Research On Microgrid Inverter And Coordinated Control Strategies Of Multiinverters

Microgrid is an effective approach to integrate the advantages of all kinds of distributed resources, to take less impact on distribution grid and improve the economic profits of distributed generations. It has become a hot topic and important direction in distributed generation area. Research on microgrid inverter and coordinated control of multiinverter was focus on in this paper. According to the behavior and control strategy of synchronous generator and control structure of utility, the mathematic model of microgrid inverter, controllers for different operation modes and hierarchical control structure of microgrid were established. The main content of this paper are as follow:First of all, a survey of the background, concept and development at home and broad of microgrid were provided; Control structures of microgrid and operation strategies of inverters were summarized based on many experimental platforms and projects in many countries.Secondly, a new kind of unified microgrid inverter was proposed—virtual synchronous generator (VSG in short); fundamental theory and implemental method of VSG were expatiated. Instantaneous reactive power theory and quasi-resonant filter were adopted to imitate the synchronous reactance; Equivalent output impedance, external characteristic, dynamic performance and power angle feature were compared relatively between synchronous generator and VSGThirdly, a digital governor and an excitation controller, extracting from the essence of governor and excitation controller of synchronous generator, were designed to embody as same power-frequency characteristic and voltage regulating characteristic as synchronous generator set. When PQ control of VSG operated in grid-connected mode, stability, dynamic performance and anti-disturbance characteristic of the active power control system were analyzed in detail relatively. A reactive power controller composing of a feedforward loop and a close loop was proposed to achieve the aim of no steady error of reactive power fast and accurately. VSG often switches in and off from grid according to the economic dispatching, so a frequency controller and voltage controller of VSG for independent operation and a resynchronous unit from independent operation to grid-connected mode were designed.Fourthly, a hierarchical control structure suitable for microgrid was presented according to the hierarchical control of utility; the possibility of employing the hierarchical frequency regulation of utility was described. Both the first frequency regulation and the second frequency regulation were focus on. Economic dispatching had a significant impact on the power distribution among the microgrid inverters, so a governor with variable frequency regulation coefficient was proposed. In this way, more generation capacity of inverters would participate in the first frequency regulation of microgrid; the second frequency regulation both in decentralized and centralized way were compared and analyzed. The reason why the decentralized way was unpractical was revealed.Then, the hierarchical voltage control was researched. A new excitation controller with adjustable voltage regulation coefficient was developed to satisfy the switching requirement between parallel control in islanded mode and reactive power control in grid-connected mode. A new online algorithm of reference voltage for the first voltage controller of VSG was proposed to solve the problem of distribution accuracy of reactive power and voltage drop; the second voltage controller of microgrid was designed. This new algorithm of the first voltage controller and hierarchical control of microgrid in islanded operation were testified on Matlab/simulink platform.Finally, the framework of microgrid based on experimental platform was introduced. The hardware scheme and software flow chart of the experimental prototype of microgrid inverter were provided. Many experiments of single inverter, such as the effect of inertia time constant, independent operation, PQ control, and two inverters in parallel control including voltage/frequency restoration were verified on experimental prototype of microgrid inverter.