| In recent years,with the continuous development of renewable energy sources and the largescale connection of DC loads to the grid,DC network technology has gradually become one of the hot spots of widespread interest at home and abroad.In DC networks,DC transformers are responsible for voltage conversion,power transmission and interconnection to the network.With the development of MMC technology in the field of DC conversion,MMC-DCT has good prospects for application in DC networks due to their high voltage withstanding capability and high reliability.At present,both high-voltage transmission networks and low-voltage distribution networks usually include single-pole wiring structures and bipolar wiring structures.Compared with singlepole wiring structures,bipolar wiring structures of DC networks have higher power supply reliability.The existing MMC-DCT are usually applied to DC networks with single-pole wiring structure.This paper proposes a bipolar output MMC-DCT topology and corresponding control techniques for the power transfer between DC networks with single-pole wiring structure and DC networks with bipolar wiring structure,and the main research contents are as follows.（1）The power transfer and control model of the MMC-DCT is derived.The internal power transfer of the MMC-DCT using a square wave modulation strategy is analysed and the mechanism of self-balancing the capacitive voltage of the sub-modules within its bridge arm is described.An adaptive balance control algorithm of capacitor voltage of sub-modules in bridge arms of MMCDCT using quasi-square wave modulation strategy is presented,and a simulation model of the MMC-HB-DCT is established to verify the effectiveness of the sub-module capacitance voltage adaptive balancing control algorithm.（2）A bipolar output MMC-HB-DCT topology and control strategy suitable for medium and low voltage DC power distribution system scenarios are proposed.The mechanism of self-balancing the voltages of the positive and negative ports on the low-voltage side of the MMC-HB-DCT is analysed,and the power transfer characteristics of the bipolar output MMC-HB-DCT with quasisquare wave modulation strategy are given based on the operating mode decomposition and transient current calculation.The current stress of the low-voltage side switching devices is studied and the parameters of the key devices are designed.The positive and negative port voltage equalisation control strategy based on adjusting the duty cycle of the switching devices is proposed to address the voltage difference between the positive and negative ports caused by the stray resistance of the main circuit devices on the low voltage side.The effectiveness of the overall control strategy is verified based on the simulation model of the bipolar output MMC-HB-DCT.（3）A bipolar output MMC-MMC-DCT topology and control strategy for high voltage DC transmission system scenarios is proposed.The power transfer expression of the bipolar output MMC-MMC-DCT with a quasi-square wave modulation strategy is given based on the operating mode decomposition of the system and instantaneous current calculations.A small-signal mathematical model of the bipolar output MMC-MMC-DCT is derived.Based on the small signal mathematical model and the stability requirements of the control system,a constant power control mode for the grid interconnection scenario and a constant voltage control mode for the new energy pooling scenario are designed,together with the corresponding controller parameters.The electrical characteristics of the bipolar output MMC-MMC-DCT under the operating condition of disconnection fault are analyzed.An inter-bridge arm phase-shift modulation strategy is proposed to suppress the dc bias of an isolation transformer by actively injecting a dc current into the primary ac side of a bipolar output MMC-MMC-DCT.A simulation model of the bipolar output MMCMMC-DCT has been developed to verify the effectiveness of the two control modes and the faulttolerant operation scheme for fault-breaking operation conditions. |
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