tripping of the line.

FINDINGS/ ANALYSIS:

The probable cause of outage is miscoordination of relays at CUSTOMER SS and our Substation. The low three-phase  fault current values registered by the SEG Overcurrent Relay is near the value of fault current transformed at the secondary side of CUSTOMER SS power transformer. Initial fault data at one of the 13.8kV feeders of CUSTOMER before tripping have the following values:

Indications of 13.8kV Feeder 1 of CUSTOMER S/S before 69kV line tripping at our substation:

Feeder 1-Lockout @ 1930H (4 minutes before the actual tripping of our 69kV Line)

Ia=   78A

Ib=   99A

Ic=1099A

Iground= 1014A

Converting the said values at 69kV primary side will yield:

Ia=   15.6A

Ib=   19.8A

Ic=  219.8A

Iground= 202.8A

The fault current values at CUSTOMER FEEDER were used as reference only. The values of Ic and Iground, 219.8A and 202.8A respectively, are near within the magnitude of the fault currents retrieved at SEG relay on the time of tripping (Ia=157A, Ib=130A, Ic=143A). Therefore, fault along CUSTOMER sub-feeders which may not be isolated by their overcurrent relays may have caused the tripping of CUSTOMER 69kV Line.

CORRECTIVE ACTION/S TO BE TAKEN:

 
          To verify if relays at CUSTOMER SS and our SS are properly coordinated.

                                                                                                           

RESPONSIBLE GROUP:

             Test and maintenance section-group, supervisors, Transmission engineers

SOLUTIONS/RECOMMENDATIONS:

            Test and maintenance section-group to conduct verification and relay coordination if needed at CUSTOMER Substation.