- Shunt Capacitor Definition: A shunt capacitor is defined as a device used to improve power factor by providing capacitive reactance to counteract inductive reactance in electrical power systems.
- Power Factor Compensation: Shunt capacitors help improve the power factor, which reduces line losses and improves voltage regulation in power systems.
- Capacitor Bank: A capacitor bank is a group of capacitors used together to provide the necessary reactive power compensation, commonly connected in shunt configuration.
- Connection Methods: Shunt capacitor banks can be connected in star or delta configurations, with grounded star connections offering advantages like reduced recovery voltage and better surge protection.
- Location Considerations: For maximum effectiveness, capacitor banks should be located near reactive loads, although practical and economic factors often influence the final placement.
What is a Shunt Capacitor?
A capacitor bank is a group of capacitor units connected to an electrical power system. Loads consume active power, measured in kW or MW, to perform useful work. Many loads also require reactive power to establish electric or magnetic fields. Transformers, induction motors, a synchronous motor, furnaces and fluorescent lighting can all contribute to inductive reactive-power demand.
The inductance of lines and transformers also affects reactive-power flow.
For a balanced inductive load, the system current lags the voltage. A lower electrical power factor requires more line current to deliver the same active power. The extra current increases conductor and transformer losses and uses network capacity.
A low power factor can also increase voltage drop. A shunt capacitor draws leading current relative to the voltage. Its capacitive reactance allows it to supply reactive power at the connection point.
This reduces the reactive current supplied through upstream equipment. It does not remove the physical inductive reactance of the load or line.
Capacitors can be connected in shunt to provide reactive-power support or in series to change line impedance. Multiple capacitor units are assembled as a bank of capacitor units to obtain the required voltage and kvar rating and to support practical protection and maintenance. This assembly is a capacitor bank.
Power-system capacitor banks fall into two connection categories:
- Shunt capacitor.
- Series capacitor.
Shunt capacitor banks are widely used for power-factor correction and voltage support. Series banks serve a different transmission-line compensation purpose.
How to determine Rating of Required Capacitor Bank
For a steady load, the required Capacitor bank rating can be estimated from Qc = P(tan θ – tan θ’):
Where:
Q is the required capacitor rating in kvar.
P is the active power in kW.
cosθ is the power factor before compensation.
cosθ’ is the target power factor after compensation. Final selection must also consider operating voltage, load variation, switching steps, harmonic resonance and acceptable overcompensation.
Location of Capacitor Bank
Placing a bank near a reactive load reduces reactive-current flow through more of the upstream network. If a switched capacitor follows that load, it can also avoid compensation when the load is off. Individual correction is not always practical because loads vary, standard capacitor steps may not match each load and many small installations increase cost and maintenance.
Medium and large loads may use local banks, while smaller loads can be compensated at feeder or substation level. Lines and each transformer also carry reactive current. A central switched capacitor bank can follow the combined demand, although it does not reduce losses downstream of its connection point. Engineers choose the location from load profiles, voltage studies, loss savings and project economics.
Connection of Shunt Capacitor Bank
A three-phase capacitor bank can be connected in delta or star, also called wye. A star neutral can be grounded or ungrounded, and banks can use single-star or double-star sections. The choice must match the protection scheme for capacitor bank, system grounding, harmonic conditions and unit voltage rating.
Large banks in an electrical substation are commonly star connected, but grounded and ungrounded arrangements are both used.
A grounded-star bank has the following characteristics and trade-offs:
- It can reduce recovery voltage across the circuit breaker after capacitor switching, but the switch must still have the correct capacitive-switching rating.
- Its grounded neutral provides a low-impedance path for lightning current and can support coordinated surge protection.
- Grounding can limit neutral displacement, but it also provides a path for zero-sequence and triplen-harmonic current.
- It can simplify insulation requirements in some designs, although total installed cost depends on switching, protection and grounding equipment.
- On a solidly grounded system, the neutral reference helps control phase-to-earth voltage. Actual unbalanced voltages still depend on the bank, fault and system impedances.
