The nature of reactive power

Alternating current systems supply two forms of energy: “Active” energy measured in kilowatt hours (kWh) which is converted into mechanical work, heat, light, etc “Reactive” energy, which again takes two forms:   - “Reactive” energy required by inductive circuits (transformers, motors, etc.),   - “Reactive” energy supplied by capacitive circuits (cable capacitance, power        capacitors, etc)

All inductive (i.e. electromagnetic) machines and devices that operate on AC systems convert electrical energy from the power system generators into mechanical work and heat. This energy is measured by kWh meters, and is referred to as “active” or “wattful” energy. In order to perform this conversion, magnetic fields have to be established in the machines, and these fields are associated with another form of energy to be supplied from the power system, known as “reactive” or “wattless” energy.

The reason for this is that inductive circuit cyclically absorbs energy from the system (during the build-up of the magnetic fields) and re-injects that energy into the system (during the collapse of the magnetic fields) twice in every power-frequency cycle.

An exactly similar phenomenon occurs with shunt capacitive elements in a power system, such as cable capacitance or banks of power capacitors, etc. In this case, energy is stored electrostatically. The cyclic charging and discharging of capacitive circuit reacts on the generators of the system in the same manner as that described above for inductive circuit, but the current flow to and from capacitive circuit in exact phase opposition to that of the inductive circuit. This feature is the basis on which power factor correction schemes depend.

It should be noted that while this “wattless” current (more accurately, the “wattless” component of a load current) does not draw power from the system, it does cause power losses in transmission and distribution systems by heating the conductors.

In practical power systems, “wattless” components of load currents are invariably inductive, while the impedances of transmission and distribution systems are predominantly inductively reactive. The combination of inductive current passing through an inductive reactance produces the worst possible conditions of voltage drop (i.e. in direct phase opposition to the system voltage).

For these reasons (transmission power losses and voltage drop), the power-supply authorities reduce the amount of “wattless” (inductive) current as much as possible.

“Wattless” (capacitive) currents have the reverse effect on voltage levels and produce voltage-rises in power systems.

The power (kW) associated with “active” energy is usually represented by the letter P.

The reactive power (kvar) is represented by Q. Inductively-reactive power is conventionally positive (+ Q) while capacitively-reactive power is shown as a negative quantity (- Q).

The apparent power S (kVA) is a combination of P and Q (see Fig. L1).

Sub-clause The power factor shows the relationship between P, Q, and S.

Fig. L1: An electric motor requires active power P and reactive power Q from the power system