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Total Power Failure of Fertilizer Complex owing to ground fault in MV cable

Total Power Failure of Fertilizer Complex owing to ground fault in MV cable

Total Power Failure of Fertilizer Complex owing to ground fault in MV cable

(OP)
Hello Everyone,

I have an interesting case study about total power failure of a fertilizer complex due to ground fault leading to phase-phase fault on one of its MV cable in network.

Details of network are mentioned below:

1- Four power generators TG-701A and TG-701B (Steam turbine driven generators), GT-703 (Gas turbine driven generator)and MG-702 (emergency diesel generator)
2- Generation and distribution voltage level is MV side 6.3kV and LV side 380V.
3- Five medium voltage distribution bus bars.

Single line diagram of complex is in attached file for your reference please.

On 16 October 2017, an electrical flash occurred in the power supply cable of SS-7 near SS-1, followed by tripping of all power generators: TG-701A/B & GT-703. Emergency diesel generator, MG-702 started and its breaker closed but it also tripped, leading to total blackout of the complex. All control & safety systems went into shutdown condition (Unprecedented!). It is pertinent to state that no damage to personnel or plant equipment & machinery occurred.

Detailed study about fault and sequence of events were carried out and report is attached.

All professionals are requested to please review attached report. Your valuable suggestions/comments will be helpful to identify week points in our system and possible remedies.

I am listing some unanswered questions below.

1. Why immediate ground fault protection didn't clear fault within time?
2. Why gitch observed in UPS output while rectifier of one of two UPS is being restarted after power restoration?

Regards;
Adeel

RE: Total Power Failure of Fertilizer Complex owing to ground fault in MV cable

Too much summary information, too little raw data to really say anything definitive. Not knowing the CT locations makes analysing the protection response somewhat speculative.

You had failure after failure after failure, none of which would have been catastrophic in their own right but together they formed the perfect storm. Start by getting your maintenance sorted out so things work as designed:
  • The failure of the protection scheme to clear the initial fault on SS-7 is difficult to understand, other than a poorly-designed and/or poorly-maintained and/or poorly-adjusted protection scheme. Earth fault protection using a core-balance or residual connection is straightforward enough.
  • Loss of turbo-alternator TG-701B to a cable fault in the location indicated is difficult to understand: is the diagram correct, and do the cables really terminate direct to the switchgear? Is the GCB inside the differential zone?
  • The GT trip was, in my opinion, avoidable: a properly maintained and tuned governor on a machine with good combusters would have handled that load rejection.
A protection coordination study certainly wouldn't go amiss. Test your protection and make sure it behaves as expected.

RE: Total Power Failure of Fertilizer Complex owing to ground fault in MV cable

What Scotty said. You need competent help, on the ground, Each failure must be analyzed.
eg: A breaker did not open.
Why?
Faulty relay?
Faulty breaker?
Improper settings?
Problems with the CTs, location? CT failure? wiring issues?
Considering the catastrophic result of the DCS failue, consider a dedicated standby generator for the UPS alone. No other loads allowed on the dedicated set.
Is ther a lot of stuff like this in the plant?

Quote (itsmoked)

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Total Power Failure of Fertilizer Complex owing to ground fault in MV cable

I'll add to what was said above: Do not leave your emergency generator dependent on a UPS. Emergency generator controls should be directly DC powered and independent of any other system.

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