Capacitor Bank Voltage

hope this helps

XXXX://store/gedigitalenergy.com/faq/documents/general/shunt.pdf

Just change the "X"'s to the usual intertube stuffs.

That link did not work for me, even after changing the xxxx to http, and changing the / between store and gedigitalenergy to a period.

This link seems to work.

I think you'll find knowing how to use the Laplace impedance of a capacitor X_c=1/(sC) will be very helpful. Oliver Heaviside was working on transmission line effects when he re-invented the Laplace transform.

a good amount of information in capacitor selection, de-tuning and related application notes is available from some reputed manufacturers - a brief list is shared here:

already shared here before - http://tinyurl.com/gepfnotes

and :

http://tinyurl.com/moreelectroniconpfnotes

http://tinyurl.com/schneiderpfnotes

if you need to know something beyond the links listed then do ask . . . i promise to try to help . . .

Simply put, the voltage marked on the capacitors is the highest SAFE voltage that the capacitor should ever see. More than this may cause it to explode.....

It is always best to use an actual voltage lower than the marking.

I used to work on heavy printers where the 50 volt caps had 48 volts on them, they did occasionally explode, but had a metal cage to contain the pieces......I was in the same room one day (30 meters away) and only one exploded in a bank, but it was louder than a hand grenade at 30 meters!!!! Probably being in a room amplified the effect to a degree......

So don't be unsafe with the voltages you apply to any caps, you could lose an eye or worse......

GA. I gave you a GA because I believe that someone who actually answers a question deserves more credit than someone who provides a Wiki-reference. Think about selecting a fuse rating. In most circumstances you would select a current rating somewhat higher than the maximum expected current. Capacitor voltage rating selection is analogous. As the voltage rating of a capacitor is exceeded, its reliability is reduced and the risk of dielectric breakdown (explosion) increases.

We should not lose sight of the fact that the voltage rating is also a function a function of the reactance and the VAr of the capacitor in question. It is in effect the main determinant of the capacitance value in farads or micro farads. Moreso, a capacitor is expected to have a leading PF which makes it imperative for it to have a higher voltage than the system voltage. It's actually a fixed voltage and frequency synchronous machine and it is expected to do the PF correction within the safe values of it's rated voltage. The higher voltage of the capacitor is purposely allowed to enable capacitors achieve the desired result of PF improvement. Just as synchronous generators will maintain steady frequency and voltage at all loads to maintain a close unity PF, capacitors have fixed voltages and at times frequency that is above the system voltage.

Your doubts would be clarified if you attend to our training programme on Reactive Power Management.

Dear Friend,

The Capacitor will have a LEADING CURRENT and hence LEADING POWER FACTOR and the Impedence Voltage will add to the LINE VOLTAGE known as TERMINAL VOLTAGE, and hence always HIGHER than the LINE VOLTAGE.

CAPACITORS in series is EQUIVALENT or SIMILAR TO THAT OF RESISTANCES added in PARALLEL in which the RESULTANT RESISTACE WILL BE LESS.

For example, If CAPPACITORS C1, C2, C3, connected in PARALLEL will add up like resistances are connected in series ( R1+R2+R3...) and NETT CAPACITANCE WILL BE 1/C1+1/C2+1/C3

If Capacitors are connected in SERIRIES nett CAPACITANCE will be C1+C2+C3.... like Resistances R1,R2, R3 as 1/R1+1/R2+1/R3=...

RAJESWARI.

I am afraid the OP's question is not about the "very marginal" increase in bus voltage when the capacitive load exceeds the inductive load in the system. The OP's question is why should a capacitor used - in a 415V system - along with a 7% reactor, be rated for 440V.

So the 6 to 7% increase in voltage due to the 3rd and 5th harmonics (assuming post #9 is correct) is not relevant?

Why?

Finally a correct answer. GA Electricalexpert65!

Most of the answers given before yours are not correct. They are based on limited understanding of the real science. Who ever use the info in the previous posts above should beware and ask for advise from a competent engineer.

The interaction between a capacitor bank and the inductive feeder is complex. PF, harmonics attenuation, parallel resonances, and voltage drop/boost are affected by so many parameters that a simple answer is not enough.

I have seen many bad use of capacitor banks that were done by the sales guy that caused more problems than it solved.

In the Northern Hemisphere the opposite applies. Maby translation problem? S.M.

Excellent post, but you completely forgot the effects of parallel and series capacitor connection on the maximum working voltage then allowed!!

Will you add it or should one of us do it for you?

I feel its important as the original question was about voltages and caps.....don't you agree?

NO! Rajeswari you have it all wrong!

Capacitances in parallel is C1 + C2 + C3...

Capacitances in series is 1 / (1/C1 + 1/C2 + 1/C3...)

Don't give answers for concepts that you don't understand.

Thank You Mr. Marcot, for correcting the comment. I acknowledge your correction.

I express my regret to the cr4 Forum Members for giving wrong comment.

rajeswari.

There is a reactance in series with capacitor. Normally it's capacity is 6% of capacitor MVAR,. Due to this L-C series combination, voltage develop across capacitor is 6%( actually it depend on the rector value.) higher than the supply voltage.As voltage develop across inductor and capacitor is of opposite sign.Hence the 440V of a capacitor is a operating voltage for 400V system.

This 6 to 7% combination is actually a third and fifth harmonic filter circuit.

7% de-tuned filter is used for filtering 5th and above and NOT 3rd.

For filtering 3rd harmonics onwards you need 14% filter.

Ok. Simpli(mind)fying an answer. In an AC cirquit concisting from at least a capacitor C and inductor L, when AC frequency goes near it's resonant frequency (which is f=1/2pi*sqrt(L*C)) strange things happen. Voltage and current on L and C goes bananas. Thankfully there is always an added or parasitic R arround to keep things under control, but that's not something to rely on. So the sugestion is to keep the cirquit detuned (unless you're planning on broadcasting). Detuning factor is refering to how close to the resonant frequency you 're safe to go at this capacitor voltage safety margin. (400V>440V) i.e. 10% These are pre-calculated limits and are more or less convensional. It's not even safe to go that close because neither capacitors nor inductors are perfect devices and other (lesser) resonances WILL happen. S.M

thank you all.................so the higher voltage selection for capacitance is due to the reactive voltage of the capacitance and detuned reactors is to prevent resonance condition. ...7% detuning is for filtering 5th ,7th harmonic and 14% for 3rd harmonic............thanks a lot...you guys have been of great help................

so for a 440v system , the voltage selection of the capacitance required must be greater thn 440v right?

For 7% detuning :

Consider a detuned filter circuit with a j100 ohm capacitor and a j7 ohm reactor( in series connection). The steady state voltage across the capacitor is :

Vc = -j100/(-j100+j7)*1pu = 1.075pu.

For 14% detuning :

Vc = -j100/(-j100+j14)*1 pu = 1.163 pu.

Therefore, it is seen that for different % of detuning filter, the voltage across the capacitor is different.

Thanks.

now to open a can of worms . . . .

so many amongst us here have answered with a probable presumption that harmonics are actually present on the bus under discussion . . . and explained the effectiveness of de-tuning reactors . . . .

however the reactor (if present) really adds nothing to the performance of the system in the absence of harmonics . . .

and the behavior of the system differs depending on the % load prersent at any time too . . . .

therefore :

i suggest that a load study (including a predictive harmonic spectrum) precede the design and installation of any capacitor bank, please.

Soebfatehi,

You are correct that a study need to be conducted for every cases as they are all different.

But, there are always some harmonics generated by "linear" loads such as transformers and motors. You also have steps and impulses voltages caused by load switching and lightening strikes on the network.

This is why, passive filters look simple but are extremely difficult to design. Many customers now use active filters to avoid most of these problems even if the equipment is more expensive, the risks are lower.

Capacitors charge to the Peak of the RMS incoming maintained at 120 degree intervals. A basic electronics text has all the formulas.