Analysis and design of active inductor as DC-link reactor for lightweight adjustable speed drive systems

Size and weight considerations are very important in certain specialized industries as offshore oil drilling and marine/subsea systems that employ adjustable speed drives (ASD). Typical ASD topology consists of a three phase diode rectifier front-end. It is also common to specify a dc-link inductor and/or ac line reactors to reduce utility line current harmonics to acceptable levels. However, the required dc-link inductor/ac line reactors contribute to additional weight and volume. In this paper an active inductor approach is explored to emulate a large size dc-link inductor. Analysis and design of an active inductor for a 1 MW ASD is proposed and analyzed. The required inductance value is commanded and current control regulates the active inductor current to match inductor behavior. It is shown that the proposed active inductor topology is similar in weight/volume as the smaller inductor while emulating a higher per unit inductance. The proposed active inductor meets the required compact and lightweight specifications of ASDs in retrofit and special applications. Furthermore, the active inductor is tunable to achieve high performance operation during faults. Experimental results from a scaled down prototype are presented. A size, weight and loss analysis is detailed and a soft switching method to reduce losses is also explained.