Power factor correction

[porky]

Hi all

I know that this question is not related to plcs but would be much obliged if you could help.


We have 360KVAr 400 Volt PFC Bank 6 steps, controled by a 6 step Varlogic controller.

The Problem that we are having is that the Circuit breaker supplying this bank trips (due to differencial current) everytime two or three substantial loads are shutdown. We have a torroid coil that feeds a Differencial relay.
This is the first time that I have seen a Differencial relay protecting a capacitor bank. Is this correct ??

What would happen to the current being drawn by the capacitors when the Power factor goes into leading as opposed to lagging ?

would this shift in current cause an imbalance in the phases causing the relay to trip ??

thanks to all
porky

 

[DonsDaMan]

I've had problems with capacitor banks tripping on voltage imbalances when they were shutting down. It's easy to see where this comes from, as the instantaneous voltages on the caps will be different at any point in a 3 phase AC cycle.

Differential current because of loads dropping? Typically, differential current should not be a problem in such cases. I'm wondering if you have a capacitor that's failing.

If you go leading on your pF, then the voltage will tend to pump up a bit. The impedance between your regulator and your capacitors will be the prime factor determining the amount of voltage rise you're going to see (of course capcitor current and capacitance (XsubC) are important; my point is the variables after the design are mainly determined by the source impedance cited).

I'm assuming you're dropping load phases simultaneously. I've seen zero-break contacts drop out each contact as it reaches zero volts, giving each phase a different time of opening, but these have been primarily on MV circuits, which yours is not.

I'd have to give it more thought, but your situation does not sound like it is characteristic. I suspect something is amiss.

I've seen PFC's that operate on each phase independently. Each phase is treated like a different source, and the pF on each corrected separately. These can have different phase currents (and thus imbalances) by design. If such a scheme is involved, then I'd remove the differential detector, because differential current is a characteristic of this type of compensation network.

If you give me more specific questions, perhaps I could be more helpful. I've only chimed in because I saw over 30 views for this topic without a response.

 

[porky]

power factor correction

Our Total current drawn by the PFC unit is as follows
R = 484 A
S = 492A
T = 509A

Is this imbalance acceptable ??

Separately the worst case of imbalance is

R = 75.2A
S = 83.8A
T = 72.8A

Each bank is rated at 60 KVAr at 400 V (Exact Voltage is 388Vac )

60000/(388 X 1.732 ) = 89.3 Amps

Is this a sign that some Capacitors are failing ??
What would you state is an acceptable difference in phase current ??

Many Thanks

 

[Lancie1]

Porky,

When you shut off motor loads, you usually get a large current at the capacitor bank. It will be unbalanced if the loads were unbalanced. If you have a differential relay, it will trip unless you set it to a higher trip point. If you must keep the differential relay, then you have to find some way to more closely balance the loads on each phase.\

You could set up a shut-down sequence, where the loads are shut off in a specified order, in small increments over several minutes. This would help to keep the capacitor currents from becoming unbalanced.

From your numbers, I don't think that you have a large unbalance. The currents actually looked fairly close. This level of unbalance should not be a problem for the capacitor banks at all. Plus or minus 10 percent is acceptable. I would check out the need for that differential relay.

 

[504bloke]

Is this an old system that has always worked and now has a problem ?

Or is this a new system that has always had a problem ?

How old are the capacitors ?

Has anything changed lately that may have caused this problem?

 

[Unregistered]

Pfc

I totally agree that the differencial relay is not really needed, but hey Im just a small fish in a big pond.... if you know what I mean... Unfortunatly Im the guy they call when the Plant shutsdown... ( we are at the limit on our supply so when the PFC trips the Transformer overloads and also trips.)


I am now currently monitoring the current on the secondary of the C.T. and then will set the relay conform with this current.

Thanks to all for the advise.

Porky