Publications

Impact of Inverter Controls in Small-Signal Stability of Power Systems Dominated by Inverter-Based Resources

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.

Power control of a wind farm with active stall wind turbines and AC grid connection (paper and poster)

A Robust Singular Perturbation Method for Decoupling of Power and Voltage Control of Grid-Forming Converters

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.

A Generic SVPWM Method of Suppressing Common-Mode Voltages for Multilevel Converters

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.

Power Semiconductors for An Energy-Wise Society

This IEC White Paper establishes the critical role that power semiconductors play in transitioning to an energy wise society. It takes an in-depth look at expected trends and opportunities, as well as the challenges surrounding the power semiconductors industry. Among the significant challenges mentioned is the need for change in industry practices when transitioning from linear to circular economies and the shortage of skilled personnel required for power semiconductor development. The white paper also stresses the need for strategic actions at the policy-making level to address these concerns and calls for stronger government commitment, policies and funding to advance power semiconductor technologies and integration. It further highlights the pivotal role of standards in removing technical risks, increasing product quality and enabling faster market acceptance. Besides noting benefits of existing standards in accelerating market growth, the paper also identifies the current standardization gaps. <br/>The white paper emphasizes the importance of ensuring a robust supply chain for power semiconductors to prevent supply-chain disruptions like those seen during the COVID-19 pandemic, which can have widespread economic impacts.<br/>The white paper highlights the importance of inspiring young professionals to take an interest in power semiconductors and power electronics, highlighting the potential to make a positive impact on the world through these technologies.<br/>The white paper concludes with recommendations for policymakers, regulators, industry and other IEC stakeholders for collaborative structures and accelerating the development and adoption of standards.

Synchronization Stability Analysis of Current-Limited GFM Converters Considering Internal Voltage Amplitude Dynamics

Synchronization stability assessment of grid-forming (GFM) converters is essential during and after large disturbances in the external grid, where the active power control (APC) loop is perceived as the key concern since it is employed to generate the voltage phase angle. However, the impacts of the internal voltage amplitude dynamics and current saturation, determined by the reactive power control (RPC) loop and current limiter, are often neglected. This paper considers dynamic interactions between internal voltage magnitude and phase angle to derives an improved power-angle curve of the circular current-limited GFM converter, which facilitates more accurate identification of the critical angle margin for evaluating the synchronization stability after phase jumps. Furthermore, the impact of the RPC coefficient is quantified to provide parameter design recommendations for enhancing the post-phase jump synchronization stability. The analytical results are validated by simulations and experimental tests.

Accelerating Online Optimization with Differentiable Predictive Control in Power Electronics

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/>

Performance Exploration of Black-Start Block Loads Pick-Up Sequences for Grid-Forming Dominated Systems

After a total or partial system outage, black-start serves as a critical measure to restore the system to the normal operation. In the later stage of restoration, various types of loads will be energized, the pick-up sequences of which may affect the success probability of the black-start. In this context, a block loading strategy for the inverter-based low-inertia systems is explored, with special consideration given to the inertial support by induction motors. Through detailed analysis, it is shown that initiating induction motors at an early stage leads to improved frequency stability during subsequent loads pick-up. The advantages of prioritizing motor energization are validated from multiple perspectives, including motor starting success rate, the timing of block load pick-up, and the overall system stability. Both simulation results and hardware-in-the-loop validations confirm these observations.

In-Situ Monitoring of Lithium-Ion Battery Health Using Acoustic Emissions

The durability and dependability of lithium-ion batteries (LIBs) play a crucial role in advancing energy storage technologies that power modern applications, including electric vehicles (EVs), consumer electronics, and large-scale energy storage systems. Effective battery health monitoring is essential not only for ensuring safe and efficient operation but also for optimizing battery performance and facilitating second-life applications. Conventional diagnostic techniques, such as voltage and current profiling or impedance spectroscopy, have been widely utilized for LIB assessment. However, these methods often face challenges, such as susceptibility to environmental influences, the need for offline analysis, and dependence on direct electrical connections, limiting their suitability for real-time monitoring. Acoustic Emission (AE) technology has gained attention as a promising, non-destructive approach for real-time monitoring of LIB degradation. By detecting mechanical waves generated during battery operation, AE provides insight into internal structural changes and helps identify key degradation phenomena, including electrode cracking, gas formation, and electrolyte leakage. These acoustic signals offer a unique perspective on stress accumulation and material transformations within LIBs, contributing to a deeper understanding of aging mechanisms. In this study, AE was employed to investigate LIB degradation mechanisms, beginning with graphite/Li and NMC/Li coin cells assembled in a glovebox using LiPF6 in a 1:1 EC:DEC electrolyte. During charging at a 0.5 C-rate, a differential wideband sensor was affixed to the coin cell surface to capture AE signals. The recorded acoustic fluctuations were analyzed, and identical coin cells were aged under similar conditions for validation. Observations revealed significant graphite exfoliation at the fully charged state, with an amplitude of 65 dB. Incremental analysis indicated that the highest amplitude occurred at the peak value, corresponding to the phase transition point, suggesting a strong correlation between AE signals and structural changes during cycling. Post-mortem analysis was conducted to establish correlations between AE signals and electrode structural changes, providing a preliminary link between acoustic signatures and degradation processes. Subsequent experiments focused on commercial LIBs, where testing was carried out within a specially designed foam enclosure to minimize background noise. Advanced digital processing techniques were applied to the AE data to filter out interference and categorize acoustic events based on coin cell spectra. Initial results suggest that AE signals, such as those associated with microcracking, provide distinct markers of electrode degradation. These findings demonstrate AE’s potential as a diagnostic tool for tracking LIB health and detecting critical aging phenomena. Future research will aim to establish more precise correlations between AE events and specific electrochemical reactions occurring within LIBs. By integrating AE with complementary characterization techniques, a more comprehensive understanding of battery aging mechanisms can be achieved. Overall, this study underscores the potential of AE technology to transform LIB health monitoring by providing a real-time, non-invasive, and scalable diagnostic solution. This advancement contributes to improving battery reliability, extending operational lifetimes, and supporting the development of more sustainable and efficient energy storage solutions for future applications.

Electrical Topologies of Wind Farms Based on Different Wind Turbines

An improved Noise Covariance Adaptable X – Unscented Kalman Filtering algorithm for accurate state of charge estimation in lithium-ion batteries adaptive to varying temperature conditions

Topological and Modulation Design of Three-Level Z-Source Inverters

Investigation and Improvement of Transient Response of DVR at Medium Voltage Level

Dynamic Voltage Restorer (DVR) is proposed for use in the power distribution network to protect sensitive loads from sudden sags in grid voltages. An area of interest for DVR research is the damping of transient LC oscillations initiated at the start of a voltage sag and at the recovery instant from a voltage sag. Nonlinear loads, with harmonic currents around the DVR LC filter resonance frequency, would continuously excite resonance oscillations. For compensating voltage sags and damping high frequency oscillations simultaneously, an investigation of the transient response of the DVR has first been carried out, with the inclusion of the source of LC resonance and factors that affect this resonance. Possible control schemes and their effects on the oscillation attenuation are also studied. Such studied control schemes are: single voltage loop control, voltage feedback plus reference feedforward control, double-loop control with an outer voltage loop and inner current loop. Subsequently an effective and simple resonance damping method is proposed by employing closed-loop (single-loop or multi-loop) control with an embedded two-step Posicast controller. Finally, the transient responses for proposed control methods have been extensively tested on a 10 kV laboratory DVR system with different loading conditions. It is shown that with a linear load, the proposed damping methods are effective for transient response improvement, while for a nonlinear load, the methods would improve both the transient and steady state performance.

Improved harmonic loss – History gated unit recycling for online state of charge and state of energy co-estimation of lithium-ion batteries for large-scale energy storage stations

Keynote speech

These keynote speeches discuss the following: Power Electronics - Trends and Research Challenges; Stability Issue for Power Systems with Multiple Electronic Converters: What Is the Appropriate Analytical Approach?; Modular Multilevel Cascade Converters for Grid Connections and Motor Drives; Series Compensation of Open-Winding PM Machines.

Generic and Flexible Digital Model of DAB Converters With Various Topologies and Control for Efficient Optimal Design

To evaluate the performance of dual-active-bridge (DAB) converters across various topologies and control schemes for optimal design, this article 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.

Virtual Synchronous Generator Stability Enhancement and Power Oscillation Suppression with Adaptive Virtual Impedance Control

Forskningsfriheden er under pres

I en nylig rapport konkluderer Danmarks Forsknings- og Innovationspolitiske Råd (DFiR), at universitetsloven fra 2003 har været en succes i forhold til at styrke universiteternes relationer til omverdenen. Inden for de sidste to år har 96 pct. af forskerne været engageret i forskningssamarbejde, rådgivning, efteruddannelsestilbud eller anden formidling. I gennemsnit har en forsker ved et dansk universitet været involveret i fire forskningssamarbejder, seks rådgivnings- eller efteruddannelsesaktiviteter og otte formidlingsaktiviteter inden for de sidste to år.