Very hot cables in our PFC unit 1

[jpcqub]

We installed a PFC unit at one of our sites as well as an active filtering unit. Now because our capacitors were not de-tuned, the PFC unit would not operate as required (or so we were told). We have now installed series inductors to detune the unit, however the cables coming from the contactor to the reactors/capacitors are very very hot! Now the cables were fine before installing the reactors, but now the cables are roasting! Has anyone on here encountered this problem before and how it may be cured?

Thanks in advance

 

[Skogsgurra]

It sounds like the detuning has turned into a tuning! This sounds very much like resonance. Measure voltage across the capacitors. If higher than mains voltage, then you have resonance. Switch off and have the experts (maybe not so much experts?) have another go on it.

Gunnar Englund

http://www.gke.org

 

[jpcqub]

So does that mean the filtering unit needs re-commissioned or that we have the wrong type/size of reactors and/or capacitors in the PFC unit?

Thanks for your help. It's off at present, and we're heading to the site early next week so we'll check the voltages then.

 

[waross]

I agree with skogsgurra;
I suspect that the first issue was that a problem with the PFC was mis-diagnosed as a "Tuning" problem. It sounds like the second problem may have been calculating a reactor value to achieve resonance rather than to avoid resonance.
It will probably be useful to skogsgurra if you describe the order of connection of the filtering unit, the power factor correcting capacitors and the sensing point for the PFC unit.
respectfully

 

[electricpete]

Just to rule out a few other possibilities - check if the cables are hotter near the reactor than at the contactor. If so it would not be a loading issue but a case of the reactors creating heat or field that influences the cables.

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[jghrist]

If the filtering unit is supposed to be filtering harmonic currents, then it should be tuned to the harmonic frequency (in resonance). It sounds like it is now tuned properly but is not sized properly for the amount of harmonics present.

 

[jpcqub]

got it sorted (we think) and thanks for all your help. We had one 4sqmm cable coming from our main fuseswitch and daisy chained to each of the four contactors! We changed it for a 16sqmm from the main fuseswitch down to a distribution block and then fed each of the contactors seperately with 6sqmm cable. Thankfully nothing to do with the reactors/capacitors (or at least for now!)

 

[Marke]

Hello jpcqub

One thing to be careful of, is the detuning reactors do give off a lot of heat compared to the capacitors on their own. If you have a lot of harmonics on the supply, then you should use very good ventillation in the enclosure. It is common to add thermostatic control of ventillation fans to keep things cool.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jpcqub]

Thanks Marke, we're looking into that at the moment.

 

[jpcqub]

Just out of interest Marke, how do you size your fans for a PFC unit. Are there ref tables? Is it acceptable to power the fans off the contactors, or better to use the thermostatic control?

Thanks for your help.

 

[Marke]

Hello jpcqub

You need to get an idea of the power dissipated in the enclosure by the reactors etc and from that, you can calculate the air flow required to achieve your desired temperature rise.

I use some software that I wrote to help with the airflow calcs, but you still need to determine the power dissipation.
You can download the software from my web site.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jpcqub]

Thanks Marke, I'll try and get a look at your site (and software) over the holiday.

John

 

[mohanlal]

I would like to know the type of cable installed. If it is single core armoured cables, then you may have to look into grounding of armours on both sides, that should not be.
Thanks,
subra