Four-port power electronic transformer based on hybrid modular multilevel converter

A four-port power electronic transformer based on a hybrid modular multilevel converter (MMC), comprising:

• a hybrid MMC, direct current (DC)/DC converters, and an inverter, wherein
• the DC/DC converter comprises a front stage part, a high frequency transformation part, and a back stage part, wherein an alternating current (AC) side of the front stage part is connected to a primary side of the high frequency transformation part, and a secondary side of the high frequency transformation part is connected to an AC side of the back stage part;
• the inverter is a three-phase four-bridge arm inverter;
• the MMC has three phases, each phase having two bridge arms, and each bridge arm comprising X first submodules, Y second submodules, and an inductor, wherein X+Y≥N, N being the minimum number of modules required when the MMC works normally;
• the first submodule comprises two half-bridge structures connected in series, comprising a DC capacitor C1, a DC capacitor C2, and four insulated gate bipolar transistors T1, T2, T3 and T4 with anti-paralleled diodes;
• the T1, T2, T3, and T4 have collectors respectively connected to cathodes of their respective freewheeling diodes and emitters respectively connected to anodes of their respective freewheeling diodes;
• an emitter of the T1 and a collector of the T2 are connected and used as an AC port A of the first submodule, a collector of the T1 and a positive electrode of the C1 are connected and used as a positive electrode port C of the first submodule, an emitter of the T2, a negative electrode of the C1, a collector of the T4, and a positive electrode of the C2 are connected and used as a port I of the first submodule, an emitter of the T4 and a collector of the T3 are connected and used as an AC port B of the first submodule, and an emitter of the T3 and a negative electrode of the C2 are connected and used as a negative electrode port D of the first submodule;
• the positive electrode port C and the negative electrode port D of the first submodule are respectively connected to a positive electrode port and a negative electrode port of the front stage of the DC/DC converter connected to a DC side of the module;
• the second submodule comprises a DC capacitor C3, a DC capacitor C4, and five insulated gate bipolar transistors T5, T6, T7, T8, and T9 with anti-paralleled diodes;
• the T5, T6, T7, T8, and T9 have collectors respectively connected to cathodes of their respective freewheeling diodes and emitters respectively connected to anodes of their respective freewheeling diodes;
• in the second submodule, an emitter of the T5 and a collector of the T6 are connected and used as an AC port E of the second submodule, a collector of the T5 and a positive electrode of the C3 are connected and used as a positive electrode port G of the second submodule, an emitter of the T6, an emitter of the T9, and a negative electrode of the C3 are connected, an emitter of the T7 and a collector of the T8 are connected and used as an AC port F of the second submodule, a collector of the T7, a collector of the T9, and a positive electrode of the C4 are connected and used as a port J of the second submodule, and an emitter of the T8 and a negative electrode of the C4 are connected and used as a negative electrode port H of the second submodule;
• the positive electrode port G and the negative electrode port H of the second submodule are respectively connected to a positive electrode port and a negative electrode port of the front stage of the DC/DC converter connected to a DC side of the second submodule;
• without considering redundancy, X+Y=N, (2X+2Y)Vc=Vdc, and vm=(2X+2Y)Vc, where Vdc is a voltage at a high-voltage DC side, Vc is a voltage of each DC capacitor, and vm is a phase voltage amplitude at a high-voltage AC side;
• considering redundancy, the following relationship needs to be satisfied:
• X+Y≥N;
• when the MMC has a DC fault ride-through capability, without considering redundancy, X and Y satisfy the following relationship:
• 3
• 2
• ⁢
• (
• 2
• ⁢
• X
• +
• 2
• ⁢
• Y
• )
• ≤
• 4
• ⁢
• Y
• ;
• in the same phase, the X first submodules, the Y second submodules and the inductor are successively connected in series to form an upper bridge arm of the phase, the inductor, the Y second submodules, and the X first submodules are successively connected in series to form a lower bridge arm of the phase, and a connection midpoint of the upper bridge arm and the lower bridge arm in the same phase is a high-voltage AC interface of the power electronic transformer in the phase;
• positive electrode ends of three upper bridge arms are connected together and used as a positive electrode P of a high-voltage DC port of the power electronic transformer, negative electrode ends of three lower bridge arms are connected together and used as a negative electrode N of the high-voltage DC port of the power electronic transformer, and the DC side of each of the first submodules and the second submodules is connected to the front stage of each DC/DC converter;
• the DC side of each submodule of the MMC is connected to a DC/DC converter, the DC side of the front stage of each DC/DC converter is connected to the positive electrode and the negative electrode of the DC capacitors connected in series in a corresponding module in the MMC, and positive electrodes and negative electrodes at DC sides of back stages of all the DC/DC converters are respectively connected in parallel and then connected to a positive electrode and a negative electrode at a DC side of the inverter.