A power electronic converter (30,130), for connecting AC and DC networks (46,44) and transferring power therebetween, comprising: first and second DC terminals (32,34) defining a DC link for connection to a DC network; wherein, in use, the DC link has a reversible DC link voltage applied thereacross; at least one converter limb extending between the first and second DC terminals (32,34) and having first and second limb portions (38,40) separated by an AC terminal for connection to an AC network, each limb portion (38,40) including a plurality of rationalised modules, each rationalised module having first and second sets of series-connected current flow control elements connected in parallel with at least one energy storage device, each set of current flow control elements connected in parallel with at least one energy storage device, each set of current flow control elements including a primary active switching element to selectively direct current through the energy storage device and a primary passive current check element to limit current flow through the rationalised module to a single direction, the current flow control elements and the or each energy storage device combining to selectively provide a voltage source to synthesise an AC voltage at the AC terminal; and a first controller to selectively switch each rationalised module in each limb portion (38,40) to control the configuration of the AC voltage at the corresponding AC terminal so as to transfer power from the AC network to the DC network in an AC to DC power transfer mode and to transfer power from the DC network to the AC network in a DC to AC power transfer mode, characterised in that each limb portion (38,40) includes one or more secondary passive current check elements 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 being connected in series with the respective plurality of rationalised modules, wherein the or each secondary passive current check element is configured to support part of a differential voltage appearing across the corresponding limb portion (38,40) and each plurality of rationalised modules is configured to support the other part of the differential voltage appearing across the corresponding limb portion (38,40) in one of the AC to DC and DC to AC power transfer modes, and wherein each plurality of rationalised modules is configured to support a differential voltage appearing across the corresponding limb portion (38,40) when the or each corresponding secondary passive current check element is forced into conduction by a reversal of polarity of the DC link voltage across the DC link in the other of the AC to DC and DC to AC power transfer modes. first and second DC terminals (32,34) defining a DC link for connection to a DC network; wherein, in use, the DC link has a reversible DC link voltage applied thereacross; at least one converter limb extending between the first and second DC terminals (32,34) and having first and second limb portions (38,40) separated by an AC terminal for connection to an AC network, each limb portion (38,40) including a plurality of rationalised modules, each rationalised module having first and second sets of series-connected current flow control elements connected in parallel with at least one energy storage device, each set of current flow control elements connected in parallel with at least one energy storage device, each set of current flow control elements including a primary active switching element to selectively direct current through the energy storage device and a primary passive current check element to limit current flow through the rationalised module to a single direction, the current flow control elements and the or each energy storage device combining to selectively provide a voltage source to synthesise an AC voltage at the AC terminal; and a first controller to selectively switch each rationalised module in each limb portion (38,40) to control the configuration of the AC voltage at the corresponding AC terminal so as to transfer power from the AC network to the DC network in an AC to DC power transfer mode and to transfer power from the DC network to the AC network in a DC to AC power transfer mode, characterised in that each limb portion (38,40) includes one or more secondary passive current check elements 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 being connected in series with the respective plurality of rationalised modules, wherein the or each secondary passive current check element is configured to support part of a differential voltage appearing across the corresponding limb portion (38,40) and each plurality of rationalised modules is configured to support the other part of the differential voltage appearing across the corresponding limb portion (38,40) in one of the AC to DC and DC to AC power transfer modes, and wherein each plurality of rationalised modules is configured to support a differential voltage appearing across the corresponding limb portion (38,40) when the or each corresponding secondary passive current check element is forced into conduction by a reversal of polarity of the DC link voltage across the DC link in the other of the AC to DC and DC to AC power transfer modes.