Contactor Vs. Circuit Breaker

[ACCOTTER]

Can anyone see any other drawbacks to using a Circuit Breaker instead of a contactor. This is being considered for MV switchgear application feeding a 3000hp Gas Turbine Motor starter. Other than a contactors ability to withstand wear and tear what other pros are there to using a contactor in place of a circuit breaker.

 

[dpc]

Contactor:

Smaller, cheaper, rated for more starting operations - requires fuse for short circuit protection so coordination options are limited.

Breaker:

Bigger, more expensive, short circuit interrupting and withstand capability without fuse. Better coordination.

If starting will not be too frequent, breaker is a good option, at least in my opinion.

 

[ScottyUK]

3000 HP is big for a contactor. What voltage are you proposing to feed this motor at?

Contactors aren't that common above 3.3kV (UK) or whatever your local equivalent voltage is. At 11kV (UK) they are rare indeed. In my opinion 3000HP is too large for an MV contactor, and too large for a 3.3kV system unless there are very strong reasons to avoid going to a higher voltage level which we're unaware of.


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If we learn from our mistakes I'm getting a great education!

 

[SACMatt]

dpc & Scotty are correct, although contactors are cheaper and are designed to endure "operational abuse" so to speak, I cant think of a contactor that could effectively support a 3500HP starter.

If it's the price that is shying you away from going with a breaker, consider remanufactured equipment. Something like a 15kV GE Magneblast or a Westinghouse VCP-W Vacuum breaker lineup would work and would be inexpensive compared to buying new equipment. These old beasts are workhorses and are pretty reliable.

Matt Brierley
Satin American Corp

http://www.satinamerican.com

 

[BJC]

If it's medium voltage is there any difference other than the relaying and control?
Medium voltage is 100-=35000 volts. Is the turbine generator being used to start the turbine??
Or do you have something like 100- 250 HP to spin the turbine up before you lite it?

 

[ScottyUK]

BJC,

I can imagine that 3000HP is probably the cranking motor for the turbine designed to bring it up to about 20% rated speed for ignition and to assist the turbine in accelerating to the point where it can sustain its own compressor load. Our machines use a 2000HP starter motor through a torque converter, and they aren't that large by modern standards.


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If we learn from our mistakes I'm getting a great education!

 

[BJC]

ScottyUK
thanks.
At medium voltage whats the difference between a starter and a compressor? In something like the 15kV GE Magneblast or a Westinghouse VCP-W Vacuum breaker it's controls and relaying. The mechanism that interrupts the current is the same.

 

[dpc]

At medium voltage whats the difference between a starter and a compressor?

So, I'm sure that's not what you meant to ask.....

 

[wbd]

Here's a link to similiar topic:

thread238-182064

 

[ScottyUK]

BJC,

Most vacuum contactors are designed to switch load currents and make fault currents. Fault clearance is achieved using MV fuses. Vacuum circuit breakers can switch load current, plus make and break fault current. They don't require fuses. The mechanisms of breakers usually aren't intended for frequent switching, or at least they require routine maintenance at more frequent intervals. Protection coordination with contactors is sometimes tricky to ensure the thermal characteristic of the relay operates the contactor but also ensure that the fuse clears the fault before the relay tries to open the contactor. Contactor clearing fault current = major problem.

As for compressors...


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If we learn from our mistakes I'm getting a great education!

 

[BJC]

ScottyUK
I knew that was the case with 480 volt gear. I have worked on medium voltage line ups both vaccume and magnetic blow out. The switchgear would have motor starters and transformer feeder side by side. The vacuume interupters were interchangable ( except maby for some of the autotransformer starts)

 

[LionelHutz]

A contactor requires a disconnect switch, fuses and a protection relay to make a complete motor starter. A breaker only requires a protection relay.

There was no voltage given but at 4-7kV a contactor can make sense. At a lower voltage such as 2.4kV it would not.

The only other advantage a contactor can have that I can think of right now is that the chopping currents can be lower compared to a breaker.

 

[tem1234]

Lionel,

What do you mean by chopping current? Is it the current needed by the trip coil?

My english is still not perfect

thanks

 

[jraef]

When you use a vacuum bottle device to interrupt current flow, there is a phenomenon called "chopping" that can happen where transients are sent down the line. Vacuum contactors use different materials in their contacts to minimize that, vacuum breakers do not because their design criteria is for emergency interruption, not frequent interruption. If you use vacuum breakers to switch loads then, you should also use surge suppression.

More informed reading:
John Lett white paper
thread237-42676
Stanford Linera Acc. white paper on chop current issues damaging transformers



By the way, 3000HP is considered middle of the road for MV vacuum contactors, it takes a 400A contactor at 4160V, and several VC manufacturers offer up to 800A.

 

[tem1234]

Thanks jraef