Circuit Breaker Breaking Current

SKS

You can only protect the load that is beyond the circuit breaker.

Example - Hydro Generator is 100 feet from breaker.

Transformer is 100 feet from breaker.

Generator to Transformer distance is 200 feet.

Only 100 feet is protected - past the breaker to the transformer.

The reason is this - even you shut off the breaker, the generator side is still energized. You cannot protect the wire coming from the generator unless you put the breaker closer to the generator.

Move the breaker closer to the generator to protect more of the wire between the breaker and the transformer - Generator to breaker 25 feet. Breaker to Transformer 175 feet. More wire on the load side is protected.

This has nothing to do with switching of the exciter circuits or the magnets in the generator, or stopping generator rotation, that would stop voltage in wire between generator and circuit breaker.

I based pune city in india countery,

my office a mnc and used ups system in office.

one day our office light current off and ups current are on and when main current return in office light than circuit breaker not working properly and ups system are blast and our office inside some fire.

so used carefully circuit breaker.

Generator on rent

Have you considered asking your supervisors in this hypothetical hydro power plant?

If they exist they will explain the system used.

thanks, i was considering if reacatance of cb should be considered.

regards,

sks

The reactance of the circuit breaker in the class you are examining is negligible and can be ignored. For current limiting circuit breakers, the reactance at time of interruption can be used to improve selectivity, essentially limiting maximum fault current downstream.

IEEE C37.010-1999 "Application Guide for AC High-Voltage Circuit Breakers Rated on a Symetrical Current Basis" gives information on adjustment of the current interrupting rating of high voltage circuit breakers when the X/R ratio is higher than the standard value (I believe it's X/R=17 for 60Hz). See particularly section 6.3.

You might also want to check out IEEE C37.04-1999 regarding asymmetrical current ratings.

The circuit breaker must trip if the fault occurs at downstream side of the breaker. The 11 KV breaker of transformer will get tripped if the fault occurs in the transformer or downside of the transformer.

Breaking capacity or interrupting rating is the current that a fuse, circuit breaker, or other electrical apparatus is able to interrupt without being destroyed or causing an electric arc with unacceptable duration.

The interrupting rating of the breaker must be sized for the worst case interrupting a fault by opening the breaker. It is possible that the fault level on the generator leads could be higher than a fault on the substation bus, in the case of a small generator on a large system. Or a fault on the substation bus could be higher, in the case of a large generator on a small system. For a fault at the breaker in question, your short circuit calculation will show you the fault level contribution from each source, you then subtract the contribution of the source that is not interrupted by opening the circuit breaker. That is the required interrupting rating.

It depends on location of CT connected to relay which is tripping the breaker. If the CT is after breaker, any fault after CT will sense the current and trip the breaker, fault cleared as there is no other source than your generator which is isolated from fault due to CB open.

if the CT is before CB (between Gen and CB), then any fault after CT, CB, relay will sense fault current and trip CB but fault will not cleared as still your generator will be feeding the fault. So you need to have CB very close to generator as additional protection to generator.

Thanks,

Vijay

The location of the CTs define the zone of protection for differential relays, and will define the protection limits for a fault within the breaker. Since the breaker is normally closed, fault level current on either side of the breaker will be measured, so the location of the CTs is irrelevant for most faults, which are external to the actual circuit breaker.

What if the fault is between CT and CB when CT is between GEN AND CB. CT will sense the fault current, buy opening the CB is of no use. So CT location is important in all cases with respect to source.

Not at all. The breaker opens for a fault sensed on either side of the breaker. If the generator breaker opens for a fault on the generator side (usually differential relays), the excitation is taken off and the prime mover shut down. If the fault is outside of the generator, the breaker still opens, and other protection clears the fault. Only in the case of a fault in the breaker itself does the CT placement matter. Normal practice is to overlap the protected zones, differential CTs on the system side, and system CTs on the generator side, 50 years and counting. The OP question was how to quantify the required interrupting rating for the breaker, which was not answered in most of the posts.