Reliability of Bus Tie breaker protection

[Newbie2000]

Hello Fellow Members,

Excuse me for my lack of knowledge, I am a project engineer and putting this topic for clearing my doubts and getting some more electrical knowledge.

Suppose I have 2 switchboards, each switchboard has 2 busbars (so in total 4 busbars). Each busbar will be powered by one generator. Under normal conditions, the switchboards and busbars will form a closed ring operation. The switchboard busbars are interlinked with breakers to form closed ring operation. This mode of operation will give flexibility in terms of generator usage, load sharing and so on.

However, if due to some reason there is a bus fault and the related protection system of bus tie breakers on either side of busbar detects the fault and will isolate the bus to protect other busbars/generator/switchboard section.

Now, my question is.
1. What if fault happens and since circuit breaker is a mechanically operating device, it surely takes a few milliseconds to open. During this milliseconds required for breaker operation, is there a possibility that fault current travels to other busbars before circuit breaker trips?
2. How can other busbar/switchboard busbar protection system with breakers identify that fault is not on our busbar, so healthy busbar breakers do not trip?

Thanks and hoping for your answer.

regards

 

[wroggent]

1. All of the generators will contribute to the fault at its inception and you will see a massive voltage dip on all of the busbars.

2. Differential scheme

 

[jghrist]

It depends on the protection scheme. If bus differential is used, the relay will get current signals from all breakers connected to each bus bar. If there is no fault in the bus bar, all currents going into the bus will add to zero. For simple case, a fault on one feeder is 1000 A and the only source is one generator. The current into the bus from the generator breaker is 1000 A. The current into the bus from the faulted feeder is -1000 A. The sum is zero. If there is a fault in the bus bar, the currents going into the bus will add up to be the fault current and the relay will trip all breakers connected to the bus.