Design and control of resonant dual-active-bridge converter for medium voltage DC networks

  • Ibrahim Alhurayyis

Student thesis: Doctoral ThesisDoctor of Philosophy

Abstract

The current extensive employment of large-scale renewable energy sources (RESs) and energy storage systems (ESSs) due to high demand expectation for energy presents the medium voltage DC (MVDC) networks as a distinguished technology to connect more distributed energy resources. In DC networks, a group of AC and DC sources are connected to a common DC bus or mesh type DC network via power electronic interfaces. This interconnection process essentially depends on dc-dc power converters. Converters with precise control strategies can ensure the optimum integration enhancing better performance and stability. Thus, providing a control scheme of DC-DC power converters is the main objective to tackle most commonly obstacles such as voltage regulation and current sharing in different applications of MVDC networks.

Among different topologies of power converters for MVDC applications, the resonant dual-active-bridge (DAB) converter can be a significant candidate because of galvanic isolation, simple structure with scalability, and soft-switching features reducing losses. Also. DAB converter can block double the voltage level when it is compared with traditional DC-DC converter, so it is perfect for medium voltage applications. Besides, it allows medium or even high switching frequency operations which leads to reduce components size and volume. Modular configuration based on DAB such as input-parallel output-parallel (IPOP) provides the feature of scaling up power and current sharing with better behavior against faults.

To control the DAB, the power calculation of dual-phase-shift (DPS) modulation is used for designing both sliding mode control based on direct power control (SMC-DPC) and super-twisting SMC (ST-SMC). They are applied to both conventional and CLLC DAB converters. They are developed to ensure fast response and stability against system variations, along with the proportional-integral (PI) controller as a benchmark. The DPS scheme is used to reduce the backflow power and current stress with acceptable flexibility in medium voltage level.

The primary contribution of this thesis involves proposing current sharing control strategies based on PI and SMC for IPOP resonant CLLC DAB converters. The aim is to enhance the efficiency of interconnecting MVDC and LVDC networks. These strategies are applicable to both non-resonant and resonant converters, such as LLC and CLLLC DC-DC converters. Effective current sharing among modules is crucial in IPOP systems to fully leverage the benefits of the modular configuration. The proposed control strategies have been developed to achieve output voltage regulation and effective current sharing control. Additionally, a technique to reduce the number of current sensors has been proposed, offering a cost-effective alternative to conventional master-slave methods. To validate the effectiveness of these control methods, an experimental prototype of a downsized CLLC DAB resonant converter with a medium power planar transformer was designed and tested, alongside computer simulations.

Thesis embargoed until 31st December 2025
Date of AwardDec 2023
Original languageEnglish
Awarding Institution
  • Queen's University Belfast
SupervisorAhmad Elkhateb (Supervisor) & Timothy Littler (Supervisor)

Keywords

  • DC-DC power converters
  • dual active bridge
  • DAB
  • resonant dual active bridge
  • CLLC DAB
  • Sliding mode control
  • SMC
  • MVDC
  • Input-parallel output-parallel
  • IPOP
  • Super-twisting SMC
  • current sharing control

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