Study On Start-up Control Strategies Of Modular Multilevel Converter-high Voltage Direct Current

Voltage source converter-high voltage direct current (VSC-HVDC) puts to usefull-controlled power electronic device IGBT to substitute converter structure ofconventional HVDC based on half-controlled SCR. The technology highlights threemajor technical characteristics of full-controlled power electronic device, voltagesource converter and pulse width modulation, which figures out various bottlenecksexisting in conventional HVDC, such as bad waveform, needing reactive powercompensation, supplying no power to passive system and so on. At present,VSC-HVDC systems which have been put in operation for the most part adopttwo-level or three-lvel converter through the way of power devices in series for highvoltage and large capacity while voltage equalizing of tandem device is tough andswitch loss is bigger.Modular multilevel converter (MMC) is a neoteric VSC topology applied inHVDC transmission and its bridge arm is in series by a number of independentsub-modules (SM) made up of IGBT and capacitor. Before the normal operation ofMMC-HVDC system, every SM needs pre-charging so as to make voltage of capacitorand DC bus rise from zero to the rated value, completing start-up of the systemultimately. Start-up of MMC-HVDC is a transient and complicated process,meanwhile an appropriate control method is critical to boost start-up speed, enhancestability and inhibit over-current and over-voltage.A charging way employing external DC power from DC side called separatelyexcited mode is expounded and its abundant shortcomings are presented in this paper.In order to make up for the deficiencies, operating principle of MMC is introduced,voltage, current and power equations are deduced, homologous mathematical modeland equivalent circuit are set up, providing necessary theoretical basis for studyingcontrol strategies and designing control system.On the basis of a detailed analysis for MMC start-up dynamic procedure, apre-charging control method of SM from AC side utilizing AC network calledself-excitation mode is brought forth in this paper and the process is divided intouncontrollable stage and controllable stage. The minimum current limiting resistor isproposed, to restrain pulse current and stack its adverse impact. End point setting islower than theoretical value, which avoids the slow course of voltage rising duringlate uncontrollable stage. Current control, voltage control and SM balance control strategies in controllable stage are presented based on carrier phase shiftedmodulation (CPSM), accomplishing fleet pre-charging, suppressing over-voltage andover-current, taking into account system balance, achieving reliable and efficientstart-up. Charging from the simulation result comparisons of five-level models builtin Matlab/Simulink by voltage and current double closed loop control and voltagesingle closed loop control, the start-up mode put up in this paper possesses excellenteffectnes and feasibility.