A power electronic converter (30), for connecting AC and DC networks (46,44) and transferring power therebetween, comprises: first and second DC terminals (32,34) defining a DC link for connection to a DC network (44); wherein, in use, the DC link has a reversible DC link voltage applied thereacross; at least one converter limb (36) extending between the first and second DC terminals (32,34) and having first and second limb portions (38,40) separated by an AC terminal (42) for connection to an AC network (46), each limb portion (38,40) including at least one rationalized module (52) having first and second sets of series-connected current flow control elements (54) connected in parallel with at least one energy storage device (56), each set of current flow control elements (54) including a primary active switching element to selectively direct current through the energy storage device (56) and a primary passive current check element to limit current flow through the rationalized module (52) to a single direction, the current flow control elements (54) and the or each energy storage device (56) combining to selectively provide a voltage source to synthesize an AC voltage at the AC terminal (42); and a first controller (60) to selectively switch the or each rationalized module (52) in each limb portion (38,40) to control the configuration of the AC voltage at the corresponding AC terminal (42) so as to transfer power from the AC network (46) to the DC network (44) in an AC to DC power transfer mode and to transfer power from the DC network (44) to the AC network (46) in a DC to AC power transfer mode, wherein each limb portion (38,40) includes: one or more secondary passive current check elements (48) to limit current flow through the corresponding limb portion (38,40) to a single direction between the corresponding AC and DC terminals (42,32,34), the or each secondary passive current check element (48) being connected in series with the or each rationalized module (52); or one or more secondary active switching elements that is connected in series with the or each rationalized module (52).