Page 211 - Renewable Energy Devices and System with Simulations in MATLAB and ANSYS

198 Renewable Energy Devices and Systems with Simulations in MATLAB and ANSYS ®
®

q
d
d
q
I DC = D × I s + D × I s (8.14)
where
U is the stator voltage
s
V is the DC-link voltage
DC
I is the current flow into DC link
DC
I is the stator current
s
D is the duty ratio
The generator torque control for WTs is well established and can be found in [73–76]. The
control algorithm implemented in this section is based on the vector control of the generators for
maximum power extraction. The control structure for both the drive trains is made up of a number
of parts.
Reference current generation—The rotational speed of the rotor is measured and used to generate
a reference torque from the maximum torque/power curve, which is based on the turbine character-
istics and design. This reference torque is then used to generate the reference current signal in the dq
frame for the generator-side converter.
Current control loop—The generated current references are compared with the measured cur-
rents in the dq frame to generate an error signal, which is then fed through PI controllers to generate
a voltage reference for the converter. It must be noted that here in the examples, the measured value
is taken from the generator model. In a real system, this value would be taken from a measurement
circuit. The PI controller is defined by the following transfer function:
 1 
G PI () = K 1 +  (8.15)
s

 τ s ⋅ 
There are a number of methods that are used for tuning the PI controllers. Here, the internal mode
control method is used, which places the zero of a PI controller on the pole of the open-loop system
[77]. Therefore, the controller parameters are given by the following equations:

K =α L s (8.16)

τ= L s (8.17)
R s

where
α is the closed-loop bandwidth
L is the inductance in the circuit (e.g., the stator inductance)
s
R is the resistance in the circuit
s
Modulation: The resulting reference voltages for the generator-side converter must be converted
into a duty ratio that will finally result in a PWM switching signal for the converter. Figure 8.23 shows
the block diagram of the modulation used in the simulation file, which is switching cycle averaged.
8.6.3.2 Grid-Side Converter and Control
The grid-side converter model is based on three differential equations (Equations 8.18 through 8.20)
that use the inductance and resistance of the grid-side filter and the voltage of the grid as inputs:
d
di g d q d d (8.18)
L f + Ri fg = ω Li fg + V conv − V grid
dt