Design And Simulation Of Three Phase Inverter Based On The Technique Of Virtual Synchronous Generator

The increasing proportion of distributed generation is one of the vital features of modern electric power system. Additionally, the inverter, a significant power electronic interface that connects distributed generators and large power grids, possesses a variety of control ways. Therefore, simulating the characteristics of synchronous generator(SG) while controlling the inverters is one of the mainstream methods of the inverter controller design. The advantage of this approach is that all power generation unit could be unified as SG model on the generating side. Thereby the total inertia of the power system increases and the control of power system is simplified based on the the existing theoretical knowledge. Such kind of inverter is referred to as virtual synchronous generator(VSG).In this paper, a new kind of VSG is presented. It has virtual governor, virtual exciter, rotor equation and electromagnetic transient equations, thus the mathematical model of VSG and SG are highly consistent. According to the simulating results, this VSG controller has similar characteristics compared to SG with same parameters.In this paper, the research objectives and design are presented in chapter one, in which the development direction of modern power systems and research status of micro-grid and distributed generation have been elaborated, the typical structure of the inverters and four traditional control methods(constant current control, constant power control, constant voltage&frequency control and droop control) were demonstrated, and two VSG controller design ideas(current type and voltage type) as well as their advantages and disadvantages were illustrated.The mathematical model of the generator and listed some important equations are analyzed in chapter two. Moreover, the corresponding virtual governor, virtual exciter and other control unit in VSG controller were constructed through imitating mathematical model and the control mode of SG, which made the control model of the inverter and SG tend to be highly consistent. Relevantly, two PI link were added in terminal voltage controller so that the characteristic of inverter and SG with similar parameters became remarkably similar. Indeed, the selection measures of key parameters were also illustrated, including inertia, and torque and the calculation of electromagnetic torque, etc. And spectacularly, the analysis on phase locking loop and islanding detection principle which are two important aspects in the inverter operation have been displayed.Chapter three was divided into four parts, which thoroughly investigated the inverter prototype design. The first part introduced the design method of output AC filter, DC support unit, driver and snubber circuit, all of which contained high-power signal path. The second part was on the signals that were below 12 V, which mainly analyzed the working principle and design of the core board, bottom board, adapter board and sample board. The third part described the software design to implement the VSG control strategy. Finally the fourth part introduced the design of the human-computer interaction port, which is combined of a display board and a communication port.In chapter four the steady and dynamic characteristics when the inverter works in off-grid mode through simulation is discussed and the difference between the controller with the PI link and without the PI link when the inverter works in on-grid mode is analyzed, thus the importance of PI link in the controller design of VSG to imitate the SG is proved. In the end, physical prototype design in chapter three is demonstrated.