In distribution system, shunt capacitor bank can be connected in ungrounded y and delta connection. Which one is more better and practically which arrangement is mostly used?

Further i mentioned, here i mean distribution system supplying electricity to residential consumers. Also please state, practically what parameters are considered in optimum shunt capacitor bank placement in distribution system, under ground as well as overhead?

Most of the correctly applied power factor correction capacitors are located on the downstream side of the feeders where they are closer to proper point of application closer to the loads or load center. Very seldom, for not ordinary reasons should they be applied at the outgoing feeders.

Anyway most capacitor banks are wye connected but there are applications where delta connected capacitors are installed. A wye connected has a lower voltage rating because the units are line-line voltage/sq root of 3, while the delta connection needs full line-line voltage rating. There are many other considerations in the selection of the type of connection specially when the multiple banks are located within some distance with the other bank.

For distribution system supplying electricity to residential consumers,Capacitors are always connected in DELTA solely due to the fact the capacitance per phase will be 1/3 rd of what is required in star connection.However for High voltage star connected capacitor banks are provided

I think its related to nominal voltage of capacitors.In Y connection you can use capacitors with lower nominal voltage.But you also notice about harmonics.Third Harmonics can rotate in delta connection. Inrush current in Y connection is lower when capacitor bank energize.

Most feeders are limited in length, except for rural feeds and they are typical of single phase at MV levels. Our utility install cap ban predominately with the feeder sub. Its operation is a function of var support to maintain the Feedere bus voltage, and yes they are wye connected systems.

For longer distribution line or issue with rural lateral lines, pole mounted capacitors are employed to alleviate the effects of voltage drops., could be single phase of three phase and typically wye conneced with fused cut-outs.

Dear Lee, thanks for ur reply..

1. Sir what level u called MV level? I read in books and literature banks are wye connected in low voltage ratings. IEEE Std 1036-1992 standard states that delta connected capacitors are generally only used at low voltages.

2. Sir what are the reasons for connecting capacitor bank in wye? U will required 1/3 of capacitance in case of delta connection.

Medium voltage from 751v to 38 kv. Typical distribution nominal levelbis between 4160 v (5kv class) to 34.5 kv (38kv class).

Capacitor banks are though about in terms of (reactive) power capability so sized in kvar or mvar, whether wye or delta is immaterial; it breaks down to the total (compensation) current that is important, and bresks down again into capacitance gor the make up if the capacitor bank

In the case of a three phase capacitor bank is driven, in terms of power factor correction may be Wye or Delta, but from the economic standpoint, the connection uses capacitors Delta 1/3 of the capacitance and is Economic much, and never grounded.
Today you must use the Electronic Control Capacitor Bank to prevent Capacitive Power Factor.
regards
William

Power factor is really a terminology that are use by the customer. Poor power factor leads to increase current demand by the customer. Utilities do not charge for this for residential customers, but for business (commercials, plants) demand metering are typical. So for a plant that continually require var support for their motors having poor power factor, the penalty is hefty enough such that addressing their system by improving their power factor is of cost benefit to them.

Utility looks at distribution in terms of voltage levels and frequency. For long laterals, cap are installed to maintain voltage support. Power factor what , you said ?

In Delta connection, for a given cell capacitance, the net capacitance across the lines will be 1.5 times the (individual) cell capacitance. So, for a given cell capacitance 'C', you get 1.5C, which is economical. But, the catch is the line-to-earth insulation, which, in this case, should be for the line voltage.

In Star, for a given cell capacitance, the net capacitance across the lines would only be 1/2 (50%) of the (individual) cell capacitance, which is uneconomical. But, the advantage is that the line-to-earth insulation can be for the phase voltage (57% of the line voltage).

In low voltage systems, below 1000 Volts, there is not much cost difference in sizing the line-to-earth insulation for phase voltage and for line voltage. Thus, in low voltage capacitors, one gets economical advantage by connecting the capacitors in Delta (big saving of higher line capacitance for a given cell capacitance; no big cost advantage in phase-voltage insulation or line voltage insulation).

Whereas, in high voltage systems, there is huge cost difference in sizing the line-to-earth insulation for phase voltage and for line voltage. In comparison, the economical advantage of higher net line capacitance for a given cell capacitance is not much. Thus, in high voltage systems, the capacitors are connected in Star, to gain a better economical advantage of insulation cost.

Certainly, the most used connection for MV Capacitors is " Y ", but now in Europe they use more and more for MV " Y - Y " connection with differential current detection, that helps to detect any capacitance failure, because any MV capacitor is internally composed of many capacitors have been connected in " series & parallel " to achieve the needed values of Capacitance and Voltage.

Delta systems require delta connections - for which one must use two bushing capacitors. Wye systems - whether single point grounded at the substation(with an insulated neutral wire (as required in California) or for multi-point systems with grounded at each distribution transformer - only require less expensive single bushing capacitor cans.