| In the field of high-power medium-voltage-drive applications,multilevel voltage source converters(VSC)are the primary choice of the inverter topologies due to their better overall performances.The modular multilevel converter(MMC)has been extensively researched for itas advantages of easy expansion,high reliability,and excellent waveform quality,and has been rapidly promoted into high voltage direct current(HVDC)applications.It also hs obvious advantages over cascaded H-bridge(CHB)when used in medium-to-high voltage high-power AC drive.But it is rarely used due to the large ripples of capacitor voltage during low-frequency operation.In the case of medium voltage(2.3-6kV)applications,the five-level topologies are more popular,which is not mature enough yet.The aforementioned reasons limit the further application of high-performance VSC in the field of high-power drives.To solve these problems,the following research is carried out:Firstly,combined with the typical operation principle of MMC,the iterative process of the internal voltage and current quantity is analyzed in detail,revealing the mechanism and characteristics of the first order pulsating power in the arms.And the composition of the capacitor voltage ripples of the submodule is clarified.The iterative process of the internal voltage and current states in combination with the typical operating principle of MMC is analyzed in this paper firstly.The formation mechanism and characteristics of the bridge arm pulsating power is analyzed,and the sub-module capacitor voltage ripples components are revealed.Three ideas for suppressing capacitor voltage ripples are proposed for the first time,and the methods of the existing literature are categorized and compared.The industrial application scenarios of suitable ripples suppression schemes with different ideas are summarized.Based on the three ripples suppression ideas,three specific suppression schemes are proposed.Scheme one proposes an improved high-frequency injection method based on the flying-capacitance MMC.By changing the configuration of the traditional carrier phase-shifting modulation angle,a higher-frequency high-amplitude voltage is formed.And a high-frequency current is formed by the auxiliary path to achieve the same effects with the traditional injection method.Also,it broadens the applicable speed range of the injection method as well as reduces the switching frequency and capacitance of capacitors.In order to further improve the commercial applicability of MMC in high power density driving applications,scheme two proposes a capacitor voltage ripple suppression strategy based on three-level three-port power channels.And the operation principle of the auxiliary power channel and the soft switching operation process is analyzed.Besides,the comparison of the characteristics of these three-level power channels and the traditional two-level ones is done in this paper detailed which turns out that these power channels can reduce costs and improve system efficiency.Aiming at solving the specific problem of relatively low efficiency of traditional high-power locomotive traction converter,based on the third ripples suppression idea,a set of MMC traction drive system with back-to-back(BTB)structure is proposed,which alters the dc bus voltage with fundamental frequency in the full speed range to maintain the ripples constant.The single-phase full-bridge MMC is used as the grid-side rectifier and the half-bridge MMC is used as the motor-side inverter.The circuit characteristics and capacitor voltage balance methods are analyzed,and a complete control strategy is proposed as well.Since the dc voltage must rise in extremely low frequency because of the stator voltage compensation in practice,the connection mode of the two-terminal modules of each phase of the MMC on the motor side is improved to form a through-submodule with parallel capacitors.Only the through-submodules are used,when the stator voltage drops need to be compensated.The control block diagram with low and high-speed transition is proposed.Compared with the traditional traction converter,this scheme has a great improvement in many performance indicators.In view of the scarcity of reliable clamped five-level topology,the latter part analyzes the evolution of clamped multilevel topology firstly and proposes several new five-level clamped topologies.And 5L-HC is chosen as the research topology.The operation principle,advantages and disadvantages of the circuit are elaborated.The capacitors in the circuit are divided into three categories,and ripple characteristics of all capacitors under various working conditions are analyzed rigorously.Then a corresponding balancing strategy is proposed.In particular,several balances have been proposed for the balance of the upper and lower capacitors on the dc side,and comparative analysis has been carried out,making the application of this topology in high-power drive feasible.In regard of the interference caused by low-switching-frequency modulation of large-capacity multilevel topologies,based on the traditional two-level SPWM modulation process,the influence of the modulation process on the main circuit operation at low-switching-frequency are analyzed,which are summarized as the harmonic-overlap phenomenon.Three mechanisms are described to illustrate the overlap phenomenon in different topologies,and the solutions are proposed accordingly.In addition,the proposed scheme is verified.The harmonic-overlap phenomenon of MMC and 5L-HC which occur under low-switching-frequency modulation are deeply analyzed and the solutions are verified.Finally,the design rules of the modulation schemes for multilevel VSCs at low switching frequency are explained.And other engineering implementation problems are discussed.The study makes the normal operation of high-power multilevel converters at low switching frequencies feasible. |
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