Research On The Parallel Technique Of Three Phase Microgrid Inverter With Droop Control

Construction of regional microgrid can effectively solve the problem of power transmission caused by the random distribution of micro source, improve the overall utilization rate, and promote the development and utilization of new energy sources. At present, the microgrid system mainly includes the master-slave structure and peer-to-peer structure. In the master-slave structure, a huge power supply capacity, such as an energy storage inverter is needed, to provide the bus voltage and frequency for the system. The system depends on the master inverter and lack of redundant capacity. In peer-to-peer structure, a variety of micro sources parallel connected by inverter modules based on droop control. These modules are independent of each other, which can be removed while working and implemented with redundancy. While, there are still some problems such as low power sharing accuracy and coupling power in the conventional droop control strategy. In order to compensate for the droop control deficiencies, improving the overall performance of the system is of great practical significance. Therefore, this paper focuses on the research of control strategy and basic characteristics of inverters in peer-to-peer structure.Firstly, a small signal model of the three-phase full bridge inverter topology based on droop control is established by dynamic vector theory. Through the analysis of droop control loop we deduce the droop coefficients range to maintain the stable operation of the system and give the optimization design method. On the basis of this, further research of the main disadvantages of conventional droop control was carried out, and derived the mechanism of power sharing accuracy.In view of the problems existing in the conventional droop control, this paper puts forward a kind of improved droop control strategy, through the introduction of AC bus voltage compensation, the improved droop control basic principle and the stability of the system, this paper gives a parameter selection basis to balance compensation affection and external characteristic of inverters. In addition, the reliability of the theoretical analysis is verified by simulation and experiment.In practical application, the connection inductance is usually used to ensure the consistency of the line impedance, resulting in the volume and cost increasing. In order to replace the connection inductance, the virtual impedance is constructed based on the SOGI method. And the SOGI virtual inductance implementation avoids the time derivative operation of the output current. Based on the characteristic of virtual impedance, this paper proposes an adaptive virtual impedance strategy to reduce the difference of the equivalent connection impedance and improve the performance of the system. Simulation results show the effectiveness of the proposed control strategy.Finally, the design scheme of the experimental platform of the droop parallel system is given; including the design of the hardware circuit, the realization of the control logic and the design of the power filter.On the basis of this, an experimental prototype of two 15 kVA three phase inverters is built.