Capacitor Connections In Capacitor Bank

Yes. It would probably increase capacitor life but that's because of the lower voltage at which each capacitor is loaded. But also effective KVAR would decrease accordingly so you must then use bigger capacitors for same PF correction. So where is the gain? S.M.

Voltage rating part advantage is OK. However, the kVAR rating will remain same irrespective of whether capacitors are Star connected or Delta connected.

Er... are you sure? If the same capacitor bank (originally connected in Δ) is reconnected in Υ, the kVAR rating will become 1/3, i believe?

Let me try explain why I felt capacitor kVAR remains same:

Capacitor being source of reactive power, defined in terms of kVAR, i.e. it depends on V and I. While for delta, V_ph=V_L, for star I_ph=I_L, thereby 'advantage' gained by delta in V gets nullified by 'advantage' gained by star in I. Thus output kVAR remaining the same i.e. capacitor rating is independent of capacitor bank connection.

An analogy - For DY transformer, transformation from delta to star doesn't change the rating of transformer.

I am eager to learn, how capacitor bank kVAR rating gets reduced in Star connection.

Oh, I missed same capacitor, whereby capacitance 'C' or capacitive reactance 'Xc' is already dead and frozen. Yes, I agree with you. The rating will go down by 1/3 as per formula given by wareagle below.

If still at design stage, kVAR rating will have no effect whether capacitor connected star or delta. Is that right ?

Still on it. Pl. see the following link:

http://www.nepsi.com/del_new.htm

Capacitor: Except for voltage rating, the capacitors in both ungrounded-wye and delta-connected banks are the same and will have the same kvar rating.

Forum experts' views are requested.

Except for voltage rating, the capacitors in both ungrounded-wye and delta-connected banks are the same and will have the same kvar rating.

Yes you are correct.But you have to raise the line voltage by √3 times, so that the capacitors would get the rated voltage.

Otherwise kvar of individual capacitor would be 1/3 times. Kvar of capacitor bank would be 1/√3 times .

Sorry, a correction.

Otherwise kvar of individual capacitor would be 1/3 times. Kvar of capacitor bank would be 1/√3 times .

The kV in formula is the system volg, which remains same, whether star connct or delta connct.

For say 6.6kV system, volg value in the formila will be 6.6 kv irrspective bank star connct or delta connect. it is not that for delta we put 6.6kv, for star we put 6.6/sqrt3 in the formula.

kVAR should remain same in both cases.

OP can provid more data, sys volg, cap design volg, kvar calculation etc.

Sorry I came in late for this topic discussion.

The KVAR consumed is directly proportional to square of the voltage applied as mentioned in this post.

Rating is related to capability. So if the Delta connected capacitors are connected in Star, their rating will not change, but their kVAR consumptions will change as the voltage across them will be different.

Quote " If still at design stage, kVAR rating will have no effect whether capacitor connected star or delta. Is that right ?"

If you at the design stage, You call for a kvar rating at the voltage you have. Eg

you need 1000 kvar at 240 volts and you get 1000 kvar. If you need 1000 kvar at

208 volts you get 1000 kvar. The units are designed to work at the voltage you call for. Does that clear up your question?

Pl. see # 8.

I truly hope some guys giving answers here are not licenced engineers, because I cannot justify an engineer's ignorance in so basic concepts. When you buy KVARs you buy KVARs at a specific rated voltage (and frequency). This voltage has not only to do to capacitor's voltage tolerance, it is a part of the KVAR value itself. A 20 KVAR capacitor rated at 440v for example if connected at 220V same frequency, will give you 5 KVAR. If you can't understand this much, you're on a wrong line of expertise, sorry. S.M.

Hi simpleMind,

I have written in the sense that,

A 20 KVAR delta connected capacitor bank at 440V would have the same KVAR rating if star connected at 440*√3= 762V, so that individual capacitors get 440V. By this way i mean that, as the system voltage is constant, the KVAR rating of capacitor bank would be only 1/3 times when star connected.

Qoute Hbadham "Capacitor: Except for voltage rating, the capacitors in both ungrounded-wye and delta-connected banks are the same and will have the same kvar rating."

I see where the confusion is now.This statement is saying that if you have 1000 kvar connected delta and 1000 kvar connected wye you will have the same kvar rating of 1000 kvar. They will have the same effect on your utility bill. But the delta kvar units will not work on a wye system. The units are designed to provide 1000 kvar at the voltage rating of the system to which they are connected. Therefore when you are designing a system to use capacitors, you must specify the system voltage to get the correct KVAR rating. On a delta system the units are connected phase to phase. On a wye system they are connected phase to neutral.

i think this thread has become a hapless victim of the semantic barrier in communication like a delta connected capacitor when confronted with the dreaded triplen currents . . . goes round and round and only creates heat in the process . . . .

to confound the matter now i suggest using a capacitor set rated 900kVAR at 718 V in delta on the same 415V bus to get the same PF benefit and increase capacitor life . . .

go drive a Beamer series 7 behind a Suzuki Swift in Madison Avenue or Zaveri Bazaar afternoon traffic and sing hakuna matata . . .

A good way to understand this is to consider wye/delta starting of motors. Emphasis must also be placed on rated vs applied voltage. With the supply voltage (line to line) the same, the voltage seen by the motor windings is dependent on how it is connected-wye or delta. Take the case of a motor rated to deliver its HP (horsepower) rating with the rated voltage applied and the motor connected in delta, if you start the motor in wye and keep it running that way, it will burn out because it is attempting to deliver its HP rating at a lower voltage across its windings- resulting in current above its rated full load current.

A "similar" reasoning applies to the capacitor banks; you get reduced voltage across the capacitors in wye and cannot expect to get the rated KVAR.

Yes. Just like AH capacity of a battery bank (analogous to capacitor rating), its full AH 'nameplate' rating is of no use if battery cannot be charged to its full capacity (analogous to capacitor bank which cannot be charged to its 'full potential' due to reduced volg. available in star). Excellent postings at # 3, 13, 16 (nice analogy) & 18.

Please remeber that in Delta connection, even a smaller cell capacitance would give a better equivalent capacitance across the two lines. Say, in a three phase delta connected capacitor, the net capacitance across lines 'R' & 'Y' would be 1.5 C, where 'C' is the individual cell cpacitance. Remember, it is a series parallel circuit. So, also for the other two lines 'Y-B' and 'B-R'. Thus, you get a better kVAR output for a smaller cell capacitance.

Whereas in Star connection, you would only get 0.5C across any two lines, where 'C' is the individual cell capacitance. (It is only a series connection now).

So, if you re-connect the alreday delta connected capacitors in star, you would get only 1/3rd of the kVAR output of that in Delta.

As for as connection in Star is concerned it is quite safe electrically but what difference it makes the system is questionable.