## 5 Load Side Converter Control  
The load side converter, a three phase voltage source inverter, is used as interface between DC-link voltage and the load. Unwanted high frequency harmonics which are generated from load side voltage source inverter based on the switching frequency. This unwanted high frequency can be eliminated by using a simple passive LC filter to prevent the problems in power quality at the customer load. The load side voltage source inverter control is responsible to regulate the voltage and frequency at the customer load. In the stand-alone wind energy systems, there is no grid, the output voltages should be controlled in terms of amplitude and frequency. Vector control strategy is used to control the output load voltage during variation of required load power or wind speed. In this paper, the specified load voltage frequency and _RMS_ value of output phase voltage are 50 Hz and 220 V, respectively. The voltages \(V_{a}\), \(V_{b}\), \(V_{c}\) and the currents \(I_{a}\), \(I_{b}\), \(I_{c}\) should be measured and transformed from stationary _abc_ reference frame to rotating _dq_ reference frame using chosen electrical load voltage frequency. The voltage equations using _dq_ transformation in the rotating reference frame are:  
\[ \begin{array}{c}\
u_{d}=\
u_{d}-L_{f}\ \frac{di}{dt}+\omega L_{f}\ i_{q}\\ \
u_{q}=\
u_{qi}-L_{f}\ \frac{di}{dt}-\omega L_{f}\ i_{d}\end{array}  \tag{10}\]  
So, the active and reactive power can be calculated by using the _dq_ transformation and given by:  
\[P=\frac{3}{2}\left(\
u_{d}\ i_{d}+\
u_{q}\ i_{q}\right) \tag{12}\]  
\[Q=\frac{3}{2}\left(\
u_{d}\ i_{q}+\
u_{q}\ i_{q}\right) \tag{13}\]  
Figure 8: The DC-DC bidirectional converter control strategy. Figure 14: Power coefficient (\(C_{p}\)). Figure 12: Estimated and calculated generator speed (rad/second). Figure 13: Percentage error of generator speed (%). Figure 15 demonstrates the performance of DC-DC bidirectional buck-boost converter controller. It has been able to charge the batteries bank when the generated power is more than the required load power, and discharge the batteries bank when the required load power is more than the generated power. Also, to maintain the DC-link voltage at constant value which equals 690 V. Figure 15(a) shows the generated, load, and batteries bank powers, where the maximum generated power is variable according to the wind speed variation. The required load power steps up at times 6.005 and 12.005 second (in worst time of change, where the instantaneous value of voltage and current of phase A is at maximum value) from 12 kW to 13.5 kW and from 13.5 kW to 15 kW, respectively. Also, it steps down from 15 kW to 13.5 kW and from 13.5 kW to 12 kW at times 18.005 and 24.005 second, respectively (also in worst time of change). Batteries bank power changes (charge and discharge) to meet power balance of the system during required load and wind speed variation. Figure 15(b) shows the DC-link voltage which is maintained at constant value at 690 V.  
The total harmonic distortion (THD), in percentage, of the output load current for different required load power situations is shown in Figure 14. Where, the THD is less than 1.6% when the required load  
Figure 15: The performance of DC-DC bidirectional converter controller. (**a**) Generated, load, and batteries bank powers; (**b**) DC-link voltage. Figure 17(a) shows that the output load current responses with a long time simulation. Figure 17(b,c) show the output load current at the instant of step up of required load power at time 12.005 seconds and step down of required load power at time 24.005 seconds, respectively. The output load voltage responses for all simulation time, output load voltage at the instant of step up of required load power at time 6.005 seconds and step down of required load power at time 18.005 seconds are shown in Figure 18(a-c), respectively. Figure 19 shows the _RMS_ value of output phase voltage, where it is maintained at 220 V as a reference.