Why Protection didnÆt TRIP 3

[deadzone]

Please I need you help as soon as possible
Last we there is S/S 11/0.415 kV at middle of markets, was burning the customer inform the Emergency. They found the station has supply not yet trip so they went to main source to disconnect.
4 panel board 11 kV (2 transformer and 2 incomer) not affected by the fire, but the LV board was completely burn and reach to the LV Cables at TR 1 side, TR 2 fire didn’t reach to it.
We think that the fire started at LV Board because of the highly damage at that side. The protection relays was only at 11 kV boards, at 0.415 kV was only fuses. The Relays tested before six month as preventive maintenance and repeated direct after the cleaning and found 100 % healthy and the grading was correctly from the SS to the Source the dc system was also healthy.
At LV Board we found that incomer 1 and the outgoings are not protected by fuses but only by links (maybe was temporally arrangement) the load brick Switch found not tripped.
The Damage was less at the LV Board (TR 2 i.e. incomer 2) because we found that they are protected by fuses, the load brick Switch found tripped.


Please I need explanation about really what happen there

 

[DanDel]

You probably need an onsite investigation with much more complete information provided.

 

[deadzone]

I know that and the investigation is going on but I am looking for different analysis maybe on of these replays will give me a hint.

Thank

 

[busbar]

Heed DanDel’s advice and get a qualified person to do an onsite evaluation of the system. You have expressed urgent need, so it is assumed you understand the very significant life-safety and property-damage aspects of nonoperational protection.

 

[RRaghunath]

One thing I can say here is, the transformer primary protection cannot be effective for fire on the secondary side.
The high set overcurrent protection on primary (if provided) acts instantaneously but only for primary faults.
The time delayed (IDMT) overcurrent protection is graded to allow fuses on the secondary circuits to blow first and hence time delayed.
Earth fault protection is not effective for secondary faults considering that the transformer would be delta-wye connected.

I think the inspite of fire, the fault was not dead three phase fault. It may be noted that the transformer can stand dead three phase secondary fault only for 3seconds.

The fuses are very important for protection against fire though many a time we don't pay so much attention to these devices.

 

[vtakev]

I suppose there was an arc between busbars in LV switchgear. Any short-circuit fault or overload would trip the CBs. If the inside air is not clean (even a close standing motorway could cause this) arc could appear and it will destroy the switchgear before any action from main CB.

 

[Bung]

I agree with vtakev. I have seen several burn-downs similar to this in which the HV side fuses did not operate for a fire on the LV side transformer terminals. But think about it - how many small welding sets are there out there that operate at 100A or so? This level of current on the LV is not going to cause any HV fuse to operate! It does not take a lot of current to give enough of an arc to start and maintain a fire, and once the transformer oil is burning, its all over red rover. Bolted faults are almost never found in real life on LV systems.


Bung
Life is non-linear...