11 KV Motor Capacitor

The best form of reactive power compensation for any inductive load is "individual compensation", where an "appropriately rated" capacitor is connected at the motor terminals itself, where the customer can reap maximum benefits as only 'real' current (and not the apparent current) would flow thro the cable upto the motor.

If you connect the capacitor at the sub-station bus, this is called "group compensation" - and this is also widely practised. Here, the benefits are moderate. But, beneath the sub-station bus, un-compensated current only flows thro the cable and there will be increased cable losses as the current flow thro the cable would be more.

But, wait! I am a bit confused! You said tat the capacitor is connected in motor incomer breaker (i.e.) in paralle with the moto. Does this mean that this capacitor would be there in line, right from the instant of pressing the star button? Normally, pf correction capacitors are connected to the motor, only in the 'run' mode (i.e.) afte the motor has fully accelerated, thro the control of an N.O. contact of the RUN contactor. Pl. clarify!

The ideal position would be next to the motor, but not always practical. Does the motor have an isolator? Connect in there would be good.

If all options are run out, a double cable box on the breaker.

I'm not sure I understand the situation well enough to comment. Are there three locations involved--the motor, the motor incoming breaker (in a motor starter, somewhere in between but not extremely close to either the motor or the substation), and the substation. Or, is the incoming motor breaker in the substation?

In any case, trying to sort of read between the lines, I think your question has to do with what will change depending on whether you put the capacitors near the motor or near the substation?

The (power factor reducing) capacitors can help you in several ways--if they are near the motor, the cables from the substation / motor starter to the motor do not have to carry the reactive power needed for magnetization currents.

Similarly, wherever the capacitors are, from that point to the power utility and within the power utility, cables do not have to carry that reactive power. The biggest saving that you get by installing pfc capacitors is when your utility has a power factor billing penalty for low power factor.

Wherever you put the capacitors (substation, motor, or near a motor starter between the two), the utility will reap the benefit of the reduced kvars on their lines (and you will reap the benefit based on a lower power factor in the billing calculation).

I suspect there is no increased economic benefit on your side from locating the capacitors closer to the motor--you generally will not be able to reduce the size of your conductors to the motor in this case, as you must design them to carry full load current. You may achieve a reduced voltage drop, and in a situation (a long cable run from motor starter to motor) where you might have had to oversize the cable to achieve a satisfactory voltage drop, you could avoid that oversizing (if the capacitors are at the motor).

This link may help to understand it:

http://www.alcorn-energy.com/pfc.pdf (try to understand Fig 4)

When capacitor is used to compensate the power factor, the reactive power consumed by the motor is supplied by capacitor. In this case, the reactive part of the motor current flows from capacitor to motor, this means motor power cable doesn't have to carry the reactive current and hence it reduces cable cooper loss and voltage drop. So, the closer the capacitor to motor, the better is the result obtained from the individual compensation.

- MS

i have a doubt....when the motor+capacitor are switched on, the capacitor takes a very high transient inrush current, being a short-circuit when in the discharged state. In LV, this is called AC6b duty, and special capacitor-switching contactors are used to attenuate this inrush.

If an ordinary CB is used, it may weld on making. The only contactors i have seen rated 11kV are Vacuum ones, which are also prone to weld-on-make i understand.

So what should the OP use to switch on this motor+capacitor combo in your opinion ?

Dear Sir,

In LV side we are using AC-6B capactor Duty contactor (due to frequent switch-on & siwtch off ).but one APFC panel supplier told that there is no need to provide AC-6 duty contactor instead of we can provide normal contactor with one size higher current rating.is it correct or not?.

then we connected HT capacitor to motor.incoming protection will be fuse.its directly connected cable chamber of the motor breaker.capacitor will be sized for 90% of the No load.

i would not advice it. Excellent engineers have researched the phenomenon of capacitor bank switching and come up with the special contactors. So please use these only.

If you have some constraints, you can use an aircored coil in series with the capacitor cable...here is an extract from NEMA ICS2 :

You may also download and study this excellent leaflet from ABB :

1sbc101140c0202.pdf

Dear Sir,

We are considering series inductor for this application to limit the harminic current.

At 11kV, your options are limited. You may get a vacuum contactor to swith the motor+capacitor on and off (try to avoid switching by the CB), and use the aircored reactor as mentioned in NEMA and the ABB leaflet (Have you downloaded it? It gives the calculation of the reactor).

On the LV side, i would once again urge you to use only AC6b capacitor switching contactors.

I fear, in an APFC Panel, the capacitor switching duty becomes more onerous, as the already charged capacitors also would feed into the capacitort being switched-ON. In such a case, the total inrush current would be very high. If normal duty (I assume, AC-3 Duty) contactors were used for such an application, then the contacts would melt during switching inrush, as the making capacity in AC-3 duty is only 10Ie. The inrush would be much more than this.

I doubt that your APFC panel supplier is not technically sound. His argument of going for the next higher size is grossly incorrect. You have to calculate the actuals inrus current with the last capacitor bank in the panlbeing switched on , with all the other capacitor anks in the panel alreday ON. Then select such a contactor, whose making capacity is much above this worst case inrush current.

By the by, I had asked earlier in the post, whether the capacitor is switched ON even when the motor is still in the starting mode or does it get connected after the motor has fully accelerated. Pl. confirm.

Dear sir,

capacitor is connected when the motor is starting & running.

It is still unclear. can you post the power & control schematic here?

i could not post my drawings