Grid Integration and Power Quality - Wind Energy Systems for Electric Power Generation - page 158

7.4.3 Audio Frequency Transmission Compatibility

Capacitors used for power factor compensation contribute to decrease the parallel
resonance frequency of the grid. Consequently, the performance of audio frequency
transmission equipment in use by the utility may be disturbed. Such transmission
systems operate a signal frequencies of 250 up to 1500 Hz, and transmitter levels of
1% up to 4% of U N . Hence the utilities require application of series inductors or trap
circuits with the capacitances.
An example is Vattenfall in Berlin who operate a system using a signal frequency
of 750 Hz. The company recommends an inductive compensation of 7%, i.e. the
series inductors should be designed for 7% component of reactive power referred to
the capacitive reactive power.
Figure 7.15 shows a simplified model of inductor-compensated capacitor. Its
impedance is:

Z = Z 0 ω

ω 0 − ω

0
ω

The inductive factor is defined as the ratio of reactive powers at fundamental
frequency, which is equal to the voltage drop relation:

C = ω 1
ω 0

2

p = U L
U c

= Q L
Q C

ω 1 2 = · L · (7.12)

where

ω 1 = 2 π f 1 grid rated frequency,

ω s = 2 π f s signal frequency,

ω 0 = 1 / √ L C

resonant circuit eigenfrequency,

Z 0 = L / C characteristic impedance.

Fig. 7.15 Flicker-relevant
voltage drop on a short-circuit
impedance