In a power system, three important measures are used: voltage, current and frequency.

The vital problem in power system is the power quality as it impacts the voltage, current and frequency. Power factor is considered a significant part in enhancing energy supply efficiency. 

A load with a unit power factor has a high output and can save a significant amount of cost. This is why several energy utilities are asking for a decrease in reactive power. In case if the demand for reactive power reduces so, the power factor will also be improved. AC electrical power is classified into three main sub-types which are apparent power, active power and reactive power. 

Active power ‘P’ (KW): 

The actual power that dissipated through the circuit is an active power. It is also known as true power or real power. This type of power is very useful for load and can be generated by the part of the current in the power system. The output voltage is always in line with the current, and the actual work cannot be performed if the output voltage goes out of sync with the current. 

Active power is measured in watts (W) or megawatts (MW) and is symbolized by ‘P’. The following mathematical equation can calculate the average value of active power. 

Active power P = VI cos Φ (kW)

Where P is the real power in watts and Φ is the impedance phase angle between the current and the voltage. 

Reactive power ‘Q’ (KVAR) 

The interaction between source and load where the power moves back and forth is defined as reactive power. It is retained in the electrical circuit and discharge by reactive elements such as a capacitor, induction motor, transformer or a solenoid. The reactive power does not use the power for the load. That is why the reactive power is often referred to as the imaginary power.

Reactive power is measured in volt-ampere reactive (VAR), rather than watts and denoted by Q. The mathematical representation of this type of power is 

Reactive Power Q = VI sinθ (kVA)

In the equation, Q represents the reactive power in watts.

Apparent power ‘S’ (KVA): 

Apparent power is an ideal power which is the product of root mean square (RMS) value of voltage and current without the reference to phase angle. The apparent power can be defined as the combination of reactive power and true power. The apparent power is measured in the unit of volts-amps (VA) and denoted by ‘S’.

The mathematical expression for apparent power is:

S = V X I (for single-phase apparent power)

S = 1.73 V X I (for three phases apparent power)

Where V is phase voltage, and I is line current.