PFC Capacitor Testing 2

[Latango]

We have an oldish (mid 90's) ABB Power factor correction system that was found in a warehouse and dropped off to us as a freebie. It has 9 50kVAR capacitors + reactors rated at 415V, and an ABB relay to allow for automatic operation or each bank can be switched in manually. The control circuit works exactly as the schematic says it should, all the contactors work and don't look like they have ever actually switched any current (low ducter readings on the contacts), and visual inspection of all connections show that the discharge resistors are intact and in good condition.

My problem is that I haven't actually used much in the way of delta connected capacitors before. According to the IEC standards they require a dielectric test, a visual inspection, electrical operation test and insulation test.

-Dielectric test for 415V is 2500V, so I am assuming that is the 3 phases linked to earth. (We do a lot of MV transformer and switchgear testing, so we have the gear)

-Insulation test is the same I take it? Say 1kV megger to earth.

But...

How do I test if they are functioning AS a capacitor bank? We don't have a large supply here, so I can't just power up a 200kW motor or something (we have plenty of transformers we could energise in the 100-1000kVA range) and I'm a little wary of charging up a big capacitor without being 100% confident in the methods.

Any ideas on how to check their capacitance, and whether I need to remove the shorting resistors to do it?

 

[Skogsgurra]

The easy and simple way is to connect to an AC source. Doesn't have to be capacitor's nominal voltage - can use much less. Then check current through capacitor with clamp-on meter.

Calculate C from I/(6.28*f*U) and var from U*I. U*I shall reflect rated var at rated voltage, reduce rated var with U squared if lower voltage is used.



Gunnar Englund

http://www.gke.org

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

 

[waross]

Don't remove the shorting resistors. Test with AC as Gunnar suggests. If the bank is rated in KVAR use rated voltage, root 3, and ohms law to calculate the current. Again as Gunnar suggests, use a lower voltage and adjust the expected current accordingly.
Each 50 KVAR capacitor should draw 59.5 Amps at 415 Volts.
If 50 KVAR @ 415 Volt capacitors are tested at 240 Volts, expect to see about 40 Amps.
This is based on three phase power factor correction capacitors, tested with three phase power.
Other factors apply when testing single phase capacitors and testing three phase capacitors with single phase power.
I am assuming three 16.67 KVAR capacitors in one case, connected in delta with three terminals brought out as a three phase, 50 KVAR unit, as is common in North America.

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

 

[edison123]

Where do you want to use these caps if you do not have the requisite power ?

Muthu

http://www.edison.co.in

 

[Latango]

Thankyou very much for the information, Gunnar and Bill, we will give them a test soon using those procedures.

After closer inspection, despite the fact the nameplate clearly says 450kVAR in 50 kVAR steps, the actual capacitor modules have 45kVAR written on them... which is a little odd. So presumably they are 3 x 15kVAR units.

We're not going to be using it, Muthu, just want to know if it's a functional unit or if we just bin it.

 

[edison123]

Do you do MV transformer short circuit test or temperature rise test ? If yes, then testing these caps with 415 V supply should not be a problem.

Muthu

http://www.edison.co.in

 

[Latango]

I WISH we could do short circuit withstand tests... We aren't a manufacturer though, and they are considered type tests (temp rise) and so-called "special" tests (sc withstand).

Unfortunately, since clients request that sort of test VERY rarely, there has been no push for us to upgrade our facilities to allow for it. Instead that work is sub contracted out. Not worth doing for this particular case.

 

[sibeen]

the actual capacitor modules have 45kVAR written on them... which is a little odd. So presumably they are 3 x 15kVAR units

Latango, remember that the kVAr ofthe capacitor is going to depend upon the applied voltage. Many PFC capactiors have, on their nameplate, different voltages and different kVAr ratings for each of these voltages.

For an example, if your capacitors were 45 kVAr at 380 volts then they would be 50 kVAr at 400 volts.

 

[rockman7892]

skogsgurra

Just out of curiousity where does the 6.28 factor come from in your calculation for capacitance?

I know the general form for current through a capacitor is i = C* dv/dt.

 

[waross]

2 times pi

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

 

[edijor]

What you saying is that I can buy dual voltage capacitors?, Do you know what company make them?

 

[davidbeach]

No such thing as a dual voltage capacitor. They all have a maximum voltage rating and one value of capacitance at all voltages. If a kVAr value is calculated, it has to be at a specific voltage and will be different for other voltages. But as long at cap is not operated above its maximum voltage it can be operated at any voltage and therefore any kVAr value associated with those lower voltages.

 

[LionelHutz]

Every power factor capacitor is capable of multiple voltages. They can handle any voltage up to their rated voltage. You can apply a 690V capacitor on 690V, 600V, 480V, 415V, 380V, 208V, etc. There is generally not much concern going between 50hz and 60hz either. The problem is you can end up with a large and expensive capacitor for the kVAR you achieve.