Limit of Capacitors Connected on a Transformer

its not safe.......total power kva= real power in kwatts+ apparent power in kvar...so according to yo specification total power s less than apparent power ..tht shouldnt happen........so either u increase kva or decrease kvar......or u do one thing , there s a software to calculate kva or kvar when u use a capacitor bank.....jus google free power factor correction software.......

Kvar does not stand for apparent power, it is Inductive Reactance (varsL), or Capacitive Reactance (varsC), Total or apparent power in a RLC circuit equals the difference between Inductive and Capacitive reactances plus your pure resistive TRUE power.

If there is more Inductance in the circuit then the power factor lags, otherwise it leads.

VA/ WATT=PF

sorry .....i meant reactive power but in a hurry i wrote apparent power ......my bad...............apparent power in kva= real power in kw+ reactive power in kvar

watt/VA=pf ....not VA/watt.................@vicman

that is true, i guess that is why we use software to do the math for us

I don't think your utility is very knowledgeable. PF correction to 0.97 ought to be deemed quite satisfactory. PF correction is kind of a "moving target" that depends on which motors are running and which capacitors are switched on. It is not practical to hit 1.00 PF exactly. Furthermore, if you overcorrect into a leading PF, I think you can get into positive feedback situations that are unstable. Thus the more usual criterion is to correct to maybe 0.95 or 0.97 PF.

I was going to answer the same...

It is not practical to correct the PF to unity or 100%, 95% is by most standards the satisfactory PF correction for circuits containing large amounts of induction. When we actually calculate this by the book it would look something like this:

VA=P/PF

VARS(L)=square root of the square of VA- the square of P

VARS(C)=VA-VARS(L)

Xc=square of E/VARS(C)

Capacitance =1/(2)(3.1416)(f)(Xc)

since transformer is major source of reactive power(lagging) its compensation particularly during no load condition is very much necessary. again system power factor correction=load power factor correction + transformer power factor correction

as you have 400 kvar capacitor banks ,are they kept off during no load ? if so then you have to be some capacitors across secondary of transformer during no load to compensate .the exact kvar depends on impedence of the transformer but generally 3-7 percent of the transformer kva should be the right choice in your case. you have not explained about second transformer?

This is not correct.

The transformer no load reactive power consumption is only the magnetising current load which is not so important and does not affect the PF so much as to need correction.(unless you have a real bad transformer...)

No need to keep a capacitor in parallel on an open transformer connection.

Dear Shekhar,

Your analysis is right. The lagging PF is due to furnace transformer inductive load.

The furnace transformer is 33kV/850V, 3000 kVA. It is difficult to connect capacitors across the primary because of cost and across secondary because of odd voltage and harmonics. When furnace is off, we are switching off 200 kVAr.

Regards

Yes It is safe and you have calculated the required KVAr to achieve unity PF in your system...It is hard but not impossible to achieve the unity,however you will get atleast 0.99.

more,you have to match your capacitor steps with your loads.I suggest you to have smaller steps,so that you will not get over capacitance.

correct me guys If I am wrong....cheers

Hi

Your 0.97 is excellent .

Don't change it .

Regards

Frome Morocco

With 400kVAr, 440V capacitors connected (8 nos. x 50 kVAr) to the distribution transformer secondary, the overall P.F. is 0.97, still short of unity but sufficient for maximum PF incentive offered by the supply company as above 0.95 they can give 2% rebate (maximum) and below 0.95 only 1% .Theoretically, to improve the P.F. to 1.0, additional capacitors required are 260 kVAr (Furnace rating is 2200kW) which can be switched on separately with furnace and close watch on PF should be kept, or we can take trial with 200 kvar capacitor and pf recorded.

thanks

MANOJ KUMAR SINGH

GENERAL FOREMAN - URANIUM CARPORATION OF INDIA LTD.

NARWAPAHAR

With 400kVAr, 440V capacitors connected (8 nos. x 50 kVAr) to the distribution transformer secondary, the overall P.F. is 0.97, still short of unity but sufficient for maximum PF incentive offered by the supply company as above 0.95 they can give 2% rebate (maximum) and below 0.95 only 1% .Theoretically, to improve the P.F. to 1.0, additional capacitors required are 260 kVAr (Furnace rating is 2200kW) which can be switched on separately with furnace and close watch on PF should be kept, or we can take trial with 200 kvar capacitor and pf recorded.

thanks

MANOJ KUMAR SINGH

GENERAL FOREMAN - URANIUM CARPORATION OF INDIA LTD.

NARWAPAHAR

Dear Manoj ji,

We are ready to take the trial but are worried about the distribution transformer safety when we connect 400kVAr + 260kVAr = 660 kVAr as the transformer rating is only 630 kVA. The load on the distribution transformer is only cranes, pumps, lights and fan totaling 200 kW

It is difficult to connect capacitors on furnace transformer secondary due to odd voltage (850V) and harmonics and on primary (33kV) because of cost.

Regds,

It would not be safe to correct the power factor to unity nor it is required. Unity power factor means adding as much capacitors as much inductance in the circuit. This would result in resonance. The voltage across transformer as well as capacitors would be high and may harm the insulation.

That is totally true, I am surprised no one mention this yet; at unity, the transformers and the capacitor banks become a huge tank circuit achieving resonance.

Dear Sunilji,

I have analysed your question and also the opinions expressed by forum members and guests. Since the electricity is being used for a critical industrial application, I recommend that you conduct harmonic analysis and energy audit which will give details about the energy consumed, active KVA, reactive kVAR, power factor of the load(when capacitors are not used) and other energy details and the harmonic analysis will give the predominant harmonic present in the system. the above two analysis will provide you the capacitor kVAR along with the % of detuned filter to be considered for each transformer.

As per you problem, you have an odd voltage of 850V. that may not be a problem while selecting capacitors. you can contact EPCOS/ABB or any other reputed capacitor manufacturer for the correct solution.

As regards energy audit and harmonic analysis, you can contact CONZERV which is based in bangalore and is a Schneider group company. Infact for one of our projects, we had taken the services for the above companies.

I do agree on Harmonic study approach. Crompton Greaves Ltd., Aurangabad has an independent Power Quality Solutions division which can help in providing a comprehensive solution through their wide range of services & equipment. Please contact CGL at 9673333523.

It is common problem in any process industry, where you have different loads and different voltage. In that Case, You need to consider Capacitor on HT side, there is no meaning of going for LT Capacitor when the PF is already maintained at the required and over saturation should not happen at any level. The power factor should be maintained at each voltage level. Necessarily, You Should consider Capacitor on Primary Side of the transformer to reduce the demand and to get incentive........

Reagrds

K.V.Imayavaramban

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