Max size of PF Capacitors on Transformer

[Handasee]

I have been told by one supplier of power factor correcting capacitors that you can not put more than 20% of the transformers kva size. (Plant voltage is 3ph 600 volts)
Assume that we are talking about fixed capacitor banks not an automatic system.
What is the maximum kvar in relationship to the utility transformer size, that can be installed at the service and what problems will occur if it is too large?

 

[Skogsgurra]

I think that something got lost in translation from tech talk to sales talk.

There is a rule-of-thumb that you shall not use PF capacitors blindly if there is more than 20 percent non-linear load (Thyristors, VFDs etc) on the transformer.

You can, of course, use PF capacitors on any transformer. But you have to do more checking and perhaps install filters or reactors to avoid resonance if you have more than 20 % non-linear load.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

I have measured the no-load KVA of a transformer. That is close to the amount of KVAR required to correct the unloaded transformer to unity. Another way of putting it is to say that the capacitors are supplying the magnetizing current. I have gone double that amount and pushed the transformer to about the same power factor leading as it was lagging before the capacitors were connected. There was enough base load to avoid problems.
Too much capacity starts to drive the voltage up. In the old days before automatic power factor correction was affordable, capacitors would be permanently connected to the main bus in a plant. In some instances when the plant shut down for the night and the capacitors were left connected the voltage would rise high enough to burn out all the lights. It was more art than science, (but based on science) to correct the power factor of a plant at the lowest cost but in such a way that the voltage would not be driven up at light loads.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Handasee]

I have found one reference in our codebook. CEC
Quote:
"26-220 Transformers supplying capacitors
The volt-ampere rating of a transformer supplying a capacitor shall not be less than 135% of the capacitor voltampere rating."
I don't know why this rule was written. Maybe someone has a technical explanation.

 

[rbulsara]

Perhaps to be consistent with required the min. conductor sizes (ampacity) for capacitors, which is also 135%.

Rafiq Bulsara

http://www.srengineersct.com

 

[collies99]

Waht was your initial rason for buying the caps ? Do you voltage fluctuation problems ? Have you done a demand monitoring over days, weeks and months and recorder as such. Hvae you done similar record for voltage levels.

For one, poor PF and over compesation via too much capacitance can incur demand charge by the utility, something to think about.

How often woul you need the cap to be connectd for the intended purpose ?

More stuff to think abou.

 

[Rodmcm]

We have put 300kVAr on a 300kVA transformer at LV to correct pF at HV in three switched banks of 100kVAr, no problems noted. Would be interested in the rational for "135%"