Modeling And Control Of Novel Traction Power Supply System Based MMC-MTDC

The current traction power supply system(TPSS) of China was adopting single-phase, power frequency, ac power supply mode, it was a asymmetric power supply mode which need the traction transformer to transforms three-phase ac power to two lines of single-phase then supply two side feeding sections of the traction substations, respectively; and exists two problems which are electrical sectioning and power quality. This supply mode constrains the development of high-speed and heavy-load electrical railway. With rapidly development of power electronic technology, the converter technology become a new feasible solution to solve the existing problems in the tradition TPSS. Since high voltage direct current(HVDC) based voltage source converter(VSC) was proposed, it has been got more attention of the academic and industrial fields with its attractive advantages, especially with the intense study of modular multilevel converter(MMC) in the recent years. To conquer the disadvantages of the current TPSS, this paper apply the VSC-HVDC technique to TPSS and proposes a novel TPSS based MMC-MTDC. The novel TPSS utilizes three-phase grid power convert to dc power and builds a dc-network, then uses single-phase H bridge MMC(SPH-MMC) supply the single-phase AC voltage to traction network after inverters from dc-network. The phase, frequency and amplitude of traction network voltage was equal, therefore the novel TPSS was not exists three-phase unbalance problem and electrical sectioning, meanwhile, the converter-based supply system could also be controlled to compensate the reactive power and harmonics of locomotive. According to the novel TPSS based MMC-MTDC, this paper mainly researches three part—operational principle and mathematic model of MMC, MMC station control strategy and system coordination control strategy, then constructs the original model of the novel TPSS. The mainly works of this paper as following:1) Basic operational principle and mathematic model of MMC was studied. First discusses the characteristics of three current prevalent sub-module(SM) of MMC, establishes its mathematic model. Then, researches the two modulation strategies of MMC, carrier phase shift PWM(CPS-PWM) and nearest level control(NLC), and draw a conclusion by mathematic means under CPS-PWM which was that the output voltage quantity was different when the phase offset between upper and lower arm carry waves was different. Thirdly, mathematically analyses capacitor voltage fluctuation and circulating current of MMC, and designs various capacitor voltage balancing controller and circulating current suppressing controller for MMC. Finally proposes single-phase H bridge MMC which applied in the traction network side of novel TPSS.2) Various station-level control strategies of MMC were presented. To begin with, the internal characteristics such as capacitor voltage and circulating current, and dc-side related characteristics of MMC were investigated of different control objectives under grid voltage unbalance conditions. Secondly two control strategies of MMC was proposed under grid voltage unbalanced, one is direct power compensate strategies which without ac current positive and negative sequence separation; another is proportional reduced-order resonant control strategy. Then, a power flat controller based differential flatness theory which is a nonlinear control theory was designed, this controller could be used in both grid voltage balanced and unbalanced conditions. Finally, doubled closed loop controller and direct power controller of SPH-MMC were designed which used to supply power to the traction network.3) Novel TPSS based MMC-MTDC was constructed, and system dc-side related coordinated control issues are investigated. Firstly, twice frequency fluctuation of novel TPSS dc terminal was studied, point out that circulating current of SPH-MMC caused the twice frequency fluctuation of dc terminal undoubtedly, and the fluctuation caused by SPH-MMC can be isolated in SPH-MMC by suppressing circulating current; A controller without phase locked loop and twice dc voltage compensator are designed to solve the dc terminal fluctuation caused by three-phase MMC worked constant dc voltage mode under grid voltage unbalanced conditions. In addition, to find a suitable dc voltage coordinated control strategy which use in novel TPSS, dc voltage coordinated control strategy in MTDC was investigated, at last a five-terminal MMC-MTDC based TPSS was built, simulation results verify the present novel TPSS can overcome some existing problems in tradition TPSS.