Novel Control Strategy For MMC-HVDC System

Novel flexible high voltage direct current (HVDC) technology based on voltage sourceconverter of modular multilevel converter (MMC) is a domestic and foreign research hot spotin recent years. Firstly, this master’s thesis elaborates the background、significance andresearch status of MMC in the beginning of the introduction. Secondly, the mathematicalmodel of MMC-HVDC is established and the control strategies adopted by MMC-HVDC atcurrent are introduced. Thirdly, the lowest modulation strategies of MMC are researched.Fourthly, the harmful influence of MMC caused by dead time is analyzed. Finally, a numberof controllers of MMC-HVDC are designed according to the mathematical model establishedin different aspects correspondingly. The major innovations of this thesis are following:1) Optimized sorting algorithm of capacitor voltage is proposed. Starting from theaspect of decreasing the switching frequency and considering the traditional sorting algorithmof capacitor voltage has no occasion to increasing the switching frequency of power devices,an optimized sorting algorithm dedicated to lowing the switching the frequency is proposed.2) Various control strategies for suppressing the circulating current of MMC are putforward. Firstly, revealing the essence of circulating current suppression of MMC and takingthe changeable total inserted sub-modules in each phase, an optimized circulating currentsuppression algorithm of MMC, which is especially suitable for the passive network of powersupplying, is presented based on the improved ladder wave modulation. This algorithm cannot only suppress the circulating current but also decrease the THD of output voltage.Secondly, combined with the optimized algorithm of capacitor voltage, an optimizedefficiency controller is designed under synchronous dq coordinate according to the circulatingcurrent suppression mathematical model. Thirdly, a novel controller for suppressing thecirculating current under static αβ coordinate is projected by adopting proportional integralresonance controller.3) The influence on MMC caused by dead time is analyzed. According to the directionof arm current and the state of sub-module, this thesis analyzes the influence on singlesub-module caused by dead time and a conclusion that produces dead voltage is obtained.Analyzing the effect on MMC caused by dead time at worst conditions according to the CPSand NLM respectively, several conclusions, which are not the same under two different modulation strategies, are reached.4) A control strategy of MMC without phase lock loop is firstly proposed. Firstly, anovel control strategy without phase lock loop, which can achieve the same effect comparedto the double closed loop control strategy under dq coordinate, is proposed. Secondly, a directpower control strategy without phase lock loop, which does not need to know the gridfrequency and inductance parameter, is established. This strategy decreases the controlsystem’s complexity, but it need to be fast and correct to extract the positive components andnegative components under asymmetrical conditions in power grid.5) A controller of MMC based on differential flatness theory is designed. Introducing thedifferential theory in nonlinear control system to MMC and testifying MMC belongs todifferential flatness system, a corresponding controller based on DFT is designed, andsimulation results show that the control system based on DFT is superior than traditionaldouble closed loop control system in dynamic performance.6) Pre-charging and shutdown control strategies of MMC-HVDC are proposed based onthe synergy of voltage and sub-modules. This thesis analyses the dynamic model of MMC inuncontrollable and controllable states and modifies the expressions of dc voltage and currentlimiting resistance in uncontrollable stage. Considering redundant sub-modules in each arm, apre-charging strategy for MMC-HVDC which solves the mismatching problem of DC voltageand total inserted sub-modules in steady state is proposed. Dividing the shutdown process intotwo parts, namely energy feed state and energy consumption state, and transplanting thepre-charging strategy mentioned before into shutdown procedure, a novel shutdown strategywhich can both decrease the voltage level and power level of discharging resistance isproposed.