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PFC Tripping problem


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Hi all



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


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Hello Porky


Welcome to the forum.


It is not normal to use a differential relay with power factor correction, but there should not be any real problem with doing so.

In regard to your problem, provided that the capacitors are in good condition, and the supply is clean, there should not be any real imbalance in current so the imbalance realy should not operate.

In reality, there are harmonics on the supply and these can cause sever current imbalances, even iff the capacitors are OK. Capacitors do age and usually, loose there capacitance due to rupturing small sections of the capacitors internally, or blowind internal/external fuses.

I would suggest that you measure the current in each phase of each bank and determine if the current is correct and if it is reasonably balanced.

If the current is low, you have some failed capacitors. If the current is high, you probably have a harmonics issue.

If harmonics are an issue, you can use detuning reactors to reduce the effect of the harmonics and reduce the harmonic current.


Best regards,

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Hi Its porky here... for some reason I could not log in again.


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



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