Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 116

4.5.1 Dips
Other than starting conditions, the major source of dips in wind-powered networks
is the variability of the wind. The discussion above has indicated how the voltage is
altered by changes in the direction of load serving flows. Some research is required
to consider the performance of mixed demand and embedded generation on real
circuits against the national standard, e.g. P28 in the United Kingdom (Electricity
Association, 1989). By and large the standards were constructed to facilitate design
where the load switching is fixed or continuously variable. Wind variability is
neither. Also, some wind turbines are IGBT switched, therefore performance
should be assessed to determine whether the variation exceeds the standard at any
point, and an assessment made as to whether this creates any customer difficulties.
If no difficulties emerge, then standards can be relaxed. The level of voltage dip
will be related to the fault level at the point of common coupling with other cus-
tomers. Most often wind farms are in remote areas where the fault level is low and
this makes the flicker problem worse.
4.5.2 Harmonics
Devices containing power converters are apt to create harmonic voltage distortion.
The AC side harmonics are related to the number of pulse units in the converter
according to m n 1, where n is the number of poles and m is any integer
multiplier. Thus a 12 pulse converter will emit AC side harmonics at 11, 13, 23, 25,
35, 37, . . . times fundamental frequency. The harmonic performance of voltage
source converters depends entirely on whether the power order and Selective
Harmonic Elimination (SHE) arrangements are met by PWM technology or capa-
citor voltage control and ancillary arrangements (see Appendix 1). Apart from the
stress that these high frequencies place on utility and users' plant (capacitors are
particularly susceptible to overload when higher harmonics are present), the har-
monics can be induced into telecommunication circuits where they cause audible
buzzing and other nuisance activity.
In general, power system planners will seek to ensure that installations do not
worsen harmonics on the power system. Standards are quoted to comply with grid
codes. There may be requirements not to exceed total harmonic distortion of the
voltage waveform and limits on the distortion due to specific harmonics.
This however is not the whole story. Power systems experience resonance.
Resonant frequency depends on network topology, connected generation and con-
nected reactive power devices. It is difficult to predict harmonic resonance condi-
tions, but simulation programmes exist. The challenge is to predict the higher
frequency characteristics of plant based largely on 50 Hz information. Without a
spectral analysis, the parameters will be approximate for higher frequencies.
As noted above, wind turbines with converters will emit harmonics, the
severity depending on the technology employed and the control algorithms. The
practice is to negate them at source or construct filters to ensure that harmonic
waveform distortion remains within limits. Nonetheless a very small voltage dis-
tortion can cause large harmonic currents at a resonant condition. The characteristic