Control of Power Inverters in Renewable Energy and Smart Grid Integration电力—ch8.pdf

8 Bypassing Harmonic Current Components As discussed in the previous chapter, the filter capacitor C can be regarded as part of the load instead of part of the inverter and the output impedance of an inverter can be changed to resistive or capacitive with a simple proportional or integral controller. In general, C-inverters offer better voltage quality than R-inverters and L-inverters. Simulation and experimental results have indicated that the voltage THD is high for R-inverters and L-inverters when a non-linear load is connected. It can be improved via reducing the filter inductor but the inductor cannot be too small. In this chapter, a voltage controller is introduced to improve the voltage quality. Moreover, it is found that the physical interpretation of the strategy is to bypass harmonic components of the load current. 8.1 Controller Design For the single-phase inverter shown in Figure 8.1, the inductor current i is measured to construct a proportional controller so that the output impedance of the inverter is forced to be resistive and so that it dominates the impedance between the inverter and the AC bus, as shown in Chapter 7 to design R-inverters. This 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. Here, it is used to force the output impedance of the inverter to be predominantly resistive with a large enough Ki . However, a large Ki results in high THD in the output voltage if non-linear loads are present. In order to improve the voltage quality, the output voltage vo can be measured and compared with the voltage reference vr to form a voltage loop, as shown in Figure 8.2. The following two equations hold for the closed-loop system consisting of Figure 8.1 and Figure 8.2: u = v − K i + K (s)(v − v ), r i R