Control and Optimization Technologies in Renewable Energy and Integrated Energy Systems

The rapid deployment of inverter-dominated generation, hybrid storage fleets, and cyber-connected automation is reshaping renewable and integrated energy systems. This Special Issue assembles contributions that advance control and optimization technologies for reliable, low-carbon operation across grid-tied and islanded contexts. These works collectively confront enduring challenges, including frequency stability with low inertia, voltage regulation under uncertainty, multi-energy coordination, forecasting for scheduling, and the security of increasingly digitalized assets. This issue also highlights gaps in interoperability, scalability, and resilience-by-design.

A consistent theme is the orchestration of converter-interfaced resources to provide grid services alongside legacy assets. Authors of contribution 1 demonstrate a multi-objective coordination of smart inverters with conventional devices to balance voltage support and ancillary services while respecting device limits. Complementing coordinated services, a survey and evaluation of virtual inertia implementations for autonomous AC/DC microgrids, clarifying design trade-offs between emulated inertia, damping, and stability margins as grid-forming/-following roles evolve, is conducted in contribution 2. Extending to hybrid storage, contribution 3 present a coordinated strategy that couples adaptive virtual inertia with virtual droop while optimizing state-of-charge trajectories, highlighting how control layers can be co-designed with energy management objectives.

Optimization-driven management under uncertainty has become highly important with the penetration of variable energy sources such as solar and wind systems. Authors of contribution 4 integrate unscented transformation with enhanced metaheuristics to improve microgrid scheduling quality and robustness under renewable variability. At the system scale, contribution 5 cast low-carbon economic dispatch for regional integrated energy systems with wind/solar uncertainty as a robust optimization, quantifying the emissions–cost trade-space and demonstrating the operational value of uncertainty sets. On the demand side, Laitsos et al. propose an enhanced sequence-to-sequence deep transfer learning framework for day-ahead load forecasting that strengthens generalization across domains—a prerequisite for dependable model-predictive scheduling contribution 6.

Some of the papers in this Special Issue target the control of energy conversion hardware and power quality. The methodology in contribution 7 develops a fast repetitive control strategy that improves tracking performance for power conversion systems with periodic disturbances, while contribution 8 introduce a fractional-delay PI multi-resonant-type repetitive controller based on a Farrow-structure filter to enhance harmonic compensation in grid-tied inverters. For hardware-in-the-loop validation, authors of contribution 9 validate flywheel energy storage control for wind-farm power smoothing, emphasizing the importance of high-fidelity testbeds for controller verification. In hydrogen-coupled systems, contribution 10 design fast frequency response strategies for electrolysis-driven plants, evidencing the flexibility value of sector-coupled resources.

Cyber–physical integration and infrastructure interfacing add further dimensions. Contribution 11 propose a distributed-ledger-enabled smart contract for total power factor management in grids with customer-owned wind farms, pointing to auditable, automated compliance mechanisms in prosumer-rich networks. Authors of contribution 12 study mitigation strategies for DC neutral-point potentials on transformers induced by metro stray currents, underscoring the need to consider transportation electrification externalities in distribution design and protection. Authors of contribution 13 envision a prosumer hydro plant network as a sustainable distributed energy depot, illustrating community-scale flexibility that complements utility-scale resources. At the converter level, contribution 14 show how modular multilevel converter control can maximize AC fault currents while maintaining grid code compliance, clarifying the dynamic limits for protection coordination in converter-rich grids.

Beyond the specific advances, the aggregated findings reveal persistent gaps. Firstly, interoperable controls that seamlessly coordinate grid-forming and grid-following resources across vendors and voltage levels remain immature; standardized interfaces for services (frequency, voltage, inertia, and fault response) are needed to reduce integration cost and commissioning risk. Secondly, scalable uncertainty-aware optimization must move from case-specific tuning toward reusable formulations with certified robustness and computational tractability for real-time deployment. Thirdly, experimental validation at scale is still scarce; controller designs that perform in simulations can falter in the field without hardware-in-the-loop, power-hardware-in-the-loop, and staged commissioning protocols. Fourthly, resilience-by-design, including cyber hardening and fail-operational behavior under contingencies, must be co-engineered with control. Finally, human-in-the-loop operability (explainable settings, safe defaults, anomaly detection, and self-tuning policies) is crucial as more distributed energy resources are integrated into the existing grid infrastructure.

Looking ahead, I see several promising directions. (i) Unifying grid-forming control with protection-aware objectives to safeguard dynamic stability amid low inertia and high fault-ride-through demands; (ii) hierarchical and distributed optimization that couples stochastic commitment with fast control synthesis, leveraging surrogate models and certified learning; (iii) hybrid storage dispatch that explicitly values ramp equity and lifecycle-aware control; (iv) forecasting–control co-design where uncertainty quantification flows directly into constraint tightening and reserve activation; (v) cyber–physical trust that enables verifiable services from prosumers and virtual power plants; and (vi) reproducible validation pathways from simulations to HIL/PHIL tests, which will shorten the time-to-field. Together, these research thrusts can deliver flexible, resilient, and low-emission energy systems that are as dependable as they are sustainable.