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To evaluate the performance of the developed WTS, a series of experiments have been conducted. The tests are focused on both static and dynamic characteristics of the WTS. The characteristics of the considered PWT are determined using the manufacturer data and wind tunnel test results. The WTS is operated under realistic wind condition, which are measured using the weather station (Davis Vantage Pro, sampled at 1 s interval) installed on the tower of the PWT.

3.1.5.1

Static Characteristics

The characteristics of a PWT and WTS are shown in Figure 3.9.

Figure 3.9: Comparison of (a) Power curve and (b) C - TSR curve between the WTS and a PWT 0 100 200 300 400 500 600 700 0 200 400 600 800 P o w er ( W ) Rotor Speed (rpm) PWT WTS:6m/s WTS:5m/s WTS:7m/s 5m/s 6m/s 7m/s 0 0.1 0.2 0.3 0.4 0.5 0 5 10 15 P o w er C o ef fi ci en t ( Cp )

Tip Speed Ratio (TSR)

PWT WTS (a)

The power speed characteristics are obtained under three wind speeds. The power coefficient (C ) is also plotted against the tip speed ratio (TSR) at a fixed pitch angle. Tests have confirmed that the WTS produces an excellent match to these characteristics under steady state conditions.

3.1.5.2

Dynamic Characteristics

The performance of the WTS can only be validated when a comparison to the dynamic performance of the PWT is made. The WTS is compared with the PWT at instantaneous and average (for 60s) wind speed for a given period. The real time measurement data of WTS includes noise in the laboratory environment, due to mechanical vibration and sensors measurement.

Figure 3.10: Comparison of WTS and PWT output power, rotor speed, torque and their errors at the measured wind speed: (I) instantaneous wind profile and (II) average wind profile

The wind speed is normalized at 9.5 m/s. The WTS output power, rotor speed, and torque are very close to the PWT, as shown in Figure 3.10. The noise in the measurement data of WTS increases with increasing wind speed, especially in the instantaneous wind profile. The average wind profile provides relatively less turbulence, which keeps the smooth operation of the simulator. In addition, the errors between the WTS and PWT outputs under the instantaneous wind profile are within 6%. This error has improved to 4 % for the average wind profile.

3.1.5.3

Simulator Protection against High Wind Speed

The WTS is protected against the high real wind profile in order to operate with limited size components and the corresponding objectives. This is, in fact, the emulation of a pitch angle control in the PWT, which is used to curtail the power beyond the rated wind speed. The control objective is to maintain the power at rated value beyond a certain wind speed.

(b) (a)

Figure 3.11: Activation of pitch angle control

The wind profile in Figure 3.11(a) considers the wind speed beyond the rated value, as indicated by the value above the red line. The rotor speed is regulated to the maximum permissible value by controlling the pitch angle in determining the reference power as shown in Figure 3.11(b). The increasing value of pitch angle decreases the power coefficient of the turbine, as shown in Figure 3.11(c). This decreasing power coefficient curtails the power from the maximum possible value. Thus the power is maintained at the rated value for higher wind speed, as shown in Figure 3.11(b).

3.1.5.4

WTS in Microgrid

The microgrid is designed based on the features of the energy sources (the considered PWT, roof top PV, and commercial battery) and programmable load. The WTS and PV systems are operated under their respective MPPT schemes, and the load is kept constant before the experiment starts. The battery storage regulates the DLV. If the WTS does not follow the PWT closely then the storage compensates the degraded performance of WTS, as discussed in Section 4. The impact of the degraded performance of WTS on the microgrid would be critical in the discharging mode of battery because the DLV can fall below the minimum operating level, 0.95 p.u. The objective is to validate the performance of the WTS in accordance to the PWT in the microgrid environment (the microgrid is designed based on the features of the PWT). Therefore, the microgrid is operated in the discharging mode of battery only. The step change in wind speed is considered in the microgrid to test the performance of the WTS and the coordination of the energy sources as shown in Fig. 12 (a).

Figure 3.12: Performance of the WTS within a microgrid environment

Starting Transients

At the start, as the WTS and PV are not fully activated until 2.13s, the battery provides power to the load. The negative battery power in Figure 3.12 (c) means the battery is discharging. Once the WTS is activated at 2.13s, it creates some degree of transients; however, the battery promptly reduces its power output to the load so that more renewable energy can be used. Furthermore, when the PV increases its power output gradually, starting from 3.23s, the participation of the battery further reduces. This can be seen from Figure 3.12 (c) between 2.13s and 8.9s. During all these processes, the DLV is maintained well within the acceptable range, as shown in Figure 3.12 (b).

Low Wind Speed

When the wind speed is dropped to 6m/s at 13.56s, as illustrated in Figure 3.12 (a), the power output from the WTS also drops accordingly. At this point, the battery picks up the power deficiency to balance the load. The DLV drops only slightly, as can be seen from Figure 3.12(b). To see the interaction between the wind and PV sources, assume that there is a sudden increase and decrease in irradiance at 14.1s and 17s respectively. Hence, the PV output power increases at first, and then decreases in proportion to the amount of irradiance. As a result, the battery cuts back its contribution at 14.1s, and then increases again at 17s to maintain the DLV. The coordination between energy sources is smooth, especially during transient periods. This can be seen from Figure 3.12(c) between 14.1 s and 17 s.

(a) (b)

High Wind Speed

When the wind speed is increased to 7 m/s at 19s, the majority of the load is supported by the WTS. The power output of the WTS at wind speed 6.6 -7 m/s is increased marginally under the MPPT scheme as shown in Figure 3.12 (c) between (19 -20)s. It is interesting to note that even when the PV output drops due to reduced irradiance at 19s, the battery still cuts back on its participation. Under these conditions, the WTS becomes the dominant power source to keep the DLV within the desired range, as shown in Figure 3.12 (c).

Thus, the WTS, together with the PV and the battery storage, can indeed participate in a microgrid system. The DLV is maintained above 0.95 p.u. during the discharging mode of operation of the battery under varying weather conditions. This demonstrates that the WTS follows the PWT in the microgrid environment and can operate as a VSDD wind system for further research work.

3.1.6 Conclusions

The thesis has utilized the actual wind tunnel test data in the construction of the WTS. An induction motor and variable speed drive have been found to offer an excellent platform for implementing vector torque mode control to achieve desired flexibility for the WTS under the real measured wind profile. The paper has presented a detailed design process and construction procedures for such a WTS. The static and dynamic characteristics of the WTS correlate well with the considered PWT. It has also shown that the designed WTS can successfully interact with other renewal resources and battery storage devices to maintain stable operation of a microgrid.