You are on page 1of 6

Reactive Power and Compensation Solution Basics

Reactive Power and Compensation Solutions

Basics For Students (photo credit:

Why we dont like reactive power

The total power, the so-called apparent power, of a transmission network is composed of active
and reactive power (Figure 1). While the power consumers connected into supply transform
the active power into active energy, the reactive energy pertaining to the reactive power is not
consumed. The reactive power at the consumer side is merely used for building up a magnetic
field, for example, for operating electric motors, pumps, or transformers.
Reactive power is generated when power is drawn from the supply network and then fed back
into the network with a time delay.
This way it oscillates between consumer and generator. This constitutes an additional load on the
network and requires greater dimensioning in order to take up the oscillating reactive power in
addition to the active power made available. As a consequence, less active power can be

Figure 1
Composition of the total power of a transmission grid

Reactive power has zero average value because it pulsates up and down, averaging to zero.
Reactive power is measured as the maximum of the pulsating power over a cycle. It can be
positive or negative, depending on whether current peaks before or after voltage.
By convention, reactive power, like real power, is positive when it is supplied and negative
when it is consumed. Consuming reactive power lowers voltage magnitudes, while supplying
reactive power increases voltage magnitudes.

Solution with compensation //

With a reactive power compensation system with power capacitors directly connected to the low
voltage network and close to the power consumer, transmission facilities can be relieved as the
reactive power is no longer supplied from the network but provided by the capacitors (Figure 2).

Figure 2 Principle of reactive power compensation using low

voltage power capacitors
Transmission losses and energy consumption are reduced and expensive expansions become
unnecessary as the same equipment can be used to transmit more active power owing to reactive
power compensation.

Determination of capacitor power

A system with the installed active power P is to be compensated from a power factor cos 1 to
a power factor cos 2 . The capacitor power necessary for this compensation is calculated as
Q c = P (tan 1 tan 2 )
Compensation reduces the transmitted apparent power S (see Figure 3). Ohmic transmission
losses decrease by the square of the currents.

Figure 3 Power diagram for a non-compensated (1)

and a compensated (2) installation

Reactive power estimate

For industrial plants that are still in a configuring stage, it can be assumed by approximation that
the reactive power consumers are primarily AC induction motors working with an average
power factor cos 0.7. For compensation to cos = 0.9, a capacitor power of approximately
50 % of the active power is required:
Q c = 0.5 P
In infrastructural projects (offices, schools, etc.), the following applies:
Q c = 0.1 to 0.2 P

Calculation of the reactive power

(Based on the electricity bill)
For installations which are already running, the required capacitor power can be determined by
measuring. If active and reactive work meters are available, the demand of capacitor power can
be taken from the monthly electricity bill.
tan = reactive work / active work
For identical meter operating times in the measurement of reactive and active work //
tan = reactive power Q / active power P with
tan = (1 cos2 ) / cos
The compensation power Q c matching the active power P can be calculated for a desired
value of cos 2.
Qc = Q1 Q2 = P F
In this case F = tan 1 tan 2

To simplify the calculation of Q c , Table 1 states the conversion factors F when a measured cos
1 is to be compensated in order to attain a power factor cos 2 in operation.

Table 1

Conversion factors F for phase angle adjustments

3 main types of compensation //

Capacitors can be used for single, group, and central compensation. These types of
compensation will be introduced in the following //

Single compensation
In single compensation, the capacitors are directly connected to the terminals of the individual
power consumers and switched on together with them via a common switching device. Here, the
capacitor power must be precisely adjusted to the respective consumers. Single compensation is
frequently used for induction motors (Figure 4).

Figure 4 Single compensation

Single compensation is economically favourable for:

Large individual power consumers

Constant power demand
Long ON times

Here, load is taken off the feeder lines to the power consumers. A continuous adjustment of the
capacitor power to its reactive power demand is not possible, however.

Group compensation
With group compensation, each compensation device is assigned to a consumer group. Such a
consumer group may consist of motors or discharge lamps, for example, which are connected
into supply together through a contactor or switch. In this case, special switching devices for
connecting the capacitors are not required either (Figure 5).

Figure 5 Group compensation

Group compensation has the same advantages and disadvantages as single compensation.

Central compensation
Reactive power control units are used for central compensation, which are directly assigned
to a switchgear unit, distribution board, or sub-distribution board and centrally installed there.
Control units contain switchable capacitor branch circuits and a controller which acquires the
reactive power present at the feed-in location.

Figure 6 Central compensation

If it deviates from the set-point, the controller switches the capacitors on or off step by step
via contactors.
The capacitor power is chosen in such a way that the entire installation reaches the desired cos
(Figure 6). Central compensation is recommended in case of:

Many small power consumers connected into supply

Different power demands and varying ON times of the power consumers

References //

Planning of Electric Power Distribution by SIEMENS

Principles for Efficient and Reliable Reactive Power Supply and Consumption by Federal
Energy Regulatory Commission