INTEGRATED CONVERTER FOR PLUG-IN ELECTRIC VEHICLES

Abstract

This thesis is focused on evolution, analysis and design of improved single-stage based integrated converters (achieving all modes of vehicle operation, i.e., plug-in charging, propulsion and regenerative braking using single-converter) for on-board applications of plug-in electric vehicles (PEVs). The research work in this thesis is divided into development of four integrated converter topologies, each one of which is briefly discussed below. The first proposed topology is a ZETA-SEPIC based integrated converter. It operates as ZETA converter during plug-in charging and regenerative braking modes (battery charging operation), and as SEPIC converter for propulsion mode. Due to ZETA and SEPIC operation, the converter has buck/boost capabilities in each mode; The battery can therefore be charged from universal input voltage. Further, the stored mechanical energy can be captured entirety to charge battery during regenerative braking of vehicle. During propulsion mode, the dc-link voltage can be controlled for wide range of battery voltages. Moreover, in this converter only one switch operates for any given mode which simplifies the control implementation. The theoretical efficiency of the converter in plug-in charging, propulsion and regenerative braking modes are computed and compared with existing integrated converters. This converter is then evaluated for voltage and current stresses on semiconductor devices during abovementioned modes of operation. It has a limitation of high voltage/current stresses (sum of input/output quantities (voltage/current)) on semiconductor devices. The voltage stress on semiconductors in propulsion and regenerative braking modes are a sum of dc-link voltage and battery voltage usually higher than plug-in charging mode (sum of grid voltage and battery voltage). The power rating of converter in propulsion mode is usually much higher than other two modes; therefore, high stresses in propulsion mode are downside for vehicle application. To overcome above-mentioned limitation of ZETA-SEPIC converter, another modified ZETA based integrated converter has been proposed, which has lower stresses in propulsion and regenerative modes. Also, the proposed converter has higher efficiency in these two modes compared to other existing integrated converters . Efficiency improvement in these two i modes leads to a longer run of vehicle. The peak efficiencies of propulsion and regenerative braking modes are 97.2% and 98.1%, respectively. To take forward this work, another integrated converter is developed from conventional SEPIC which has also lower stresses (in propulsion and regenerative braking modes) same as the modified ZETA based converter. Moreover, this converter utilizes a nonlinear carrier control method which saves voltage sensor requirement for power factor correction (PFC) in continuous conduction mode (CCM) operation. Reduction of feedback circuitry enhances compactness of the converter making it more suitable for on-board charger (OBC). The control scheme for PFC gives high power factor (PF) and low total harmonic distortion (THD) for both simulation and experimental validation. Further, converter stress and loss analysis have been carried out for selection of proper rating of semiconductor devices. The total losses of the proposed converter in ac/dc (plug-in charging) and dc/dc (propulsion and regenerative braking) stages have been found close to their conventional counterparts. It is due to the fact that both in ac/dc and dc/dc stages, one additional mechanical switch in the current path compared to conventional converters, and calculated loss in mechanical switch is negligible compared to semiconductor devices and passive components. An integrated converter has been derived from conventional two-switch buck/boost converter capable of operating for charging, propulsion and regenerative modes have been further investigated. The main advantage of this integrated converter is a peak efficiency improvement in propulsion boost and regenerative buck modes compared to an existing integrated converter which is most competent to the proposed converter, and low voltage/current stresses in each mode. Efficiency improvements in aforementioned modes are desirable in vehicle applications because the occurrence of these modes is more common than propulsion buck and regenerative boost modes. Moreover, this converter has two inductors but the size of the second inductor, i.e., L2 is approximately reduced by more than 25% compared to existing integrated converter. All the proposed converters are verified through simulation and experiment undertaken using laboratory prototype. The waveforms obtained from simulation and experiment are presented in this thesis.