The objective of this paper is to share the RCA (Root Cause Analysis) of the power failure incident of 11kV substation fed from upstream 33kV SWGR through 2 nos. 33/11kV transformers at Company operating Plant.
Both interconnecting 33/11kV transformers tripped simultaneously causing black out of certain process train at the plant. However, all other downstream substations fed from the same 33kV SWGR did not trip
Maintenance team performed preliminary investigation and found that event records of protection relays of both 33kV outgoing feeders indicated that downstream 11kV incomers were racked-out.
Since, no physical rack out of both 11kV incomers, and after quick review of the relevant schematics, it was thought that simultaneous tripping could be attributed to mal-operation of Arc flash flap Switch at incomer/coupler circuit breakers of 11 KV Switchboard.
As such, in order to restore the power supply and resume production, the following has been made:
Switchgear cubicles along with VT and circuit breakers compartments were inspected and cleared from major findings.
TC (Temporary Concession form) had been filled and Risk Assessed to temporary disable Arc flash flap Switch at incomer/coupler circuit breakers of downstream 11 KV SWGR – till further investigation.
Finally, 11KV Power supply was restored after approximately 2 hours.
Company formed investigation team lead by the author of this paper to find out the root cause, which is concluded as follows:
This particular 11kV substation among all other substations is 1500 meter far from main upstream 33kV substation. As such, the interconnecting control cable is long enough to carry undesired stray voltage due to well-known phenomena of capacitive effect.
11kV CB rake-out trip signal was directly wired to upstream outgoing CB protection relay and powered from 110VDC control voltage of upstream substation. Due to long distance, stray voltage of 15VDC was measured in the cable but it was not enough to trigger the trip signal until 110V DC system disturbance took place causing enough stray voltage above 25VDC (experimentally verified) to initiate the trip.
Nevertheless, why other substation did not trip even though all of their 33kV outgoing feeders are tapped from the same affected110VDC source? This is what we will explain in the next pages.