Cascaded Current-Voltage H Repetitive Control - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 123

6
Cascaded Current-Voltage H ∞
Repetitive Control
Both the THD of the microgrid voltage and the THD of the current exchanged with the grid
need to be kept low according to industrial regulations. It has been shown that it is not a
problem to obtain low THD for either the microgrid voltage (Chapters 4 and 5) or for the
current exchanged with the grid (Chapter 3). However, it has been a challenge to obtain low
THD for both the voltage and the current simultaneously. In this chapter, the advantages of the
voltage and current controllers based on H ∞ and repetitive control techniques, presented in
Chapters 3 and 5 respectively, are brought together to achieve low THD for both the microgrid
voltage and the current exchanged with the grid. It is not a simple combination of the two
control strategies but a complete re-design after realising that the inverter LCL filter can be
split into two separate parts to design the controllers. The LC part can be used for the voltage
controller design and the grid inductor can be used for the current controller design. The
voltage controller is responsible for the power quality of the local load voltage (microgrid
voltage), power distribution within the microgrid and synchronisation with the grid. The
current controller is responsible for the power exchanged between the grid and the microgrid,
the power quality of the current exchanged with the grid, and the over-current protection.
When the microgrid is connected to the grid, both controllers are active; when the microgrid is
not connected to the grid, the current controller works with zero current reference. Hence, no
extra effort is needed when changing the operation mode of the microgrid, which considerably
facilitates the smooth operation of grid-connected inverters (Yao et al. 2010). It also shows
that there is no need to consider the processing delay W d considered in Chapter 5.
6.1 Operation Modes in Microgrids
Microgrids are normally operated in the grid-connected mode. However, it is also expected to
provide sufficient generation capacity, controls, and operational strategies to supply at least a
part of the load after being disconnected from the distribution system and to remain operational
as a stand-alone (islanded) system (Chen et al. 2010; Hatziargyriou et al. 2007; Katiraei et al.