shortening of one of transformer load (fault propagates at transformer pri )

[usman74]

hello ENGINEERS
i have some query about protection breakers.
two days ago, in plant during normal operation, one of the load at 380V bus (heater) shorted. heater has MCB for its protection. that 380V bus is fed from 6.3KV through step down transformer. that transformer has breaker at both primary & secondary side. breaker hierarchy is as follows:
MCB for load, T/F secondary side breaker above the 380V bus, T/F Primary side breaker below 6.3KV bus. when this short circuit happened, it causes to operate T/F primary side breaker and 380V bus lost eventually & supply to all other loads have been lost. while MCB and T/F secondary side remain intact. my question is that why this fault propagates, why not other two breaker operates.

 

[waross]

Poor co-ordination. Sometimes it is difficult to achieve good co-ordination under all possible conditions.
Another possibility is improper settings of the breaker parameters.

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

 

[dpc]

Another possibility is that the secondary breakers are non-functional. Have they ever been tested?

 

[arrehman1]

Is there a neutral ground resistor on the transformer?

I was looking into an investigation once with kind of similar circumstances. There was a ground fault downstream of 13.8kV/4.16kV Delta Wye-grounded transformer with a 2 ohm neutral resistor. There was an overcurrent electromechanical relay (CO relay) monitoring the neutral of the transformer for ground faults. A ground fault occurred downstream of the transformer where a 4.16kV breaker should have obviously tripped. But the neutral ground relay on the transformer operated before the breaker. It turned out that the neutral ground relay was actually coordinated for a high-resistance ground system and NOT a low resistance ground... So for two decades, the plant operated correctly until there was a ground fault... and BAM! mis-coordination.

http://GeneralPAC.com

- Awesome video tutorials for power systems PAC!
The Wye Wye Transformer Connection video:

http://gpac.link/1GdGRSQ