Advanced control of grid-connected converters in microgrids with enhanced performances

Abstract

This thesis presents a study on some advanced control strategies for grid-connected converters (GCC) to improve both the power quality and the power flow performances of microgrids. First, a survey on the fundamental topologies and control of GCC for both the demand side and the supply side of microgrids are conducted. At the supply side of microgrids, conventional proportional-integral (PI) control performs poorly in dynamics as the operating conditions change. Hence, a sliding mode control, which shows better dynamic tracking performance and robustness in maximizing the power transfer, is applied to a wind energy conversion system to increase the harvested energy from the wind. Then, some control strategies are designed for electric spring (ES)-based smart loads (SL), DC electric spring(DCES)-based SL, wireless power transfer (WPT) systems, and uninterrupted power supply (UPS) systems at the demand side of microgrids to overcome the issues made by the conventional control. For the ES-based SL, a voltage and frequency (V&F) control is proposed to achieve better voltage and frequency regulation performance than the conventional control. For the DCES-based SL, an enhanced digital PI control comprising a state-variable feedback loop, aimed at optimizing both the steady-state and transient performance of the SL, is proposed. Compared with the conventional PI control, faster transient performance of DCES that can rapidly tame the possible fluctuations of RES is achievable with the newly designed control. For the WPT system, a discrete sliding mode control (DSMC) scheme is presented to achieve fast maximum energy efficiency (MEE) tracking and output voltage regulation. Compared to the conventional control scheme for the series-series compensated WPT system, the proposed scheme displays better dynamic regulation of the output voltage during MEE tracking. Such an improvement prevents the load, e.g. a charging electric vehicle (EV), from suffering undesirable overshoot/undershoot during transient states. For the UPS system, an adaptive reference model predictive control (ARMPC) is proposed in response to the issue of non-negligible steady-state errors of the conventional model predictive control (MPC) when a model mismatch of the system occurs. The ARMPC renders a consistent attenuation of offsets without significant sacrifice of transient performance. All these advanced control strategies for the GCC at the demand side enhance the power quality performance of microgrids. Finally, global control methods for multiple SL are proposed. A coordinated tuning method of the local PI controllers for ES-based SL is given to ensure the stability of multiple SL in AC microgrids. The coordinated control method unveils the fact that as the number of ES in a weak AC microgrid increases, more stringent tuning-coefficient values of the local PI controllers are required. In addition, a centralized model predictive control (CMPC) scheme is validated to extend the existing functions of the DCES in the microgrid. The control scheme coordinates the DCES to mitigate distribution power loss in the DC microgrid, while simultaneously providing their original function of DC bus voltage regulation.