FACTS technology - Wind Power Integration: Connection and System Operational Aspects - page 268

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Figure A.4 Converter system for DFIG

device with a remarkably fast and flexible response. Figure A.4 shows one phase of
a converter system which can be applied to the rotor of a DFIG device.
Wind farms often require reactive compensation for voltage regulation under
normal conditions, and to assist ride-through under fault conditions. The relevant
devices are:

Static VAr compensator (SVC). This device, which has been in use since the 1970s,
is a combination of thyristor-switched capacitor banks and thyristor-controlled
inductor banks. At low system voltage conditions the device generates reactive
power, that is, delivers capacitive current, whereas during high voltage conditions it
absorbs reactive power, behaving like an inductor. Commonly the devices have a
steady-state rating and a transient rating (in the absorbing direction only) with the
aim of rescuing a dangerously high voltage condition and allowing time for other
system action. It has two basic uses: to regulate bus voltage at a node, and, placed
in the mid-point of a line, to allow the line impedance to be compensated, hence
extending its rating. A problem with the device is that its capacitive contribution is
most effective at higher voltage. Below 0.9 pu voltage, it falls off linearly to 0 at
zero voltage, as shown in Figure A.5.
This is because the reactive power capability of the capacitors decreases with
voltage. It therefore must be seen as a device that is useful within a normal voltage
range. Obviously where the problem is related to fault ride-through of wind gen-
erators, the voltage will fall much below this range and, to be really useful, the
capacitors must be charged until the disturbance is cleared. Taken alone, the device
has no way of discriminating between supplying reactive power to the system
disturbance and to the wind generator.

Static compensator (STATCOM). This device, formerly known as the advanced
static VAr compensator (ASVC), is based on a voltage source converter rather
than a thyristor-controlled capacitance. The voltage source converter DC terminals
are connected to a small capacitor which is maintained at some voltage level (see
Figure A.6).
The output voltage of the device lags the system voltage by a small angle to
maintain the capacitor charged. The angle is varied to adjust the voltage of the
capacitance, which determines the reactive power (MVAr) injection into the system.
The output is achieved by creating the current wave with a 90

phase shift to