Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 52

1.4

1.2

v w1

1

v w2

0.8

0.6

v w3

0.4

v w4

v w5

0.2

v w6

0

0

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

Turbine speed (pu)

Figure 1.44 Desired operational power curve

is the optimal turbine speed corresponding to the wind speed v w . In order to extract the

maximum power from the wind, the turbine should always be operated with λ opt via controlling

the turbine speed.
A typical desired operational power curve corresponding to different wind speeds is shown
by the thick line in Figure 1.44: it increases in proportion to

ropt when the rotor speed is
below the rated speed and is maintained at the rated power to protect the wind turbine system
when the rotor speed exceeds the rated speed. If the wind speed reaches the cut-off speed,
then the turbine should be shut down. This is not shown in Figure 1.44. An MPPT technique
should be applied to extract the maximum power from the wind turbine when the rotor speed
is below the rated speed and a rotor power control mechanism should be activated when the
rotor speed exceeds the rated power.
MPPT controllers can be classified into three types (Thongam and Ouhrouche 2011): tip-
speed ratio (TSR) control, power signal feedback (PSF) control and hill-climb search (HCS)
control. For TSR control, both the wind speed and the turbine speed are measured or estimated
to maintain the TSR at

ω

λ opt so that the maximum possible power is extracted. For PSF control,
the power is measured and controlled to reach the maximum power, according to the power
curve of the wind turbine obtained through either simulation or off-line experiments. The
HCS method is different from the TSR and PSF control methods, in which the maximum
power point is continuously looked for, according to the current operating power point and the
relationship between the changes in power and speed.

1.4.4.3 Grid Integration

Once the power is extracted, then it is important to feed it to the grid. Apart from the SCIG-
based topology, power electronic converters are involved in the grid integration of wind power.
How to control power electronic converters so that the integration of wind power into the grid
can be done in a grid-friendly manner and imposes minimum impact on the grid is the main
subject of this topic and will be discussed in detail.