Ungrounded wye banks do not permit zero sequence currents, third harmonic currents, or large capacitor discharge currents during system ground faults. (Phase-to-phase faults may still occur and will result in large discharge currents.) The neutral, however, should be insulated for full line voltage because it is momentarily at phase potential when the bank is switched or when one capacitor unit fails in a bank configured with a single group of units.
One Unit Phase to Neutral
For systems with line-to-neutral voltages corresponding to available capacitor ratings, wye connected capacitor banks with a single series group per phase may be used. The capacitor bank neutral may be ungrounded in order to avoid the need for power fuses to interrupt the system short-circuit fault current or to minimize the flow of triplen harmonics, which can cause telephone interference.
This design requires that single-bushing capacitor units be mounted on an insulated rack. If two bushing capacitor units are used with a grounded rack, a fault to the case will be a system line-to ground fault. High system fault currents may lead to case rupture.
For externally fused capacitor banks, the fuses should be selected to interrupt the available phase to ground short-circuit current. If lower rated fuses are used, a fault to the capacitor case may not be cleared by the capacitor-unit fuse and will require that the fault be cleared by the capacitor bank overcurrent protective devices.
Ungrounded wye capacitor banks usually do not require current-limiting capacitor-unit fuses because current through a faulted capacitor unit is limited to three times normal phase current. However, caution needs to be exercised when re-fusing a bank of this type because faulted capacitors in different phases could result in a phase-to-phase system fault.
Multiple Units in Series Phase to Neutral—Single Wye
Wye banks with multiple series groups may also be ungrounded. Such a bank does not provide a path to ground for a surge voltage and provides no path to ground for third harmonic currents. The entire bank, including the neutral, should be insulated for full line voltage.
Capacitor units with external fuses, internal fuses, or no fuses (fuseless or unfused design) can be used to make up the bank. For unbalance protection schemes that are sensitive to system voltage unbalance, either the unbalance protection time delay shall be set long enough for the line protections to clear the system ground faults or the capacitor bank may be allowed to trip off for a system ground fault.
Multiple Units in Series Phase to Neutral—Double Wye
When a capacitor bank becomes too large for the 4650 kvar per group maximum for expulsion fuses and is large enough to meet the minimum units per group requirement as outlined in 4.2, the bank may be split into two wye sections.
When the two neutrals are ungrounded, the bank has some of the characteristics of the ungrounded single-wye bank. These two neutrals may be tied together through a current transformer or a voltage transformer. As for any ungrounded wye bank, the neutral instrument transformers should be insulated from ground, as should the phase terminals, for full line-to-ground voltage.