Sensorless Control of Wound Field Synchronous Machine Drive Using Current Source-Based Modular Multilevel Converter

This article proposes a wound field synchronous machine (WFSM) drive using a double star chopper cell (DSCC)-based modular multilevel converter (MMC) for high power medium voltage variable speed applications. A controlled current source delivers power to this DSCC-MMC. This current source is derived using the field winding of WFSM and a 12-pulse full-controlled thyristor rectifier. The 12-pulse rectifier draws power from a three-phase grid and delivers power to the field winding and the DSCC-MMC. Thus, the field current of WFSM becomes the input dc current of the DSCC-MMC, and no separate power supply is required for the field winding of WFSM. The input current to the DSCC-MMC is kept constant at all operating speeds of the WFSM for a given value of torque to keep peak–peak voltage ripple constant across the submodule capacitor of the MMC. The field current is varied proportionately with the average torque to avoid the saturation of the machine at light load torque. This variation of field current is essential for the successful operation of DSCC-MMC. The small current ripple in the field current, caused by the 12-pulse thyristor rectifier, is utilized to extract the rotor speed information for sensorless operation of vector-controlled WFSM drive. Simulation results are presented in this article to verify the proposed configuration and their control strategy at medium voltage. A hardware prototype has also been built in the laboratory to verify the same results experimentally at low voltage and low power.