Control of Power Inverters in Renewable Energy and Smart Grid Integration电力—ch5.pdf

5 Voltage H∞ Repetitive Control with a Frequency-adaptive Mechanism In the previous chapter, an H ∞ repetitive controller is designed with the feedback of the output voltage and the output current. In this chapter, this is further developed after simplifying the model of the inverter and taking the voltage only as the feedback for the repetitive controller. The feedback of the inductor current is adopted to form an inner-loop controller. The control plant for the repetitive controller is then reduced to a single-input-single-output (SISO) one and the complexity of the control design is considerably reduced. The stability evaluation of the system becomes easier as well. Moreover, a frequency-adaptive mechanism is embedded into the internal model and, hence, the controller is able to cope with grid frequency variations in the grid-connected mode. This mechanism allows the controller to maintain very good tracking performance over a wide range of grid frequencies. Extensive experimental results are included to demonstrate the control strategy. 5.1 System Description The general idea of the control strategy is to adopt an individual controller for each phase in the natural frame, under the assumption that the system is implemented with a neutral point controller (Zhong et al. 2006). The overall control structure of the system is shown in Figure 5.1. It consists of three cascaded controllers: a power controller to generate the voltage reference uref while regulating the power exchanged with the grid, a voltage controller to track uref , and a current controller to generate the PWM signals to drive the power switches while tracking the current reference generated by the voltage controller. The current controller is chosen as a simple proportional controller Kc with the inductor current as feedback. It can be equipped with current protection as well. It does not affect the tracking accuracy of the outer voltage loop