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

 

[waross]

I saw this issue solved with cables.
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

 

[tem1234]

Thanks waross,

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.

 

[dpc]

I've dealt with reactors in similar situations. They will work as long as the necessary current reduction is not too significant.

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.

 

[tem1234]

quote: "They will need to be on the low side. "

Can i ask why?

Thanks

 

[dpc]

An oversimplification, but it will be hard to get sufficient current limiting for low side faults if the reactor is on the high side. Primary source impedance doesn't have nearly as much to do with low fault current as the transformer impedance and any additional low side impedance.

But do some calcs - I may be spouting utter nonsense - it happens a lot.

 

[VTer]

What is your available fault current on the primary 6.9kV side? 3000kVA @ 5.2%Z is roughly 55kA assuming infinite primary source. Maybe you have enough impedance on primary. According to a quick calc, on a 3MVA base if your primary available fault current is 15kA or lower you will be ok with 42kA rated CBs. Just a thought.

"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

 

[tem1234]

Hi Vter,

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.

 

[waross]

After 100-200 ms, their contribution will be close to none in my opinion.
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

 

[dpc]

But the breaker still has to withstand the SC current, even if it is not trying to actually interrupt it. If these are molded case circuit breakers, no delay is possible. If they are power circuit breakers, there will be maximum allowable current and delay time.

 

[tem1234]

The short time current of the breaker is 50 kA, so it's close but "OK". they have trip unit with setting that we can modify.

 

[hmchi]

What about using current-limiting fuses in lieu of reactors, such as ABB ISLimiter ?

 

[davidbeach]

The fault current would still be there, and unless there is a listed series combination the current-limiting fuse wouldn't do anything of value.

 

[MikiBg]

Looks like your CBs can cope with sc current from grid. Are you sure about motor contribution? To take all motor currents into account, they all need to work simultaneously and direct on line or star-delta. Is that so? Do you have some motors on VFD or soft starter?

 

[jghrist]

You might need to be concerned with voltage drop. A 4000A load with an 85% power factor would have a 6.3 volt drop across a 0.003 ohm reactor. This is a 1.8% voltage drop.

 

[hmchi]

A current limiting fuse would be able to avoid the voltage drop issue, plus the iron and copper losses of the reactor 24/7.

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.

 

[davidbeach]

True statement about the operation of a current limiting fuse in absence of anything else on the circuit that also begins to interrupt the current. Current limiting fuse in series with a breaker with a magnetic trip element changes the operational characteristics of the fuse. The only way to know whether or not you have sufficient limitation is through the use of listed series rated combinations. You can't just pick out a current limiting fuse and slap it in to the circuit ahead of the breaker and assume that you have protected the breaker.

 

[hmchi]

The IS Limiter type of CL fuse is a 'system' type of fuse application, very different from the CL fuse embedded in molded case breakers. The IS Limiter would be used far upstream of the low voltage systems.

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.

 

[davidbeach]

I'm not talking fused circuit breakers, I'm talking about fuse upstream of the circuit breaker. The fuse can only provide any benefit if it is part of a listed, tested, combination. You can't just pop in current limiting fuses willy-nilly and say that your downstream fault duty problems are magically solved. That thinking was quite common 30-40 years ago, but has been thoroughly repudiated since. The problem is that the breaker begins to operate in that first 1/4 cycle and that changes the system impedance and can impact the interrupting characteristics of the fuse.

 

[hmchi]

You are right that this CL Fuse solution has been succesfully implemented many times and many places all over the world --- Why I recommended it ... The product was specifically developed for this purpose. Perhaps you should read up on this first .