Island Operation Control And Stability Analysis Of Microgrid System Based On Mmc Half-bridge Series Structure

In recent years,the growth in energy demand and the increasing of environmental protection have promoted the development of distributed generation,which has become an important driving force for the development of power system.In order to overcome the adverse impact of distributed generation on power grid,take advantage of its flexibility and easy control,and improve power supply reliability and quality,scholars put forward the concept of microgrid(MG)at the beginning of the 21 st century.MG is a power generation and distribution system which assembles distributed generation,energy storagesystem and load.It has high output power quality,flexible operation mode and high penetration rate of renewable energy.Under the background of multi/energy complementary distributed generation,better structure and performance of MG are required.Since the development of MG,there are mainly three types of topological structure: AC,DC and AC/DC hybrid.There are some problems to be solved in different degrees in each grid structure.A MG based on the half bridge series structure of modular multilevel converter(MMC-MG)is constructed in this dissertation which combined with the advantage of modular multilevel converter(MMC).The wind turbine,photovoltaic,microgas turbine and other microsources(MS)are connected to the DC side of its sub-module.The island operation output voltage control,microsource power coordinated control,phase to phase power balance control and small signal stability of the MMC-MG are studied,and the major work can be summarized as follows:1)The topology of MMC half bridge series connected microgrid is introduced.Based on the system equivalent circuit of islanding operation,the mathematical model of system output voltage is established.On this basis,the characteristics of system output voltage,frequency and circulating current are analyzed in detail.Finally,the corresponding solutions are proposed for the problems of output voltage fluctuation,power balance and microsource output power coordination.2)For the fluctuation of DC link voltage caused by random MS output power,a hybrid energy storage device is used to suppress the voltage fluctuation.In view of the characteristics of high system coupling and strong nonlinearity,the feedback linearization is used for accurate decoupling.Sliding mode control law is designed for the decoupled system to realize output voltage stability control.Considering the system output voltage characteristics,voltage fluctuation compensation is added according to the system structure characteristics to reduce the influence of MS voltage fluctuation on the system output voltage.3)A coordinated control method of MS output power based on adaptive variable carrier cascade modulation is proposed.The output power of each carrier is analyzed,and its average power is derived according to the input time within a cycle.Combined with the voltage fluctuation standard,the low-frequency components of the output power of random MS are extracted by variational mode decomposition as the effective output power of each MS.The equivalent duty cycle of each generation unit is adjusted to realize the output power adaptive control according to the effective power.4)In order to solve the problem of three-phase power imbalance caused by the output power difference of MS in different phases,a phase to phase power balance control strategy is proposed.Three kinds of phase to phase power transmission modes of islanding system are analyzed,and the mathematical model of three-phase output power is established.The mechanism of DC circulating current to achieve interphase power flow is described.The DC circulating current controller is designed and its output signal is used as the power coordination control signal,which superimposed on the modulation wave of each arm to achieve phase to phase power control.The DC circulating current reference value is obtained through the output power and load power of each phase MS,and compensate it according to the state of charge of the energy storage device of each phase power generation unit to realize the system phase to phase power balance control.5)Based on the dynamic phasor method,the state space model of system inverter is established,and the state space model of islanding operation system is obtained by combining the state space model of control system and line.The accuracy of the state space model is verified by comparing the simulation results of electromagnetic transient model.The small signal model is obtained by linearizing the state space model of the system.Based on this model,the influence of the changes of system line parameters and voltage and current double closed loop control parameters on the small signal stability of the system is analyzed.6)The experimental platform of three-phase MMC half bridge series structure microgrid is built,and the system output characteristics,MS power coordination control strategy,phase to phase power balance control strategy are verified.The experimental results are consistent with the simulation results,which verify the correctness of the theoretical analysis results and the effectiveness of the proposed control strategy.