Isolated soft switching DC-DC converters for HVDC and renewable energy applications

Abstract

Power electronics converters are essential in both wind/solar grid connection as well as energy storage applications. Specifically, isolated DC-DC converters can provide functions like galvanic isolation, voltage level conversion, load/line regulation, etc. Traditionally, isolation is provided by bulky line frequency transformers. The disadvantage is that magnetic components have considerable volume and weight. By incorporating isolation in medium to high frequency DC-DC converter, the size of magnetic components can be greatly reduced. However, switching loss in semiconductor devices will also increase proportionally with operation frequency. Thus, soft switching techniques that can greatly reduce the switching loss is usually applied in high frequency converters. The special challenge for renewable applications is how to maintain high efficiency over large load variation.

Steady state and small signal analysis are conducted for all proposed topologies. Design process and performance investigation are provided. The converters are simulated in PLECS and Saber RT. Experiments are carried out using LabVIEW and cRIO system.

The focus of this work is on designing and analyzing of transformer isolated soft switching DC-DC converters suitable for renewable applications. In this dissertation, the specifications are taken from wind and battery storage applications, but the design principles can be applied to related areas such as microgrid, solar inverters, electric vehicles, etc. First, a secondary-phase-shift full bridge (SPSFB) DC-DC converter consisting of full bridge inverter and active rectifier is proposed. The converter can achieve zero voltage switching (ZVS) for all primary switches and zero current switching (ZCS) for all secondary switches. Next, input-parallel-output-series connected modular converter based on the SPSFB converter is presented. The modular converter has additional benefits including higher voltage gain, lower input current stress, interleaving, module shading capability and online redundancy. Fault tolerant control strategy is developed for the converter. The IPOS converter is especially suitable for high voltage applications such as HVDC connected wind farms. A three-phase converter with similar operation principle is then introduced, which has tripled harmonic frequencies of input current and output voltage, lower RMS current through switches and diodes, and reduction in transformer size compared to SPSFB converter. Lastly, a bidirectional converter based on half bridge and current doubler is proposed for energy storage system.

With the growing concern about fossil fuel depletion and environment issues including global warming, renewable energy is playing more and more important roles internationally. Currently, over half of renewable energy is consumed to generate electrical energy. Compared to other types of renewable energy sources, wind and solar energies have the advantages of zero emission and unlimited resources. This makes them the fastest growing forms of renewable electricity generation. With increasing penetration of renewable power generation, the intermittent nature of wind and solar energy will impact the stability of power grid. There has been extensive research on high voltage direct current (HVDC) transmission system for connecting offshore wind farms, where wind are stronger and steadier. Additionally, energy storage systems (ESS) based on batteries and supercapacitors are considered a good solution to help shape the load and stabilize power grid.