A novel Modular Multi-level Converter topology with Voltage Correcting Modules (M2LC-VCMs)

Modular Multi-level Converters (M2LCs) are usually controlled with PWM based schemes to minimize circulating currents but at the expense of increased complexity of a controller and switching losses. A new approach that uses a modified conventional M2LC topology to minimize circulating currents as well as variations in capacitor voltages is proposed. The proposed technique incorporates a low power rated module, referred to as Voltage Correcting Module (VCM), into each arm of the converter to compensate for the voltage difference in capacitors and the dc-bus. The proposed solution simplifies the control, allows for the operation at lower switching frequency and is unaffected by the number of modules of the converter. The paper describes the simplified control concept and demonstrates its validity through simulated performance of a three-phase five-level, 2 MVA M2LC system. A comparison with a conventional topology is also carried-out to show the improved performance of the proposed topology.