Current limiting reator
Current limiting reator
(OP)
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
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






RE: Current limiting reator
A 13kV lineup was feeding Power Distribution Centers that included step down transformers. The Available Short Circuit Current exceeded the rating of the PDCs.
The impedance of the feeder cables was calculated and it was found that 100 feet of feeder cable would reduce the ASCC to the rating of the equipment.
All PDCs were fed with a minimum of 100 feet of cable.
I know that this solution is not always possible but where it is possible it may be the cheapest and easiest solution.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Current limiting reator
i thought about this one. The breaker are just next to the transformer and indoor. I don't know where we can pass the run of cable, but i will take a look at this, to know how many feet of cables i need.
RE: Current limiting reator
They will be big and heavy, and put out some heat.
Reactors are a viable solution and used to be installed fairly often back before the big increase in interrupting ratings for molded case circuit breakers.
They will need to be on the low side.
RE: Current limiting reator
Can i ask why?
Thanks
RE: Current limiting reator
But do some calcs - I may be spouting utter nonsense - it happens a lot.
RE: Current limiting reator
"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic — and this we know it is, for certain — then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". – Nikola Tesla
RE: Current limiting reator
You're right. In fact, the fault current is more in the range of 48.5 kA. The available fault current at 6.9 kV is 34.5 kA.
The current that come from the transformer is 38.3 kA, and the 3 others MCC contribute for the rest (this is one example). There's approxymately 3000 hp of motors in total on the 3 others MCC.
I made a quick calculation and it appear that the reactor at 6.9 kV should be 0.4 ohm approximately, maybe a little bit high, i think you're right dpc.
At 600 V, the reactor should be 0.003 ohm. What would like this reactor, a beast? Maybe we don't need 4000 A nominal since the transformer aren't and will not be full load.
An other option that i'm not sure about it is to remove the instantaneous trip, so that we can neglect a part of the motor contribution since they are asynchronous in the range of 100-200 hp. After 100-200 ms, their contribution will be close to none in my opinion.
RE: Current limiting reator
This may not be a safe assumption. If you have a high percentage of high inertia loads that take awhile to slow down, the motors may act as induction generators and back feed for longer than expected. A fault will load the motors-turned-induction-generators and contribute to the deceleration.
It depends.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Current limiting reator
RE: Current limiting reator
RE: Current limiting reator
RE: Current limiting reator
RE: Current limiting reator
RE: Current limiting reator
RE: Current limiting reator
The current limiting fuse by definition limis fault current by interrupting very fast [typically within 1/4 cycle] and drive the fault current to zero with or without going through current zero --- the 'let-thru' current is a small fraction of the available fault current, which is what you want --- a smaller fault current 'tet through' to do less harm to the system downstream.
RE: Current limiting reator
RE: Current limiting reator
Yes, you lose fault coordination. However the school of thought here is that when you have that big a fault, knocking off the source might be the better of the bargain.
Most fault calculations are based on faults having very low impedances in the faulted location. In actual system, these are usually the result of failure to remove ground wires or ground switches. Many other faults are of the high impedance variety that would not have triggered the operation of the CL fuse setting in the first place.
RE: Current limiting reator
RE: Current limiting reator
RE: Current limiting reator
We install and use this produt all over the world for more than 10 years ..
RE: Current limiting reator
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)
RE: Current limiting reator
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 ..
RE: Current limiting reator
RE: Current limiting reator
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)