| Micro grid is a small distribution power system, which integrates distributed generation, load, energy storage device, inverter and monitoring protection devices. With the development and popularization of micro grid technology, permeability of micro grid in the distribution network is improved gradually. Whether it is to meet the needs of micro grid operation, or need access to the distribution network, or need for distribution network to support, need to control strategy and corresponding devices. This paper start from storage system and solid-state transfer switch which are set in the Point of Common Coupling of the micro grid and the distribution network, proposed and analyzed the control strategy of the micro grid flexible access into distribution network. The strategy is based on the safe and stable operation of the microgrid, which is a flexible, friendly, positive way access to distribution network. This paper also designed the hardware and software of the flexible access-integration-control device, then produced a prototype to verify the strategy by experiments. The main contents of this paper are as follows.First of all,the research background of the topic, as well as the research status of micro grid’s flexible accessing technology to the distribution net is introduced, which illustrates the necessity for researching on a set of control methods and corresponding devices to manage the interaction between micro grid and the distribution net.In addition, the engineering application background is also introduced.Secondly, the essence of the control strategy for micro gird to flexibly access to the distribution net and the basis to realize the strategy are elaborated. During this part, the whole framework of the strategy is systematically introduced. Then the design method and control strategy of energy storage system and the solid state switch, which are the two executors of the flexible accessing strategy, are anaylized. Afterwards, the simulation model is established in PSCAD to verify the correctness of the strategy on either the grid-connected or islanding operation state.And then,for three different operation states of micro grid, integrated control strategies of the energy storage system and the solid state switch at the point of common coupling(PCC) are analyzed. The strategies include:(1)against to power fluctuation and power quality problems at grid-connected state, PCC power flow control methods and compound control methods of power quality are researched, and the evaluation method of voltage limit violation of the distribution net and method of micro grid-support-distribution network are raised. Methods and strategies above have been validated through simulations. (2)aimed at problems such as voltage oscillation and impact caused by abrupt changes of PCC power, which are probably occurred during the transition from grid-connected to islanding state, stability control methods based on variable parameter PI control and planed grid-disconnecting strategies based on least disturbance are put forward; (3)specific to the impact during the transition from islanding to grid-connected state, active pre-synchronization strategy is proposed;Finally, according to functional requirements put forward by the flexible accessing strategy, a controlling integration prototype for micro grid to flexibly access to the distribution net is designed and made, which consists of the main circuit, the master controller based on DSP-FPGA framework,the sampling circuit and the equipment cabinet. The disigning scheme considers the most perfert functional requirements and the need for function extension of the master controller in the future.The prototype is applied in a micro grid laboratory of an enterprise located in Jiangning District of Nanjing, and some of its funcions have been verified through experiments. |
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