Research On Variable Frequency Motors' Characteristics And Its Impact On Transient Of Power Grid

With the continuous improvement of industrial and agricultural production requirements for speed regulation and the rapid development of power electronics and computer control technology,frequency conversion speed regulation technology is becoming more and more mature.Because of the high performance and excellent energy-saving characteristics of inverter,the traditional asynchronous motor,The DC speed regulation system is gradually replaced by the frequency conversion speed regulation system.In China,more and more frequency conversion motors are connected to the power network,and the scale of grid connection is further improved.Large-scale variable-frequency motor brings challenges to the safe and stable operation of power system.It is very important and urgent to carry out relevant research on the transient stability of power grid caused by large-scale variable-frequency motors.In this paper,based on the development status of frequency converter,three kinds of mainstream variable-frequency motors on the market are modeled in detail.The response characteristics of speed and power to voltage and frequency changes are studied by simulation analysis method.In view of the load characteristics of large-scale variable-frequency motor connected to grid,the safety and stability of power network with large-scale frequency conversion motor accessed is analyzed,and the corresponding suggestions and control measures are put forward.Firstly,the main electrical equipment and motor types in various industries are analyzed,and the load ratio of frequency conversion motor is estimated.At the same time,the system structure,mathematical model and control strategy of frequency conversion asynchronous motor,permanent magnet synchronous motor and brushless DC motor is analyzed.Through the analysis of the influence mechanism of voltage sag on inverter,the method of start-up simulation and transient characteristic simulation,the model of frequency conversion motor is verified.Then a physical test platform is built to verify the effectiveness of the simulation model by comparing the voltage and frequency response characteristics of the induction motors and the variable frequency motors.This paper analyzes the operation characteristics of frequency conversion motor,and extracts the characteristic quantity suitable for the group equivalent of frequency conversion motor in view of the response characteristics of frequency conversion motor to voltage change.According to the group aggregation method of induction motor and the influence of frequency converter on the polymerization error of induction motor,a single machine equivalencemethod of frequency conversion motor group is put forward,and the equivalent model of frequency conversion motor in the system is proposed.A power grid model of single-machine driving concentration load is established,and a series of simulation studies are carried out based on the established power grid model.The variation rules of bus power,voltage,system frequency under transient condition is analyzed by using MATLAB/Simulink simulation platform.The transient voltage characteristics of power grid connected by high proportion variable frequency speed motor are simulated and analyzed.By comparing the transient characteristics of power system under different load models,the influence of future large scale variable frequency motor on the transient characteristics of power grid is discussed.It verifies the rationality of updating the load model in the future grid stability calculation.