Wind Power in Power Systems

Wind Power in Power Systems 103
voltage and current time series of the wind turbines are the input to a simple grid model
(see Figure 6.2). For different grid impedances, especially grid impedance angles, time
series of voltage fluctuations are calculated for this fictitious grid. The simulated time
series of instantaneous voltage fluctuations are the input to the standard voltage flicker
algorithm in order to generate the flicker emission values P st. The algorithm is described
in IEC 61000-4-15. Such P st values are calculated for a larger number of measured time
series over the whole power range of the turbine. Weighted with standard wind-speed
distributions, a 99 percentile of the Pst values is calculated, which ensures that for 99 %
of the time, the flicker of the wind turbine will be within this 99 percentile. Last, a flicker
coefficient is calculated from this 99 percentile of the Pst values. The flicker coefficient
gives a normalised, dimensionless measure of the flicker, independent of the network
situation and thus independent of the selected short-circuit apparent power of the
fictitious grid. It determines the ratio between short-circuit power and generator-rated
apparent power, which is necessary to achieve a long-term flicker level, P lt, of 1. The
flicker coefficient c is defined as:
where
Sk
cð k; VaÞ ¼Plt
Sn
ð6:1Þ
c( k, Va) is the flicker coefficient, dependent on the grid impedance angle k and on
the annual average wind speed Va.
Sk is the short-circuit power of the grid at the point of common coupling (PCC).
Sn is the apparent power of the wind turbine at rated power .
Plt is the long-term flicker emission.
This flicker coefficient can be used by utilities to calculate the flicker emission of a wind
turbine or a wind farm at a specific site.
The procedures of IEC 61400-21 (IEC, 2001) and the German guideline are similar.
The only difference is that the German guideline requires 1-minute time intervals and
thus 1-minute Pst values, and the IEC guideline requires 10-minute intervals. This leads
to different results, with the flicker coefficients of the German guideline in general
slightly exceeding those of the IEC guideline.
Grid impedances
Ideal voltage supply
Wind
turbine
Figure 6.2 Simulation model for flicker measurements
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