Grounded wye capacitor banks, composed of series- and parallel-connected capacitor units per phase, may require overvoltage/unbalance protection. Grounded capacitor banks provide a low impedance path to ground for lightning surge currents and give some protection from surge voltages.
Some banks may be operated without surge arresters, taking advantage of the capability of the capacitors to absorb the surge.
Grounded capacitor banks also provide a low-impedance path to ground for triplen harmonic currents. These harmonic currents may cause communication facility interference, if such circuits parallel power lines.
Additionally, an open phase produces zero sequence currents that may cause ground relay operations. Harmonic resonance may also be a problem.
When the neutral is grounded, recovery voltages encountered during switching are reduced; however, careful consideration should be given to the effect of inrush current on protective relay performance and the method of grounding.
One Unit Phase to Ground
There is no overvoltage across the remaining capacitor units if a failed, externally fused capacitor unit is isolated with a fuse; therefore, unbalance relay protection is not required. The individual external capacitor fuses shall be capable of interrupting the system available fault current.
Internally fused and unfused capacitor banks should have unbalance protection to avoid excessive element voltages and to trip the bank for severe unbalance conditions.
Multiple Units in Series Phase to Ground—Single Wye
Grounded wye externally fused capacitor banks above 34.5 kV are commonly composed of two or more series groups of parallel connected capacitor units per phase. The presence of multiple series groups of units limits the maximum fault current so that individual current-limiting fuses are usually not required unless the parallel kilovar exceeds 4650 kvar or the level specified by the manufacturer.
Internally fused capacitor units should be arranged in the bank design to ensure that energy in the parallel group does not exceed the interrupting capability of the internal fuses. Relatively small banks can be designed with internally fused capacitor units and still use large capacitor units for a simple design.
The fuseless design is not usually applied for system voltages less than about 34.5 kV. The reason is that there shall be more than 10 elements in series so that the bank does not have to be removed from service for the failure of one element because the voltage across the remaining elements would increase by a factor of about E/(E− 1) where E is the number of elements in the string.
Multiple Units in Series Phase to Ground—Double Wye
When a capacitor bank becomes too large, making the parallel energy of a series group too great for the capacitor units or fuses, the bank may be split into two wye sections. The characteristics of the grounded double wye are similar to a grounded single wye bank.
The two neutrals should be directly connected with a single connection to ground. The double wye design allows a secure and faster protection with a simple uncompensated relay because any system zero sequence unbalance affects both wyes equally, but a failed capacitor unit will be detected in the neutral unbalance.