Reactive Power

Indonesia while on long holiday like Idul Fitri or New Year, MVAR indication in generator capability curve always under excitation (leading) but at normal day, MVAR indication always over excitation (lagging), why?















500kv grid line conductor is tend to capacitive so it tends to increase the voltage. If during long holiday with load grid is low, the load is tend to capacitive (over voltage) so our generator MVAR normally will set negative for compensation.

But in the normally day with load consist motors, lamp, TV etc which load tend to inductive (under voltage), our generator MVAR will set to positive for compensation.
Lets take a look reactive power as below...


Reactive load such as inductor and capacitor dissipate zero power, yet the fact that they drop voltage and draw current gives the deceptive impression that they actually do dissipate power.

This is called reactive power and its measure in a unit called Volt-Amps-Reactive (VAR). The symbol of reactive power is Q. The actual amount power being used, or dissipate in circuit is called true power, symbolized P.

The combination of reactive power and true power is called apparent power. Apparent power is symbolized S measured in the unit Volt-Amps (VA).


Reactive power only becomes important when an electrical load contains coil or capacitor. If the electrical load behaves purely as a resistor, such as a heater or incandescent bulb, then device consumes real power only. Reactive power and power factor can be ignored.


When a coil or capacitor is connected to an AC power supply, the coil or capacitor stores electrical energy during one-fourth of an AC cycle.

But then during next quarter-cycle, the coil or capacitor dumps all the stored energy back into the distant AC power supply.

If the electrical appliance contains inductance or capacitance, then electrical energy will periodically return to the power plant and it will flow back and forth across the power lines.

This lead to an extra current which heat the power lines, but which isn’t used to provide energy to appliance.



The actual resistance of a conductor at any frequency is called its effective resistance. The resistance of inductor XL = 2 x 3.14 x f x L is proportional both to its inductance and to the frequency.

Capacitive reactance XC = 1 / (2 x 3.14 x f x C) is inversely proportional both to capacitance and the frequency. If XL > XC the current is said to lag the voltage. If XL < XC the current is said lead the voltage.

Example a 600 ohm resistor is in series with a 0.5 henry inductor and 0.2 mf capacitor. Compute the impedance of the circuit with frequency 400 Hz and 600 Hz.






Answer at 400 Hz
XL = 2 x 3.14 x 400 x 0.5 = 1256 ohms
XC = 1 / (2 x 3.14 x 400 x 0.2 x 0.000001 ) = 1990 ohms

XL < XC the current is said lead the voltage. Answer at 600 Hz XL = 2 x 3.14 x 600 x 0.5 = 1885 ohms XC = 1 / (2 x 3.14 x 600 x 0.2 x 0.000001 ) = 1328 ohms XL > XC the current is said lag the voltage.