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

 

[jgoebel]

peebee, the discrepency is not whether current limiting fuses and VFDs work together to bring down fault current, but whether reactors and VFDs/not can be used etc.


I factored in motor contribution. This bumped up the original fault current from 36kA to 38KA, ish. So we were still a bit over.

Wrapping feeder is certainly not the ELEGANT solution to this. And I'm not even convinced its the cheapest. At approx 60$/ft, it wouldnt be cheap. A fuse would be cheapest if possible.

 

[jgoebel]

Anyone very familiar with SKM reply please -

I am trying to model putting a particular fuse in my system to see if SKM dappor/captor short circuit simulation is somehow adjusted if you put a particular fuse in the system. I'm just putting busses on the same side. I still don't quite understand though I don't expect SKM to pop out lower numbers for the short circuit, but yet someone's telling me here and from Littlefuse that the fault will be cleared immediately for anything below 200kA of a fault .

I don't intuitively understand the relationship of a fault, the breaker coordination to make sure tripping order works (whats tripping order) , and this ultimate relationship with your fault current values etc. I can see a relationship forming, so that I will understand this stuff REALLY WELL, but the synapsises just haven't started firing quite yet....



jimbo

 

[jgoebel]

I just tested placing the Littlefuse in my SKM model and put a bus on either size of the fuse to test available short circuit current at both sides.

Identical.

I don't get it, I think I need to be thinking at this more from a time of trip/break the circuit perspective...

please shed light for the love of all that is good

 

[davidbeach]

The fault calculation module in SKM does not evaluate protective devices, it only calculated the prospective fault at each location. For current limiting devices you would have to calculate the fault at the motor, see if the fuse is in its current limiting range (if it is below the current limiting range, the fuse will not clear instantaneously) and then use the fuse data to try to determine the peak current.

Unless the fuse and the breaker are a listed series rated combination the calculation of peak current is just an exercise, you can't do anything with it, it does not change the fact that the breaker is over its rating. If the fuse and the breaker are a listed combination, then the calculation of peak current is also irrelevant and you can use the combination as long as your fault current does not exceed the combination rating.

While looking at the interrupting rating of components in the motor circuit, the motor contribution is just a red herring, forget about it. For any fault devices on that circuit your device will only have to interrupt either the fault current from the supply or the motor contribution, but never both. Contribution from your motor becomes important when evaluating any other circuit as it then combines with the supply fault current.

 

[JensenDrive]

The default report from SKM is the total fault current available at each bus, normally. This cannot be used for finding fault current contributions from each source (system vs motor vs total fault current) for a specific fault. If you want to see the contribution from each source for a specific fault:

Click on Run/Balanced System Studies
Click on SC
Click on Setup

Under report specifications for Branch Currents, select "Second Branch from fault" or "All Branch Currents"

Under Faulted Bus, select the bullet "Selected Buses" and then click "Select" and select the bus on one side of the fuse or the other.

Now run the fault study.

SKM does not make it all that easy to see the individual contributions. To do this, you either
a) study the SC.rpt, or whatever you named it. It can be a tad confusing to follow what the report is saying, but it will show the currents in all the reported branches for the specified fault.
b) You can create a custom Data Block Report that shows currents in all cables right on the one line.
c) Create a Probe Data Block where the probe displays branch currents (enable the probe via View/probe, and then click on the cable of interest to see the current in the cable).

An annoying issue I have with SKM does not have a "Close in fault" function. If you want to see the current going to a close in fault on a feeder that has a motor at the other end, and want to see the current from the bus and from the motor for a fault on either side of your fuse or breaker, you have to create a fake bus a few feet outside the main bus, and attach the motor cable to the fake bus, and then create a short (3ft?) cable between the main bus and the fake bus. Then you fault the fake bus. Clumsy as heck and the pits in terms of user friendliness.