Research On Control Strategy For Microgrid Inverter With Droop Control

A systematic organization of distributed generation systems forms a microgrid which make full use of renewable energy for the local load power supply or in parallel with the grid operation. So it can improve the security and reliability of power system and enhance the ability to resist natural disasters of the grid which are of great practical significance for the grid and even the entire national security. The inverters are key interfaced to the grid or load through power electronics and energy storage systems.Firstly, the stability of he three phase inverter parallel system based on the droop control method is chosen to be researched. The small signal model of the droop control method is carried out in this paper. On this basis, the stability of the active power frequency system and the reactive power amplitude system are analyzed in depth concluding that the stability and dynamic of the system is mainly depend on the droop codfficients and the low power filter design. Furthermore, optimum design method of the droop coefficients and low power filter is given. In addition, simulation and experimental platform are carried out to theoretical analysis and experimental verification.Then, considering the line impedance of the microgrid is not mainly inductive the virtual impedance using SOGI algorithm is given in this paper. The virtual impedance using SOGI algorithm are proposed to make the line impedance inductive and to ensure power-sharing stability with less actual inductance. The virtual inductance implementation avoids the time derivative operation of the output current, which reduces the harmonic of the virtual voltage and it is not sensitive to noise.This paper deduces the implementation method of virtual impedance based on SOGI in DQ rotating coordinate system. In addition,the design method of inverter line impedance is given and variable virtual impedance is forward to reduce the impact current for inverters parallel system.Finally, a new hybrid control strategy was proposed for microgrid(MG) system with master-slave structure. In grid-connected operational mode, the droop control strategy is adopted for the main converter for the case of non-intention islanding, while in stand-alone operational mode the droop gain would decrease to zero, becoming a conventional v/f control to make sure the local AC bus is more rigidly with the overall performance greatly improved. Control sequence for dynamic transferring between droop control and v/f control was also proposed, ensuring the hybrid control to be practical. Furthermore, a new pre-synchronization method is proposed for droop control strategy. Both simulation and experimental verifications were carried out, the results show that the seamless transfer between grid-tied and stand-alone operational mode can be easily realized.