In the present power conversion device, when a fault occurs in a power system (1) and DC voltage (VDC) of a capacitor (15) included in a unit converter (5) exceeds a protection level (VH), the operation of an inverter (10) is stopped. When the DC voltage (VDC) decreases to a recovery level (V1) or lower, DC voltage control of the inverter (10) is resumed to quickly decrease the DC voltage (VDC). When the DC voltage (VDC) decreases to a recovery level (V2) or lower, reactive power control of the inverter (10) is resumed. Thus, even when a fault occurs in the power system (1), the operation of the inverter (10) can be quickly resumed.

Provided is an active power filter-based modular multilevel converter, relating to the field of power electronics. According to the converter, an active power filter circuit is provided between upper and lower arms of each phase in a modular multilevel converter. The active power filter circuit includes two switch power devices, two submodules, a capacitor, and an inductor. The upper and lower bridge-arms are connected in series by means of two submodules, the two switch power devices are connected in series and then connected in parallel to ends of intermediate submodules which are connected in series, and the capacitor and the inductor are connected in series and then connected in parallel to two ends of the switch power device connected to the lower bridge-arm. The defect of large capacitance of submodule in conventional modular multilevel topology is overcome.

In the present power conversion device, when a fault occurs in a power system (1) and DC voltage (VDC) of a capacitor (15) included in a unit converter (5) exceeds a protection level (VH), the operation of an inverter (10) is stopped. When the DC voltage (VDC) decreases to a recovery level (V1) or lower, DC voltage control of the inverter (10) is resumed to quickly decrease the DC voltage (VDC). When the DC voltage (VDC) decreases to a recovery level (V2) or lower, reactive power control of the inverter (10) is resumed. Thus, even when a fault occurs in the power system (1), the operation of the inverter (10) can be quickly resumed.

A power converter is electrically connected with an AC power supply via a switch, and is configured with a plurality of unit converters (5) connected in series. A drive circuit (40, 42) drives a plurality of switching elements (11 to 14) of a main circuit (30). An interface circuit (48) outputs a detection value of a voltage sensor (46) to a control device (4). If a bypass switch (7) is turned off when the power converter is activated, each of the plurality of unit converters (5) charges a capacitor (15) using power supplied from the AC power supply. When the power converter is activated, a power supply (50) supplies a power supply voltage to the voltage sensor (46) and the interface circuit (48), prior to the drive circuit (40, 42). The control device (4) turns off the switch when the detection value of the voltage sensor (46) of at least one of the plurality of unit converters (5) is more than or equal to a predetermined value.

A modular multilevel converter (MMC) converter including an auxiliary sub-module provided between an upper converter arm and a lower converter arm of the MMC converter is provided. An MMC converter includes an auxiliary sub-module including: an energy storage unit storing a DC voltage in the sub-module; a first semiconductor switch connected to the energy storage unit; a second semiconductor switch connected to the first semiconductor switch; a third semiconductor switch connected to the second semiconductor switch; and a switching controller turning ON/OFF the first to third semiconductor switches, wherein a mid-point between the first and second semiconductor switches is connected to the sub-module of the upper converter arm, a mid-point between the second and third semiconductor switches is connected to a load connection terminal, and a mid-point between the third semiconductor switch and the energy storage unit is connected to the sub-module of the lower converter arm.

The present invention proposes a hybrid converter branch operating mode for a Modular Multilevel power Converter MMC with MMC cells in distinct subsets operating according to a pulse blocked cell operation mode with DC cell voltage increase or according to a bypass cell operation mode without DC cell voltage increase. Repeated cell subset assignment and corresponding alternation of cell operating mode allows to reduce or at least manage a mean deviation of the cell capacitor DC voltages of the converter cells. The invention also reduces no-load losses of the MMC in standby mode and a charging voltage in an MMC charging mode.

A power converter is electrically connected with an AC power supply via a switch, and is configured with a plurality of unit converters (5) connected in series. A drive circuit (40, 42) drives a plurality of switching elements (11 to 14) of a main circuit (30). An interface circuit (48) outputs a detection value of a voltage sensor (46) to a control device (4). If a bypass switch (7) is turned off when the power converter is activated, each of the plurality of unit converters (5) charges a capacitor (15) using power supplied from the AC power supply. When the power converter is activated, a power supply (50) supplies a power supply voltage to the voltage sensor (46) and the interface circuit (48), prior to the drive circuit (40, 42). The control device (4) turns off the switch when the detection value of the voltage sensor (46) of at least one of the plurality of unit converters (5) is more than or equal to a predetermined value.

A modular cell for a phase leg of a Modular Multilevel power Converter (MMC) is disclosed. The cell includes a power storing device, a plurality of semiconductor switches, a cell controller, and a current sensor. The current sensor is connected between a current conducting line in the cell and the cell controller for measuring a current through the cell and signaling information about the measured current to the cell controller. The cell controller includes an optical communication interface and is configured for forwarding the information about the measured current to a higher level controller over the optical communication interface.

A reactive power compensation device is connected with an AC power system via a switch, and includes an arm circuit having a plurality of sub-modules connected in series, and a central control protection device. Each of the sub-modules includes a DC capacitor, and a bridge circuit for switching whether or not to output a voltage held in the DC capacitor. The central control protection device is configured to shift to a standby mode in which all semiconductor switching elements constituting the bridge circuit of each of the sub-modules are set to an opened state, with the switch being set to a closed state, when the AC power system has a power failure during operation of the reactive power compensation device, and to shift from the standby mode to a stop mode in which the switch is set to an opened state.

In a power conversion device, an inverter (10) of each of three arms (A1 to A3) is controlled such that circulating current (Iz) of three arms (A1 to A3) follows a reference (Izrt) in a test period (times t3 to t4) in which a power system (1) is cut off from the three arms (A1 to A3), and whether the power conversion device is normal is determined based on circulating current (Iz) in the test period. Whether the power conversion device is normal therefore can be determined without affecting the power system (1).