Voltage and Current H Repetitive Control - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 90

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 both in combination. The local generator can supply the microgrid and export power to
the grid. Two isolators, S c and S g are provided to facilitate this and a grid interface inductor is
provided to allow separation of the (sinusoidal) microgrid voltage and the (possibly distorted)