Multiport DC-DC Converters for Hybrid Energy Systems

Immanuel Ninma Jiya will defend this PhD thesis Multiport DC-DC Converters for Hybrid Energy Systems 22 September 2023.

Summary of the thesis

Renewable energy sources (RESs) like solar and wind have gained attention for their potential to reduce reliance on fossil fuels and mitigate climate change. However, integrating multiple RESs into a power grid is challenging due to their unpredictable nature. Power electronic converters can manage hybrid energy systems by controlling power flow between RESs, storages, and the grid. Conventional single input dc-dc converters have limitations such as low efficiency, bulky designs, and complex control systems. Multiport dc-dc converters (MPCs) have emerged as a solution for hybridizing multiple sources, storages, and load systems by providing a common interface. Existing MPCs have limitations such as high component count, limited operational range, complex control strategies and restrictions on the number of inputs to list a few. Thus, there is a need to develop new MPCs that combine the advantages of existing designs while overcoming their limitations. Isolated MPCs with unipolar or bipolar outputs are needed that can accommodate any number of inputs, offer high voltage gain, use fixed magnetic components for galvanic isolation (regardless of the number of ports), and have a simplified control strategy. Additionally, new non-isolated MPCs with unipolar or bipolar outputs are required, featuring reduced component count, simultaneous power transfer and power flow between input ports, high voltage gain, low control complexity, and modular design allowing for arbitrary increase in the number of input ports. There is also an opportunity to apply MPCs in the integration of RESs and storages to ac grids through multilevel inverters for low component count, high efficiency, low harmonics, and higher power density. Further, advances in bipolar MPCs provide the chance to balance the dc bus without requiring a complex control system.

In this dissertation, five novel MPC topologies (TA to TE) of non-isolated (TA, TB and TD) and isolated (TC and TE) configuration with unipolar (TA, to TC) and bipolar outputs (TD and TE) were developed and verified for various hybrid energy system applications. All these contributions were made in eight publications (Papers I – VIII), including three journals and five conference papers. These papers are listed in Chapter 1, highlighting the details of their specific contributions, respectively. Further, the ac grid integration through integrating some of the MPCs (TA and TD) with MLIs were explored and validated. The integration of the bipolar MPCs to bipolar dc transmission/distribution infrastructure with the possibility of supplying a critical unipolar dc load was also verified. The initial idea of TA, a new non-isolated MPC with unipolar outputs was presented in paper I with more detailed analysis and the experimental validations presented in paper II. Paper III presents TC, an isolated MPC topology with unipolar outputs, along with key results. Paper IV, presents TD, a novel family of non-isolated MPCs with bipolar outputs, and key results including the ac grid integration using an MLI is verified in this paper. In paper V, a modified unidirectional version of TA was used to achieve the unipolar dc to ac grid integration, verifying the use of an auxiliary circuit and control-based approach for balancing the dc link voltage. Paper VII presents TB, an improvement to TA, which allows for power flow between the RES ports and the energy storage ports. The initial idea of TE, a new isolated MPC with bipolar outputs was presented in paper VI with more detailed analysis, and the experimental validations were implemented in paper VIII. The verification of integrating TE to a bipolar dc distribution infrastructure supplying a critical unipolar dc load was also presented in paper VIII. The steady state operation of these new MPC topologies was analysed mathematically and verified using detailed simulation and validated on an in-house hardware-in-the-loop (HIL) platform and on an experimental test rig, respectively. The novelty of the proposed MPC topologies is highlighted through detailed com energy practices. This dissertation has five main chapters: the first presents the introduction and background to MPC, the second reviews the state of the art while the third and fourth presents the new MPCs proposed herein and the main results, respectively, and a concluding Chapter 5 at the end, highlighting the key findings and the limitations of this PhD study. parative studies, underscoring some important metrics such as component count, modularity, and voltage gain to list just a few. The development of these converters can significantly contribute to the integration of RESs and storages into the power grid and promote sustainable energy practices. This dissertation has five main chapters: the first presents the introduction and background to MPC, the second reviews the state of the art while the third and fourth presents the new MPCs proposed herein and the main results, respectively, and a concluding Chapter 5 at the end, highlighting the key findings and the limitations of this PhD study.

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