Study On Control System And Application In Five-Level Converter Based On Three-level Topoloty

As the modern electric power industry developing rapidly, the problem of electricpower system frequency modulation and peak-trough difference of load is becomingmore and more significant. Being able to achieve peak clipping, trough filling, phaseand frequency modulation, pumped storage power stations turn into economical andefficient technique of conditioning for power system. With huge single-machinecapacity, which can reach hundreds of megawatts, synchronous motors used in pumpedstorage power station cannot be started up easily, and direct starting will producetremendous impact on power grid. As a result, large synchronous motors are notallowed to be started up directly. In order to ensure safety and reliability and reduce theinfluence of the power grid, power electronic devices are used to start up the motors.The study object of this paper is large synchronous motors in pumped storage powerstation, and research work completed can be listed as follows.Based on the start-up system for pumped storage power station, the mathematicalmodel of synchronous motor generator is presented. In view of research status of highvoltage variable frequency speed control technology, five-level converter based onthree-level topology is described in details, including circuit composition and operatingprinciple. With simple structure, operational reliability, low switching frequency, lowharmonic content, five-level converter is appropriate for start-up of large synchronousmotor generators.The theory of modulation strategy for three-phase multi-level inverter is introduced, and three-segment algorithm for five-level voltage space vector modulationmethod is chosen. On the one hand, the flexibility and adaptability problem oftraditional algorithm are solved; on the other hand, the switching loss of power switchis reduced effectively. Five-level inverter circuit model is built withMATLAB/Simulink platform, and simulation results verify the validity of modulationalgorithm.Vector transformation closed-loop peed control method used for varying frequencystart-up devices for large synchronous motors is introduced, and mathematical model offlux orientated observer is derived, resulting in decoupling control of flux and torque.Different flux observers based on gap flux field orientated are analyzed from the pointof response speed, motor parameters and measurement errors. Motor model is built byusing MATLAB/Simulink platform, and simulation results verify the validity of speedcontrol strategy.A set of large capacity variable frequency start-up device prototype is designedand built based on mathematical model and circuit simulation analysis, and main circuitpower module, DC phase-shifting transformer, modulation and speed regulation controlmodule design are completed. Starting test, non-load test and open-loop load test areexecuted after main circuit test and control panel test. Test result show that themodulation method is corrent and the start-up system is reliable.