Paralleled generators protected with fuse AND breaker??

[tumbleweed1]

First time poster…certainly wont be the last either!

I have multiple 4.16kV generators in a parallel operation. The customer has spec’d that there be a “lockable fused non-loadbreak disconnect switch” between the generator and the withdrawalable circuit breaker that we have spec’d. So the configuration goes GEN->FUSED D/C SWITCH->BREAKER->SHARED GEN BUS. Of course there is a tie breaker between the shared gen bus and the utility bus.

What’s the purpose of 2 protective devices??

I mean, I could understand that someone would want a means of isolating the generator as close as possible to the unit for testing purposes. But having 2 protective devices in series with no load between them seems illogical and a possible coordination issue.. I suppose the customer could use a non-fusible link in the disconnect switch.

Any insight?
Thanks!

 

[ScottyUK]

Possibly the fuses have a better breaking capacity than the breaker. Faults close in to a generator or group of generators impose fairly severe breaking conditions on the circuit breaker, and a lot of common breakers aren't up to it. If the breaker is a general-purpose distribution-class design then the fuses may be required to provide backup protection in the event of a heavy fault.

The breaker is almost certainly there to allow other protective functions to disconnect the machine from the system and perhaps to provide automatic synchronising capability. Manual synchronising is possible, but is a nerve-wracking experience with a big set especially if you are standing in close proximity to the breaker. Not as nerve-wracking as an out-of-sync closure from a bad manual sync, but that's a different story.

 

[tumbleweed1]

Yea that was my thought as well- that the fuses would have higher current capability. Just seems odd, never seen such stuff.

Being a jerk back in college I enjoyed the grinding sound of briefly hitting the sync switch with the lab volt motors...figured it's better to learn there what it looks like then in the real world!

 

[ScottyUK]

Try it with a 300MW set. An entire transmission substation growling and vibrating when you're standing 10' away from the breaker is frightening. I am reliably told that being at the machine was much worse, and that the whole massive structure supporting the turbo-alternator lurched and shook. I recall a lot of white faces that night.

 

[catserveng]

I am seeing this more frequently in the size units I deal with most of the time, mainly 500 to 4000 kW. In most cases when I ask the design engineer or customer specifying the local disconnect, it is usually to provide a local disconnecting means for safety during service that doesn't require their plant staff to rack out breakers from a live bus.

It appears to be more common when the engine/generator is some distance from the switchgear room. Lately a number of the Solar turbine based cogen units I get around are also using a local disconnect, usually non-fused or with solid links.

I guess the best answer is probably going to come from the customer as to why he wants it.

Mike L.

 

[davidbeach]

The withstand rating of an unfused disconnect can be unrealistically low. It may be that the only way to have a disconnect is for it to include fuses.

 

[squeeky]

As you add alternates to the network the fault level rises. It may reach a point where it exceeds the rating of the rating of the breaker. If the breaker is now required to open up under fault conditions, it mat destroy the breaker and equipment along with anyone standing next to it. The fuses have a lower let through current and thus limit the fault level and allow the breaker to open when the current is reduced or stopped. The breaker protection will see the fault and start to operate. As it is slower than fuses, the fuses will blow before the breaker contacts start to open and thus limit the fault current. Obviously this is only on major faults. Lower level faults will be cleared by the breaker while the fuses stay in tact.

 

[davidbeach]

But only if the breaker and fuse are part of a listed series rated combination.

 

[tumbleweed1]

I asked the electrical contractor why they added the fuse with the disconnect. Said they could have done without the fuse, otherwise still never got a clear answer on that. He agreed that there would be no coordination.

Davidbeach not sure what you mean unrealistically low, if the switch is rated as a loadbreak at 41kA it should be able to interrupt that. But really, the breaker will take care of the clearing. Not sure what you mean by a rated combination either. Is this a predefined combo that has been chosen to coordinate with each other?

 

[davidbeach]

If you're not familiar with series rated combinations, you're probably not in NEC land. Different rules apply in different places.

Unfused disconnect is likely to have a 5kA withstand rating, although 5kV rating may be different, will the exactly same disconnect with fused might have a 200kA withstand rating.

 

[squeeky]

In S.A. a fuse disconnect is an off load isolator. We would never use it to interrupt the load. The kA rating is not the switch rating but the fuse rating inserted into the body work of the holder with arc chutes and flash barriers.

 

[davidbeach]

Withstand rating. Not switching rating. The ability to physically withstand a fault of that magnitude further downstream. Even non-load interrupting devices have a withstand rating.

 

[1capybara]

squeeky said, "In S.A. a fuse disconnect is an off load isolator. We would never use it to interrupt the load. The kA rating is not the switch rating but the fuse rating inserted into the body work of the holder with arc chutes and flash barriers."
he's right! the fuse disconnect is non-load break, the breaker is load breaker.
thats the intent of the customers request, "The customer has spec’d that there be a “lockable fused non-loadbreak disconnect switch” between the generator and the withdrawalable circuit breaker that we have spec’d."