Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 50

grid but the real power in the rotor circuit can flow bidirectionally, from the grid to the rotor
or from the rotor to the grid, depending on the operational condition. Ignoring the losses, the
power handled by the rotor circuit is (Tazil et al . 2010)

P rotor =−

·

s P stator ,

where s is the slip, and the power sent to the grid is

P grid = P rotor + P stator =

−

s ) P stator .

(1

Since most of the power flows through the stator circuit, the power processed by the rotor
circuit can be reduced to roughly 30%. This means the great advantage of a sufficient range
of operational speed can be achieved at a reasonably low cost.
DFIGs are often applied in variable speed wind turbine systems with a multi-stage gearbox,
as shown in Figure 1.43(b). Its basic operating principle is the same as an SCIG-based system
but the rotor active power is controlled by the power electronic converters so that a speed
range of 30% around the synchronous speed can be obtained. The choice of the rated power

for the rotor converter is a trade-off between cost and the desired speed range. Moreover, the
converter compensates the reactive power and smooths the grid connection.
Although a DFIG offers a sufficient range of operational speed and many other merits,
it is very sensitive to voltage disturbances, especially voltage sags. Abrupt voltage drops
at the terminals often cause large voltage disturbances on the rotor, which may exceed the
voltage rating of the rotor-side converter (RSC), make the rotor current uncontrollable, and
even damage the RSC. Many strategies are available to improve the low-voltage ride-through
capability of DFIGs; see (Guo et al . 2012).

±

1.4.3.3 Permanent Magnet Synchronous Generator (PMSG)

A PMSG adopts a permanent magnet to generate the magnetic field needed for electricity
generation. Hence, there is no need to provide an external power supply for excitation and
there is no need to have a rotor circuit. This simplifies the structure and reduces the maintenance
cost. PMSGs are more efficient than induction generators and the power factor can be made
unity or even leading. Moreover, PMSGs have very high power density and are becoming
cost-effective because the price of rare-earth magnets has reduced by more than an order
of magnitude in the last 10 years. As a result, PMSGs are becoming increasingly popular
for wind power applications. A PMSG runs at the synchronous speed and the frequency of
electricity generated is directly in proportion to the mechanical speed and hence the slip is
zero. This could be used to eliminate the need for a mechanical sensor to measure the speed of
the turbine.
PMSGs are often applied in variable speed wind turbine systems, which are direct-driven
or with a single stage gearbox, as shown in Figure 1.43(c). Full-scale back-to-back converters
are often used for the AC-DC-AC conversion. It is also possible to use an uncontrolled rectifier
cascaded with a DC-DC converter and an inverter.
Because permanent magnets are used, care should always be taken to avoid possible demag-
netisation caused by too high currents and/or too high temperature.