Network integration of wind power - Wind Power Integration: Connection and System Operational Aspects - page 98

Table 4.1 Grid code reactive power requirements, October 2003

Country Absorbing reactive power Generating reactive power

Rated power Lower Limit Rated power Lower Limit

Australia 1.0 pf Proportional 0.95 pf Proportional

Denmark 1.0 pf Proportional 1.0 pf Proportional

Germany 0.95 pf Proportional 0.925 pf Proportional

Ireland 0.93 pf 0.7 pf, 0.4 fl 0.85 pf 0.35 pf, 0.4 fl

Scotland 0.85 pf Proportional 0.95 Proportional

In the case of a connection to the 33 kV system, the worry will be that the
voltage will fluctuate wildly. In the United Kingdom, a standard, P28, or more
widely IEC 60868, is used to transparently state the allowable voltage regulation
(Jenkins et al ., 2000). Other countries have similar approaches. The relationship
between voltage variation and frequency of occurrence set out in P28 may be
summarised as follows:

Voltage variation (%) Time between each

change (s)

0.4 1

0.8 10

1.6 100

3.0 1,000

The standard makes reference to different patterns of voltage change, e.g. step,
ramp and motor starting. For most equipment other than the fastest load cycling
systems a 3% voltage step is seen as acceptable. As stated above, very infrequent
occurrences giving rise to a step change up to 6% may be acceptable.
If the wind farm can go from large output to small under gusting conditions
then a reasonable test is whether the voltage at the 33 kV connection bus varies by
more than 3%. (It is always possible to control the rate of increase of power but not
the rate of decrease, although if bad weather prediction is accurate enough, and
evasive action taken, it may be possible to allow a 6% step change for a full gust
induced shutdown.) To explore this, a study would take a minimum and maximum
load on the 33 kV bus and the variation in voltage created by minimum and max-
imum wind.

4.3.1.3 Steady-state operation of a wind farm - example
We will examine the operation of a wind farm consisting of 20 660 kW WTGs
connected to a large 50 Hz system by a 33 kV line, as shown in Figure 4.2. This
could equally well consist of a smaller number of larger WTGs. The large utility
power system consists of a transmission network to which the line is connected by a
transformer in a substation. As far as our example is concerned, we may assume