Current Limiting Reactors 7

[mpparent]

Hi All,

I have a situation, where I'm looking at adding a 4th gen., to an existing system because of increased, anticipated load. The only problem is, my initial studies indicate that I would exceed my existing fault current levels. My thought is to add the 4th gen, and put a current limiting reactor in series with its feeder to limit the fault current. This is a 600V system, and I do not have any experience with installing said equipment. Are there any conditions I should be aware of? Any and all advice is appreciated.

Thanks,

Mike

 

[alehman]

It's best if the per-unit impedance of the generators match. You may wish to consult the supplier to be sure the added impedance for one generator will not cause difficulty with voltage regulation and load balancing.

 

[RRaghunath]

mpparent,

I think the reactors create more problems by way of voltage drops etc. and are not relaly helpful in most cases.

I would prefer

1) segregation of buses / loads,

2) selecting the new generator with higher impedance or

3) transformer in series with the generator

in that order depending on which suits the given system best.

raghunath_n00@rediffmail.com

 

[Bung]

Another suggestion is to use reactors between the bus sections to which the generators are connected. This way the bulk of the load current does not flow in the reactors, saving on losses and volt drop, but under fault conditions the reactors limit the contributions from the other genrator busses.



Bung
Life is non-linear...

 

[dpc]

I second Bung's recommendation. You probably don't want to put the reactor in series the generator where it will carry the full generator all the time and give you corresponding losses. If you can put the reactors on tie feeders or tie sections between buses, this will still help limit SC current but the reactors will generally carry much less current on a normal basis.

Breaking up the common generator bus is another option.

 

[mpparent]

Thanks for all of your responses,

When I get to work on Monday (5/23), I'll attach a one-line diagram to give you guys an idea of what I'm looking at. I'm not quite sure how I'd split the bus, but maybe you all will have a better clue than I.

Thanks again,

Mike

 

[mc5w]

Another solution is to install a 1% or 2% impedance reactor in series with each of the 4 generators if what you have is a single bus.

International Exposition Center in Cleveland, Ohio has a quad bus 4,800 volt distribution system that has four 10 MVA transformers that step down from 138,000 volts to 4,800 volts. There is a 5th bus with 4 reactors on it that in turn is connected to 4 tie breakers, 1 on each or the 4 main buses. Since they are no longer manufacturing stuff there they normally use only 1 of the 10 MVA transformers to save wear on the load tap changers. The other 3 buses run just fine with their power flowing through the 4 reactors.

 

[mpparent]

Well...

I can't do what cuky does and paste in a pic. Basically, our current configuration is 3 gen's all hitting a common bus. Each gen has an input breaker (all in the paralleling gear), a load bank breaker, and an output breaker, which closes in upon utility failure once all the gen's have sync'd. I'm really not sure where you could put in a current limiting reactor as far as on the paralleling gear bus goes. This is why I thought about putting it in-line with the new gen feeder.
We've also talked about breaking up loads, and that has been done somewhat. We may have to add another substation to get that to work (yuck).
Anyway, if you have any more feedback I'd appreciate it. Thanks for all of your replies.

Mike

 

[stevenal]

See

http://www.gwelec.com/product_current/commutat.cfm

They show a solution using a current limiting fuse and reactor in parallel, in a bus tie position. Not sure how your generator controls would handle them in the main lines though. Drop a fuse and current starts circulating. Mc5w's solution might be the best.

 

[alehman]

One other possibility might be to up-rate your switchgear if possible. That might be as simple as replacing the breakers, but some careful investigation would be needed obviously.

Reactors at each generator may be less expensive.

Not sure how the G&W scheme would work on LV.

 

[mpparent]

Well all...lucky me.

I was finally able to find some information on the bus rating of the gear in question. I wasn't able to find it before, so I had to assume the lowest rating of my compliment of breakers (65kA). The bus rating is good for up to 100kA. I should be able to easily replace my existing breakers, AKR-75's, with AKR-75H's, and that will get me to 85kA. All I have to do now is double check my equipment downstream. Thanks for all of your help guys, I think I have this under control now.

Mike

 

[windie]

Don't forget to check that your grounding system provides safe touch and step potentials at the increased fault current.

 

[alehman]

I think the 75H uses limiters (fuses). If so, that could cause some coordination problems. You may want to check that.

 

[mpparent]

alehman,

Yes..that did cross my mind. I'll be sure to check that as well.

Thanks,

Mike

 

[mc5w]

A second substation and bus might also be more effective for a number of reasons:

1. Not having all of your eggs in 1 basket.

2. Transformers generally run cooler, have greater efficiency, live longer, and have lower voltage drop when running at 1/2 of the forced cooled rating rather than flat out. A common configuration for large hydroelectric plant is to arrange the generators and transformers in pairs that use 3 circuit breakers in the breaker and a half configuration. The transformers run at 50% of the pedal-to-the-metal rating under normal conditions and only run hot when both generators need to be on the same transformer for maintenance or transformer or circuit breaker outage.

3. Busbars, circuit breakers, wires, and so forth also run cooler and have fewer other problems.

4. Avoidance of increased short circuit current.

5. You can keep your existing switchboard and circuit breakers.

 

[robthecob]

Before considering current limiting breakers, review NEC 240.86(C). I had a similiar problem and was going to use Cutler-Hammer DSL breakers to reduce the fault current to the downstream MCCs, but was prohibited by this requirement. This requirement first appeared in the 2002 NEC apparently due to misuse in some applications where the motor short circuit contribution added significant fault current through the downstream underrated device.
If your loads downstream from the current limiting device are mostly non-motor, than you might be able to use them.
If I used the current limiting breakers, I would have dropped my arc flash energy level to class 1 PPE from class 4. My short circuit study would have proved that the downstream MCCs would still have adequate margin to trip even with the added contribution from the motors, but the NEC does not allow engineering supervision on this one.

 

[mpparent]

Rob,

Do you have the 2005 NEC? I currently have 2002, and do not see a (C) subsection. My particular case does not involve a series rated situation. The breakers I'm looking at are fully rated at 85kA. My concern now is the addition of a gen, and its subsequent addition of fault current to the remainder of the system. Of all of my infrastructure (except a few instances), everything is fully rated.

Mike