Generally in an electric circuit, the rate of energy flow in a specific point of the circuit is called power. In Alternating Current (AC) circuits, there are energy storage elements such as capacitance and inductance which can periodically reverse the direction of energy flow.
Active Power (P) is known as the part of power that averaged over the whole cycle of the AC waveform results in a net transfer of energy in one direction. On the other hand, another part of power related to the stored energy, which returns to the source in each cycle is defined as reactive power (Q). As shown in Figure 2-1, complex power (S) is the total power in the circuit and consists of active and reactive power as real and imaginary components respectively.
Apparent power is known as the absolute value of complex power (|S|). The general expression for the (complex) apparent power is:
Figure 2-2 illustrates voltage and current vectors when current is lagging and leading voltage. Since current is conjugated in equation 2-1, considering counter clockwise direction as positive, the negative φ in Figure 2-2 corresponds to positive Q in Figure 2-1, and vice versa.
According to Figure 2-1, the following equations are applicable:
Power factor (pf) is defined as the ratio between active power and apparent power which represents the efficiency of a power distribution system by showing how much of apparent power (S) is active power (P); i.e. how much of total power is transferred. Hence,
In summary, reactive power flow is unavoidable in an AC transmission system in order to support the transfer of active power over the network. As mentioned, the periodic reversal of energy flow direction in AC circuits results from the temporary energy storage in inductive and capacitive elements. The part of energy that can be used is active power; which is in fact, the portion of power flow remaining after being averaged over the complete AC waveform. On the other hand, reactive power is the portion of power flow that is temporarily stored in the form of magnetic or electric fields and is returned to the source.
Inductive elements which store energy in the form of magnetic field could generally be categorized as reactors consisting of a large coil. As the voltage is applied to the coil, the magnetic field builds up, and it takes a period of time for the current to reach its final value. This causes the current to lag the voltage, and hence these devices are said to absorb reactive power (positive Q for an inductive load).
Capacitive elements which store energy in the form of electric field are generally categorized as capacitors. As a current is driven through a capacitor, it takes a period of time for the charge to build up and make the full voltage difference. Since the voltage across a capacitor in AC network is always changing, the capacitor opposes this change which causes the voltage to lag behind the current; i.e. the current leads the voltage, and hence these devices are said to generate reactive power (negative Q for a capacitive load).
The stored energy in capacitive or inductive elements of the network increases the reactive power flow. Reactive power flow strongly influences the voltage level in the network. To allow a power system to be operated within acceptable limits, voltage level and reactive power flow must be carefully controlled and supervised.