Decentralized Coordinated Control Strategy of Islanded Microgrids

Facing the challenges brought by the traditional large power system concerning the environmental and economic issues, along recent years distributed generation is considered as an alternative solution to provide clean energy in a local manner. In this context, Microgrid which performing as a local small size grid, is emerging as a promising concept to flexibly control both sides of power generation and consumption as an integral controllable unit. Regarding whether there is main grid participating in the power regulation, the operation of Microgrid can be mainly classified as two modes: grid-connected mode and islanded mode. In the islanded mode operation, since there is no energy back-up from the main grid, the coordinated control of power balance between generation and consumption encounters more challenges and this thesis is mainly focused on this scope.

Hierarchical structure can be applied on the control of Microgrid which analogic compared with the conventional frequency regulation of large power system. This thesis starts from the investigation of a coherent primary control level that can be generally implemented on distributed units to achieve global active/ reactive power distribution, as well as grid voltage/frequency regulation. In order to enhance the reliability of overall islanded Microgrid operation, basic functions of coordinated control which taking into account the state of charge (SoC) limitation and power availability of renewable energy sources is implemented in a distributed level without additional communication facility assistance within this thesis. Here two bus-signaling method (BSM) methods are proposed. The power line is then not only taken as physical connection among the distributed units, but also the signal carrier to inform Microgrid components changing their control actions. Moreover, this BSM control principle is applied on both AC and DC islanded Microgrids which utilizing bus frequency and bus voltage as signaling variables respectively.

Apart from the primary coordinated control, the distributed secondary control is introduced on the top of the coordinated control strategies in this thesis, in order to promote the decentralization of the overall system. Especially the consensus algorithm based secondary level is investigated in the thesis in order to simplify the communication configuration which only flood information through the neighboring units. The detailed developing steps for this distributed secondary control based on consensus algorithm are introduced in this thesis.