Research On Equivalent Electromagnetic Simulation Model And Average Value Model Of MMC Using Controlled Voltage Source

Since 2010, high voltage direct current transmission(HVDC) based on modular multilevel converter(MMC) has been developing rapidly in China and all over the world.Electro-magnetic simulation is an important tool to investigate the technologies related to MMC-HVDC. An MMC is composed of a large number of power electronic devices. So how to improve the computational efficiency of the simulation worths studying. Although the MMC and its similar technologies have replaced the conventional 2-level/3-level voltage source converter(VSC) technology in the HVDC field, in lower voltage DC systems such as wind power generating system, conventional 2-level/3-level VSC are still commonly used. How to improve the simulation speed of conventional 2-level/3-level VSC is also a research topic that worth deep study. This thesis takes improving the simulation speed as a start point and has completed following research:Operating principles of MMC and 2-level VSC are summarized, and the generic equivalent simulation model for MMC and average value model for 2-level VSC are proposed using controlled voltage sources. Compared with the detailed electro-magnetic simulation models, the simulation results demonstrate the accuracy and the improved computational efficiency of the proposed generic equivalent model. Simulation and verification are carried out under normal operation, AC fault and DC fault are verified.The above-mentioned equivalent simulation model of MMC based on controlled voltage sources is further improved, and the equivalent electro-magnetic simulation model of MMC that are able to simulation the dynamics under start-up and blocking IGBTs is proposed. On this basis, the averaged value model of MMC that is able to simulate dynamics of MMC under different typical operating scenarios is proposed.The proposed equivalent electro-magnetic simulation model of MMC is applied to the research on the starting up of the MMC. A series of starting strategies are investigated,including the single terminal MMC, two-terminal MMC-HVDC. The impact of the time of bypassing the starting up resistor, the starting up sequence and the DC voltage reference setting point during start-up on system transients is studied.The proposed averaged value model of MMC is applied to the start-up of front-to-front type DC/DC converter. The pre-charging strategy of the sub-modules offront-to-front DC/DC and corresponding start-up sequence are designed. Simulations verified effectiveness of the designed starting sequence.The major indices measuring the harmonic performance in the AC system are summarized according to the standard IEC/TR62001. An equivalent harmonic circuit for MMC is then derived. Based on the ladder-wave characteristics of the inner EMF of MMC,a simple method of treating the ladder-wave as multiple superimposed square-wares is derived for calculating the harmonic characteristics theoretically. To overcome the time-consuming problem of analysing MMC harmonic using detailed switching model, a method using fast numerical simulation model for harmonic analysing is proposed.Comparisons of the simulations on PSCAD/EMTDC and the theoretical results from Matlab verify the correctness of the proposed methods.