Reliability enhancement and power management of hybrid micro-grid system with centralized supervisory control scheme

This thesis presents a centralized supervisory control scheme for the reliability enhancement and power management of multiple power converters based ac-dc hybrid micro-grid system. The ac-dc hybrid micro-grid system consists of fixed speed wind turbine induction generators, photovoltaic system, multiple parallel connected power converters, battery energy storage system, utility grid, ac loads, and dc loads. Power management of the ac-dc hybrid micro-grid is performed under three cases: local mode, grid mode and islanded mode. Central supervisory unit (CSU) generates command signal to ensure the power management during different modes. In local mode and islanded mode, the dc loads in the ac-dc hybrid system can be controlled by the CSU. In case of grid mode operation, power flow between the utility grid and micro-grid is controlled. A novel feature of this thesis is the incorporation of the multiple power converters. The generated command signal from the CSU can control the operation of the multiple power converters during local, grid and islanded modes. Depending on the load consumption and generation in the micro-grid system, the power converters can be switched from single to multiple operations. Control of multiple power converters by the CSU ensures partial loading or operation of the micro-grid system in case of any failure or damage of individual units. An additional feature of this thesis is the incorporation of sodium sulfur battery energy storage system (NAS BESS) which is used to smooth the output power fluctuation of the wind farm. Real wind speed pattern is used to verify the smoothing operation of the sodium sulfur battery energy storage system. The effectiveness of the centralized supervisory unit is also verified using real load pattern. The islanding and grid reconnection operation of hybrid micro-grid system is performed with control strategy of the power converter and the power management by central supervisory unit. The control strategy of the power converters maintains the terminal voltage of the micro-grid system constant during islanded mode. The central supervisory unit performs the power management during transition from grid mode to the islanded mode and islanded mode to grid reconnection mode which keeps the micro-grid system stable and reliable. The reliability enhancement of multiple converter schemes during transient fault is also presented in thesis. During transient fault, additional power converter is turned on by the control strategy to provide necessary reactive power to withstand the severe voltage dip hence fulfilling grid code requirements. For the different cases, the reliability enhancement and power management of the multiple power converters based ac-dc hybrid micro-grid system is verified by the simulation results performed in PSCAD/EMTDC.