A capacitor bank is very essential equipment of an electrical power system. The power required to run all the electrical appliances is the load as useful power is active power. The active power is expressed in KW or MW. The maximum load connected to the electrical power system is mainly inductive in nature such as electrical transformer, induction motors, synchronous motor, electric furnaces, fluorescent lighting are all inductive in nature. In addition to these, inductance of different lines also contributes inductance to the system.
Because of these inductances, the system current lags behind system voltage. As the lagging angle between voltage and current increases, the power factor of the system decreases. As the electrical power factor decreases, for same active power demand the system draws more current from source. More current causes, more line losses.
Poor electrical power factor causes poor voltage regulation. So to avoid these difficulties, the electrical power factor of the system to be improved. As a capacitor causes current to lead the voltage, capacitive reactance can be used to cancel the inductive reactance of the system. The capacitor reactance can be used to cancel the inductive reactance of the system. The capacitor reactance is generally applied to the system by using static capacitor in shut or series with system. Instead of using a single unit of capacitor per phase of the system, it is quite effective to use a bank of capacitor units, in the view of maintenance and erection. This group or bank of capacitor units is known as capacitor bank. There are mainly two categories of capacitor bank according to their connection arrangements.
- Shunt capacitor.
- Series capacitor.
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How to determine Rating of Required Capacitor BankThe size of the Capacitor bank can be determined by the following formula : Where, Q is required KVAR. P is active power in KW. cosθ is power factor before compensation. cosθ' power factor after compensation.
Location of Capacitor BankTheoretically it is always desired to commission a capacitor bank nearer to reactive load. This makes transmission of reactive KVARS is removed from a greater part of the network. Moreover if capacitor and load are connected simultaneously, during disconnection of load, capacitor is also disconnected from rest of the circuit. Hence, there is no question of over compensation. But connecting capacitor with each individual load is not practical in the economical point of view. As the size of loads extremely differs for different consumers. So various size of capacitors are not always readily available. Hence proper compensation can not be possible at each loading point. Again each load is not connected with system for 24 × 7 hours. So the capacitor connected to the load also can not be fully utilized.
Hence, capacitor, is not installed at small load but for medium and large loads, capacitor bank can be installed at consumer own premises. Although the inductive loads of medium and large bulk consumers are compensated, but still there would be considerable amount of VAR demand originated from different uncompensated small loads connected to the system. In addition to that, inductance of line and transformer also contribute VAR to the system. On viewing of these difficulties, instead of connecting capacitor to each load, large capacitor bank is installed at main distribution sub-station or secondary grid sub-station.
Connection of Shunt Capacitor BankThe capacitor bank can be connected to the system either in delta or in star. In star connection, the neutral point may be grounded or not depending upon protection scheme for capacitor bank adopted. In some cases the capacitor bank is formed by double star formation.
Generally large capacitor bank in electrical substation is connected in star. The grounded star connected bank has some specific advantages, such as,
- Reduced recovery voltage on circuit breaker for normal repetitive capacitor switching delay.
- Better surge protection.
- Comparatively reduced over voltage phenomenon.
- Lesser cost of installation.
- In a solidly grounded system the voltage of all 3-phases of a capacitor bank, are fixed and remain unchanged even during 2 phase operation period.