Page 196 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

Power Electronics and Controls for Large Wind Turbines and Wind Farms 183

8.3.2 Strict Codes from Grid Side to Be Connected
The fluctuation and unpredictable features of wind energy are not preferred for the grid operation.
Most countries have strict requirements for the behavior of WTs, also known as “grid codes,” which
are updated regularly [13–17]. Basically, grid codes are always trying to make the WTS to act as a
conventional power plant seen from the electrical utility point of view. That means the WTS should
not only be a passive power source simply injecting available power from the wind but also behave
like an active generation unit, which can wisely manage the delivered active/reactive power accord-
ing to the demands and provide frequency/voltage support functions for the power grid. Examples
of the state-of-the-art grid supporting requirements are given in the following. They are specified
either for the individual wind turbine or for whole wind farm connected, for example, the transmis-
sion system.
According to most grid codes, individual WTs must be able to control the active power at the
point of common coupling. Normally, the active power has to be regulated based on the grid fre-
quency, for example, in Denmark, Ireland, and Germany, so that the overall grid frequency can be
maintained [15, 17]. Similarly, the reactive power delivered by the WTS has also to be regulated in
a certain range. This leads to larger MVA capacity of the WTS when designing the whole converter
system. The TSO normally specifies the reactive power range of the WTS according to the grid
voltage levels.
Besides the normal operation, TSOs in different countries have issued strict grid supporting
requirements for the WTS under grid faults. Figure 8.6 shows that the boundaries with various grid
voltage dip amplitudes as well as the allowable disturbing time are defined for a wind farm. It has
become a need that the WTS should provide reactive power (up to 100% current capacity) to con-
tribute to the voltage recovery, when the grid voltage sag is present, as shown in Figure 8.7. Future
grid codes have even requirements for large WTs to inject underexcited reactive power when the
grid voltage is 20% higher than normal condition, and the reoccurred or multiply voltage dips could
also become demands for large WTs [18].
The requirements for more grid support functions by WTs have on one hand increased the cost
per produced kWh but on the other hand have enabled wind energy to be better utilized and smoothly
integrated into the power grid. It can be predicted that stricter grid codes in the future will keep chal-
lenging the WTS and also continuously pushing forward the demands for the power electronics
technology.

Voltage(%)
100
90
Germany—type 1
70

Germany—type 2
China
Denmark
20 Keep connected
above the curves
Time (ms)
0
150 500 700 1000 1500 2000

FIGURE 8.6 Grid voltage profile for low-voltage ride-through capability of WTs by different countries.