Generators - Wind Energy Systems for Electric Power Generation - page 38

3.2.3 Reactive Power Compensation

Asynchronous machines develop their flux by drawing reactive (magnetizing) power
via the terminals. In case of sinusoidal voltages and currents the power factor is:

= P 1

P 1
P 1 2 + Q 1 2 cos ϕ

S 1 =

(3.14)

Here Q 1 is the fundamental reactive power and S 1 the apparent power. In grid
supply cos can be improved (increased) by a compensation device. In case of

stand-alone operation all the required reactive power must be supplied indepen-
dently.
In grid-supplied a.c. drives it is well known to compensate part of the inductive
reactive power by using capacitors parallel to the stator winding, to achieve a power
factor of 0 , 9 ... 0 , 95. In a three-phase star-connected circuit, using a per-phase ca-
pacitance C , the reactive power is:

ϕ

U 1 2 Q = 3 · · ω 1 C (3.15)

A stepwise adjustable reactive power can be obtained by using capacitor banks.
To obtain a continually adjustable compensation, different concepts are applicable.
Figure 3.10a indicates the basic circuits.

1. phase-controlled a.c. inductive load and capacitor.
The voltage across the inductance is adjustable by phase-control, yielding ad-
justable reactive (inductive) power. Together with the set-value reactive (capaci-
tive) power of the capacitors the resultant reactive power is adjustable and may
be designed to cover both signs of reactive power (a1).

Fig. 3.10 Compensation device concepts ( a ) basic circuits; ( b ) admittance characteristics