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 ...