Study On The Stable Operation Control Of Microgrid Inverter Under Unbalanced Working Conditions

Driven by the dual carbon target,the share of renewable energy distributed generation in the grid is increasing.Microgrid as a carrier of distributed micro-sources,distributed power sources can be made grid-friendly through suitable microgrid inverter control strategies.Most of the distributed micro-sources in micro-grid are connected to the grid through power electronic interface converters,but there are problems of inertia and insufficient damping,and a large number of grid connections will affect the stable operation of the system.Virtual Synchronous Generator(VSG)technology has been widely studied as a microgrid inverter control technology that can provide inertia and damping to the system.However,unbalanced operating conditions such as unbalanced load access during off-grid operation and unbalanced grid voltage during gridconnected operation can affect the stable operation of microgrid inverters based on VSG control.Therefore,this paper focuses on the study of load imbalance under offgrid and voltage imbalance under grid-connected.The details are as follows:First,the structure of VSG-controlled microgrid inverter system is introduced,and the VSG-controlled active loop,reactive loop and two kinds of underlying dual-loop control are designed.The power-frequency dynamic characteristics of the VSG in gridconnected and off-grid states are analyzed,and the simulation is verified in MATLAB/Simulink in grid-connected and off-grid states.Second,to address the problem of grid voltage imbalance during VSG grid operation,we analyze the impact of unbalanced voltage on the output of conventional VSG,and then design a balanced current control strategy based on split-sequence control to control the negative sequence component of the output current to achieve current balance.Considering the impact problem of current transition process when the grid voltage is unbalanced,we analyze the amount of fault current change and power angle stability and then improve the VSG power loop based on the balanced current control to suppress the inrush current of the transition process and improve the stable operation of VSG under the grid voltage unbalance condition.Then,for the problem of unbalanced VSG output voltage under the unbalanced load of the island,the improved VSG control strategy of adding negative sequence voltage control loop is designed to directly control the unbalanced component of the output voltage caused by the unbalanced load;at the same time,the unbalanced voltage drop of the line impedance makes the load voltage unbalanced,and the negative sequence virtual impedance is designed to compensate the unbalanced voltage drop of the line impedance to achieve the load voltage balance and improve the stable operation of the VSG under the unbalanced load condition.Finally,the VSG control system is established through the experimental platform of the group,and relevant experiments are conducted to verify the effectiveness of the VSG algorithm.