Voltage Balancing Control for Y-Connected Modular Converter in MV Drive Application

The Y-connected modular converter is getting popular in medium voltage applications. The Y-connected modular converter is essential structure in various industrial products: H-bridge cascaded multilevel converter, Modular multilevel converter, and so on. Moreover, using the isolated DC/DC converter, the Y-connected modular converter can connect the MV AC voltage source with AC load and/or DC load. Because this Y-connected modular converter has many module capacitors, the control of these capacitor voltages is a very critical issue. The basic operation of the modular converter is possible assuming that the DC capacitor voltages of all modules are constantly controlled. In particular, the capacitor voltage balance should be guaranteed under some module failure conditions as well as AC voltage unbalance conditions. This paper proposes the voltage balancing control method for Y-connected modular converter. Because this method uses the common mode voltage, any other AC characteristics does not deteriorate. And this method is generally applied into any application of the Y-connected modular converter structure. To verify the feasibility of the proposed method, the AC unbalance condition and the module fault condition was considered.