Recent Advances in Power Quality Analysis and Robust Control of Renewable Energy Sources in Power Grids: 2nd Edition

The continuous integration of renewable energy sources and power electronic converters into modern power grids has improved power quality and control engineering. Although these technologies offer flexibility, higher efficiency, and decarbonization benefits, they also introduce challenges such as harmonic distortion, voltage instability, and complex generator–converter interactions. The six contributions in this Special Issue address key aspects of these challenges, provide experimental evidence and methodological advancements, and highlight remaining research gaps.

The authors of paper no. 1 present a comprehensive experimental comparison of direct and indirect control strategies for active power filters (APFs). Their work shows that direct control achieves a faster dynamic response, while indirect control provides greater robustness under varying load conditions. After individually testing FPGA-based APFs, they observed that indirect control resulted in the lowest supply current distortion, achieving the minimum current total harmonic distortion (THD) at a 230 V phase-to-ground supply voltage. Indirect control proved to be the most effective solution for harmonic mitigation, delivering superior performance compared to other control strategies in reducing supply current distortion.

Paper no. 2 presents the design and experimental validation of a low-voltage test bench for implementing and testing shunt Active Power Filters (APFs) to mitigate current harmonics and improve power quality. Both direct and indirect control strategies are described, with the study focusing on the indirect control method due to its simplicity and minimal sensor requirements. Simulation and hardware results using an FPGA-based platform demonstrate effective harmonic reduction, achieving supply current THD within standard limits (approximately 3%). The conclusion highlights the test bench’s suitability for future research and the practical advantages of indirect control in real-world three-phase systems.

Paper no. 3 presents an advanced mathematical model of a switched reluctance generator (SRG) that incorporates mutual coupling between phases, iron losses, and remanent magnetic flux—effects which are typically omitted in conventional models. The model relies exclusively on data obtained through DC excitation, eliminating the need for machine design details such as material properties. It represents iron losses using current components from look-up tables and accounts for induced electromotive forces due to remanence. Validation against experimental results from an 8/6 SRG (1.1 kW) shows improved accuracy, with the advanced model reducing the error between simulated and measured input power compared to conventional models. The authors conclude that the model is effective and applicable to various control strategies.

A novel grid connection method for a squirrel-cage induction generator (SCIG) is described in paper no. 4; this uses a partial power electronic converter in parallel with a capacitor bank to reduce inrush current and improve power quality during grid synchronization. Unlike direct connection or full-rating back-to-back converters, this configuration supplies only the generator’s reactive power requirement, reducing the converter rating and increasing robustness. The control system regulates generator voltage build-up and synchronizes connection to minimize grid disturbances. Experimental results confirm the effectiveness of this approach in maintaining grid voltage quality and reducing connection transients.

The authors of paper no. 5 propose a harmonic state estimation (HSE) method for distribution grids to assess power quality as inverter-based distributed generation increases. Because traditional monitoring of all nodes is costly and impractical, the authors develop a physics-aware neural network that combines network topology knowledge with data-driven learning to estimate harmonic magnitudes at unmeasured locations. Training data are generated using OpenDSS simulations across diverse load profiles. Results show that the proposed approach achieves high-accuracy estimation of harmonic orders from 1 to 20, demonstrating feasibility for near-real-time monitoring with sparse measurement infrastructure and potential for enhanced grid monitoring.

Finally, paper no. 6 proposes a goal-function-based decentralized control strategy for microgrid primary control that simultaneously regulates voltage and frequency and compensates for voltage harmonics using grid-tied inverter current injection. Unlike traditional droop methods, the proposed control does not assume specific network X/R ratios to derive control laws, making it applicable in both inductive and resistive conditions. Priorities within the control can be adjusted to reflect user preferences. The method is independent of network topology, suitable for both islanded and grid-connected microgrids, and is scalable. Detailed simulation results confirm improved harmonic mitigation and enhanced microgrid stability under nonlinear loads.

Collectively, these six studies reveal several emerging trends and persistent gaps in the field of power quality and converter-based renewable systems. First, experimental validation is essential, as simulation-based assessments alone cannot capture converter nonlinearities, switching transients, and hardware-specific effects. Second, integrating unconventional generators, such as SRGs and SCIGs, introduces dynamic interactions that require coordinated control and co-design with interfacing converters. Third, adaptive, data-driven, and multi-level control strategies are increasingly necessary to maintain stability and mitigate harmonics in weak or distributed grids. Fourth, there is a pressing need for standardized performance metrics that include both steady-state and dynamic quality indicators, enabling consistent evaluation across topologies and control approaches.

In conclusion, the contributions in this Special Issue provide a coherent overview of current advancements in experimental validation, control design, and generator–converter integration. By directly addressing real-world implementation challenges and highlighting gaps in both methodology and system-level coordination, these papers collectively chart a roadmap for future research toward resilient, adaptive, high-quality power systems that are capable of supporting the continued expansion of renewable energy sources.

I thank all the authors for their valuable contributions to this Special Issue, as well as the reviewers for their role in ensuring the high quality of this publication.