**5. Simulation results and discussion**

The system described above was simulated using MATLAB Simulink for the 5.6 kW turbine described in the Appendix, initially for a wind speed is 10 m/s. Figure 10 shows results for aerodynamic starting where the generator torque is zero up to the λopt. Then the generator torque closely matches the aerodynamic torque and power production starts. In Figure 11, the turbine is motored to λopt. The significant reduction in starting time is obvious.

As shown in Figure 11 10, the current is held positive and constant at its rated value during motoring. It is then changed to a negative value corresponding to the wind speed to run as a generator while for aerodynamic starting the current is kept zero then increased in negative direction to the value which matches with the aerodynamic torque. As shown in Figure 10 and 11, the rotor takes 13.29 s to reach λopt when starting aerodynamically, which is consis‐ tent with the field test data in Fig. 4, while it takes 1.9 s for a motored start. The significant difference results in a significant gain in energy harvested.

In both starting cases, the turbine runs at the maximum Cp of 0.476 after λ reaches λopt. The MPPT would re-establish this Cp if U subsequently changes. Whether in motoring or gener‐ ating mode, the grid current is sinusoidal due to the operating characteristics of the VSMC and its phase angle can be controlled depends on the direction of power flow and if desired unity power factor can be achieved to insure the power quality.

**Figure 10.** Aerodynamic starting of the 5.6 kW wind turbine for a wind speed of 10 m/s.

**Figure 8.** Starting strategy flow chart.

168 Advances in Wind Power

**Figure 9.** Block diagram of the generator-motor FOC technique.

**5. Simulation results and discussion**

The system described above was simulated using MATLAB Simulink for the 5.6 kW turbine described in the Appendix, initially for a wind speed is 10 m/s. Figure 10 shows results for As shown in Fig. 12, for all wind speeds, motor starting reduces the starting time. Fig. 12 also shows the starting time when power production commences at a fixed fraction of the rotor maximum speed which is common operational practice for small wind turbines. Fig. 13 indicates that there is a net gain in energy delivered to the grid for motor starting com‐ pared to aerodynamic starting. As seen in Fig. 13, provided the turbine is rotating sufficient‐ ly fast, the shaft speed at which the controller switches from motor mode to generator mode does not significantly alter the energy gain compared to aerodynamic starting. This is valua‐ ble for the practical implementation of the proposed approach since the wind speed does not need to be measured to determine the length of duration of the motoring mode. Motor starting can also reduce the starting wind speed, allowing the turbine to start and then pro‐ duce power down to 1 m/s.

**Figure 11.** Motor starting of the 5.6 kW wind turbine for a wind speed of 10 m/s.

**Figure 12.** Starting time for aerodynamic and motor starting.

At this point, it is too early to determine the cost effectiveness of the motor starting ap‐ proach partly because we have not yet done any simulations to test its effectiveness in realis‐ tic low-wind conditions. It is also difficult to ascertain the net energy gain over a substantial time (e.g. one year of operation). Nonetheless, as power electronics continue to quickly drop in cost [5], the use of a wind turbine in a variable speed mode of operation with an accompa‐ nying power converter seems increasingly attractive, and techniques such as motor starting may be a part of the role that power electronics can serve.

**Figure 13.** Energy gain by motor-starting to λopt.

ble for the practical implementation of the proposed approach since the wind speed does not need to be measured to determine the length of duration of the motoring mode. Motor starting can also reduce the starting wind speed, allowing the turbine to start and then pro‐

**Figure 11.** Motor starting of the 5.6 kW wind turbine for a wind speed of 10 m/s.

**Figure 12.** Starting time for aerodynamic and motor starting.

duce power down to 1 m/s.

170 Advances in Wind Power
