Reactive power is the magnetic field energy required in generators, transformers, power transmission lines, motors and other loads. Reactive power energize the magnetic field. The reactive power is consumed by the overhead lines, transformers, and the loads. Reactive power generation and control is important in maintaining the system voltage under normal and abnormal conditions in the power system and to reduce the system losses in the system. Cost per kVAr of the reactive power generation varies between 2 to 4 percent per kW installed generation cost and will rise with increase in the use of automatic and regulation controls.
Transmission lines consumes reactive power (I2X) depending upon the series reactance (X) and load current (I). The series reactance of the line is proportional to the conductor self-inductance, which decreases as the spacing between the conductors decreases. The minimum spacing is kept such as to prevent the flash-over in bad weather conditions. The underground cable generates reactive power (Ic2Xc) depending upon the capacitive reactance (Xc) and charging current (Ic) at the rated voltage.
Increase in the line voltage at transformers and motors above the rated voltage will increase the consumption of the reactive energy. Generally increase of 10 percent of the rated voltage will result in about 20 percent reduction in the power factor.
In distribution system supplying composite loads, which includes some elements of lighting and domestic loads, motor load, inverter and rectifier load as well as transformer and line losses. Reactive power is a function of series leakage and shunt magnetizing reactance of transformers, motors and other elements of the lines and loads. while the series leakage reactance component is load dependent and varies with the square of the load current, the magnetizing reactances vary with the voltage
The reactive power consumption takes place in the series leakage and the shunt magnetizing reactances. The second component is voltage dependent while the first component is proportional to the square of the transformer current. A completely unloaded transformer will be very inductive and has very low power factor. The reactive power used by the transformer at full load and at rated voltage is approximately 7 to 9 percent of the rated power of the transformer. When unloaded, the reactive power remains between 3 to 4 percent of the rated power.
Loads have varying reactive power requirement. The low power factor can be result of the equipment as in the case of welders. Lightly loaded transformers and induction motors operate at low power factor and require more reactive power.
Reactive Power Compensation:
Mainly capacitors are employed at the load center to deliver the reactive power which is consumed by the loads. In the case of capacitor compensation at loads, power factor is corrected and maintained constant (near to unity) under all the operating loading conditions. Suitable capacitor banks are provided at the power grid and at substations to feed the reactive power to the lines, transformers and other loads. Optimum correct compensation should be provided for each feeder for improving the power factor and voltage profiles.
In the case of capacitor over-compensating, the voltage rises and capacitive kVAr will export (it will flow to the reverse side of the load). Under certain conditions, dangerously high transient voltages may occur on the power lines. Torque surges will be transmitted to the motor shaft under rapid restart conditions. Over-compensation at the consumer premises is not desirable.
The capacitors are automatically controlled by per-programmed micro-processor based control unit on the basis of VAr, current, voltage of feeder. According to the load, the switching operation of the capacitor units are done in steps so as to closely follow the load curve and proper compensation is provided for different loading conditions.