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

 

[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