Abstract: A power conversion device includes a plurality of leg circuits and a control device. The plurality of leg circuits correspond to respective phases of an AC circuit and connected in parallel between common first and second DC terminals. Each leg circuit includes a plurality of chopper cells each including an energy storage and cascaded to one another and at least one inductance connected in series to the plurality of chopper cells. The control device controls an operation of only at least one chopper cell included in each leg circuit based on a circulating current which circulates through the leg circuits.

Abstract: When switching operations of converter cells are stopped due to temporary voltage reduction in an AC system, a voltage determination unit performs determination regarding a voltage of each DC capacitor after a predetermined period has passed, and a charge control unit transmits a charging gate signal, via a gate control unit, to select a converter cell at a voltage level at which a self-feeding circuit of the converter cell is operable. Thus, a DC capacitor of a converter cell whose voltage is lower than the voltage level at which a self-feeding circuit is operable is charged, thereby enabling switching operations of all the converter cells.

Abstract: Since a power converter including a modular multilevel converter uses a large number of cells each combining a plurality of switching elements and a DC capacitor, there is a problem of conduction loss due to the switching elements. The conduction loss is reduced by connecting a bypass circuit between terminals of each of the cells, controlling to open and close the switching element, and controlling to short-circuit the bypass circuit connected to the cell controlled to output zero voltage.

Abstract: In a control device of a power conversion device, an AC control portion generates a first voltage command value representing an AC voltage component to be output from a plurality of chopper cells of each leg circuit. A DC control portion generates a second voltage command value representing a DC voltage component to be output from the plurality of chopper cells of each leg circuit. A circulating current control portion generates a third voltage command value to be output from the plurality of chopper cells of each leg circuit in order to suppress a circulating current. The circulating current control portion performs a non-linear operation with the first, second, and third voltage command values. The plurality of chopper cells of each leg circuit operate in accordance with a result of the non-linear operation.

Abstract: In a multilevel converter, three circuit breakers are respectively connected between three arms and three reactors. One circuit breaker is a DC circuit breaker configured to interrupt direct current when a short circuit accident occurs between two DC power transmission lines. Each of the two circuit breakers is an AC circuit breaker configured to interrupt alternating current when the short circuit accident occurs. When the short circuit accident occurs, the two AC circuit breakers are brought into the non-conductive state and then the DC circuit breaker is brought into the non-conductive state, thereby interrupting the short circuit current.

Abstract: A power conversion device includes: a positive arm and a negative arm each of which is formed by connecting a plurality of converter cells in series, the converter cells each being composed of a plurality of semiconductor switching elements and a DC capacitor; and a control circuit. An arm balance control unit in the control circuit calculates a first voltage adjustment value for balancing voltage of the DC capacitors in the positive arm and voltage of the DC capacitors in the negative arm. The control circuit adjusts an AC control command using the first voltage adjustment value, thereby calculating an AC voltage command for AC voltage to be outputted to an AC line for each phase.

Abstract: In a multilevel converter, three circuit breakers are respectively connected between three arms and three reactors. One circuit breaker is a DC circuit breaker configured to interrupt direct current when a short circuit accident occurs between two DC power transmission lines. Each of the two circuit breakers is an AC circuit breaker configured to interrupt alternating current when the short circuit accident occurs. When the short circuit accident occurs, the two AC circuit breakers are brought into the non-conductive state and then the DC circuit breaker is brought into the non-conductive state, thereby interrupting the short circuit current.

Abstract: A power conversion device includes: a positive arm and a negative arm each of which is formed by connecting a plurality of converter cells in series, the converter cells each being composed of a plurality of semiconductor switching elements and a DC capacitor; and a control circuit. An arm balance control unit in the control circuit calculates a first voltage adjustment value for balancing voltage of the DC capacitors in the positive arm and voltage of the DC capacitors in the negative arm. The control circuit adjusts an AC control command using the first voltage adjustment value, thereby calculating an AC voltage command for AC voltage to be outputted to an AC line for each phase.

Abstract: When switching operations of converter cells are stopped due to temporary voltage reduction in an AC system, a voltage determination unit performs determination regarding a voltage of each DC capacitor after a predetermined period has passed, and a charge control unit transmits a charging gate signal, via a gate control unit, to select a converter cell at a voltage level at which a self-feeding circuit of the converter cell is operable. Thus, a DC capacitor of a converter cell whose voltage is lower than the voltage level at which a self-feeding circuit is operable is charged, thereby enabling switching operations of all the converter cells.

Abstract: In a multilevel converter, three first rectifying elements are respectively connected between three arms and a negative voltage terminal. Three second rectifying elements are respectively connected to the three first rectifying elements in antiparallel. During a normal operation, current flows in the three first rectifying elements and the three second rectifying elements. When a short circuit accident occurs between two DC power transmission lines, the three first rectifying elements are brought into the non-conductive state, thereby interrupting and quickly attenuating inter-arm direct current flowing in four arms and the like.

Abstract: In a multilevel converter, three switches are connected between three arms and three reactors, and three resistive elements are connected to the respective three switches in parallel. The three switches are configured to be in a conductive state during a normal operation. The three switches are configured to come into a non-conductive state when a short circuit accident occurs between two DC power transmission lines, whereby an inter-arm direct current flowing in four arms and the like is quickly attenuated by the three resistive elements.