Research On The Control Strategies Of Islanded And Grid-Connected Microgrid Based On Distributed Generators

A microgrid is a system that can combine distributed generator(DG),line,control device,load and protection device to form a single controllable unit.A microgrid can operate in both grid-connected and islanded mode.Compared to the traditional grid,the microgrid,the microgrid reveals obvious advantages,one of which is that it can accept new energy sources,and effectively improving the efficiency and reliability of power system.However,the microgrid has the characteristics of small inertia,strong randomness and strong fluctuation of DGs.Therefore,the microgrid depends on communication networks to achieve good control effects when it operates in islanded mode,which greatly increases the cost and complexity of the control system.Among the traditional control modes of the microgrid,the centralized control mode and distributed control mode can not get rid of the constraints of the communication.The decentralized control mode can be realized without communication,but it is difficult to achieve the function of frequency modulation and economic dispatch.In addition,the fluctuations of DGs and random load in microgrid can be stabilized by the main power grid when the microgrid operates in grid-connected mode.And the big interference in main power grid also will affect the stability of microgrid.So it is important to analyze how to bring beneficial effects to the system when microgrid operates in grid-connected mode.This paper does some research around the issues mentioned above and completes some works as follows:1.The self-optimizing control strategy based on the decentralized idea for F-P type DGsIn the islanded microgrid,the frequency-power droop type(F-P type)DGs can be called current controlled voltage source DGs in terms of the active power control.This paper proposes a decentralized self-optimized active power control strategy without microgrid centralized controller(MGCC)and communication for these DGs.The proposed control strategy can be divided into two parts of the target frequency control and active power command control.The target frequency control calculates the target frequency and makes DGs share loads according to the equal increment principle(EIP).The active power command control makes the actual frequency track the target frequency and continuously modifies the active power reference value of the inverter.The control strategy not only makes the system maintain good static and dynamic performance,but also realizes the economic dispatch in the microgrid.2.Research on stability of the grid-connected microgrid and distribution network with hybrid energy storage systemThe hybrid energy storage system consists of the storage battery and super capacitor model.The model and the corresponding control strategy can maintain the stability of bus voltage,and speed up the frequency recovery after a large disturbance.In addition,the microgrid operates stably in the grid-connected mode with a large scale of hybrid energy storage systems.The microgrid can provide voltage and frequency support when the point of common coupling is short-circuited,which can weaken the adverse effects of a large disturbance.The proposed self-optimizing control strategy in this paper can get rid of the communication and make DGs have complete plug&play of characteristics,which improves the flexibility and self-organization of microgrid and has a good application prospect in future.And the research on stability of the grid-connected microgrid can be a good reference.The research take advantage of microgrid,and give a new idea to improve the reliability of power supply in the regional distribution network.