The Research On The Low Voltage Ride Through Performance Based On Variable Frequency Speed Regulation System

In recent years,with the rapid development of power electronic technology,the inverter widely used in auxiliary equipment of power plant with its significant advantages of energy saving.However,due to the inverter is very sensitive to the fluctuation of the grid voltage.When the voltage sags or interruption caused by power failure,if the inverter does not have the capability of low voltage ride through,it will trigger inverter into low voltage and cause it tripping.In turn,the safety of the power plant unit is threatened,which seriously affects the safe and stable operation of the power grid.Therefore,it is very important to research the performance of the low voltage ride through based on variable frequency speed regulation system.This thesis describes the related concepts of the low voltage ride through and dual PWM inverter.It analyzes the present situation of the research on the low voltage through technology of the power plant variable frequency speed regulation system.The design of dual PWM inverter modulation method and the implementation of control system are given in detail.And completed the simulation of dual PWM inverter.Then,based on the variable frequency speed regulation system of fire power plant,the low voltage ride through performance of two different type inverter are evaluated by field test.The main work of this thesis is as follows:(1)Starting from the basic structure of inverter,analyzes the topology structure and working principle of grid side converter.The mathematical model of the three-phase static coordinate system and two-phase rotating coordinate system are established.It illustrates the working principle of the grid side inverter according to the formula of mathematical model and the topology.(2)Focus on the SVPWM algorithm of three level inverter through the main circuit of three level inverter.The switching state of each phase is analyzed under the condition of steady state operation,and an algorithm is presented to synthesize the reference voltage vector according to the nearest three vector principle.By judging the location of the reference voltage vector to select the basic vector,it will get the action time of each basic vector and the corresponding sequence.The operation time of the basic vector is assigned to the corresponding vector state so as to effectively control the switch tube's turn on and turn off.(3)The design for the control system of dual SVPWM inverter's network and load side converter are completed both by dual-close-loop control.The grid side converter is controlled by voltage external loop and current inner loop.The load side converter is controlled by the power outer loop and current inner loop.And the parameters of the PI regulator are determined by the corresponding transfer function.It establishes simulation model of the dual SVPWM inverter through the MATLAB/SIMULINK platform.The simulation's results show the effectiveness of SVPWM algorithm and the feasibility of control system's design.The quality of the output voltage and current waveform are better,improved the conversion efficiency of the rectifier and the inverter.(4)In Anhui,a power plant auxiliary frequency control system(powder feeder)as an example.For different types of powder feeder inverter randomly chooses one for low voltage ride through performance test and analysis the test results.For the auxiliary inverter that does not have the capability of low voltage ride through,this thesis discusses several schemes to solve the problem and improve the reliability of power system security and stability.