- Types of Capacitor Bank Definition: Capacitor banks are defined as groups of capacitors connected together to improve the power factor in electrical systems, available in three main types: externally fused, internally fused, and fuse-less.
- Externally Fused Capacitor Bank: Each capacitor unit has an external fuse; if a unit fails, the fuse blows, allowing the bank to continue operating.
- Internally Fused Capacitor Bank: Features internal fuses for each capacitor element; the bank can still run even if multiple elements fail, but may require full replacement if many fail.
- Fuse-less Capacitor Bank: Consists of capacitor strings without fuses, reducing costs and space but needing advanced control systems to handle faults.
- Fault Management: Fuse-less banks require immediate fault clearing by tripping circuit breakers and sophisticated relay systems to detect and manage faults.
A high-voltage shunt capacitor bank combines capacitor units in series and parallel to obtain the required voltage and reactive-power rating. Units are commonly single-phase, while the complete three-phase bank may use grounded or ungrounded wye, double-wye, H-bridge or delta arrangements. The list below describes three common capacitor-unit fusing constructions, not every possible bank topology.
- Externally fused capacitor bank.
- Internally fused capacitor bank.
- Fuseless capacitor bank.
The fusing construction affects failure behaviour, voltage stress, unbalance protection, fault location, maintenance and spare-unit requirements.
Externally Fused Capacitor Bank
In this type of capacitor bank, each capacitor unit has its own external fuse. A unit failure causes that fuse to isolate the complete unit, reducing the number of units in its series group while the bank may remain energised if voltage and unbalance limits are still acceptable.
Removing one parallel unit reduces the group’s capacitance and redistributes voltage across the remaining units and phases. The resulting voltage stress depends on the bank connection, units per series group and number of series groups. Bank design and relay settings must keep this stress within the capacitor ratings.
A blown external fuse usually gives a visible indication of the failed unit, simplifying inspection of an externally fused capacitor bank.
Capacitor-unit kvar ratings vary with voltage, manufacturer and bank design; there is no single rating range for every externally fused bank.
The main trade-off for this type of capacitor bank is that one fuse operation removes an entire unit. That creates a larger step in bank unbalance and remaining-unit voltage stress than isolation of one small internal element.
Internally Fused Capacitor Bank
An internally fused capacitor unit contains many series and parallel elements, with a small fuse connected to each protected element. When an element fails, its fuse opens and removes that element rather than the whole unit. The unit can remain in service, but the voltage rises across the elements that remain in the affected series group.
Unbalance protection detects accumulating element failures and must alarm or trip before remaining elements exceed their permitted voltage. The complete sealed unit, rather than one internal element, is replaced during field maintenance.
Internal fusing reduces the unbalance step from one element failure and eliminates exposed unit fuses, but the failed element is not normally visible. Fault location therefore depends on relay data and electrical measurements.
Fuseless Capacitor Bank
In a fuseless capacitor bank, capacitor units are arranged in series strings without internal element fuses or individual external unit fuses. A failed element commonly becomes a short circuit. This bypasses the associated parallel element group and raises voltage stress on the remaining series sections. The effect on string current may be small, so sensitive unbalance measurement is needed. The bank may remain in service only while the number of failed elements and the calculated overvoltage stay within the design’s alarm and trip limits.
Advantages of Fuseless Capacitor Bank
The main advantages of fuse less capacitor bank construction are:
- Removing individual fuse assemblies can reduce component count, losses and maintenance.
- The arrangement can use a compact footprint for suitable voltage and bank ratings.
- Fewer exposed fuse connections can simplify wildlife protection, although insulated wiring, clearances and physical barriers are still required.
Disadvantages of Fuseless Capacitor Bank
The main disadvantages of fuse less capacitor bank construction are:
- A bushing, tank-insulation or other earth fault in a capacitor unit is not cleared by an individual unit fuse. Bank protection must trip the associated circuit breaker fast enough to limit damage.
- A replacement unit must match the required electrical rating and capacitance-balancing criteria. Site spares should follow the bank manufacturer’s replacement procedure.
- A failed element or unit may have no visible indication. Fault location can require capacitance measurements, relay records and section-by-section testing.
- Sensitive, dependable unbalance protection is required to detect element failures before voltage redistribution leads to cascading damage. The protection also needs supervised trip-circuit and control-power design appropriate to the substation.
- Reactors may be required for switching-inrush control, detuning or harmonic filtering, but this depends on the network study and is not unique to fuseless construction.
