The dominating kind of wind power generation is asynchronous, this since it is robust and cost effective. Induction generators, however, do not contribute to regulation of grid voltage, and they are substantial absorbers of reactive power. Ideally, they need to be connected to stiff grids in order not to influence power quality in a detrimental way. This is usually not the case, however. Quite on the contrary, wind power is usually connected far out in the grid, on sub-transmission or distribution levels, where the grid was not originally designed to transfer power from the system extremities back into the grid.
The reactive power balance of asynchronous generators can be improved to a certain extent by use of the doubly-fed rotor concept. To keep this technology within reasonable cost margins, however, rotor converter ratings must be kept limited to steady-state requirements only. During transient occurrences in the grid, the performance of doubly-fed induction generators (DFIG) may well prove inadequate to safeguard primarily voltage stability of the grid. Here, dynamic reactive power compensation plays an important role in supporting DFIG. This is where FACTS is coming in.
Sea cables: a key issue
For off-shore wind, comprehensive AC sea cable networks add another dimension, calling for additional elaborate reactive power control. The overall scope of reactive power control should encompass the wind farm just as well as the sea cables, to bring about a well regulated reactive power balance of the whole system, answering to the same demands on reactive power regulation as any other medium to large generator serving the grid.
FACTS for wind power comprises SVC, SVC Light® and Dynamic Energy Storage.
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