Control of Power Inverters in Renewable Energy and Smart Grid Integration电力—ch4.pdf

4 Voltage and Current H ∞ Repetitive Control In the previous chapter and Chapters 15–17, several control strategies are presented to inject currents into the grid. These inverters are current-controlled VSIs and their output voltages are maintained by the grid. As a result, these strategies are not the best for stand-alone operation without a grid because the voltage is determined by the load. In this chapter through to Chapter 6, several control strategies are presented to maintain clean and stable output voltages. Such VSIs are voltage controlled and can be operated in both the stand-alone mode and the grid- connected mode. Moreover, the operation mode can be changed between stand-alone and grid-connected without changing the controller. In this chapter, the repetitive control technique is applied to design a controller for inverters based on the H ∞ control strategy. Both the output voltage and the load current are adopted for feedback. The controller contains an infinite-dimensional internal model discussed in Chapter 2 and is able to reject all periodic disturbances having the same period as the grid voltage. This leads to a very low harmonic distortion for the output voltage, even in the presence of non-linear loads and/or grid distortions. 4.1 System Description In this chapter, a three-phase system is considered. It consists of local loads, a grid interface inductor, an (external) power grid and a 4-wire-3-phase inverter that consists of IGBT bridges, an LC filter to attenuate the switching frequency voltage components and the controller. This forms a microgrid, of which the load contains both linear and distorting elements, lumped together into one linear load and a current sink which generates the harmonic components of the load current. Figure 4.1 shows the single-phase representation of the inverter system to be controlled. The microgrid can be supplied solely by the local generator, or solely by the grid, or by