Research On Control And Energy Management Strategy Of Micro-grid Composed Of Multi Voltage Source Type Micro-sources

At present, the distributed renewable energy generation has drawn more and more attention worldwide. However, the integration of large scale distributed generation will dramatically influence the operation, control, protection and dispatch of the power grid, especially the distribution network. The emergence of micro-grid provides new approach to utilize distributed generation. Micro-grid can effectively coordinate system energy allocation so as to achieve the integration and complementation of the distributed generation system and the traditional power system, which will provide a broad space for the development and utilization of distributed renewable energy.Currently, micro-grid is still in the stage of laboratory research and small-scale demonstration application. Most of the established micro-grid demonstration projects are master-slave controlled small scale micro-grids in which a single energy storage unit or conventional generator is the power balancing device. With the development of economy and society, the demand of micro-grid continues growing and the scale of micro-grid continues to expand especially the power supply capacity of islanded micro-grid in remote areas reaches the order of MW, in which the energy and power sources are various. Due to the restrictions such as the capacity of single micro-source converter, the application of master-slave control in large scale multi energy complementary micro-grid has some limitations. Therefore, the parallel operation of multi power balancing micro-sources in micro-grid should be studied and solved.The control and energy management technology of micro-grid composed of multi voltage source type micro-sources are studied in this dissertation, which includes the control strategy of dispatchable micro-source based on hybrid energy storage system, the improved mode adaptive droop control of micro-source converter adapting for multi operation modes of micro-grid, the hierarchical control of micro-grid and the energy management technology for islanded micro-grid. The main contents are as follows:(1) A control strategy of dispatchable distributed resources comprehensively considering the state of charge (SOC) of various energy storage devices and the lifespan of batteries is presented in which the battery/supercapacitor hybrid energy storage system and renewable energy generation system are connected in parallel using dc bus. According to whether the SOCs of energy storage devices are within the limits, low pass filtering algorithm with compensating coefficient and a novel mode switching control based on different voltage zones of dc bus are presented respectively. To avoid the transient shock brought by frequent switching between various control modes, the pre-control of supercapacitor terminal voltage is proposed to prevent the impact of frequent switching on the lifetime of batteries. Through above control strategy, the operation characteristics of distributed resource are improved.(2) An improved mode adaptive droop control of micro-source converter with virtual complex impedance is presented. The control principle of virtual impedance method to change line impedance characteristics is analyzed and a design method of virtual complex impedance in dq rotating coordinate system is proposed. Then, an improved mode adaptive droop control of micro-source converter with virtual complex impedance is presented, which can realize stable operation of micro-grid both in islanded and grid-connected modes and the smooth transition between two modes.(3) In order to solve the hard problem of reasonably sharing of reactive power and ensuring the power quality in a micro-grid composed of multi droop controlled micro-sources, a novel hierarchical control strategy of micro-grid is proposed according to different time scales. In short time scale, larger droop gains in primary control are adopted to improve power sharing accuracy. In larger time scale of secondary control, the no load frequency and voltage set points of droop characteristics are adjusted to improve power quality. A digital physical hybrid simulation platform for micro-grid based on NI-PXI (PCI Extensions for Instrumentation, PXI) is established and the hierarchical control technology of micro-grid is verified on the platform.(4) In the islanded micro-grid with wind, solar, diesel, and energy storage generation systems composed of multi voltage source type micro-sources, a multi time scale coordinated controlled energy management strategy is proposed which includes three stages. In day-ahead unit commitment optimization stage, the whole life cycle convert cost, power regulating margin of energy storage devices and demand response (or demand side management) model are taken into consideration to reasonably plan the unit commitment of every components. In with-in day economic dispatch stage, an online rolling energy management strategy based on model predictive control (MPC) is proposed to revise day-ahead optimal dispatching results. In real-time scheduling adjustment stage, some measures are taken to ensure the power regulating margin of energy storage devices. Through the cooperation of three stages, the safe and economic operation of micro-grid is realized.