It is clear from the above analysis, that the advantage of both shunt and series compensation of an AC capacitor can be made use of. In shunt compensation, the capacitor is used to compensate the individual induction generators. In series compensation, the capacitor is used to compensate the line impedance. The overall result will be an improvement in the voltage and lower losses on the transmission line.
To know the effect of the combined series and shunt compensations, simulations are carried out for two different combinations of series-shunt compensation. The same 10 wind turbines are connected at bus 9. Figure 7.21 illustrates the real power generated by WECS versus the voltage at the WECS terminals with various combinations of series-shunt compensations. It is seen that the voltage profile is high compared to individual series or shunt compensation. In fact, there is over compensation with series capacitor equal to 75 % of line reactance and shunt compensation of 200 kVAR.
Figure 7.22 shows that reactive power demand decreases significantly when compared to those with series compensation alone. As shown in Figs. 7.20 and 7.23, a comparison between the line current for series compensation and series-shunt compensation, it is observed that the transmission line current is reduced significantly with series-shunt compensation.
Comparing Figs. 7.12, 7.18 and 7.21, which show the variation of voltage with power for different types of compensation, it is evident that series-shunt
compensation provides a very good voltage profile at WECS terminals and increases the wind power penetration, as more turbines can be connected.
From Fig. 7.24, it is observed that series-shunt compensation with 50 % series compensation and 150 kVAR shunt compensation gives better voltage profile than that obtained by strengthening the grid with 5 parallel lines. Also, for such capacitor combination, the reactive power demand and transmission line current are reduced significantly as seen from Figs. 7.22 and 7.23. Thus it can be concluded that series-shunt compensation is an economical alternative to grid strengthening.
Table 7.2 Summary of results