Salvatore DaArco

Power Hardware-In-the-Loop (PHIL) testing is a powerful approach that combines the flexibility of numerical simulations with the high fidelity of hardware tests to allow experimental validation of electrical equipment in a close-to-reality laboratory environment. This is enabled by using specially designed software and hardware that serve as an interface between the modeled environment and the equipment under test. Using PHIL for testing and validation can help significantly reduce time-to-market for novel energy solutions by both reducing the number of design/prototyping iterations, which in turn reduces the development costs, and the need for timely and costly field tests later during the system validation stage. The PHIL as a technology has been studied over the last 20 years, focusing mostly on the software/hardware interfaces and on system stability challenges. However, there has not been a consensus yet on how a PHIL system should be designed, validated and operated to maximize its benefits. The present work provides answers to these questions. Starting from the existing research experience and use cases, the present paper introduces practical guidelines for PHIL modeling, design, laboratory implementation and validation, and it identifies a few remaining open topics that need to be addressed to enable wider adoption of PHIL for testing and validation of electrical equipment in industrial environments.

Accurate grid impedance estimation is essential for the stability and efficiency of modern power systems, especially with high renewable energy penetration. This paper introduces a signal-based on sum of sine waves (Multi-Sine) approach for broadband impedance estimation, offering superior spectral efficiency and adaptability compared to traditional methods like Pseudo Random Binary Sequence (PRBS) and White Noise. The proposed method ensures precise characterization while minimizing grid disturbances. Validated experimentally on a 60 kW VSI connected to an emulated grid, the approach demonstrates reliable performance under varying conditions, including weak grids and dynamic loads. These findings highlight the practical applicability of Multi-Sine signals, advancing grid management and stability in renewable-dominated systems.

This paper explores the use of the recently proposed dual grid-forming control applied to modular multilevel converters and enhanced by embedded energy storage. Dual grid-forming technique enables the converter to form both ac and dc grids by leveraging its internal energy, while the embedded energy storage provides damping to the grid, thereby improving its stability margins. The application of dual grid-forming in a modular multilevel converter is characterized by using its internal power balance as a synchronization mechanism. The addition of controllable and decoupled energy storage provides a new and promising degree of freedom. On this basis, two innovative damping provision methods are proposed and compared with the existing solutions by means of small-signal eigenvalue analysis and time-domain numerical simulations. Results show enhanced system stability margins, with an increased damping ratio of the critical eigenvalue from 10% to 60%, and a lower perturbation of system variables when using the proposed strategies.

Realizing the necessity and benefits of integrated operation of multiple microgrids (MGs), this article proposes a unified controller for the interconnection of MGs energized with a variety of distributed generators (DGs), including both rotating machine-based and inverter-based interfacing systems. The developed controller adopts an embedded loop approach that corrects the frequency and phase errors adaptively, thereby eliminating the need for multiloop sequential controllers, and accelerating the alignment of the phase and frequency of incoming MG with that of the existing/reference MG system. The controller tuning process is simplified with the incorporation of rate of change of frequency and an aggregated modeling approach in the design, which enhanced the flexibility and robustness. Care has been taken such that the developed philosophy shall be generic, so that it can be scalable and can be applied to the interconnection of any two MGs, irrespective of the type and number of DGs within each MG, and their time-varying responses. The performance of the developed controller is verified through various numerical simulations and is validated through experiments on a power-hardware-in-loop-based test bed for seamless interconnection, undisturbed power deliveries, and stable post-interconnection operations.

Dual-port grid-forming control enables simultaneous ac and dc voltage regulation for modular multilevel converters (MMCs) connecting the HVDC transmission line to the ac network. However, its ability to provide damping and inertial power is inherently constrained by the limited internal energy capacity of the converter. To address such limitation, this paper proposes the coordinated operation of a static synchronous compensator with energy storage (E-STATCOM) in parallel with a dual-port grid-forming HVDC-connected offshore wind farm, which enhances converter stability and performance while preserving optimal wind farm operation. Additionally, a new stabilization mechanism is introduced within the MMC's dual-port grid-forming control. This improves the dynamic performance and ensures stable operation even in the absence of additional support. In the numerical example considered, the proposed system enhances the rate of change of frequency after an ac load connection from -0.64 Hz/s to -0.32 Hz/s. It also reduces the standard deviation of the power output fluctuations by 57%, filtering oscillations above 0.5 Hz. Moreover, it decouples dc voltage control from MMC internal energy, ensuring partial firewalling of the dc grid during ac contingencies. Experimental validation demonstrated good agreement with simulations, confirming the potential of coordinated control strategies to enhance stability, inertia provision, and flexibility in future offshore wind power systems.

When the stability assessment of a power electronic-dominated system is performed on a single coordinate, for example, through Bode plots in $dd-$ and $qq-$axis, accuracy is compromised in frequency ranges where the frequency coupling is strong. Thus, the stability of such systems is normally analyzed using a multi-input-multioutput (MIMO) framework, which involves employing generalized Nyquist plots. However, if the frequency coupling in these systems is quantified and used to correct the diagonal elements of the MIMO system, the stability evaluation could be greatly simplified using single coordinate Bode plots. To achieve that, this article introduces a corrective factor based on a quantified frequency coupling parameter, referred as coupling strength. This approach allows MIMO systems to be analyzed within a single-input-single-output framework. The corrective factor ensures simpler and precise analysis using single coordinate Bode plots. In addition, this method provides an accurate estimation of resonance frequencies and frequency coupling-based stability margins by direct inspection of the Bode plots. Simulation and experimental results are presented to validate the proposed stability assessment technique.

The integration of conventional STATCOMs with a battery energy storage system (BESS-STATCOM) has been gaining popularity recently. A BESS-STATCOM is interfaced to the grid via a power conversion system (PCS). This PCS is often a single-stage dc-ac converter directly connected to the BESS without an intermediate dc-dc converter to reduce costs and size. In this configuration, BESS's voltage drops associated with a lower State of Charge (SoC) can reduce the capabilities of the BESS-STATCOM to inject reactive power. This paper introduces a control strategy to boost the reactive power capabilities of a BESS-STATCOM with a single-stage PCS. The proposed strategy disconnects the battery pack from the power conversion system (PCS) and charges the dc link capacitor to its full rated voltage from the grid. The transition into the boost mode and vice versa is seamlessly performed at zero battery pack's current, eliminating the need for a dc circuit breaker.The effectiveness of the proposed control strategy is validated through a Matlab/Simulink simulation using a 5 MVA BESS-STATCOM, followed by an experimental validation using a 32.5 kVA scaleddown PCS. The results show that when the battery pack's SoC is at 20%, the proposed strategy increases the reactive power capability approximately threefold compared to the scenario without the strategy.

The need of efficient electric transmission systems, the rise of renewable energy production, and the development of battery storage systems have driven the adoption of Modular Multilevel Converters (MMC), which offer better performance than conventional converters. However, the diagnosis of electrical faults in MMC is more challenging because more components are involved in the fault circuit. Particularly, ground faults (GF) are the most common type of faults in power systems, yet their diagnosis has not been extensively studied for AC-DC converters, where most industrial relays fail. This paper proposes a GF location method for MMCs based on the use of a high-value grounding resistor placed at the DC midpoint or AC neutral point. By measuring the voltage between its terminals and conducting frequency analysis, the fault position and fault resistance can be accurately located. The novelty of this method lies in its ability to locate the GFs on the AC side, DC side, or within a faulty submodule (SM) of the MMC. Numerous simulations were performed on a half-bridge three-phase MMC converter, yielding excellent results. Additionally, experimental tests were performed on a 60 kVA 12-level full-bridge three-phase MMC, validating the method.

This paper presents a design methodology for integrating an electrical energy storage unit into a hardware-in-the-loop (HIL) test rig for wave energy converters (WECs). Typically, the power production from WECs is characterised by pronounced fluctuations at low frequency and high peaks compared to the average. Wave energy test rigs should be able to reproduce these variations to impose realistic conditions to the device under test. Thus, the grid connection of the rig must be sized to cope with high peaks, and additional measures may be required to avoid disturbances on nearby loads and negative effects on voltage quality. The integration of electrical energy storage can smoothen power fluctuations and mitigate these drawbacks, while resulting in lower installation and operating costs. The design methodology indicates how to effectively size the storage unit and which technology to favour based on the type and duration of test campaigns. Numerical simulation results are presented for a dual HIL test rig and operational profiles of three different WEC technologies. For designs with energy storage lifetime shorter than the calendar life, sensitivity analyses indicate that the rig's annual utilisation rate and the level of accelerated testing have a significant effect on the storage energy requirements.