Reactor Design and Fault Current Levels for a Chiller

[rjgoebel]

This was an interesting issue I ran into while doing a fault study for a hospital. We turned out to be about 3kA over the 35kA AIC rating of an existing chiller. This rating was mostly constricted by a couple parts within the starter , (CB and contactor) that were only rated at 35kA. I was looking into different ways to get this issue resolved...

1 - to reduce the available fault current seen by the chiller

2 - to increase the AIC of the chiller.

Our recommendation as a firm is going to end up being to supply new VFD starters for these chillers. After looking into using a line reactor to reduce the fault current seen by the chiller, an app engineer told me that these reactors can only be used when a VFD is existing and will reduce the seen fault current by max of 5%. I really have no feel for these reactors and what they do, and when you would want to use them by and large.

I have heard many times of line/load reactors. Is the premise of these to simply control the impedance seen by the source so as to allow for more power quality, and reliability? or to correct power factor etc on a load by load basis?

input appreciated

 

[dpc]

I think you and the VFD engineer are talking about three different applications for reactors:

1) Current-limiting reactors are used to limit fault current to downstream equipment. This would seem an entirely appropriate application for the situation you have.

2) Reactors used on input of VFD to limit or smooth harmonic currents drawn by the VFD during normal operation and limit the harmonic impact of the VFD on the power system.

3) Output reactors on the drive to help control high voltages that can occur at motors when fed from VFDs, especially on long feeders.

At first glance, your proposed solution seems like a serious waste of a VFD, unless it is needed for some other reason.

Have you considered simply putting a current-limiting fuse upstream of the chiller starter? If there is a US-made circuit breaker in the chiller, there should be no problem in finding a UL-tested combination of upstream CLF and downstream breaker that will limit fault current to something the downstream breaker can handle.

A current-limiting reactor would be another option and it will definitely work, especially since you need only a slight reduction. But it will be big and you'll have to find somewhere to put it, deal with heat produced, etc.

 

[jraef]

A less expensive option might be to just replace the chiller starter and breaker (depending on size of course). Many of the starter manufacturers now have coordinated SCCR ratings on their own combinations of devices, so for instance you may have a breaker rated 50kA, a contactor rated 10kA, and an overload relay rated only 5kA, but together as a tested system, they may have passed at 50kA.

By the way, chances are really good that without current limiting fuses, that VFD is only going to have a courtesy 5kA rating anyway.

 

[rjgoebel]

A current limiting fuse would appropriately isolate the circuit/fault in a short circuit condition? Because any damage was done to equipment?

What code covers this , and fault studies in general?

 

[dpc]

Yes, the current-limiting fuse will clear the fault.

As for codes, we'd need to know what country the project is in.

 

[rjgoebel]

Florida, USA

 

[dpc]

In the US, the National Electrical Code (NEC) is the most important code for electrical installations. But virtually all components are tested and listed by Underwriter's Lab (UL), so equipment needs to be applied in accordance with UL requirements.

In this case, the fuse and the downstream breaker must be a "listed combination" which means the fuse and the breaker have been short circuit tested together. Most molded case circuit breakers have been tested with a variety of upstream fuses, so you should not have much problem finding a listed combination. Check with the breaker manufacturer.

 

[rjgoebel]

Who are some manufacturers of these Current limiting fuses?

 

[dpc]

Bussman and Ferraz-Shawmut are two major suppliers in the US.

Virtually any low-voltage fuse you can buy in this size range will be current-limiting.

 

[itsmoked]

rjgoebel; They will all have some fancy slogan that addresses the current limiting aspect. You will know it when you look at the specs. Like "Low-Peak" Meaning "current limiting".

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[slavag]

Hello.
As DPC saied:
2) Reactors used on input of VFD to limit or smooth harmonic currents drawn by the VFD during normal operation and limit the harmonic impact of the VFD on the power system.

Could you please send me some link about this application of reactor.
We have up to 40% 5-th harmonic in 690V AC system and we think it's because not good choice of reactor.
Regards.
Slava

 

[JensenDrive]

Since your calcs are coming so close to the rating, you might check a couple things:

1) Is the 38kA total fault current (power system + chiller contribution)? If so, that is not the right number to look at. The CB and contactor in the chiller see current either only from the chiller or only from the system, and will not see total fault current.

2) Revisit your cable impedances. Where did you get your cable impedances from? If you go to 3 different resources for cable impedance, you will find 3 different impedances. Did you use a resource that is maybe too conservatively low?

3) Revisit your utility source impedance. Did you get a source / utility impedance that erred on the high current side?

4) Revisit assumptions of X/R for the utility and step down xfmr. However, revising this can make your calclulated fault duty rise, but you likely need to be sure you have them right.

 

[peebee]

rjgoebel -- an old trick for ratings as close as yours are to available fault current is to run the feeders around the perimiter of the room once or twice. The extra feeder length might be just enough to drop your fault current down to 35kA.

 

[peebee]

Slavag -- is your 40% 5th on current or voltage?

 

[slavag]

Hi Peebee.
on current.
Regards.
slava

 

[dpc]

slavag,

I've always found these guys to be helpful regarding reactors or harmonic filters:

http://www.transcoil.com/

 

[peebee]

Slavag -- I'm no expert, but my understanding is that your drive acts like a harmonic current source, so reactor choice should have little impact on the harmonic current levels.

Harmonic voltage is a function of harmonic current times source (including reactor) impedance, so the reactor will impact harmonic voltage.

This description may be an oversimplification. But in my opinion, I'd be looking at your drive rather than your reactor if you want to figure out where that 5th harmonic current is coming from.

 

[jgoebel]

I thought of the lengthening the feeder thing. My manager thought it was 'idiotic,' though. I checked and it would only take about an additional 35 feet of feeder..

DPC:
Your solution 1) (at top) seems great. This is what my manager suggested looking into. But after digging and talking to the reactor manuf., it seems that he indicated that you can only limit fault current in this manner if you are feeding a VFD..

please expand if possible -

Theres supposed to be a LittleFuse rep calling me tomm so X fingers.. The fuse solutions going to be a lot cheaper than 2 new VFDs..

 

[dpc]

The reactor has no idea what is downstream of it. It is just a series inductance in the circuit. The idea that it can only limit current for a VFD makes absolutely no sense to me. It's no different than adding a few hundred feet of extra cable to the circuit.

The reactor will work, but I still think the CLF will also work. You really need to be working with the circuit breaker manufacturer as opposed to the fuse supplier. They can tell you if the breaker has been tested with upstream fuses. Fuses are all the same basically, despite what the Littlefuse guy is going to tell you.

 

[peebee]

Current-limiting fuses are definitely good for many more applications than just VFD's.

There are a couple tricks, though:

1) As previously mentioned, you NEED to make sure the fuses are LISTED for use with the devices they are intended to protect. See NEC 240.86(B).

2) Motor contribution (which is often eliminated when using VFD's) can complicate their application. A quick reading of NEC 240.86(C) might convince you that your vendor is right, that you can't use current-limiting fuses to protect your starter (aka, "series rating"). But when you realize that the motor is connected to the load side of all devices involved, you'll also realize that you're in compliance with (C).

The worst part may be 240.86(A). If I were you, I'd make sure your fuse vendors stamp went onto that required documentation rather than my own.


Of course, if I was really you, what I'd really do is tell your manager that HE'S the idiot and then proceed to wrap the feeder around the wall, and forget about all this current-limiting fuse stuff.