A series-connected multi-level power conversion device (1, 1A, 1B) comprising: a multi-winding transformer configured to distribute AC power input into a primary winding to m×n secondary windings (12a to 12f), wherein n and m are coprime; and a power conversion unit comprising m×n single-phase power converters (21a to 21f), wherein each of mxn single-phase converters (21a to 21f) is connected to one of the mxn secondary windings (12a to 12f), and m output phases, wherein each of the m output phases is constituted by the serially connected outputs of n single-phase power converters (21a to 21f) out of the m×n single-phase power converters (21a to 21f); wherein the multi-winding transformer has a first relationship that, for each of the m output phases, each one of the secondary windings (12a to 12f) connected to the n power converters in the respective output phase has a different voltage phase and differs 60/n degrees from at least one of the secondary windings connected to the remaining n-1 power converters in the respective output phase; characterized in that considering n groups of secondary windings (12a to 12f), wherein each one of the groups is formed by m secondary windings (12a to 12f) and each of the secondary windings (12a to 12f) within one group is taken from a different one of the m output phases and wherein each of the mxn secondary windings (12a to 12f) is assigned exclusively to one single group among the n groups, the multi-winding transformer has a second relationship that, for each group, each one of the m secondary windings (12a to 12f) in the respective group has a different voltage phase and differs 60/m degrees from at least one of the remaining m-1 secondary windings (12a to 12f) in the respective group. a multi-winding transformer configured to distribute AC power input into a primary winding to m×n secondary windings (12a to 12f), wherein n and m are coprime; and a power conversion unit comprising m×n single-phase power converters (21a to 21f), wherein each of mxn single-phase converters (21a to 21f) is connected to one of the mxn secondary windings (12a to 12f), and m output phases, wherein each of the m output phases is constituted by the serially connected outputs of n single-phase power converters (21a to 21f) out of the m×n single-phase power converters (21a to 21f); wherein the multi-winding transformer has a first relationship that, for each of the m output phases, each one of the secondary windings (12a to 12f) connected to the n power converters in the respective output phase has a different voltage phase and differs 60/n degrees from at least one of the secondary windings connected to the remaining n-1 power converters in the respective output phase; characterized in that considering n groups of secondary windings (12a to 12f), wherein each one of the groups is formed by m secondary windings (12a to 12f) and each of the secondary windings (12a to 12f) within one group is taken from a different one of the m output phases and wherein each of the mxn secondary windings (12a to 12f) is assigned exclusively to one single group among the n groups, the multi-winding transformer has a second relationship that, for each group, each one of the m secondary windings (12a to 12f) in the respective group has a different voltage phase and differs 60/m degrees from at least one of the remaining m-1 secondary windings (12a to 12f) in the respective group.