Presynchronous Grid-Connection Strategy of Virtual Synchronous Generator Based on Virtual Impedance

Guanfeng Zhang,Haixin Wang,Jia Cui,Junyou Yang,Kai Wang

doi:10.1155/2020/3690564

Abstract

The virtual synchronous generator (VSG) technology of inverter is widely used to provide the inertia and damping support for power system. However, an additional measurement device PLL (phase-locked loop) is required in the virtual synchronous generator grid connection to track the voltage phase, amplitude, and frequency, which restricts the flexible output of the distributed power generation system. To tackle this challenge, a method for grid-connected control of virtual synchronous generator based on virtual impedance is proposed. It is assumed that there is a virtual power exchange between the synchronous machine and the power grid when the virtual synchronous generator is off-grid, the virtual impedance is developed to calculate the virtual current, and when the virtual current is zero, the output voltage of the VSG can be synchronized with the voltage of the power grid, thereby seamlessly switching between off-grid and grid-connected VSG. A semiphysical simulation platform is built based on RT-LAB; simulation and experimental results show that the proposed grid synchronization control strategy of the VSG can achieve seamless transform between different VSG modes, which is simpler than the conventional synchronization control, while having a good active and reactive power tracing performance.

Highlights

With the rapid economic development, the problems of energy crisis and environmental pollution have become increasingly prominent
A large number of intermittent and random distributed power sources are connected to the grid, which greatly increases the complexity and difficulty of management and control of the grid and has a major impact on the safe, reliable, and economic operation of the grid. e existing distributed power generation system uses power electronic devices to be integrated into the grid, which is more flexible than traditional power generation systems
If the virtual synchronous generator is equivalent to a voltage source, the equivalent circuit of single-phase virtual synchronous generator grid connection system is shown in Figure 1, where e is the excitation voltage of a virtual synchronous generator, E is the virtual synchronous generator excitation voltage amplitude, i is the output current of virtual synchronous generator, XS is the line impedance, ug is the grid voltage, Ug is the amplitude of grid voltage, αg is the phase angle of the grid voltage, and α is the phase angle of the excitation voltage of VSG

Summary

Introduction

With the rapid economic development, the problems of energy crisis and environmental pollution have become increasingly prominent. Ese research results have the same basic principles at the level of the inverter-synchronous generator control strategy, the difference lies in the application It can operate stably in both off-grid and grid-connected operation modes, and switch of operation mode can be realized without changing the controller structure. An improved virtual synchronous generator grid-connection method is proposed to solve the problem that the extra measuring device in the traditional gridconnection restricts the flexible output of the distributed generation system, avoid the problems such as nonlinear, slow response and difficult parameter design of the PLL, and realize the more concise and effective VSG smooth grid connection; a semiphysical verification method is given.

Equivalent Model of Virtual Synchronous Generator On-Grid

Simulation Analysis

Experimental Verification

Conclusion