Use of Nonlinear Inductors in Source-Interface Converters to Compensate for Instability in DC Distribution Networks

Description

Driven by the depletion of traditional fuel resources and advances in technologies for next generation energy systems, the power industry is undergoing a paradigm shift, from large centralized power plants to small green distributed generation systems located at the point of consumption. Such transformation has the merits of enhanced energy efficiency, power quality and system reliability, optimized asset utilization, and increased flexibility and capacity. DC systems are becoming more and more popular due to their advantages, such as reduction of power processing stages, high energy efficiency, no reactive power circulation and synchronization, high system reliability, and easy to integrate DC-based devices, such as renewable energy sources, energy storage systems, home appliances, etc.The latest developments in power electronics technology have accelerated the penetration of loads with constant power characteristics. Typical DC distribution networks contain source-interface converters (SICs) for supplying energy to constant power loads (CPLs), which exhibit negative incremental impedance. However, if the interaction between the output impedance of the SICs and the input impedance of the CPLs does not satisfy stability criteria, local and/or global instability may occur. With the increasing necessity, the research on CPL instability compensation has attracted the attention of thousands of researchers around the world. Various approaches, such as shaping converter or network characteristics with added circuit modules, applying sophisticated control laws, and introducing passive or active damping, have been proposed to enhance system stability.This project aims to study a new technique for improving the stability of DC distribution networks. The idea is based on replacing the fixed-value inductor in the output filter of the SIC with a nonlinear inductor exhibiting soft-saturation characteristics. Preliminary studies conducted by applying this simple technique in a buck converter reveal that it can immediately extend small signal and large-signal stability regions without the need to add additional circuits in the power conversion stage or modify the control scheme. The physical size of the nonlinear inductor is also comparable with that of the fixed-value inductor in the output filter without sacrificing ripple current requirement. Hence, the results lay a solid foundation for further research on the use of nonlinear inductors in other types of converters having different performance characteristics. Traditionally, core saturation is avoided in the converter design. It is expected that this research project will open a new perspective for the power electronics community, that is, the use of magnetic core saturation to enhance the stability of DC distribution networks.