Wind power technology - Wind Power Integration: Connection and System Operational Aspects - page 78

3.6.1.3 Voltage control
A basic implementation of the DFIG voltage controller is shown in Figure 3.22b. In
this scheme the difference in magnitude between the terminal voltage reference,
V s ref , and the actual terminal voltage, V s , is manipulated to generate the reference of
the rotor current in the d -axis, i dr ref . The reference current, i dr ref , is compared with
the actual value of the rotor current in the d -axis, i dr , to generate an error signal,
which is then processed by a standard PI controller. The required rotor voltage v dr
is obtained as the addition of the PI controller output and a compensation term used
to eliminate cross coupling between torque and voltage control loops.

3.6.2 Wide-range variable-speed synchronous generator
wind turbine
The wide-range variable-speed wind turbine based on a synchronous generator is
shown schematically in Figure 3.23. The aerodynamic rotor and generator shafts
may be coupled directly (i.e. without a gearbox) in which case the generator is a
multi-pole synchronous generator designed for low speeds, or they can be coupled
through a gear box, which allows for a generator with a lower number of poles. The
generator can be either an electrically excited synchronous generator or a perma-
nent magnet machine (Heier, 1998). To permit variable-speed operation, the syn-
chronous generator is connected to the grid through a variable frequency power
converter system, which completely decouples the generator speed from the grid
frequency. Therefore, the electrical frequency of the generator may vary as the
wind speed changes, while the grid frequency remains unchanged. The rating of the
power converter system in this wind turbine system corresponds to the rated power
of the generator plus losses.
The power converter system consists of the grid-side and the generator-side
converters connected back-to-back through a DC link. The grid-side converter is a
pulse width modulated-voltage source converter (PWM-VSC), and the generator-
side converter can be a diode-based rectifier or a PWM-VSC.

Synchronous
generator

Inductive
reactor

Generator-
side
converter

Grid-
side
converter

Step up
transformer

Power converter

Common
collecting point

Turbine rotor

Figure 3.23 Wide-range variable-speed wind turbine based on a synchronous

generator