In AC circuits, any Star connection has an equivalent Delta connection, and vice versa. Thus, the actual connection is immaterial electrically, but there are practical conveniences involved.

AC Capacitors upto about 500volts AC rating are cheap and commonly available. Thus, on LT side, connection in Delta meets the voltage need of each capacitor while generating maximum kVAR of compensation per uF used. Capacitors can be easily paralleled to achieve the requirement.

On the HT side, capacitors have to be connected in series. Star connection gives the lower voltage rating requirement of each limb (line voltage divided by 1.732), which is created by series connection of minimum capacitors. The less the number in series, more is the reliability against failure, requiring the smallest number of capacitor of a given uF to achieve the requirement. However, note that series connection of capacitors reduce the overall uF value of the series string, thus individual capacitors need to be of higher uF value !

In AC circuits, any Star connection has an equivalent Delta connection, and vice versa. Thus, the actual connection is immaterial electrically, but there are practical conveniences involved.

AC Capacitors upto about 500volts AC rating are cheap and commonly available. Thus, on LT side, connection in Delta meets the voltage need of each capacitor while generating maximum kVAR of compensation per uF used. Capacitors can be easily paralleled to achieve the requirement.

On the HT side, capacitors have to be connected in series. Star connection gives the lower voltage rating requirement of each limb (line voltage divided by 1.732), which is created by series connection of minimum capacitors. The less the number in series, more is the reliability against failure, requiring the smallest number of capacitor of a given uF to achieve the requirement. However, note that series connection of capacitors reduce the overall uF value of the series string, thus individual capacitors need to be of higher uF value !

Higher the voltage rating higher is the cost of capacitor. the cost does increase with increase in capacitance value. But a major factor is the voltage rating. On LV side, both star and delta connections can be used. But on HV side, Star connection is more preferable from the cost point of view as, one can use capacitors of  voltage rating which is 1.732th of the HV line voltage, if the capacitor are connected in star.

Dear Dhaval,

I got your point. Does the capacitance value has something to do with in-rush currents into the bank as well ?

Regards,

Arun. 

Dear Sujit Sir,

Thank you for answering. My doubts are quite clear now. Would it be possible to share some literature about this ?

Regards,

Arun. 

Dear Arun Khilnani, as the price of capacitors go up with the voltage rating, it is economical to use low voltage capacitors in delta connection. In order to use the same cheap capacitors for HV systems, we connect the capacitors in series and in star arrangement  to have smaller voltage across each capacitor. As you know the voltage for a phase of a star is 1 over 1.73 the line voltage.

Dear Mr. Mirsalim,

Thank you for your reply. That explains a lot. 

Regards,

Arun. 

Dear Arun Khilnani, as the price of capacitors go up with the voltage rating, and in star voltage rating is divided by 1.732  so in HV system less voltage and hence become economical.

In LV system voltage is already less where capacitor are available in low cost and you can generate more KVAR in this.

secondly, in delta third harmonic current can be circulated and hence harmonic current can not pass towards utility. whereas in star we need to take care of harmonic current through proper neutral grounding.

In both cases series and parallel resonance must be checked while designing circuit.

I think the grading of the capacitors also has a role to play in the star connection at the HT side.  Also, with an earthed neutral, this aspect might be important on HV side.