Publications

Optimal design of power electronic converters requires extensive simulations, data collection, and performance comparison under various configurations of operating parameters, power devices, and control strategies. To improve design efficiency, an optimal design tool (ODT) for dual-active-bridge (DAB) converters is developed based on an explicit digital model and an optimal operating framework, which provides accurate performance prediction and fast optimization determination. The circuit-power model and loss-thermal model are integrated into the ODT with a generic closed-loop control structure. Therefore, the major performance indicators of the DAB converter, including dynamic and steady-state voltage and current waveforms, power losses of power semiconductors and passive components, and junction temperature, can be predicted by the proposed ODT. The evaluation results support efficiency/reliability-oriented design. Based on the ODT, a case study for efficiency improvement in terms of power semiconductors and control strategies is demonstrated. A database of qualified power devices and various control strategies is established, and the efficiency prediction under all potential combinations is executed with a tailor-made automated configuration-sweeping method. This enables comprehensive performance assessment and fast identification of the loss-minimizing configuration without manual adjustment of device models or control schemes. Thus, design efficiency can be improved especially when a large number of power semiconductors and modulation strategies are under evaluation. Simulation and experimental results confirm that the proposed ODT achieves both high accuracy and fast execution.

The design of control algorithm parameters has a significant impact on the reference tracking accuracy and efficiency of power electronic converters. Therefore, it is important to relate the control parameter design to the converter's performance metrics and operating conditions in order to achieve optimized performance. In conventional implementations, these parameters are precomputed and remain nearly constant throughout the converter's operating range. This can lead to sub-optimal performance when the metrics are highly dependent on the converter's loading conditions. Moreover, the relationship between control algorithm parameters, operating conditions, and performance metrics can be complex and difficult to model. In this paper, the Kolmogorov-Arnold Network (KAN) is proposed to derive an analytical expression that maps control parameters to converter performance under varying load conditions. The approach is demonstrated through the design of weighting factors in a finite-set model predictive control (FS-MPC) algorithm. Experimental validation confirms that the resulting analytical expression effectively controls the average switching frequency under different loading conditions. The proposed method offers high accuracy with low computational complexity.

To evaluate the performance of dual-active-bridge (DAB) converters across various topologies and control schemes for optimal design, this paper proposes a generic explicit digital model applicable two-level (2L)/three-level (3L) half-bridge (HB), full-bridge (FB), and three-phase (3P) topologies with a unified control structure. Voltage and current expressions for a single bridge leg considering both 2L and 3L topologies are developed in a generalized manner, and thus the model of DAB topologies with different number of legs can be constructed by superimposing the corresponding number of individual bridge leg models. Subsequently, the digital model, i.e., discrete-time model of DAB converters with different topologies and control configurations, is derived based on the circuit differential equations and a unified closed-loop control system by applying backward Euler method. Therefore, both steady-state and dynamic performance of DAB converters can be predicted in the proposed model. Owing to the analytical formulation of operating parameters and generic expressions for different topologies and control, the calculation speed of the proposed digital model can be accelerated compared to commercial simulation platforms such as MATLAB/Simulink and PLECS, and the design efficiency can be significantly enhanced, especially when it eliminates the need for frequent manual adjustments of topologies, modulation, and loss-thermal models of power devices. Simulation and experimental tests validate that the digital DAB model can achieve high accuracy under various topologies with fast execution speed.

Screening small-signal stability risks in converter-dominated power systems comprising multiple black-box converter-based resources is challenging, since a high-dimensional operating point (OP) space is involved to identify the small-signal instability boundary (SSSB). This paper introduces a SSSB generation method that combines balanced truncation and eigenvalue perturbation theory within an eigenvalue-based analysis framework. The method systematically identifies the critical mode(s) that can become unstable under OP variations and efficiently computes the corresponding SSSB. Its effectiveness is demonstrated on a converter-based energy island type system, where the identified SSSB is validated against electromagnetic transient simulations. The computational efficiency of the proposed method and its expandability to large-scale power systems is quantified by comparison with an existing SSSB generation method, showing a substantial reduction of up to 99.6% in computational time.

This paper proposes a new technique for grid synchronization under unbalanced and distorted conditions, i.e., the dual second order generalised integrator - frequency-locked loop (DSOGI-FLL). This grid synchronization system results from the application of the instantaneous symmetrical components method on the stationary and orthogonal alphabeta reference frame. The second order generalized integrator concept (SOGI) is exploited to generate in-quadrature signals used on the alphabeta reference frame. The frequency-adaptive characteristic is achieved by a simple control loop, without using either phase-angles or trigonometric functions. In this paper, the development of the DSOGI-FLL is plainly exposed and hypothesis and conclusions are verified by simulation and experimental results

Voltage sags are a major problem in present distribution systems. Therefore, different solutions have been examined to compensate these sags to avoid production losses at sensitive loads. In particular, Dynamic Voltage Restorers (DVRs) can realize this goal. Presently, a system wide integration of DVRs is hampered because of their high cost, in particular, due to the expensive DC-link energy storage devices. The cost of these DC-link capacitors remains high because the DVR requires a minimum DC-link voltage to be able to compensate a sag. As a result, only a small fraction of the energy stored in the DC-link capacitor can be used, which makes it almost impossible for the DVR to compensate relatively long voltage sags. Present control strategies are only able to minimize the distortions at the load or to allow a better utilization of the storage system by minimizing the needed voltage amplitude. To avoid this drawback an optimized control strategy is presented in this paper, which is able to reduce the needed injection voltage of the DVR and concurrently to mitigate the transient distortions at the load side. In the following paper a brief introduction of the basic DVR principle will be given. After this, an optimized control strategy will be briefly described. Finally, experimental results using a medium-voltage 10 kV DVR setup will be shown to verify and prove the functionality of the presented control strategy in both symmetrical and asymmetrical voltage sag conditions.

The operation of water electrolyzers with renewable energies means that long-term operation also involves several start-up/shut-down cycles of the system. In particular, for alkaline water electrolyzers (AWE) it is known that shut-down leads to reverse currents, which causes electrochemical degradation and thereby reducing the efficiency. In this work, a modeling framework is proposed and implemented for the electrochemical degradation of nickel electrodes in AWE stacks under start-up/shut-down (SU/SD) operation, and it is achieving good agreement with experiments. Such a model can be used for digital twins of electrolysis plants as well as optimizing its operation.

This paper describes modeling of wind farm controllers. The paper contributes to the IEA Annex XXI “Dynamic models of wind farms for power system studies” and was presented at the Annex XXI workshop in EWEC 2006. The wind farm controllers are developed with features, which make the wind farms able to participate in the control of the power system. The modeled wind farms provide active and reactive power control in the point of common coupling of the wind farm, which can be combined with automatic frequency and voltage control with droop and dead band.

This paper proposes a single-stage bridgeless buck-boost power factor correction (SSBLBB-PFC) converter with two-terminal outputs. By adopting a minimized loop inductance design and using only one high-frequency switching device in both the boost and buck modes, the circuit loss has been significantly reduced. Additionally, the twoterminal output structure is more versatile than the conventional three-terminal output structure, and it can also reduce voltage stress. Additionally, the single inductor is reused to achieve power conversion in the positive and negative input voltage, achieving 100% utilization of the DC inductor. The dead beat control (DBC) is used to control the inductor current to achieve PFC. A detailed analysis was conducted on the working modes and control strategies of the converter. The simulation results under the 220 V power grid prove that the proposed inverter has a better dynamic performance.

The paper deals with a wound rotor induction generator (WRIG), also known as the doubly-fed induction generator. A complete experimental set-up is presented and analyzed. It is composed of: WRIG, two power electronics converters connected in the rotor side of WRIG, a line filter, and the data acquisition and control system (dSpace DS 1103). Both converters are commercial units and are vector controlled using appropriate interfaces. They are back-to-back connected, sharing the same DC bus, one supplied through a line filter from the power grid, and the other one with the output on the rotor of the generator. The stator of the generator is directly connected to the power grid. Two stator flux observer topologies were investigated and compared, one with the voltage model in parallel with the current model and the other one with both models connected in series. A speed estimation strategy, which works also during the synchronization procedure, was implemented and tested. It is based on model reference adaptive system (MRAS) principles. All control strategies, the flux observers and the MRAS, were developed in Matlab-Simulinkreg and implemented using a dSpacereg DS1103 single-board control and acquisition interface. Different tests were performed, and sample results are presented and discussed in the paper. The schemes used are illustrated in the paper, and the experimental results are shown and analyzed

In microgrids, DC-side short circuits can produce high fault currents that threaten converter safety. Conventional converters often struggle to limit these currents effectively. This paper presents a bipolar single-phase AC/DC converter designed for microgrids, featuring inherent output short-circuit current limiting. To reduce fault current, a small DC bus capacitance is used, which poses challenges in suppressing voltage ripple. To address this, a ripple suppression control strategy is proposed, leveraging the converter's operating modes to maintain stable output under small capacitance conditions. The proposed solution improves system protection and enhances overall reliability.