Research On Voltage And Frequency Hierarchical Control Strategy Of Low-voltage Microgrid

With the large number of distributed generations(DGs)accessed in the power system,it has a great negative impact on the voltage quality and power supply reliability of the power system.In order to solve the access problem of the distributed generation and improve the comprehensive utilization of energy,the microgrid(MG)was proposed and received extensive attention.However,in a large-capacity low-voltage MG,the line impedance is generally resistive,and the line impedance between the DGs is different,resulting in the traditional droop control cannot achieve the decoupling control of active,reactive power and the reactive power sharing,and even leading to a serious reactive circulation between DGs.Moreover,it cannot solve the effect of uncertain time delay on system stability by using PI control in the control system.Therefore,this thesis focuses on the voltage and frequency stability control,the power sharing problem of low-voltage microgrid.The main research works are as follows:1)A voltage and frequency hierarchical control system for islanded microgrid is established,including primary control and secondary control.Primary control includes traditional droop control,virtual impedance loops and Quasi-PR voltage and current controllers.In order to make the droop control applicable to resistive circuit and suppress the reactive current effectively,a virtual negative impedance control loop of positive sequence current is designed in the primary control system to avoid the decrease of output voltage due to the voltage drop generated by the negative sequence current on the virtual impedance,while the decoupled control of the active and reactive power can be realized.The secondary control adopts different algorithms to design the controllers to generate corresponding voltage,frequency and power compensation signals,which are sent to the primary control to adjust the deviations.2)In order to solve the limitation of the influence of the unknown communication time delays on the system frequency by using the traditional PI control to design the secondary control,a frequency secondary control strategy based on the model predictive control method is proposed.In this control method,the stability of the designed control system is analyzed by establishing a small signal model and participating factor analysis,and the proposed control method is verified by a simulation model.The simulation results show the proposed control method has strong robustness to uncertain time delays in the system.3)To solve the problems of voltage,frequency deviations and reactive power sharing in islanded microgrid system,a distributed hierarchical cooperative control method is proposed.In this method,discrete distributed consensus algorithm is used to design the state observer to obtain the global average value,and then the global consistency regulates voltage,frequency,and reactive power deviations to realize the stable control of voltage,frequency and the average sharing of reactive power.Finally,through the simulation model of Matlab/Simulink and the semi-physical simulation experiment platform of MG based on d SPACE,the control strategies proposed in this paper are verified by simulation experiments.Through analysis of simulation and experimental results,we can see that the the dynamic performance of the distributed secondary control system is better.Simultaneously,it has a strong robustness to time delay,and the DGs can be “plug and play”.Simulation and experiment verify the effectiveness and feasibility of the proposed control method.