Help with capacitors

[eskimo39]

Hi guys. I am currently working in a large warehouse doing some investigating after originally doing an energy survey. When I first got here I isolated every submain but still got 30a on each phase. I originally thought someone was illegally tapping off but now think it could be something else.

I have taken apart a main busbar chamber and can see it feeding 4 boards. However I have also traced another set of wires to a large switch controlling a bank of capacitors. On testing this is drawing 30a per phase.

Why would there be a large bank of capacitors now a days. The warehouse has all new lighting with capacities built in for correction and other than that it is mainly only ring mains. Did older lighting require these banks of capacitors or were/are they used for something else?

Also when I turn them off, I am only saving 70a so would this mean they are been used somewhere although not correctly.

Thanks in advance for any help

 

[davelerave]

was it a factory at one stage?

not too familiar - i think the PF on the old fluorescent lighting was ok with the internal capacitors

 

[accordfire]

Hi guys. I am currently working in a large warehouse doing some investigating after originally doing an energy survey. When I first got here I isolated every submain but still got 30a on each phase. I originally thought someone was illegally tapping off but now think it could be something else.

I have taken apart a main busbar chamber and can see it feeding 4 boards. However I have also traced another set of wires to a large switch controlling a bank of capacitors. On testing this is drawing 30a per phase.

Why would there be a large bank of capacitors now a days. The warehouse has all new lighting with capacities built in for correction and other than that it is mainly only ring mains. Did older lighting require these banks of capacitors or were/are they used for something else?

Also when I turn them off, I am only saving 70a so would this mean they are been used somewhere although not correctly.

Thanks in advance for any help

Seems to me as if they were/are being used for phase correction - probably for lighting. Flourescents used to be connected in this way on large installs, and other plant requiring power factor correction too.

Capacitors are funny animals - in that you may well get current readings from the dissipation of current once you've isolated them - are you sure you're not reading discharge current, in this case, rather than supply current? The way to tell is to identify which way the current is flowing - e.g. are you getting a negative figure, or a positive one, and which way round is your ammeter connected (or are you getting current readings via a clamp meter)?

The other (obvious) thing to suggest, is that clearly you've got phase connected to the capacitors in some way - where do the capacitors feed on to? Can you trace the wiring at all?

Just some basic thoughts.

 

[benji]

could it be powerfactor correction ??

 

[Jas0n7]

If you decide to take them out be careful as capacitors can hold a monster charge and will blow your hand off. you can short them to blow the energy or if you leave them long enough they will lose it on their own.

 

[pushrod]

can be used for PFC in sets of motors as well i believe

 

[Guest123]

If they're only connected to the lighting system, then maybe once upon a time they may have had something to do with the emergency lighting system???


Or maybe there was a lot of industrial machinery present at one time.....

 

[eskimo39]

I learnt about power factor correction at college but unfortunalty remember bery little of it. There have never been any motors there only really lighting & sockets. Could it be perhaps they were there to somehome help balance the 3 phase supply when it was designed. Now the original design is long gone, could they be causing an opposite effect?

 

[accordfire]

Woweee.

PFC is a complex subject to discuss. There's many reasons it could be used.

As you say, it may no longer be needed.

Essentially, power factor correction works by balancing the real power flowing in a circuit with the apparent power (harmonics and such).

I think the best I can do is link you here to WIkipedia for a better explanation that I can manage.

Seems, either way you're dealing with passive power factor correction, and probably for lighting after all.

Simple way of checking if it is still needed at all is to take it out of circuit (safely) and see what effect it has on the installation.

 

[pushrod]

I learnt about power factor correction at college but unfortunalty remember bery little of it. There have never been any motors there only really lighting & sockets. Could it be perhaps they were there to somehome help balance the 3 phase supply when it was designed. Now the original design is long gone, could they be causing an opposite effect?

Yes. Inductive loads (from motors and fluorescent lighting) can cause voltage to lead the current. Capacitors have the opposite effect so they are put in to get the voltage and current back in phase and have a power factor closer to 1 (unity). If some or all of the inductive load is now missing the current could now be leading the voltage. Leading to too much current being drawn in the circuits concerned.

 

[Des 56]

Yes. Inductive loads (from motors and fluorescent lighting) can cause voltage to lead the current. Capacitors have the opposite effect so they are put in to get the voltage and current back in phase and have a power factor closer to 1 (unity). If some or all of the inductive load is now missing the current could now be leading the voltage. Leading to too much current being drawn in the circuits concerned.

That would be my conclusion as well
Lagging power factor at some distant time with condensers to compensate,but situation is now different and the current being read is charge and discharge of those condensors
They are probably now un required and would be economical to dis connect

 

[eskimo39]

Thanks for all the help guys. I am going there tomorrow to start changing lights etc for energy efficient ones. I am quite convinced that they are no longer required as I shut them off at around 3pm and there were no ill effects noticable. Only real difference was that the total load drawn went from 180A across all theree phases to just under 110A. Nothing of any real load ever been used in there.

The only bad side is that this seems to have been going on for the last 6 years, wasting around 70A 24/7 (although this could be more as the total load in the morning with all boards isolated was 90A) I have done a little calculation and it would seem at 17P per KWh it is costing over £2k per month. Again I have been told that their average electricity bill has been £9K per quarter.

I am going to look at taking them out in the future but will just isolate them totally tomorrow and leave it a while to see if they notice any changes bad or good.

Thanks again for all your help.

 

[Golden_Boy]

Thanks for all the help guys. I am going there tomorrow to start changing lights etc for energy efficient ones. I am quite convinced that they are no longer required as I shut them off at around 3pm and there were no ill effects noticable. Only real difference was that the total load drawn went from 180A across all theree phases to just under 110A. Nothing of any real load ever been used in there.

The only bad side is that this seems to have been going on for the last 6 years, wasting around 70A 24/7 (although this could be more as the total load in the morning with all boards isolated was 90A) I have done a little calculation and it would seem at 17P per KWh it is costing over £2k per month. Again I have been told that their average electricity bill has been £9K per quarter.

I am going to look at taking them out in the future but will just isolate them totally tomorrow and leave it a while to see if they notice any changes bad or good.

Thanks again for all your help.

You will only notice the drop in power consumption. The Power factor correction equipment was probably installed at the INSISTANCE of the power provider. without it the meters would not have measured the true power consumption, as to to this properly the voltage and current must not be phase shifted with respect to one another. In the past it was often the case that a factory owner would intentionally disconnect the PFC kit to save money. Some even reorted to removing the PFC capacitors from ALL of the flourescent fittings as well. Must have saved a fortune,

 

[buzzbar4]

Could have been used to hold in starter motor coils in the event of total power failure, seconds before emergency generator kicked in.

 

[pushrod]

My understanding of power factor is that if it is bad ie particularly leading or lagging then you have a larger apparent power meaning that a larger current than necessary is flowing. This has implications for cable sizes and protective devices but not for the energy consumed. It also is a big problem for generating companies as they have to supply larger currents than are actually used which is very wasteful for them and this why they can insist on PF correction measures. I didn't think that there would be particularly more energy used other than the extra heat lost in the cables due to the larger current flowing. ie the customer is being charged for the true power (kW) and not the apparent power(kVA) . Or have i already forgotten what i learnt at college?

 

[Jimmy Boy]

Yea with Pushrod Lead and Lag, nearer to unity you get keeps the leccy bills down, I learned all this a college but TBH I expect all motors and equipment are now power factor corrected at source

 

[eskimo39]

Thanks for all the input guys. After some research I found that the warehouse used to be a large factory in the 80's & 90's with lots of motors etc. Unfortunately these seem to have been connected even after the motors were all removed so goodness knows how long they have been burning energy. If I remember correctly with power factor correction you have inductive power (lagging) and capacitive loads (leading) and you are trying to get as close to unity with active (usable) power. If the circuit is mainly inductive (motors, solenoids, coils etc) you would find that the current is lagging the voltage and causing wasted energy. PFC is then used by adding capacitors to try to bring back that unity to make less energy waste. Effectively you are adding a device which causes leading (capacitor) to combat the effects of something that lags.

I think what has then happened is that after the motors etc had been removed the circuit became mostly capacitive causing the current to lead the voltage, hence the massive waste of energy. I think the only 2 ways to resolve this would be to add inductive resistance back or remove the capacitors!! I have disconnected them from the supply completely now but have never learned how to discharge them. I know leaving them should cause some loss but am unsure just how much. They are about the same size as a large microwave so am been quite hesitant with these until I find out more info. Currently they are isolated but ideally I want them removing, mainly as my old lecturer would like one for class demo purposes.

 

[Jimmy Boy]

I know of someone that had his whole shoulder ligaments torn from the bone after disconnecting supply and grabbing a Capacitor backed circuit..I believe he was out of work about a year, he couldn't let go of it.

Nasty

 

[eskimo39]

Oh thanks. I am even more scared now.

 

[johnboy6083]

isolate the caps. and then isolate them further downstream too. you now have a cable you can work on that is completely dead from both sources. install resistors between the phhses, and reconnect caps to that cable. It should short them out, and make them 100% safe. Newer PFC caps have these built in, so it happens automatically when the circuit is opened. as to what resistorsto use, im not 100% sure, but im pretty sure any rated to 400v will do

 

[Des 56]

Electrolytic capacitors can explode,the electrolyte vapourises, builds up preasure and bang
(As mining apprentices, we used to deliberately charge them up and leave them on the cluttered work bench for the unsuspecting)
They should be discharged with a bleed resistor,there is,apparantly, danger, especially given the size of them,for shock or fatality
Pour resin over the terminals,or short the terminals out from a distance on November 5th

 

[pushrod]

Unfortunately these seem to have been connected even after the motors were all removed so goodness knows how long they have been burning energy. .

Agree with most of what has been said but thought that my brain was failing me on the energy use thing so got "Scadden" out and checked and did a few searches. Was actually remembering it correctly

Inductances and capacitors do not use power. The extra current flowing in a circuit with low power factor is not all used up. Although a little of it will be dissipated as heat.
Power companies would love it if all the extra current flowing was chargeable to our bills but it is not! That is why they insist on power factor correction because without it, it ties up energy causing them to have generate more and causes them to have to use larger than necessary distribution cables, generators and transformers. The main impact for the consumer is larger than necessary cables and protective devices (hence all the threads on here about buzzing mcbs and fluorescent switching problems) the extra energy use is fairly negligible.

The reviews on the bottoms of the first few pages of THIS are interesting.

A few quotes from a wiki article on power factor

"Engineers care about apparent power, because even though the current associated with reactive power does no work at the load, it heats the wires, wasting energy. Conductors, transformers and generators must be sized to carry the total current, not just the current that does useful work".
.
"Reactive power does not transfer energy, so it is represented as the imaginary axis of the vector diagram. Real power moves energy, so it is the real axis".

"Since reactive power transfers no net energy to the load, it is sometimes called "wattless" power"

"It's a practical measure of the efficiency of a power distribution system. For two systems transmitting the same amount of real power, the system with the lower power factor will have higher circulating currents due to energy that returns to the source from energy storage in the load".

"These higher currents produce higher losses and reduce overall transmission efficiency".

Hope it is of some interest