Control of Power Inverters in Renewable Energy and Smart Grid Integration电力—ch20.pdf

20 Robust Droop Control with Improved Voltage Quality In Chapter 19, a robust droop controller is presented to achieve accurate sharing of both real power and reactive power for different types of inverters while maintaining excellent capability of voltage regulation. Apart from C-inverters, the THD of the output voltage is high when the load is non-linear. In this chapter, the strategy described in Chapter 8 is combined with the robust droop controller to improve the voltage quality. As a result, all the three major problems associated with the parallel operation of inverters, that is, sharing accuracy, voltage regulation and voltage THD, are solved with a compact controller. Experimental results are provided to verify the analysis and design. In this chapter, R-inverters are taken as an example but the concept can be extended to L- and C-inverters. 20.1 Control Strategy For a single-phase inverter depicted in Figure 20.1, the controller presented in Chapter 8, as shown in Figure 20.2(a), is able to bypass the harmonic components in the load current and hence the output voltage THD is low even when the load is non-linear. It consists of a proportional feedback of the inductor current (Zhong 2012c), which is also widely referred to as a virtual resistor (Dahono 2003, 2004; Dahono et al. 2001; Li 2009) to dampen the resonance of the LC filter. The output impedance of the inverter is forced to be predominantly resistive with a large enough K i . In addition to the current feedback through the virtual resistor K i , the voltage error vr − vo is added to vr through a block KR (s), which is able to reduce the output impedance of the inverter at harmonic frequencies. As a result, the high voltage THD caused by a large K i when nonlinear loads are present is reduced. The block KR (s) in Figure 20.2(a) can be chosen as the resonant harmonic compensators (Castilla et al. 2009; Shen et al. 2010)