400 Volt Capacitors with Harmonic Filters

What does the Voltage Rating on a Capacitor Mean?

The capacitor's rated voltage is its withstand voltage. One normally selects a capacitor whose rating is greater than the expected maximum voltage that it may or will encounter in service. This prevents sudden release of the smoke that is encapsulated therein.

from the provided link...."Take note that a capacitor's voltage rating is not the voltage that the capacitor will charge up to, but only the maximum amount of voltage that a capacitor should be exposed to and can store safely."

also..."Some say a good engineering practice is to choose a capacitor that has double the voltage rating than the power supply voltage you will use to charge it." I concur with that rule of thumb. 520V sounds too little in your case.

Calculate the required capacitance based on the actual voltage you want it to charge to.

It's a capacitor, its capacitance is a physical attribute based upon the surface area of the plates, the thickness and the dielectric constant of the material between the plates. Voltage plays no part in calculating its capacitance any more than voltage does in a resistor's value.

The only effect that voltage has is on the dielectric strength/breakdown voltage of the material between the plates. This means that a 1Farad capacitor with a working voltage of 500 Volts has exactly the same capacitance regardless of the voltage across its terminals until the dielectric is overstressed and conducts.

The reactance does change, but only as an inverse function of frequency, so in the case of capacitors used in a power system, it is important to know the magnitude and frequency of the harmonics that it will be subject to.

Do not confuse working voltage with capacitance as was done in these statements: "...must I use this 520 volt value to calculate the effective capacitance value?...", and "...Calculate the required capacitance based on the actual voltage you want it to charge to....", they are unrelated. You use the maximum working voltage to determine the capacitor's voltage rating, and you use the desired reactance at a particular frequency, to determine the capacitor's rating in fractions of a Farad.

It doesn't matter whether we're talking power frequencies or radio frequencies.

RAM what you say is true for harmonic (or any other) filters and tank arrangements not for PFC....

eg:

Single phase power factor correction capacitor's capacitance calculation:

C(F) = 1000 × Q(kVAR) / (2πf(Hz)×V(V)2)

V is a required parameter.

OP, the units in this calculation are Farads (F), and as RAM said this will be less than 1, a lot less than 1. _{(the calculator I linked to provides PFC cap value in µF)}

This from Eaton explains more on PFC and even has tables so OP can look up what size and rating PFC caps are most suitable for their observed operating conditions.

They recommend Dielectric strength to be Twice the rated AC voltage (or a DC voltage 4.3 times the AC rating for nonmetallized systems). looks like OP is still a bit shy off that mark....

Now the OP needs to determine how his PFC caps are going to interact with the other reactive network elements and what filtering he may still need. He has some happy reading time ahead.

thank you all for your kind explanations. I wonder in reality, what happens to the voltage if there is really a p= 7 frequency harmonics in the system. I mean what should be the reactive power needed to supply to the system? for calculating this value must we accept that design conditions are enough? Here for a 400 volt I think in normal cases terminal voltage rises to 430 volt with the capacitors, but What if there is a high and continuous resonance conditions (the frequency which we assume dominant in our case , about 189 Hz ) , then what must we do? . In short what if harmonics are there? I guess calculations from this side are related to normal conditions but not talk about a pfc operation under resonance condition..

I think there must be a different technique to calculate pfc capacitors with the resonance event...

calculation of capacitors with harmonic filters (http://www.hilkar.com/harmonicfilteringreactors.html) (Sorry, link no longer available.)

looked at the link you posted and it was devoid of useful background theory that might help you understand and then predict what might happen with the 7th harmonic running amok. It is more of a sales brochure.

That Eaton link I posted is a technical bulletin and has a load of well explained back ground theory in it. It covers both PFC and harmonic filtering and their interaction with each other. It will help you understand.

You may have to google for academic papers, and read them, that will also help with your ability to predict the outcome of a scenario you have described _{(tried to describe).}

You are bouncing back and forward between PFC and filtering in your posts here. To me, this suggests some confusion or a lack of instinctive understanding.

So, what is your question?

my friend, I am not bouncing to and fro, I just want to learn if I use a pfc system with harmonic filter, than what should be my capacitor voltage,taking into account the harmonics , since without considering the 189 hz harmonics, we can easily calculate the capacitor voltages from the link I gave, since this is normal conditions, but with harmonics involved I guess terminal voltages of the capacitor will be very huge am I right?

If you just want to learn then start reading I guess.

To answer your question directly:

That would depend on the amplitude of the harmonics.

GA and ditto that. Resonance and a bit of feed back and things can "take off" _{(parasitic oscillations)} with very high I_rms

If additive interaction of the fundamental and this bright kaleidoscope of harmonics occurs then I suppose that voltages could be pumped up. _{(happy to eat crow on that btw).}