Research On Load Virtual Synchronous Machine Based On Silicon Carbide Power Devices

Typical controllable loads such as electric vehicles and variable frequency air conditioning in power grid are connected with power grid through power electronic interface.If the power electronic interface can simulate the mechanical inertia and the external characteristics of synchronous motor,the frequency and voltage stability of power grid can be improved.This technology is called Virtual Synchronous Motor technology or Load Virtual Synchronous Machine technology.As a link between load and power grid,converter not only needs improvement of control strategy,but also the material it used directly affects the operation efficiency of equipment.Among the new power device materials,SiC materials have attracted much attention due to their excellent properties.The static and dynamic characteristics of SiC power module and Si IBGT module are analyzed,and a three-phase PWM rectifier loss model is established.The back-to-back module of SiC and Si materials is built and the efficiency of the whole machine is tested.The synchronization mechanism of three-phase PWM rectifier is analyzed,which provides the basis for the feasibility of introducing virtual synchronization control.The power loop is designed to simulate the inertia,electromagnetic and voltage regulation characteristics of synchronous motor.Use the output of DC voltage loop as mechanical power,which connects active power control loop,and stabilizes DC voltage.Power of low voltage side of the DC/DC is controlled.Add the power command responsed to the frequency of the power grid,to provide effective support for the power grid.Virtual inertia is also introduced in the control of DC/AC interface.At the same time,the output voltage frequency is adjusted according to the frequency change of power grid,so as to carry out active power response.In view of the unbalanced voltage of power grid,the virtual synchronization control is improved to separate the positive and negative sequence of unbalanced voltage of power grid.The structure of the original balance control strategy is unchanged,and the positive sequence component control is adopted,while the negative sequence current loop is added.Different negative sequence current instructions are designed according to the different control objectives of AC side current and DC voltage.The simulation of two virtual synchronizers for DC and AC loads is carried out.A back-to-back system made of SiC material is built,and a bidirectional DC/DC converter is connected to its DC bus.The characteristics of the device in DC loads,AC loads,power response and unbalanced voltage are tested,and the correctness of the control method is verified.