Control of Power Inverters in Renewable Energy and Smart Grid Integration

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 ...