Simplified medium/high frequency transformer isolation approach for multi-pulse diode rectifier front-end adjustable speed drives

Typical medium voltage (MV) adjustable speed AC drive (ASD) systems employ a line frequency transformer isolation front-end followed by a `multi-pulse' rectifier (12-pulse, 18-pulse, etc.) system to improve input current quality compared to a 6-pulse rectifier. This paper proposes a medium/high frequency transformer isolation approach that can serve as a direct replacement to existing bulky line frequency transformers in high power multi-pulse rectifier system. The paper presents a simplified strategy that can significantly improve power density of MV ASDs. In the proposed approach, the line frequency utility AC voltage is converted to medium frequency (MF) or high frequency (HF) voltage via AC-AC converters. The three 1-phase MF or HF outputs from AC-AC converters are fed to the primary sides of three 1-phase multi-winding transformers. The secondary and tertiary output windings from the MF or HF transformers are in star-delta configuration (for 12-pulse rectifier), which are connected to the inputs of the 3-phase diode rectifier system similar to the front-end of a typical ASD. Overall, the proposed topology improves power density by reducing weight/volume of the required input isolation transformer while maintaining the same utility input current quality. This paper explains the proposed concept using mathematical modeling, simulation and experimental results from a scaled down laboratory prototype system.