Research On Control Strategy Of Low-Voltage Microgrid In Islanded Mode

It has become one of the most difficult problems faced by today's electric power industry how to combine new energy power generation and control with existing technology and economic structure. With the development and maturity of the infrastructure and control technology of microgrid, this problem is gradually being solved. The flexible controls of multiple distributed generations strengthen its reliability and power quality, in the meantime eliminate the impact of unstable power supply to our grids. The main goals to ensure stable and efficient operation of microgrid are frequency and voltage regulation and power control. Microgrid can operate in grid-connected and islanded modes. When in islanded mode, droop control is adopted. Based on the relationship between active power and frequency, reactive power and voltage, inverters can control the output to realize proper power sharing.This paper gives a brief introduction about the occurrence and operating characteristics of microgrid islanded mode. Frequency and voltage droop control relationships are deduced and multi-loop feedback regulation based droop control structure is designed according to the analysis of microgrid inverter parallel system architecture. Among which inner voltage and current control loops and external real and reactive power control are emphatically studied. Sinusoidal Pulse Width Modulation is adopted to generate on-off signals of DG inverter. At last, droop control module of microgrid islanded mode is built in Matlab/Simulink according to the design of main circuit and control structure.Due to the line impedance impact on islanded low-voltage microgrids, traditional droop control cannot guarantee the proper power sharing between distributed generation units. In the meanwhile, droop control would cause voltage and frequency deviation. Small-signal modeling is taken in this paper to analyse the impact of line impedance angle ?, reactance X, droop slope and filter's cut-off frequencyc? to system stability. The traditional droop control can not achieve a reasonable distribution of power, because of the large R/X ratio of short-range and low-voltage transmission line. A redesigned virtual impedance loop is added to improve the reactive power coordination sharing problem. The simulation results indicate the validity of the proposed control strategy.By the end of the paper, an enhanced droop control strategy with frequency autonomous restorable regulation is proposed to eliminate the voltage and frequency deviation of traditional droop control strategy. Reactive power sharing problem is solved by adding a redesigned voltage compensation loop, which ensures PCC voltage in nominal range. To the problem of frequency deviation, autonomous frequency restoration loop is added. Small signal model of frequency control is established to analyze the stability. In circumstance of frequent load switching, the frequency can properly restore to nominal value, with smooth and stable transient process. The simulation results show a good performance of the proposed control strategy.