Research On Hardware Platform Of Flexible HVDC System

As a new type of dc transmission technology based on modular multilevelconverter, flexible dc transmission can achieve multilevel voltage output with a lowlevel of step change, lower the amplitude and gradient of voltage change, andeffectively relieve the electrical stress on valve sets. At the same time, its outputs havegood waveform quality and low harmonic content, even can consider scrapping the useof filter, so flexible dc transmission has the incomparable advantages in urbandistribution network reconstruction, new energy connecting to the grid and supplypower to passive network. As a key research project of National Power Grid Company,this subject aims to develop a set of flexible HVDC system prototype to provideexperimental platform for related theories, and to lay foundation for the follow-up highvoltage flexible dc transmission engineering application. The research contents of thispaper mainly include the following aspects.Firstly, the principle of modular multilevel converter is analyzed. In allusion to akind of open loop control algorithm, a digital simulation model is built inMatlab/Simulink, and the simulation results verify the efficiency of modulationalgorithm and voltage average control algorithm. A digital simulation model ofmultilevel back-to-back flexible HVDC system is built, and constant ac voltage controlmethod and constant dc voltage method are introduced to control it. The simulationresults show that the control method can make the system has a good dynamicresponse effect and steady state control accuracy.Secondly, participate in the design and manufacture of a three-phase multilevelback-to-back flexible HVDC system main circuit prototype, and in allusion to theconstant ac voltage control method and constant dc voltage control method, somehardware in loop simulation experiments using RT-LAB real-time simulation platformare done. The experimental results not only verify the correctness of the simulationresults, but also show that the control method can control the main circuit well.Finally, based on the above theoretical researches and related simulation tests, according to the requirements of two hundred and one level back-to-back flexibleHVDC system, participate in the design of a set of general controller, this controllerhas the characteristic of modular, and can be extended by cascading to control anylevel main circuit.All the relevant control strategies proposed in this paper can meet therequirements of system in steady state operation.