Current limiting reator 2

[tem1234]

Hi guys,

Attached is the one line of the situation.

All the 600 V breaker are rated 42 kA, and the calculated Short circuit current is 50-55 kA approximately.

I am wondering if it is a good solution (economically and practically) to add current limiting reactor to limit the short-circuit current under 42 kA.

There's 3 place I see where i can put the reactor:

- at 600 V (location 3), but the nominal current of the reactor should be 4000 A, so i don't know if it's a good way. Also, maybe more expensive compare to replace the breaker.

- at 6.9 kV (location 1 or 2). At location 1, I only need 3 reactor (1 per phase) instead of 9, but the nominal current will be 1200 A instead of 400 A (or less, the transformer aren’t full load).

Maybe there will be a voltage drop at full load on the reactor, but we can change the tap on the transformer to adjust the voltage at full load.

Do you have any experience with current limiting reactor and it is a good way to solve this problem. Changing all the breaker will cost a lot of money, but maybe add reactor will be expensive too. What do you think of this?

Thanks

 

[hmchi]

breaker contact parting in 1/4 cycle [blow apart contacts] are LV breakers. IS-Limter work in medium voltage. The transformer will isolate any of the issue you cited and allow the unfettered operation of the CL Fuse. Perhaps you should read up on this product first ..

We install and use this produt all over the world for more than 10 years ..

 

[dpc]

hmchi,

I agree with David Beach. I think you need to re-read what he wrote. To assure that an upstream current-limiting fuse will protect a downstream molded-case circuit breaker, they must be tested as a combination. The OP was talking about 600V breakers, not medium-voltage. It is unlikely that a medium-voltage current-limiting can protect a circuit breaker on the low side of the transformer from high fault currents.

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)

 

[hmchi]

Where do you think the LV current came from ? if not from the MV side ?

Asking to type test the fuse with each LV breaker is excessive --- This may be a ill-advised rule or standard borne out of some old tragedy in the past .. now becoming an unreasonable 'bureaucrratic' requirement for no good reason ... next one could ask to test a breaker with a certain transformer just os that someone can sleep at hight ..

Apparently someone misapplied a CL fuse with the old Westinghouse mccb blow apart contact type construction and carried a grudge all these years ..

OK, I am tired of this .. If you guys are not willing to listen .. good luck ..

 

[davidbeach]

Thanks Dave. I was beginning to feel like a voice crying in the wilderness. Rather strange logic, to think that CL fuses are the answer to everything, like when all you have is a hammer everything starts to look like a nail. I'd never rely on a fuse on the delta side to clear ground faults on the wye side. Not that I'm much in favor of high side fuses to clear low side faults anyway, that pesky inrush pushing up the high current end of the curve.

 

[dpc]

hmchi,

Look at a time-current curve showing the primary fuse, the secondary breaker curve and the maximum bolted fault current for a low-side fault.

You will find that the fuse clearing time for a low side fault is generally much longer than 1/2 cycle, which means that the fuse is doing no current-limiting when the breaker is trying to open. The only way a fuse limits current is by melting.

As for the testing requirement - it is basically the law in the United States for non-utility systems.

David,

Yes, it's tough to protect much on the LV side with CL fuses. Expulsion fuses can do a little better sometimes.

Cheers,

Dave

"Theory is when you know all and nothing works. Practice is when all works and nobody knows why. In this case we have put together theory and practice: nothing works... and nobody knows why! (Albert Einstein)