Research On Improving Pre-Synchronous Grid-Connected Control Of Virtual Synchronous Generator

In recent years,new energy generation technologies such as wind and light have been vigorously developed,effectively alleviating the problem of fossil energy shortage,in line with the purpose of low-carbon energy in China.The distributed generation system mainly based on new energy has the characteristics of weak immunity and high volatility,and cannot maintain its own security and stability.In order to strengthen its immunity,power electronic devices such as inverters are introduced as the power transmission carrier and control center between it and the grid.The electronic power system lacks the inertial support ability and the ability to slow down the frequency change of the traditional power grid system,but the virtual synchronous generator（VSG）can effectively solve the above problems by simulating the characteristics of the synchronous generator and making the inverter "synchronous mechanization".Based on this,this thesis uses VSG control technology to study the problems related to the grid-connected inverter.The contents are as follows:（1）This thesis analyzes the main circuit of the inverter and the control strategy of the virtual synchronous generator in detail,including VSG active ring and reactive ring design and reactive ring improvement,and improves the reactive power loop.In this thesis,the quasi-PR control is proposed,and compared with the performance of the voltage and current double closed-loop control,the LC filter parameters of the main circuit and the virtual inertia and virtual damping of VSG controller are determined,and the power output curve of VSG grid-connected process is optimized,and the stability of VSG grid-connected operation is guaranteed.（2）Considering that VSG inverter directly connected to the grid brings large impact current,this thesis analyzes the grid-connected conditions of virtual synchronous generator inverter.According to the adjustment requirements of the phase difference,amplitude difference and frequency difference between the inverter output voltage and the grid side voltage,pre-synchron ous control is adopted before VSG inverter connected to the grid.In view of the disadvantages of complex structure and slow response of pre synchronization control based on phase-locked loop（PLL）,a pre-synchronization grid connection control strategy based on virtual power is designed in this thesis,that is,assuming that there is virtual impedance at PCC,the virtual power formed on the virtual impedance is controlled to generate virtual angular frequency and v irtual amplitude,and the value is compensated to VSG active loop and reactive loop respectively,The smooth grid connection of VSG inverter is realized..（3）In this thesis,the parameter detection part of pre-synchronization control is improved.The traditional detection method of pre-synchronization is PLL.This method is closed-loop detection,including PI controller,which requires multiple comparisons to accurately capture voltage parameters,and the pre-synchronization time is long.Based on this,an improved pre-synchronization control method using open loop phase frequency（OLPF）detection is proposed in this thesis.The open-loop mathematical model of voltage amplitude and phase is built and the phase is linearized.The open-loop output voltage is three elements.The main advantage of this strategy is open-loop detection,which can quickly grasp the voltage parameter changes and shorten the pre-synchronization time.（4）The article has set up a simulation model for the VSG pre-synchronous interconnection and hardware in the loop experiment,analyses the load mutation and virtual moment of inertia,damping effect the stability of the system change.The results show that the pre-synchronous grid-connection technology based on virtual power can reduce the overshoot of VSG output power,shorten the time of grid-connection instability,and improve the grid-connection stability.Finally,the effectiveness of the improved pre-synchronization control is verified by comparing the traditional pre-synchronization control and the improved pre-synchronization control through the simulation and experiment of VSG network connection.