Primary and Secondary Control on Autonomous Microgrid operation

Abstract

The energy industry sector is moving into an era where electrical grids are more intelligent and flexible. However, production of energy from large number of small geographically distributed sources is the main trend now. Microgrids realize the aforementioned trends. They comprise different technologies such as power electronic converters, distributed generation, energy storage systems, and communications. These new power generation paradigms rely much on their control strategies for proper operation. Hierarchal control strategy is employed, in which control functions are divided into three levels: Primary, Secondary, and Tertiary. Microgrids are operated either in grid connected mode or in stand-alone (Islanded) mode. In grid connected mode, most of the system-level dynamics and microgrid frequency are dictated by the main grid. The control objective of the microgrid in this operational mode is that DG units residing in the microgrid share the active and reactive power injected into the main grid according optimal operation conditions, thereby, the microgrid works as a single point providing active power and ancillary services to the grid. That is the role of the tertiary control. In stand-alone mode, the system dynamics are dictated by DG units, by their interaction, and by the network itself. In stand-alone mode, the most critical control objective is to regulate micro-grid voltage magnitude and frequency and the system remains intact operating stably. This is achieved by sharing active and reactive power among different DG units through primary control. In this thesis, a hierarchal control strategy for ac microgrid is presented. First, a mathematical model and a voltage frequency control scheme for inverter interfacing grid forming DG unit is introduced. Hence, the DG unit can operate in isolated mode. The introduced control scheme utilizes the well understood dq-frame current control scheme that is used with inverter interfacing grid following DG units. Minimum modifications are made to the control software of the grid following units to convert them to grid forming units. Hence, allowing grid isolated operation of the DG with black-start capability. The introduced control scheme is characterized by fast disturbance rejection, start-up transients, and mitigation of dynamic coupling between control loops and load dynamics. Second, conventional static droop characteristics are built on top of the voltage frequency control scheme to establish the primary control strategy which permits sharing of active and reactive power among DG units in parallel operation during isolation mode.