Mid Sweden University

This study investigates the feasibility of utilizing camshaft-driven contactor in 48 V battery systems for future electric vehicles (EVs). Most EV powertrains operate at high DC voltages, which introduces several challenges. These include high costs associated with safety regulations, increased component expenses for galvanic isolation requirements, isolation monitoring devices, and heightened safety risks. In contrast, the proposed 48 V design could reduce safety risks, lower production and maintenance costs, and simplify operation without compromising performance. The proposed Reconfigurable Battery System (RBS) integrates contactors designed to manage voltage differences between modules during operation. These contactors, of which the RBS requires a large number, are essential for maintaining system reliability and energy efficiency, as they minimize wear and losses during extended periods of non-use. Their quality is crucial to ensuring the overall reliability of the system. This paper presents a feasibility study of a camshaft-driven contactor design for RBS. The design was tested to assess its reliability and performance beyond the system's operational parameters. Experimental results, based on repeated engagement period testing under specific conditions and varying voltages, indicate promising outcomes. 

This study investigates a Cascaded H-Bridge converter with Electric Double-Layer Capacitors as integrated energy storage components. As the DC-link voltages are variable, the modulation index and number of submodules contributing to the active power delivery vary according to state of charge. The nearest level control algorithm for this application is studied, and expressions for the duty cycle of conventional Nearest Level Modulation are derived. A modification of the sort and select algorithm to determine which submodule is to be inserted and bypassed when using the Nearest Level Control algorithm is proposed to distribute the activation time and the experienced RMS current of the submodules. Expressions for the duty cycle of each inserted submodule for the proposed algorithm is presented and compared to the conventional. Simulation experiments of current sharing between submodules under active power delivery for the conventional and proposed Nearest Level Control is conducted for an 11- level, 41-level and 61-level converter. Simulation experiments show a reduction in RMS current for the submodule experiencing the highest thermal stress. Over the course of power delivery and increasing modulation index, the peak RMS current increase for the conventional nearest level modulation while it is kept constant for the proposed modulation scheme. 

An intrinsic feature of EDLCs is their voltage dependency on the state of charge. This voltage variation is typically addressed by introducing a DC-DC interface between the energy storage components and the switching bridge for the integration in power electronic inverters. While this additional converter interface ensures a nominal voltage output, it introduces additional losses, cost and may reduce system reliability. This paper proposes a solution to mitigate the need for an additional inverter interface by utilizing redundant submodules in the Cascaded H-Bridge converter and scaling the initial modulation index to produce the desired output at varying voltage levels of the EDLCs. A reduced state model predictive control scheme combined with a sort-and-select algorithm is employed to implement the control and balance. A simulation study of a 41-level converter injecting 0.5 MW of active power into the grid is presented. A proof-of-concept prototype converter with direct integrated EDLCs is presented.

This article presents a review of the empirical core loss models for symmetric flux waveforms. These empirical models are based on Steinmetz and Bertotti loss models. The principles, advantages, and limitations of the existing models are explained and discussed. The models are evaluated based on four main criteria: 1) physics background, 2) complexity, 3) accuracy, and 4) flexibility and generality to include multiple effects. This article also discusses some scientific issues in existing works regarding the characterization of relaxation loss.

Existing works to increase the power density of LLC transformer are mainly based on the MHzapproach. This paper proposes a new LLC transformer design based on the recent introducedconcept called the unbalanced-flux magnetics. The transformer was optimized using Genetic algorithm and iterative optimization approach. The unbalanced-flux transformer was designed and experimentally tested in 1 kW 400/48V LLC converter. The LLC converter is built, and the transformer is successfully up-to 750 W without heatsink and 1 kW with heatsink. The realized realizing power density is 224W/cm3 which is approximately 3x higher than the results in the state-of-the-art. 

High-power-density magnetics are often designed through the MHz-frequency approach, but this method is constrained by excessive core losses, which restrict operation to very low magnetic flux density B, typically lower than 70 mT. Since magnetic power scales with the square of B, this fundamentally limits achievable power density. In practice, the average power density of state-of-the-art MHz magnetic designs remains around 70 W/cm3, far below the requirements of emerging applications. To overcome these limitations, we propose the unbalanced-flux concept, a new magnetic architecture that replaces the traditional balancedflux design paradigm. This approach reduces core loss by approximately 70%, enabling operation at substantially higher B. As a result, the concept achieves a threefold increase in power density compared to MHz-based designs. The approach is experimentally validated in a 1 kW, 380 V/48 V LLC transformer prototype. With natural air cooling, the transformer delivers 750 W at a record 224 W/cm3. With a heatsink, it achieves 190 W/cm3 at 1 kW, with a peak efficiency of 94.6%. These results demonstrate that unbalanced-flux magnetics provide a scalable pathway beyond the MHz, low-B limitations toward the next generation of ultra-high-power-density magnetic components. 

Existing equations to calculate the air gap for DC inductor are ambiguous and includes redundant variables. This paper presents a new design method of the DC inductor based the fundamental air gap equation. This later is basically Ampere’s law applied for the design of DC inductor. Furthermore, this paper investigates the inter-reaction between the inductor’s variables such as the number of turns, the core’s cross section and the air gap. The proposed method has several advantages over the existing area product method. First, it gives a good understanding of the interdependence between the inductance variables. Furthermore, the number of turns is calculated before and without the need to know the required magnetic core. Eventually, the filling factor can be accurately determined and finally the optimal is selected accordingly. 

This paper presents an experimental investigation on switching transients in a submodule of a cascaded H-bridge converter with the aim of investigating effects of direct integration of supercapacitor modules in switching converters. Departing from simulation studies focusing on overvoltage transients and effects from the Equivalent Series Inductance of the supercapacitor module. A passive absorption circuit is introduced to mitigate the transients on supercapacitor and switching components. The simulations are compared to experimental results on a submodule prototype. An operating point of the submodule corresponding to an output power of 6.2kW is investigated in a double pulse test. The experimental results show that effects of the series inductance across the switches are mitigated. The relationship between the equivalent series inductance of the supercapacitor and the rate of change of current is identified as causing overvoltages across the supercapacitor. Methods to handle the rate of change of current and high frequency voltage ripple across switches are suggested.

The increasing adoption of battery storage units alongside private PV systems may prove to be a new challenge for distribution grid operators. This study explored the potential impact of marketing aggregated battery discharge power as a Fast Frequency Reserve (FFR) and its effect on the distribution grid stability. We investigated the efficacy of Line Voltage Regulators (LVRs) in mitigating voltage surges caused by simultaneous battery activation. For this purpose, a simulation was developed via Matlab (Version R2023a) to simulate the voltage at the nodes of an arbitrary distribution grid, using the feed-in and consumed power of the customers as the input. We applied the model to a distribution grid section in Sundsvall (Sweden). The results confirmed that LVRs can amplify voltage surges when their adjustments are not synchronized with the FFR activation. This study underscored the need for proactive measures to address the voltage maintenance challenges arising from the integration of battery storage units and renewable energy sources.

This paper presents a new design concept to increase the efficiency and the power density of planar magnetics. In contrast to the existing magnetics, which are built using the balanced magnetic flux density design, the proposed design concept generates unbalanced magnetic flux density across the different parts of the magnetic core. The theoretical analysis shows that the core loss of the unbalanced-flux design can be reduced by more than 50% compared to the existing one. The core loss reduction brings several benefits to planar magnetics such as: high power capability, better thermal performance and wider safe operating area (SOA). The proposed design is experimentally evaluated and compared with the balanced-flux design. The experimental results are in good consistency with its theoretical counterparts. The measured core loss are decreased by more than 50% and the power density is increased by more than 250%.