The optimum arrangement for capacitor bank depends on the best usage of the available voltage ratings of capacitor units, fuses, and protective relaying. Nearly all substation units are linked wye/star. Distribution capacitor units, nevertheless, may be linked wye or delta. Some units utilize an H arrangement for capacitor bank on every phase with a current transformer in the associated branch to discover the unbalance.
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Units connected in grounded Star or wye
Grounded wye capacitor units consist of series and parallel-linked capacitor units per phase and allow for a low-impedance path to the ground. Common arrangement for capacitor banks are shown below
The benefits of the grounded capacitor units are:
Low-impedance path to ground which allows for underlying self-protection for
lightning surge currents and protection from surge voltages. Shunt capacitor units can function without surge arresters as the capacitors can absorb the surge.
Low-impedance path for high-frequency currents so they can be utilized as filters in arrangements with high harmonic content, and
Diluted transient recovery voltages for circuit breakers and additional switching devices.
Some disadvantages of grounded wye shunt capacitor banks are:
Larger interference on telecom circuits due to harmonics;
Potential disorder and/or over-operation on protective equipment due to circulation of inrush currents and harmonics; and
Need for phase-connected series reactors to decrease voltages coming out on the CT secondary due to the result of high frequency, high amplitude currents.
Multiple units connected in series phase to ground – double Wye
Once a capacitor bank gets too big, thereby building the parallel energy of a series group too big (above 4,650 kvar) for the capacitor units or fuses, the bank may be divided into two wye parts. The features of the grounded double wye are similar to a grounded single wye bank. The two neutrals need to be linked with a common link to the ground.
The double Wye arrangement for capacitor bank provides a safe and quicker unbalance protection with an uncomplicated, uncompensated relay since any zero sequence component system impacts both wyes evenly, but a broken down capacitor unit will come out as an unbalanced in the neutral. Time coordination may be needed to grant a fuse, in or on a broken-down capacitor unit, to blow. If it is designed without a fuse, the time delay may be adjusted short since no fuse coordination is needed. If the current through the string outperforms the uninterrupted current capability of the shunt capacitor unit, more chains need to be lent in parallel.
Units connected in ungrounded wye
Common bank systems of ungrounded wye shunt capacitor banks are presented in Figure below. Ungrounded wye units do not allow zero sequence currents, third harmonic currents, or big capacitor discharge currents during system ground failures to flow. Another benefit is that over-voltages coming out at the CT secondary sides are not as high as in the situation of grounded arrangement for capacitor bank. Nevertheless, the neutral needs to be insulated for full line voltage, since it is instantly at phase potential when the shunt capacitor bank is turned on, or when one capacitor element breaks down in a bank set up with an individual group of units. For units above 15kV this arrangement may be costly.
Multiple units connected in series phase to neutral – single wye arrangement for capacitor bank. Shunt capacitor bank units with external fuses, internal fuses, or no fuses can be utilized to form the bank. For unbalanced protection systems that are sensitive to system voltage unbalance, either the unbalance protection time delay needs to be set sufficiently long enough for the line protections to remove the ground failures or the capacitor bank may be granted to trip a system ground fault.
Multiple units connected in series phase to neutral – double wye arrangement for capacitor bank. Once a capacitor bank gets too big for the maximum 4,650 kvar per group, the shunt capacitor bank may be divided into two wye parts. Once the two neutrals are not grounded, the bank has some of the features of the ungrounded single-wye shunt capacitor bank. These two neutrals may be linked together through a current transformer or a voltage transformer. Same as for any ungrounded wye bank, the neutral current transformers should be insulated from the ground for full line-to-ground voltage. The same applies to the phase terminals.
Delta arrangement for capacitor bank
Shunt capacitor bank units that are connected in delta arrangement are typically utilized only at distribution voltages and are set with a single series group of capacitors rated at line-to-line voltage. With only one series group of units, no overvoltage occurs across the remaining capacitor units from the isolation of a faulted capacitor unit. Therefore, unbalance detection is not required for protection but may be used to detect the outage of units within the bank. No zero-sequence or third harmonic currents can flow into a delta-connected capacitor bank.
Where one series group of capacitors is used, the individual capacitor fuses should be capable of
interrupting the system short-circuit phase-to-phase fault current. This design may necessitate current limiting fuses.
If internally fused capacitor units are used, then unbalance detection is required to detect a capacitor with failed elements. There is no visible indication of a blown fuse. Two or more series groups are normally required in each capacitor unit to enable the internal fuses to interrupt the fault current.
Static var compensators usually use the delta connection for the thyristor-switched capacitors (TSCs) to simplify the controls and optimize the use of the thyristor valve.
H arrangement for capacitor bank
A few bigger shunt capacitor bank units use an H arrangement in each phase with a current transformer linked between the two legs to cross-compare the current down each leg. As long as all capacitors are fine, no current will go through the current transformer. If the capacitor’s fuse trips, the current will go through the current transformer. This bridge arrangement can be very sensitive. This configuration is utilized on big units with many shunt capacitor bank units connected in parallel.
