A STATCOM that helps wind to meet grid codes
A suitable STATCOM can be used to ensure that wind turbines comply with increasingly tight grid codes, particularly the low voltage ride-through requirement. It also needs to be compact and flexible.
Over the last 20 years, wind power has asserted itself as an increasingly important part of the energy production landscape. The 2011 World Wind Energy Report projected that by the end of that year, total capacity would be around 245,000 MW and meet 3 % of global electricity demand. Since then, the Global Wind Energy Council states that by 2020, 8-12 % of global electricity could be supplied by wind power.
The rise of wind power has emerged as a challenge to the technology used to generate it and, as more and more renewable energies flow into the grid, grid operators have grown concerned about stability. Accordingly, they are introducing ever more stringent grid codes to ensure that grids can integrate renewable energy smoothly. Not all wind turbine technologies can comply with such demanding strictures, however. One that cannot on its own is the doubly fed induction generator (DFIG). Until recently, DFIGs were the most widely used variable-speed wind turbines. A DFIG’s stator is directly connected to the grid while the rotor is connected through a power electronic converter, which controls the generator’s speed and power factor. The converter is rated at around 30 % of the turbine’s nominal capacity, which was ample for wind turbine applications until grid codes required turbines to stay connected to the network during voltage drops to ensure low voltage ride-through (LVRT).The drawback of DFIGs is that, in the event of a voltage dip, the voltage of the grid connected stator changes suddenly.
In the event of a sudden load change or low voltage incident like a short circuit in the grid, a STATCOM responds fast.
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The rotor voltage is too low to compensate and so a disturbance current flows through the stator and rotor, damaging the converter. The DFIG’s original control strategy was to trip, but grid codes now require generators to ride through voltage drops and feed reactive power into the network to stabilise it. The only alternative is to enlist the support of a shunt-connected dynamic reactive power source. The most widely used is the thyristor-based static VAr compensator (SVC). But SVCs do not fit the requirements for (i) fast dynamic response, (ii) short overload capability, or (iii) the ability to provide maximum reactive output current during sharp voltage drops. However, one dynamic reactive-power compensator that does deliver on all three counts, and so ensures ride-through in the event of a network fault, is the static synchronous compensator, or STATCOM.