Patent ID: 9716425
Date: 2017-07-25
CPC Classifications: H02M

Claim:
1. A multilevel voltage source converter configured to convert between alternating current and direct current, comprising: a plurality of phase legs connected between two DC poles, each phase leg comprising an upper phase arm and a lower phase arm, at least one of the upper phase arm and the lower phase arm comprising a number of cells between one of the two DC poles and an AC terminal, said cells comprising at least one hybrid full bridge cell for fault current handling operation, wherein each cell in the at last one of the upper phase arm and the lower phase arm is controllable to contribute to meeting a reference voltage for obtaining a waveform on the AC terminal of the phase leg, said hybrid full-bridge cell consisting of: a first cell connection terminal for coupling to the one of the two DC poles; a second cell connection terminal for coupling to the AC terminal; and an energy storage element having a positive and a negative end; a first group of series connected semiconducting units, wherein the first group of series connected semiconducting units is connected in parallel with the energy storage element, wherein the semiconducting units of the first group of series connected semiconducting units comprises first and second semiconducting units, wherein the first semiconducting unit only comprises a first active switching element with a first anti-parallel connected unidirectional conducting element, wherein the second semiconducting unit only comprises a second active switching element with a second anti-parallel connected unidirectional conducting element, wherein the first semiconducting unit and the second semiconducting unit are connected in series, and wherein a junction between the first semiconducting unit and the second semiconducting unit forms one of the first cell connection terminal and the second cell connection terminal; and a second group of series connected semiconducting units, wherein the second group of series connected semiconducting units is connected in parallel with the energy storage element as well as with the first group of series connected semiconducting units, wherein the semiconducting units of the second group of series connected semiconducting units comprises third and fourth semiconducting units, wherein the third semiconducting unit only comprises a third active switching element with a third anti-parallel connected unidirectional conducting element, wherein the fourth semiconducting unit only comprises a fourth unidirectional conducting element, wherein the third semiconducting unit and the fourth semiconducting units are connected in series, wherein a junction between the third semiconducting unit and the fourth semiconducting unit forms the other one of the first cell connection terminal and the second cell connection terminal, and wherein the third active switching element of the third semiconducting unit in the second group of series connected semiconducting units is configured to always be in on state in normal operation of the converter.