21-05-2013, 04:37 PM
Power Quality Standards for Wind Turbines
ABSTRACT
Since the beginning of the development of the wind power industry in the eighties the rated
capacity of the wind turbines has increased from some tens of kW to today’s MW turbines.
At the same time the trend has moved from installations including a single or a few wind
turbines to planning of large wind farms ranging from some tens of MW to over 100 MW.
It is evident from this, that whereas the earlier small installations could quite easily be
connected to the existing grids, it may not be quite so straightforward to hook up today’s
major wind farms to the grid. This is often because of the impact the wind turbines may
have on the voltage quality, i.e. slow voltage variations, flicker, voltage dips and
harmonics, being the focus point of this chapter. To assess the effect, knowledge about the
electrical characteristics of the wind turbines is needed or else the result could easily be an
inappropriate design of the grid connection. To this, the application of power quality
standards plays an important role for securing a rational assessment. In particular IEC
61400-21 provides basis for consistent specification of the power quality characteristics of
wind turbines. These characteristics are specific for the wind turbine type, but not site
specific. This means that having the actual parameter values for a specific wind turbine, the
expected impact of the wind turbine type on voltage quality when deployed at a specific
site, possibly as a group of wind turbines, can be calculated. The methodology for this is
explained and illustrated by case studies considering a 5x750 kW wind farm on a 22 kV
distribution feeder. The detailed analysis suggests that the wind farm capacity can be
operated at the grid without causing unacceptable voltage quality. For comparison a
simplified design criterion is considered, assuming that the wind farm is only allowed to
cause a voltage increment of 1 %, i.e. a criterion commonly used in the early days of the
wind power development. According to this criterion, only a very limited wind power
capacity would be allowed. Measurements confirms however the suggestion of the detailed
analysis and it is concluded that a simplified design criterion like the “1 % rule” should not
be used for dimensioning the grid connection of a wind farm.