Circuit breaker rating in GIS substation 3

[leoliu]

Hi, we are evaluating the short circuit current interrupting capability of circuit breakers in 115kV and 345kV phase-insulated GIS substations. The three phase to ground or two phase to ground fault currents in the station exceeded the breaker ratings. I'm wondering if multiple phase fault types, i.e., three phase to ground, two phase to ground, and phase to phase ungrounded fault, should be considered in breaker duty study in these GIS substations, as theses GIS are phase isolated design i.e.not three phases in one enclosure. I didn't find any references or standards to answer the question. Appreciate it you could help to answer it or point me to some references.

 

[dpc]

The fact that the GIS is phase-isolated does not eliminate the possibility of three-phase or line-line faults elsewhere in the system that the breaker might have to clear.

I would consult with the few manufacturers of this type of equipment regarding their breaker ratings and your short circuit duties.

Normally (in the ANSI world, at least), breaker ratings are based on three-phase faults. See ANSI C37 standards for the testing protocols.

 

[leoliu]

Thanks for the reply! Understand the breaker has to clear the network fault, i.e., the fault outside of the substation. But do we need to rate the breaker considering the multiple phase to ground fault inside the GIS substation e.g. bus fault? Or do you refer to the substation as the "elsewhere in the system"? Manufacture's brochure does say the phase isolated GIS eliminates the possibility of phase to phase fault. If true, why do we consider multiple phase to ground fault on the substation bus to evaluate breaker duty?

 

[davidbeach]

How far away do you have to get before a multi-phase fault is plausible? If that's just 250ft outside the station why not just calc the in-station multi-phase fault.

When one this sentence into the German to translate wanted, would one the fact exploit, that the word order and the punctuation already with the German conventions agree.

-- Douglas Hofstadter, Jan 1982

 

[waross]

Rate the breaker for the maximum current that it may have to clear.
If the bus is feeding switch-gear or a transformer, consider a phase to phase fault at the end of the bus inside the transformer or switch-gear.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[dpc]

This isn't a situation where you want to get clever or try to economize. I would calculate the maximum possible fault current and if you get to within 90% of the breaker rating, I'd be very cautious. Again, the breaker manufacturer will know more about the breaker ratings than anyone else.

 

[stevenal]

I would add a few words to waross' statement above. Rate the breaker for the maximum current it may have to clear in its lifetime. I don't think you'll want to be changing the equipment if the source impedance changes.

 

[bacon4life]

The most common example of three phase ground faults inside substations is when the equipment is energized with the protective shorts/grounds still connected. I think it is very it is critical to be able to clear a 3 phase fault.

A single phase L-G fault will increase the L-G voltage on the other two phases, increasing the likelihood that a second phase could flash over elsewhere within the switchgear. Even with independent phases, there still may be a possibility for simultaneous loss of gas pressure in all three phases.