There is a series of accessories and components Oust as incase of low voltage capacitor banks) which are assembled with high voltage capacitor units to form a composite equipment, which functions efficiently, when connected to power system net work for power factor correction (reactive power compensation). This assembly is designated as Capacitor bank.
Capacitor banks are either FIXED type or AUTOMATIC and are complete with equipment for protection against malfunctioning when connected to power system net work. The selection of the type of capacitor bank and its configuration, totally depends on cost factor. This document discuss the various types of banking configurations and components fitted, to serve as a guideline for easy selection of best type of equipment, which would meet the requirement of a particular power system.
There are two types of banks, depending upon the mode of connection. First is 3 phase single star connected, which is suitable, where voltage is preferably low, say up to 6,6kV. Other is 3 phase double star connected bank assembled with minimum 6 Nos of single phase units, preferably for systems where voltage is higher, say 11 kV and above.
Banks with 3 phase units
This type of bank is assembled with 3 phase units 1 to 4 Nos. connected in parallel. Units are 3 phase internally star connected and thus the bank in totality is in single star configuration. The bank is having following components (to suit the system requirements):
Sheet metal enclosure.
High Rupture Capability Fuses for protection of bank
Contactor for operation (optional).
Inrush Current inductors.
Capacitor unit! units.
Practically the components required is depending upon customer requirements and type of power system. Normally this type of bank is connected to power system net work where highest voltage is 12 kV, and kvar requirement is 50 to 1600 kvar for 6, 6 kV system and up to 2400 kvar incase the system voltage is 11 kV. We can offer more power at higher voltage, because maximum current rating for fuses that we use is 315 A.
Three phase double star connection
In this case, single phase capacitor units are assembled to form, two-star connected banks (star groups) side by side, and two neutrals are then joined together through a neutral C. T. to have a common ungrounded neutral. Defect, if any occurs, in any of the star group, the unbalance current in the neutral is detected by the C. T. to initiate an alarm or a trip, before greater damage could occur.
The possible causes of unbalance are, a defect in the capacitor unit due its ageing, harmonics, or a defect developed in other connected components. The combination of internally fused capacitor units and neutral unbalance C. T. forms the best mode of protection and performance of power factor correction scheme. The number of capacitor unit in the bank, depends on the power rating (kvar) of bank and system voltage. The number of units are in multiples of 3, however, there is minimum 6Nos in any case.
Assembly of High Voltage capacitor banks comprises the following equipments, depending on the customer requirement or nature of power system
Sheet metal enclosure.
Vacuum contactor or SF6 circuit breaker (optional).
Inrush current reactors.
Neutral Current Transformer.
Neutral unbalance current relay (optional).
Normally, banks with three phase units connected in parallel are used for systems up to 12 k V where kvar requirement is not high.
Three phase units are not available above 12 k \I. Therefore, for higher voltage rating and power (kvar) requirements, double star connected capacitor banks with single phase units, is used. With this combination, we are able to achieve any voltage level and kvar output, which would give a technically and economically viable capacitor bank design.
Fixed Capacitor Banks (For permanent connection)
There is a variety of fixed banks with different types of enclosure/ mounting rack, and equipments included.
50 kvar to 4800 kvar, (other ratings, on demand from customer).
Power system rated voltages 7,2 kV to 36 kV.
Banks with parallel connected 3 phase units (internally star connected with internal element fuses).
Banks with single phase units connected in double star.