Research On Frequency Control Strategy Of Cascaded Microgrid In Island Mode

Because it is convenient for the access of DC distributed power,the cascaded island micro-grids with the advantages of modularization and expandability,which has broad prospects in places such as islands and schools that are far away from large power grids,however,cascaded island micro-grids face great challenges in terms of frequency stability and frequency dynamic response characteristics.In this paper,the island mode micro-grid with cascade structure is studied,focusing on the frequency control strategy of suppressing overshoot and accelerating dynamic response.The specific research contents are as follows:(1)The system modeling and frequency characteristic analysis of cascaded island micro-grids are obtained.Firstly,a mathematical model of the cascaded island micro-grids based on the control of virtual synchronous machine as the basic strategy is constructed,and its output characteristics are analyzed.Then,an appropriate approach for the voltage-current loop is selected in island mode,Finally,establishing a small-signal model of the control system,the influence of virtual inertia and damping coefficient on the frequency characteristics of the system is analyzed.(2)A control strategy combining fuzzy control and virtual synchronous machine control is proposed to solve the problem of excessive output frequency overshoot caused by active power mutation in island mode.By analyzing the variation law of the angular frequency and angular frequency deviation of the virtual synchronous machine,a fuzzy control rule table is designed to adjust the virtual inertia and damping coefficient.This strategy can effectively reduce the output frequency overshoot and speed up the frequency recovery.The simulation results further verify the effectiveness of the proposed method.(3)A secondary frequency modulation control strategy is proposed in the adjustment unit of power-frequency of the above-mentioned fuzzy virtual synchronous machine.Aiming at the frequency deviation caused by load fluctuation,using the principle of secondary frequency regulation of power system for reference,proportional-integral control is introduced to reduce the frequency deviation,and the feasibility of using this control strategy for single VSG and multiple cascaded VSG is deduced.Furthermore,the influence of integral and proportional coefficients on frequency overshoot and frequency recovery speed are analyzed,and a reasonable coefficient design is carried out to improve the dynamic response performance of frequency.The simulation results show the effectiveness of the control strategy.(4)A cascaded island micro-grids experimental platform was built,and the above frequency control method was implemented on the experimental platform.The experimental results show that the dynamic response of the frequency can be improved under the premise of the islanding mode power sharing based on the frequency control strategy proposed in this paper.