Grid Integration and Power Quality - Wind Energy Systems for Electric Power Generation - page 143

Using the generator and short-circuit apparent power values, the voltage devia-
tion is calculated:

u aV = Δ

U aV
U N

= S nG ·

cos( ψ kV − ϕ )

Δ (7.4)

S kV

For specifications, the cos-function is set to 0,1, in case the calculated value
is below.

7.1.3 Power Variation and Grid Reaction

Grid reactions created by wind energy systems appear in different forms. The wind-
turbine accounts for:
•

Power variations due to wind gusts
Power variation is defined as the difference between the largest and the lowest
power values during 8 periods within one minute. Single machine systems may
encounter values between 0,6 and 0,9 rated power (see Fig. 7.2); in wind parks
evening out occurs between systems, so that resulting variations are of magni-
tudes 0,25 up to 0,4 of rated power.
Operation with as little variations as possible is intended by controlling the sys-
tem on the turbine and/or on the generator side.

• Power variations due to tower shadow effects

These periodic power variations can only be leveled out by a fast acting control.
Torque variations lead to dynamic torsional stress in the drive train which are espe-
cially relevant in systems for constant speed. Also unwanted are short-time power
variations on the grid side.
The electrical system is subjected to:

• Switching operations

Switching-on and off the generator may cause voltage fluctuations at the feed-
in point. To limit inrush-currents in systems with directly coupled asynchronous
machines phase-controlled thyristor-circuits are used, as mentioned. On the other