Multi-input power electronic converters for hybrid energy storage systems

Abstract

With the proliferation of Hybrid Energy Storage Systems (HESSs) i.e., integration of different energy storage devices like secondary-batteries, fuel cells and Electric Double Layer Capacitors (also referred to as super-capacitors or ultra-capacitors), multi-input power electronic converters gain tremendous potential. HESSs employ two or more types of energy storage devices to utilize each of their superior traits while also compensating for their drawbacks. For example, Li-ion batteries and super-capacitors form an ideal pair for an HESS as super-capacitors with relatively higher power density, lower energy density and faster dynamic response is integrated into a system with Li-ion batteries exhibiting relatively higher energy density, lower power density and slower response. An HESS finds extensive applications in renewable energy based modern electric grids and electric vehicles. In this thesis, power converters with multiple inputs, assuming each voltage input to be a different energy source or energy storage devices, are proposed. A family of multi-input Switched-Capacitor Multilevel Inverters (SCMLI) as front-end power sources for high frequency AC power distribution systems is proposed. It employs asymmetric DC voltage sources with a common ground which makes it ideal for renewable energy farms and modern electric vehicles. The proposed topologies produce staircase waveforms with higher number of output levels employing fewer components compared to several existing switched capacitor multilevel inverters in the literature. These topologies are beneficial where asymmetric DC voltage sources are available e.g. in case of renewable energy farms based AC micro-grids and modern electric vehicles. Utilizing the available DC sources as inputs for a single inverter solves the major problem of connecting several inverters in parallel. The inverter inherently solves the problem of capacitor voltage balancing as each capacitor is charged to a finite value every cycle. In-depth study on two staircase modulation strategies, namely selective harmonic elimination and minimum total harmonic distortion scheme is presented with study on the variation of switching angles and THD with modulation indices under both schemes. State analysis, losses and the selection of capacitance are examined. Simulation and experimental results at different distribution frequencies, power levels and output harmonic content are provided to demonstrate the feasibility of the proposed family multilevel inverter topology. A multi-input bi-directional synchronous-buck based structure is used to derive several multi-input DC-DC converters. These converters are useful in DC power distribution and in DC motor drive applications with multiple asymmetric sources. The steady state output voltage expressions for each of the unfolding converter i.e. multi-input buck, multi-input buck-boost, multi-input flyback, multi-input forward and multi-input full-bridge converter is presented. The multi-input buck converter is analyzed in detail with a generalized average and small signal model for control. Experimental results of a three-input buck converter operating in open and closed loop with average current control is presented. The design of body integrated super-capacitor panels and a bi-directional interleaved DC - DC converter is presented. Super-capacitor panels and the developed converter are interfaced to an existing Li-ion battery-powered drive train of an electric car. The analyses of a super-capacitor-battery HESS including a simple model of Li-ion battery, super-capacitor and the bi-directional DC-DC converter are performed. Experiments are carried out to determine their internal resistances. Experimental results demonstrating the performance of the developed converter and the car is presented with waveforms captured during the running of the car on a dynamometer. The thesis proposes several novel asymmetric multi-input DC-AC and DC-DC converters, presents simulation and experimental results of the converters which validate the theoretical findings. The DC-AC converters, based on the switched-capacitor technique, are designed keeping high-frequency AC applications in mind. The multi-input bi-directional buck-derived DC-DC converters and the multi-input SCMLIs proposed can be employed in plethora of applications that utilizes multiple power sources. A practical implementation of a HESS and experimental results of the designed bi-directional interleaved converter for an electric car powered by Li-ion battery and body-integrated super-capacitor (forming original shapes like the bonnet and door) is presented.