Checking PFC capacitors.

[Tiny Spark]

Hey guys,

Hoping someone can point me in the right direction here.

So I have aquired the responsibility of looking after the PFC units on site, Problem being nobody knows anything about testing the caps including me!

So basicly they are 3 phase units with anywhere between 5 and 12 stages depending on the unit.

The caps are all the same ABB 50 kvar 15kv 415v units.

They are conected through a contactor and 100A fuses.

Now checking fuses and contactors is fine but I need to know how to properly test the caps.

A rather unhelpfull contractor just told me to calculate the expected uF value and test it with my meter.

I found a formula:
Mf = kvar/(kv x kv x .314)

Which gave me 0.7.

But if I test the caps phase to earth my meter is reading around 2nF.
If I test between phase I just get OL which I assume is because of the shorting resistors on the caps.

So yeah ime in need of some guidance please guys

 

[Lucien Nunes]

I would suggest contacting ABB for information as they probably have a recommended test procedure specific to that model of PFC unit. E.g. to check capacitance and insulation resistance of each bank / step being within a certain tolerance according to age etc. It may be necessary to disconnect the discharge resistors and/or subdivide the banks to determine whether the individual capacitors in the unit are good, all of which they should be able to advise.

 

[Tiny Spark]

Thanks Lucien, must admit I never considered calling them!!

 

[LankyWill]

What's a PFC unit?

 

[Lucien Nunes]

Power Factor Correction. The control unit monitors the load power factor (assumed to be lagging) and adds more or less shunt capacitance to increase the pf to near unity.

 

[westward10]

What's a PFC unit?

Most large sites/factories have them and in some cases factories can use synchronous motors which have the same effect.

 

[Tiny Spark]

Basicly PFC is correction to increase the efficiency of the system.

I work in a large manufacturing plant so we have some very large and current consuming kit.

All about the cost saving really lol

 

[LankyWill]

I remember now, capacitance reactance is used to correct the sine wave lead or lag. This occurs in installations with lots of motors and lighting. The cap is inserted in parallel with the motor to help combat its inductive nature.

 

[Lucien Nunes]

Parallel capacitance can only correct lagging pf, but a typical industrial load is always significantly lagging. Unless, as per Westward, there are large synchronous (not induction) motors that can be run overexcited to give leading pf, or installed without a load to act as variable capacitors (controlled by the excitation). They are still referred to as 'synchronous condensers'. There's a pic of a nice 125MVAR unit here: Synchronous condenser - Wikipedia - https://en.wikipedia.org/wiki/Synchronous_condenser#/media/File:Templestowe_Synchronous_Condenser_1.jpg

 

[Andy-1960]

The way I do it is is measure the current in each phase feeding each capacitor. A 50 kvar capacitor connected to a 415v supply should draw approximately 70 Amps per phase.
You will find the capacitors will degrade depending on the age of the units, (generally they have a life of about 10 years) and depending on the power factor of the system you will then have to determine if they need replacing. Based on the current reading you are getting you will be able to determine the percentage of life left in each stage.
Usually the relay on the front of the panel will tell you the power factor of the system.
Be careful whe taking the readings as unfortunately it does mean doing live testing.

 

[Tiny Spark]

Thanks for that reply Andy.

I had read something about this method but was not overly keen on the idea as the bus bars that feed the caps are fully exposed down each side of the cabinet and its not a big cab to start with!!

May have to look at a remote testing method.

 

[mache]

Or get ABB in to survey and report???

 

[Bobster]

Or get ABB in to survey and report???

Could do that as an initial start point, but wouldn't be economical to do as a ppm.

 

[Tiny Spark]

Or get ABB in to survey and report???

Thats what they have done in the past, trouble is they only do it once in a blue moon due to the cost so want us to do it in house on a more frequent basis.

 

[Cadgey123]

Thanks for that reply Andy.

I had read something about this method but was not overly keen on the idea as the bus bars that feed the caps are fully exposed down each side of the cabinet and its not a big cab to start with!!

May have to look at a remote testing method.

Always going to be a bit dodge testing with the power on.
You can get wireless loop ammeters now which you could stick on each phase of the capacitors on load to see what they are pulling.
As Andy said they should pull around 70 amps give or take a couple of amps on each phase.
If you have any imbalances at all I would change the cap.

Ps always be safe when doing these kind of tests.