Publications

Grid-forming converters (GFMCs), which provide superior grid formulation and supportive services, have been identified as the enabler of more-electronics power systems. Control of GFMCs mainly comprises outer-power and inner-voltage loops. Although existing literature assumes that the two loops can be independently analyzed and designed, we reveal that the converter may be unstable even with two individually stabilized control loops, thereby implying their strong coupling effect. To realize the decoupling design of power and voltage control loops, we propose a robust singular perturbation method, which comprises a marriage of robust control and singular perturbation theory. Specifically, we construct the converter model as an interconnection of slow and fast subsystems, and derive two quantitative criteria regarding system stability and performance, respectively. Through the proposed method, we successfully quantify the coupling effects between the two loops at each frequency point, enabling the identification of the maximum coupling and the critical coupling frequency. Accordingly, we provide insightful guidelines for simplified controller design. The correctness of the theoretical analysis and the effectiveness of the proposed decoupling method are validated by simulation and experimental results.

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

Initiatives to decarbonize contemporary power systems will lead to power systems dominated by inverter-based resources (IBR). These IBRs are broadly classified into grid-following (GFL) and grid-forming (GFM) inverter control regimes. In this paper, a small-signal state-space model is formulated for a 100% IBR-dominated power system. This formulation is then used to perform stability analysis on the IEEE 9-bus system. The analysis thoroughly investigates the impacts of varying GFM inverter penetration on system stability. The paper also discusses the system behavior in the wake of varying shares of different GFL inverter control schemes. The cause of instability is ascertained by investigating the participation factor of each component. Moreover, the paper examines the effects of phase-locked loop (PLL) and power synchronization loop (PSL) parameters on determining the minimum GFM penetration necessary for ensuring stable system operation. It has been found that the inverter type with the highest capacity is not always the biggest contributor to instability. Also, increasing the proportional coefficient and decreasing the integral coefficient of the PLL of GFL inverters allow the system to be operated under lower GFM penetration. Similarly, increasing the moment of inertia or damping coefficient of the GFM inverter’s power synchronization loop has a similar effect. The conclusions and the underlying theoretical analysis are verified by time-domain simulations of electromagnetic transient (EMT) models in Matlab/Simulink.

Multilevel converters are widely used in various industrial applications. One of the critical challenges in these systems is the suppression of common-mode voltage (CMV). Due to the intuitive nature of the space vector diagram, space vector PWM (SVPWM) can effectively regulate CMV. However, as the number of voltage levels in multilevel converters increases, the complexity of the SVPWM method also grows. Existing simplified SVPWM techniques, which are helpful, have not been fully successful in suppressing the CMV. To address the above issues, this paper proposes a generic SVPWM (GSVPWM) method. Unlike conventional approaches, the GSVPWM is based on the natural abc coordinate system, eliminating the need for coordinate transformations. In addition to effectively suppressing both the magnitude and frequency of the CMV, the GSVPWM further reduces the dv/dt values of the CMV by subdividing the equilateral triangular sectors into obtuse triangular sectors. The proposed method operates in real-time, without requiring look-up tables or complex calculations. It is easy to implement across different multilevel converters. Simulations and the experiments are conducted in a five-level converter to verify the effectiveness of the proposed GSVPWM.

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.

The lifetime of a high-speed Integrated Motor Drive (IMD) is highly dependent on its power converter, where power devices are the most prone to failures. This paper studies the reliability of SiC-MOSFET-based high-speed IMDs, highlighting its critical role in determining the overall lifetime of the system. The proposed reliability model estimates the End of Life (EoL) of power devices by analyzing the junction temperature, which is influenced by the load conditions based on the applied mission profile. The applied mission profile, which covers all operating conditions and a wide speed range, including maximum torque per ampere and field weakening strategies, is translated into the motor current and voltage. Then, the power losses of power devices and electrical motor are obtained through three-phase currents and voltages and applied to the thermal model of the IMD. In addition to the self-thermal effect of power devices, the mutual thermal effect of the electrical motor must be considered in the IMD's thermal model. To handle a long mission profile, the average loss model is adopted instead of using a heavy simulation. The proposed reliability model can help in designing heatsink and electrical motor cooling methods, contributing to enhanced thermal management in order to meet the lifetime requirements of specific applications for IMDs.

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

In this paper, a common ground three-terminal rectifier with active power decoupling ability is proposed. The proposed topology establishes a direct connection between the AC and DC ground, thereby suppressing common mode leakage currents and eliminating the demand for an isolation transformer. Additionally, the active power decoupling function enables the double line frequency ripple power to be absorbed by the decoupling circuit, significantly reducing the DC-link capacitance, which in turn enhances the power density and operational lifespan of the converter. Moreover, compared to existing decoupling topologies, the proposed design imposes lower voltage stress on the decoupling components, reducing the performance requirements for these devices. Finally, the feasibility of the proposed AC/DC converter is validated through simulation studies.

This paper presents an overall control approach for a grid connected photovoltaic (PV) system based on three-level DC-DC boost converter (3LBC) on the front of a three-level t-type inverter (3LT <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> I) acting as active power filter. The neutral-point (NP) voltage balancing is ensured by a new control strategy applied for 3LBC; this avoids the need to change the conventional three-level space vector modulation (SVPWM) algorithm or to add additional components. The capability of the proposed system to control the current injected into the grid, compensate the harmonics and the reactive power demand, and also guarantee the DC-link capacitors voltages balancing is demonstrated through variety of simulations.

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.

Predictive control is widely used in power electronic converters for its simplicity, clear objectives, and ease of constraint implementation. However, its computational demand increases with the pursuit of higher accuracy and performance, potentially compromising real-time operation due to the fast dynamics of converters. To address this challenge, this paper applies a novel differentiable predictive control method for real-time control of power converters. By integrating predictive control principles into the neural network framework, the method transforms the network into an automated predictive control solver through gradient descent optimization offline. Unlike supervised learning-based neural networks, this approach eliminates the need for pre-labeled data or extensive training, offering an efficient solution for achieving precise and timely control. This new method enhances the implementation feasibility of predictive control in real-time applications, addressing computational challenges while still maintaining high performance in power electronic systems.<br/>