Nonlinear control of grid-tied inverters for stabilising future smart grids

Power systems stability has been an active area of research for many decades. However, recently, the large number of renewable energy sources being integrated to the power network has started to significantly affect the stable and reliable operation of the grid. This has created essential requirements for the grid-connected units to contribute to the stability of the network by assisting in the regulation of the grid voltage and frequency via adjusting the injected real and reactive power. Hence, control design methods for grid-tied inverters will play a key role in the future smart grid architecture, which will be dominated by inverter-interfaced units.

Although several control methods have been developed for grid-tied inverters to achieve power regulation or provide ancillary services (contribute to the voltage and frequency regulation), the stability properties of the resulting closed-loop system have not been adequately exploited, especially for a system with multiple inverters, such as in the case of a smart grid. Most of the existing approaches to analyse stability are based on small-signal model and linearization methods. However, the inherent nonlinear dynamics of the power inverters together with the nonlinearities that arise from the power expressions in the control loop make nonlinear analysis essential to achieve global stability results.

This project will focus on the development of nonlinear control methods for grid-tied inverters that are based on a strong mathematical background and guarantee nonlinear closed-loop stability. These methods are required to: a) have simple structure, b) be independent of the system parameters and c) fulfil the technical requirements of the grid. Hardware-in-the-loop and experimental implementation of the developed methods are essential to verify the theoretical development.