Grid Integration and Power Quality - Wind Energy Systems for Electric Power Generation - page 144

hand the generator power should be controlled to zero before disconnecting (ex-
cept emergency breaks).

• Reactive power

Reactive power issues were discussed in chap. 3 for generators and in chap. 4 for
phase-controlled inverters. The active factor cos reflecting the ratio of (funda-

mental) reactive and active power can be improved by compensation measures,
such as fixed capacitor banks or controllable compensation devices.

ϕ

• Flicker

Voltage fluctuations of low frequency caused by power variations are called
flicker, They give rise to lightness fluctuations of incandescent lamps and also
of fluorescent lamps. In a band of around 1000 variations per minute they are
experienced extremely inconvenient for the human eye.

• Harmonics due to inverters

Grid-controlled inverters create current harmonics which give rise to voltage har-
monics. The lowest order is determined by the pulse number of the inverter cir-
cuit. In six-pulse circuits, as in the three-phase bridge connection, these are the 5
and the 7 harmonic. In self-controlled PWM inverters the pulse frequency and its
side-bands are prominent in the harmonic spectrum. Note that transistor inverters
may be operated with pulse frequencies up to and exceeding 20 kHz, so that the
audible components are above human hearing.

Voltage fluctuations in the connection point are influencing the grid voltage qual-
ity. Figure 7.3 shows voltage waveforms under typical distortions, as they occur due
to WES or to other causes.
Standards and regulations set up by utilities describe the issues of power quality,
and establish limits to be kept by WES feeding into the grid.

Fig. 7.3 Periodic and non-periodic voltage distortions. (1) oscillatory transient; (2) voltage sag;
(3) voltage swell; (4) momentary interruption; (5) voltage flicker; (6) harmonic distortion; (7) volt-
age with interharmonics; (8) voltage with notches