Why aren't VTs fused?

[mbrooke]

PT (wound) and CCVT failures are usually explosive in nature due to the voltage and fault current available. As had been mentioned above, the zone of protection provides the primary protection for the VT. Also, usually if the VT fails it means your protection is degraded with it and it's not desirable to have the element in service without the VT also in service. Thus, no real reason to go to the trouble of primary fuses at transmission and sub transmission voltages.

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In agree, but in the case of a bus mounted VT, you clear the whole bus.

 

[epimetheus]

Even if you did fuse the VT primary, I suspect the bus differential would operate anyway in the case of fault. Relay detection time is usually no more than a cycle or 2.

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[rian0201]

Wouldn't buss differential cutout first?... At least it would here in the US where nearly all POCO substations have either low impedance or high impedance buss differential relays dedicated to bus protection. Remote zone 3 or a reverse MHO curve is used as a very last resort.

Depends if there is bus differential applied.

 

[Bugman1400]

Do they make fuses that can interrupt that much MVA?

 

[mbrooke]

Do they make fuses that can interrupt that much MVA?

Up to 25ka:

http://www.sandc.com/products/fusing-outdoor-transmission/smd-power-fuse.asp


But in all honesty a breaker is a better option. But, lets pitch it like this... why not use a breaker?

 

[Tony S]

Up to 25ka:

http://www.sandc.com/products/fusing-outdoor-transmission/smd-power-fuse.asp


But in all honesty a breaker is a better option. But, lets pitch it like this... why not use a breaker?

Expulsion fuses are not fuses in the conventional sense as they rely on the arc gap being extended by mechanical means.

 

[Bugman1400]

Up to 25ka:

http://www.sandc.com/products/fusing-outdoor-transmission/smd-power-fuse.asp


But in all honesty a breaker is a better option. But, lets pitch it like this... why not use a breaker?

So, the answer is no....I asked about MVA, not kA. The answer to why not using a breaker, if you can technically get a away with it, should be obvious......$$.

 

[Bugman1400]

Expulsion fuses are not fuses in the conventional sense as they rely on the arc gap being extended by mechanical means.

I think the Englishman has got it.

 

[mbrooke]

So, the answer is no....I asked about MVA, not kA. The answer to why not using a breaker, if you can technically get a away with it, should be obvious......$$.

So your telling me kA can not be converted to MVA? 69,000x25,000x3 sqrt=2,988MVA for a symmetrical 3 phase fault. A typical breaker is in the range of 20 to 65ka, so a fuse covers about half the applications a breaker would.

 

[Bugman1400]

So your telling me kA can not be converted to MVA? 69,000x25,000x3 sqrt=2,988MVA for a symmetrical 3 phase fault. A typical breaker is in the range of 20 to 65ka, so a fuse covers about half the applications a breaker would.

I agree.....which is why you will see fuses for some 69kV applications. I think it has more to do with extinguishing the arc.....its obviously easier the lower in voltage you go.....not so much when you get into the EHV range.