INSTRUCCIONES PARA RESPONDER AL CUESTIONARIO

3.2.1 Generator-Side Converter Control

The parameters of the synchronous machine on the test rig are different to those of the 1 MW machine used in the simulations of Chapter 2; therefore, it was necessary to re-tune the controllers. Furthermore, the output from the 1 MW turbine model was scaled to make it compatible with the 1.2 kW generator on the test rig. A block diagram showing how the generator-side controller was implemented is in Figure 3.4.

Figure 3.4: Hardware in the loop implementation of generator-side control strategy.

Depending on the flow speed being experienced by the TST a speed reference is generated. The generator used in the hardware loop has a higher rated speed and lower rated torque than the 1 MW generator used in simulation. Therefore, to test the 1 MW TST model using the hardware available on the test rig the speed reference was scaled by a factor and the torque demand was scaled by a factor as shown in Figure 3.4. The scaling factors were determined as given in (3.4) and (3.5):

(3.4)

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where and are the scaling factors for the speed and torque. 1800 rpm and 3.9 Nm are the rated speed and torque of the synchronous machine used on the hardware rig. 13 rpm and 734753 Nm are the rated speed and torque of the generator used in simulation. All parameters for the respective machines are in Appendix C.1 and Appendix A.3.

After scaling the speed reference is used by the Unidrive to control the motor, which then drives the generator. The generator speed and rotor position are measured using an incremental encoder. These variables are then fed back into the dSPACE®_{. The measured speed}

is read into a lookup table which outputs a torque demand according to equation (2.8). This ensures that the turbine follows an optimal torque vs. speed curve that maximises the output power. The rotor position is used when converting the three-phase generator currents to and axis currents. The -axis current is used to calculate the generator

reaction torque which is inputted to the two-mass model in order to balance the hydrodynamic torque . The -axis current reference is maintained at zero so the maximum torque to

current ratio is obtained.

Generator-Side Controller design – Current Loop

To tune the -axis current loop for the generator-side converter the same procedure detailed in Section 2.2.2 of Chapter 2 was used. The design criteria are given as follows:

 Closed loop response should have zero steady state error.

 Maximum overshoot of the step response should be limited to less than 5%.

 The bandwidth should be high enough to ensure that decoupling with the mechanical system is achieved.

The transfer functions are as follows:

(3.6) (3.7) (3.8)

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where is the sampling delay of the PWM converter, is the generator stator resistance and

is the generator stator inductance in the q-axis. All parameters are given in Appendix C.1.

Through bode shaping a response that meets the criteria, was achieved with a PI controller given as follows:

(3.9) The frequency response curves in Figure 3.5 (a) show the performance of the controller. It can be seen that the performance is similar to that achieved by the controller designed in Chapter 2. The phase margin is 85 degrees, meaning that the overshoot is well within the 5% limit defined in the criteria. This is reflected in the step response of Figure 3.5 (b). The bandwidth is 120Hz leading to a settling time of 8 ms which can be seen in the step response of Figure 3.5 (b).

Figure 3.5: (a) Bode plot showing the frequency response of the generator-side -axis current control loop

in both closed loop and open loop for hardware in the loop experiment. (b) Generator-side current control loop step response.

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The same control parameters were used for the -axis and the -axis control loops. The control parameters are summarised in Table 3.1.

Controller

17.5 4500

17.5 4500

Table 3-1: PI controller parameters for the generator-side converter of hardware in the loop test rig. 3.2.2 Grid-Side Converter Control

A block diagram showing how the grid-side controller was implemented on the experimental test rig is given in Figure 3.6.

Figure 3.6: Hardware in the loop implementation of grid-side control strategy.

The three-phase currents are read into to the dSPACE® _{and used by the controller}

to generator the error terms and . The three-phase voltages are also read into

the dSPACE® _{and used to generate the feed-forward voltage terms and . The grid phase}

angle is used to ensure synchronisation and is obtained using a PLL. The PLL was

implemented using a standard block from the SimPowerSystems library in Simulink®_{. Reference}

voltages are used to generate the PWMgate pulses which are sent to the grid-side

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Grid-Side Controller Design – Current Loop

The grid-side controller was designed using the same procedure described in Section 2.2.3 of Chapter 2. The inner current loop was tuned first followed by the outer voltage loop. The bandwidth of the inner current loop was designed to be 20 times faster than that of the outer voltage loop to ensure that the inner loop was decoupled from the outer loop. The design criteria are summarised as follows:

 Closed loop response should have zero steady state error.

 Maximum overshoot of the step response should be limited to less than 5%.

 The bandwidth should be high enough to ensure that the inner current loop is decoupled from the outer voltage control loop. Thus a minimum bandwidth of 250 Hz was specified.

The transfer functions of the inner current loop are given as follows:

(3.10) (3.11) (3.12) where is the sampling delay of the PWM converter, and are the inductance and

resistance between the converter and grid. All parameters are given in Appendix C.1. Through bode shaping a response that meets the criteria, was achieved with a PI control structure given as follows:

(3.13) The frequency response curves in Figure 3.7 (a) show the performance of the controller. The phase margin is 81 degrees, meaning that the overshoot is well within the 5% limit defined in the criteria. This is reflected in the step response of Figure 3.7 (b). The bandwidth is 245Hz leading to a settling time of 4 ms which can be seen in the step response of Figure 3.7 (b). The same parameters used for the -axis current loop were used for the -axis.

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Figure 3.7: (a) Bode plot showing frequency response of the grid-side current control loop

in closed loop and open loop (b) Grid-side current control loop step response.

The outer voltage loop was designed to be 20 times slower than the inner current loop. When tuning the outer loop, the inner loop was approximated as a unity gain as shown in Figure 2.16. The transfer function relating the -axis current to the DC link voltage is given as:

(3.14)

where is the DC link capacitance and is the amplitude modulation ratio. Parameters are given in Appendix C.1.

The PI controller used in the outer voltage control loop is given as follows:

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The frequency response curves of Figure 3.8 (a) show that the controller gives a good response that meets the design criteria. The phase margin is 85 degrees and the overshoot is within the 5% limit. This is reflected in the step response of Figure 3.8b. The system achieves a bandwidth of 12.25 Hz, which is 20 times slower that of the inner current loop (Figure 3.7 (a)).

Figure 3.8: (a) Bode plot of the grid-side voltage control loop (b) Voltage control loop step response.

The parameters for the grid-side converter are summarised in Table 3.2.

Controller

4.6 20

4.6 20

0.12 0.8

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