Page 183 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

170 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
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The relationship between the TSR and the power coefficient of the wind turbine for different
rotor speeds has been discussed. For a given wind speed, there is an optimum rotor speed that gives
the optimum TSR in order to achieve the maximum power, and the following equation expresses this
peak power point optimization:

w opt R
λ opt = ω , w (7.17)
w v

The maximum power occurs at this optimum speed for each different wind speeds (see
Figure 7.12). As the wind speed changes from one point to another, the optimum power point
changes to a different value. To do so, a controller must be designed to follow the reference speed.
Different techniques can be applied either using a search technique that does not need the measure-
ment of the wind speed or simply using the direct relation given in Equation 7.17. Such a control
scheme can be implemented as shown in Figure 7.13. When the wind speed goes through the
) is applied to the PMSG. The wind speed is measured (e.g.,
wind turbine, a mechanical torque (T w
with an anemometer), and by using Equation 7.17, the optimized rotor speed for maximum wind
power conversion is achieved, if the turbine power coefficient is fully characterized. The speed of
the rotor is measured to compensate the controller error. The reference of the direct axis current
is zero. The proportional–integral control for each d-q axis can be designed using small signal
modeling. The d-q reference voltages are transformed by Clarke and Park transformations in order
to form the three-phase reference voltages and command the converter switches using PWM (e.g.,
the SPWM method). It is assumed that the DC-link voltage is maintained constant controlled by
the grid-side converter.
The purpose of the grid-side converter is to deliver to the grid the power produced by the genera-
tor with an acceptable power quality. Moreover, the DC-link voltage control is also controlled by
the grid-side converter. Figure 7.13 shows the α-axis control loop used in the grid-side converter
and a similar loop is used for the β-axis. The grid converter output current is controlled by the
inner loop and has a faster response than the DC-link voltage loop. Therefore, the inner loop is
considered unitary when the DC-link voltage control is designed. The block “power calculation”
makes use of active and reactive power references to produce a current reference, which in turn
will be multiplied by the DC-link controller output signal. The resulting signal is the α-axis current
reference. The power transfer is designed in order to ensure stability for different scenarios. The
dynamic load should be provided with power all the time. This is achieved by the power from the
wind generator or from the grid only when the wind power is less than the load demand. The grid
could send a reference signal to the controller requiring active power, and a dummy load is con-
nected in the system through a converter to provide full control capability of the whole system. The
excess wind power can be sent partially or completely to the dummy load. A power transfer strategy
chart is shown in Figure 7.14.
The grid-side converter is controlled in such a way to provide reactive power during voltage
sags according to grid standards and codes. The power algorithm takes the reference of P and Q
to generate the required VAR to support the output voltage. Injecting reactive power increases the
line current, and the reactive power should be maintained within the current capability of the con-
verter in order to avoid disconnecting the wind energy system from the grid due to some protection
operation.
In the complete system of Figure 7.13, the machine-side converter is controlled to extract the
maximum power from the wind using a vector control, while the grid-side controller is designed to
control the power flow using α–β reference frame. The power transfer algorithm controls the power
flow between the wind turbine and the grid. The turbine supplies a primary load that is variable in
nature and a control dummy load to manage the power flow in the system considering the state of
the grid and the load demand. The excess power is injected in the grid.