Please use this identifier to cite or link to this item:
http://hdl.handle.net/10266/6525
Title: | Investigations on Multilevel Converter Interface for Electric Vehicle Charging |
Authors: | Jain, Anekant |
Supervisor: | Jain, Sanjay K. Gupta, Krishna Kumar |
Keywords: | Bidirectional;Buck-boost rectifier;Electric vehicle;Grid-to-vehicle;Multilevel converter;Power factor correction;Switched capacitor;Vehicle-to-grid;Wide output voltage range |
Issue Date: | 28-Jul-2023 |
Abstract: | Increasing concerns over the pollution caused by the tailpipe emissions from the internal combustion engine based vehicles and the limited availability of fossil fuels have greatly paced up the adoption of Electric Vehicles (EVs). Recent advances in battery technologies, power electronics, digital controllers, electric machines and sensing technologies have laid the foundation for the development of the next generation EV technology. As such, power electronics interface plays a pivotal role in EV battery charging. The power electronics interface for both on-board and off-board EV charging generally comprises two stages: (a) AC-to-DC conversion stage with Power Factor Correction (PFC) and regulation of the intermediate DC link voltage; and (b) DC-to-DC conversion stage for regulating the charging current for the EV battery. This work deals with novel PFC converters for the AC-to-DC conversion in the futuristic and emerging EV charging systems. Multilevel Rectifiers (MLRs) have been specifically investigated as they offer numerous advantages, such as: utilization of low voltage power switches, highly improved harmonic profile of the alternating voltage at the input terminals, low dv/dt stress, modularity and so on. The major features identified for such multilevel PFC rectifiers for the upcoming two-stage EV charging systems are: (a) wide range of the output DC voltage so that a large spectrum of EV-battery voltages (typically between 72V to 800V in the commercially available EVs) can be addressed with minimal strain on the downstream DC-to-DC converter; (b) possibility of bidirectional flow of power so that Vehicle-to-Grid (V2G) mode of operation can be attained; (c) easy realization and extension of the power converter for both single- and three-phase systems; (d) possibility of multi-output operation so that multiple EVs can be simultaneously charged using a common central PFC rectifier; and (e) possibility of self-balancing of the capacitors in the MLR so that complexity in the overall control is minimized. The conventional multilevel topologies, when operated as inverters, work with unity voltage gain. As a result, while performing AC-to-DC conversion, they work as boost rectifiers, thereby severely limiting the range of output voltage. Difficulty in voltage balancing of capacitors and extension to three-phase systems are other additional limitations of many of the MLRs proposed till date. Thus, this work begins with the examination of topological and operational features of switched-capacitors based multilevel topologies, which have been mainly proposed as inverters (SCMLIs), in a standalone mode of operation (i.e. without being connected to the grid). SCMLIs offer inherent boost (i.e. the voltage gain is greater than unity in the inverter mode) and sensor-less self-balancing of capacitors. The question raised (and answered subsequently) in this work is: can switched-capacitors based multilevel topologies perform PFC rectification while offering wide output range (by virtue of inherent gain) and preserving self-balancing of capacitors’ voltages. Therefore, to begin with, a novel grid-connected 13-level switched-capacitors’ based inverter is presented. The topology is realized through 12 power switches, 2 diodes, and 3 self-balanced capacitors. It offers an overall voltage gain of six. The capacitors remain self-balanced for all values of the modulation index. The presented topology is validated experimentally and is found to be highly modular and cost-effective in comparison to other similar SCMLIs. Study of such a grid-connected SCMLI sets the stage for studying the ‘role reversal’ so as to perform rectifier operation. The work on the development of a novel PFC five-level rectifier utilizes self-balanced switched capacitors. Each leg of the presented topology comprises five power switches and one switched capacitor, where the voltage ratings of power switches are equal to the output DC voltage. It does not require an additional filter capacitor on the DC side, as the load always appears in parallel with a switched capacitor of one of the legs. The five-level operation with continuous conduction leads to the elimination of the capacitive filter on the AC-side and inductive filter on the DC-side. The performance of the presented topology is discussed through operating principle, modulation strategy, closed-loop control, and design aspects and it is validated through experimental results. Its features such as buck operation with a wide output regulation, the possibility of bidirectional flow of power needed for V2G systems, and easy realization of its three-phase version by simply adding one more leg make the topology suitable for EV charging application. While this topology establishes the proposed theory of role reversal in switched capacitors based multilevel topologies, it synthesizes only five levels with a voltage gain of two. In order to further increase the number of levels and voltage gain, two seven-level rectifiers with voltage gain of three are conceptualized. The working principle, modulation strategy and applications of the proposed topologies have been discussed and are validated through experimental results. The first of these seven-level topologies use reduced number of components, but a few of the power switches experience a voltage stress equal to thrice the output DC voltage, and hence it is not extended to its three-phase version. The second seven-level topology uses power switches with equal voltage stress and is highly modular, and hence it is investigated for three-phase operation too. This three-phase topology, however, becomes complex to be used for multiple outputs, as it requires a large number of capacitors. Finally, a novel five-level SCs based self-balancing buck PFC MLR is proposed with a wide output voltage range, which can operate with different loads at the output terminals and also operate in both single and multiple output modes. Moreover, load voltage balancing is feasible even when the multiple loads have different values. The proposed rectifier performs buck operation and offers a wide output voltage regulation, which is suitable for EV battery charging. The performance of the presented topology has been investigated through operating principle, modulation strategy, closed-loop control and experimental validation for the operation with single-/multiple-output load variations. The load voltage stabilizes at reference DC voltage when the grid current is raised, unity power factor mode of operation is preserved. The proposed rectifier exhibits high techno-economic feasibility when compared against other such structures. |
URI: | http://hdl.handle.net/10266/6525 |
Appears in Collections: | Doctoral Theses@EIED |
Files in This Item:
File | Description | Size | Format | |
---|---|---|---|---|
Thesis_Anekant_901904009.pdf | 37.64 MB | Adobe PDF | View/Open Request a copy |
Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.