Research On Optimal Control Strategy Of Three-Phase Inverters In Microgrid

Microgrid is a small distribution power system, which integrates distributed generation, load, energy storage device, inverter and monitoring protection devices. It is also an autonomous system which can control, protect and manage itself and operate in grid-connected mode or in intentional islanded mode. The control strategy of three-phase inverters which act as the power electronic converters of many controllable microsources such as energy storage device and uninterruptible power supply (UPS) is particularly important for the operation of microgrid. This paper has analyzed the operation modes, topological structures and control characters of three-phase inverters, on basis of which the corresponding optimal control strategies of three-phase inverters to the different operation modes of microgrid are proposed and then the rationality and validity of the proposed control strategies is analyzed in-depth. The main contents of this paper are as follows.First of all, the research background of the topic, as well as the prospect of microgrid is introduced. The research status of microgrid is introduced in three major areas:the system control of microgrid, the control of distributed generations (DGs) and the operation of microgrid and then illustrates that the reliable and flexible control strategies are necessary for the stable operation of microgrid. In addition, the engineering application background is also introduced.Secondly, the topological structure of microgrid is detailed, on basis of which the types and the corresponding structure and control characters of DGs’inverters are introduced systematically. Then the existing and mature control strategies of inverters are analyzed from two aspects:the outer loop control and the inner loop control. The theoretical basis of the control strategies and their topological structures are given in this paper.Then, the optimal control strategies of inverters are designed separately when microgrid operates in grid-connected mode and islanded mode. The proposed strategies include:(1) analyze the control characters of inverters when microgrid operates in different modes and build the relevant mathematic models and select the appropriate filter parameters on the consideration of relevant constraint condition such as resonant frequency, total inductance value and the harmonic suppression. (2) the optimal control strategies of three-phase inverters under stationary frame (αβ coordinates) are proposed on the basis analyzing the drawback of that under rotating frame (dqO coordinates). The proportional resonant (PR) and quasi proportional resonant (QPR) are introduced in the proposed control strategies to eliminate the static error of signal control at specific frequency points. Then the load current observer (LCO) and grid voltage observer (GVO) based on the Luenberger observe theory are introduced to estimate the load current in islanded mode and the grid voltage in grid-connected mode precisely. The estimated state variables act as the feedforward of the proposed dual-loop control strategies, which can effectively enlarge the stability margin of the control system and improve the dynamic response to load disturbance and grid uncertainty. (3) the parameters of PR and QPR regulators are analyzed and selected through the root locus method and the stability of the whole control system is evaluated and analyzed.At last, aiming to test and verify the research content of this paper, a kind of MATLAB/Simulink simulation platform was constructed and an experiment platform based on DSP TMS28335 was established. The results of simulation and experiment verified the correctness of the mathematical model established in this paper and the effectiveness of the control method proposed in this paper.