arXiv subjects

Nicolaos A. Cutululis

Publications and source records attributed to Nicolaos A. Cutululis.

13 recordsLinked to original sources

arXiv2026

Fast Frequency Services from HVDC-Connected Offshore Wind Power Plants: A Review in the European Context

The rapid expansion of offshore wind energy is central to the European Union's climate-neutrality targets, with High Voltage Direct Current-connected offshore wind power plants (HVDC-OWPPs) becoming increasingly important for integrating gigawatt-scale renewable generation over long distances. Simultaneously, the phase-out of conventional synchronous generators, together with the gradual withdrawal of renewable-energy subsidies, are creating growing demand for ancillary service provision from HVDC-OWPPs as alternatives. This paper presents a comprehensive review of fast frequency services from HVDC-OWPPs in the European context, focusing on inertia support, fast frequency reserve (FFR), and frequency containment reserve (FCR). The review covers both mandatory grid-code requirements and commercial service products, including the latest developments from ENTSO-E, ACER, NESO, TenneT, and Energinet. Furthermore, the paper discusses the key technical gaps and challenges in the practical implementation of fast frequency services from HVDC-OWPPs, highlighting the limited fast response capacity, complexity of stacking optimization, asymmetric provision of inertia, and overall coordination of an HVDC-OWPP system.

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arXiv2026

Clustering algorithms for multivariate wind farm SCADA data filtering

During wind farm operation, Supervisory Control and Data Acquisition (SCADA) systems record numerous anomalies, transients, and specific operational modes, leading to large datasets. However, for a wide range of applications, only measurements corresponding to normal operation are required and, therefore, the SCADA data must be filtered. For this purpose, several methods have been proposed to automate and replace manual filtering conducted by experts via visual inspection of the data. In this paper, we compare the filtering accuracy of multiple clustering algorithms against manual filtering, introducing evaluation metrics that are suitable for unlabeled data and robust across potential applications. Based on the results, we provide recommendations for generalizing model calibration to different datasets and discuss potential use cases for each model. The models are applied to the SCADA data of three turbines of an existing offshore wind farm, using 10-minute statistics across multiple data channels. In addition to the anomalies and operational modes typically recorded, the dataset presents a large number of non-evident outliers due to several field tests. Overall, the results highlight the importance of extending the analysis beyond the power curve, both in feature selection and in the design of evaluation metrics. In most cases, cluster-based methods are able to detect both evident and subtle outliers, achieving higher accuracy than manual filtering. However, the accuracy and the amount of data retained vary considerably depending on the model, and expert involvement remains necessary, though to a reduced extent compared to manual filtering.

cs.LG↗

arXiv2026

Transient Stability of Offshore Energy Hubs

Offshore energy hubs (OEHs) use grid-forming modular multilevel converters (MMCs) to enable large-scale offshore wind integration and multi-terminal HVDC operation. In HVDC-connected offshore wind farms and OEHs, the offshore grid-forming HVDC converters absorb active power from an offshore AC grid supplied by the wind farms and convert it to DC power for transmission to the onshore grid. Converter current limiting under different fault types in this setting is an understudied topic in the literature, which mostly focuses on power-injecting converters. This paper proposes a unified current-limiting strategy that combines a variable virtual impedance (VVI), based on a smooth threshold function, with a novel virtual-power (VP) mechanism derived from the power dissipated in the virtual resistance. The VVI ensures current limitation during fault-induced overcurrents while preserving voltage-source behavior, whereas the VP mechanism adds a compensating power term into the synchronization loop, enabling automatic power redistribution among converters. P-delta analysis further shows that a more resistive VVI can improve the transient stability of power-absorbing converters, while the proposed VP mechanism further enlarges the stability margin. EMT simulations validate that the combined VVI-VP strategy limits fault currents, maintains synchronism during severe faults, and achieves coordinated post-fault power sharing in fully converter-based OEHs.

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arXiv2026

Small-Signal Analyses Using Analytical IBR Models and Frequency-Dependent Thévenin Equivalents

This paper investigates whether component-level studies can capture additional interactions through Small Signal Analysis (SSA) when the network connected to the Voltage Source Converter (VSC), typically modeled as a simple Thevenin Equivalent, is a more complex IBR-based network. The research investigates cases ranging from basic analytical to an IEEE 9-Bus EMT model, with and without Inverter-Based Resources (IBRs), synthesized as State-Space elements. The study identified that spurious poles at 50Hz related to dq-frame conversion can hinder the accuracy of participation factor analysis. A potential approach involves a two-step process: first, applying Henkel reduction to remove most spurious poles, followed by manual elimination of any remaining ones.

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arXiv2026

Systematic Gray-Box Identification Methodology for Voltage Source Converters

This paper introduces a systematic gray-box identification framework for voltage-source converter models based solely on terminal time-series data. The proposed approach combines a physically informed white-box standard model with iterative time-domain calibration to estimate controller parameters that mimic the behavior of the black-box model in electromagnetic transient (EMT) simulations. Unlike conventional frequency-domain identification methods, the framework leverages time-domain data more effectively to better constrain the surrogate model across a broader operating range and capture reference-signal dynamics. To evaluate the accuracy of the identified model, the paper presents additional frequency-domain validation metrics based on Nyquist analysis and singular value decomposition, allowing for both quantitative assessment of model divergence and qualitative classification of mismatch types. The methodology is tested on cases with increasing structural uncertainty, from exact parametric recovery to an actual detailed EMT black-box model. Results demonstrate that the proposed framework can accurately recover parameters when the internal structure is known, adjust for moderate structural mismatch with extra degrees of freedom, and offer a reliability measure for small-signal stability analysis of converter models protected by intellectual property

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arXiv2026

Estimation of Equivalent SCR for Offshore Wind

The integration of offshore wind power plants (OW-PPs) into weak grids can pose stability challenges due to the interaction between inverter-based resources (IBRs), Flexible AC Transmission Systems (FACTS) and the grid. In this context, long HVAC transmission systems, relatively common for OWPPs, can exacerbate the stability challenges. Therefore, this paper introduces a novel methodology for estimating the equivalent short-circuit ratio (ESCR) at the offshore point of connection (PoC), combining analytical two-port network (TPN) modeling with electromagnetic transient (EMT) simulations. The approach derives the Thevenin equivalent impedance for passive and active components, enabling accurate ESCR computations without complex derivations. Limitations of traditional SCR metrics are addressed by incorporating the dynamics of the converters, such as static synchronous compensators (STATCOMs), into a hybrid EMT-TPN method for synthesizing equivalent impedances. The process is then verified on the CIGRE OWPP benchmark and is found to capture ESCR variations with cable lengths, shunt reactors, and grid strength. Additionally, the results emphasize the correlation between the ESCR and voltage stability, highlighting the role of STATCOMs in supporting voltage stability in weak grids. This modular framework aids in OWPP design and stability analysis for converter-dominated systems.

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arXiv2026

Enhancing Offshore Wind Simulations: Interpolated Switching via DLL Black-Boxes

The modern power system, increasingly composed of Inverter-Based Resources (IBR) from multiple manufacturers, requires new study and design techniques that balance accuracy with the need to protect the Intellectual Property (IP) of various stakeholders. One possible method to support detailed electromagnetic transient (EMT) simulations is to convert the original equipment manufacturers (OEM) models into shareable black-box versions using dynamic link libraries (DLLs). This technique prevents IP violations while potentially maintaining simulation accuracy by embedding the original components within the shareable DLL. Thereby, this work aims explicitly to enhance simulation fidelity by translating full-switching models of offshore wind turbines (OWTs). In this context, the paper offers valuable recommendations, including how to convert interpolation-based elements, preserve simulation speed, recognize limitations, and outline future improvements

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arXiv2026

A Methodology for Impedance-based Stability Margin Analysis for Interconnected Offshore Wind Clusters

With recent developments in offshore grid architectures, power park modules (PPMs) such as clusters of offshore wind power plants (OWPPs) are increasingly interconnected offshore. Consequently, it is necessary to assess how integrating a new OWPP affects the stability margins of an existing OWPP at the point of connection. Although impedance-based methods are widely used for small-signal stability assessment of interconnected converter-based systems, many studies rely primarily on Nyquist encirclements and do not explicitly quantify stability margins. Thus, this paper proposes a general impedance-based methodology to (i) evaluate the stability margins of an existing connection after a new PPM is integrated and (ii) derive a maximum allowable impedance for the new connection such that the minimum stability margin requirements specified by system operators are satisfied and stable operation is maintained. In addition, new Nyquist-based stability regions are introduced to complement the generalized Nyquist criterion, providing analytical indications of margin compliance and headroom. The proposed method is validated through case studies using vendor-based frequency-domain models of two interconnected OWPPs and HVDC system.

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arXiv2026

Holistic Grid-Forming Control to Enhance the Frequency Support from HVDC-Connected Offshore Wind Power Plants

To address the frequency stability challenges posed by the rising penetration of power electronics in power systems, HVDC-connected offshore wind power plants (OWPPs) are increasingly expected to provide inertial response and frequency containment reserve (FCR). In this paper, an improved holistic grid-forming (GFM) control is proposed, aiming to enhance the frequency support by coordinating the GFM controls implemented at all AC and DC terminals of an HVDC-OWPP system, without requiring communication. Firstly, the model of a typical HVDC-OWPP system is developed for control design. Accordingly, the proposed controllers are formulated, followed by an analytical tuning method, where the upper bound of the bandwidth at each AC or DC terminal is identified. Finally, simulations are conducted to verify the functionality and compare the performance with that of representative control configurations. The results show that the proposed holistic GFM control achieves faster response and thus more effective frequency support, while the utilization of the inherent energy storage of each converter is minimized, thereby supporting a new design philosophy for converter control in converter-dominated systems.

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arXiv2025

Holistic Grid-Forming Control for HVDC-Connected Offshore Wind Power Plants to Provide Frequency Response

HVDC-connected offshore wind power plants (OWPPs) are expected to provide inertial response and frequency containment reserve (FCR) to help address the frequency control challenges caused by the growing penetration of power electronics in power systems. Initially dominated by communication-based and grid-following (GFL) control, recent efforts have shifted towards incorporating communication-free and grid-forming (GFM) control into HVDC-OWPP systems to enhance their frequency response capability. This paper proposes a holistic GFM control based on dual-port GFM control to improve the coordination across the entire AC-DC-AC dynamics. A frequency response model of a typical HVDC-OWPP system is developed for GFM control design. Then, dual-port GFM control and virtual synchronous generator control are implemented respectively on the HVDC system and OWPP of the typical system, where the asynchronism of onshore and offshore frequencies is revealed. Next, holistic GFM control is proposed to improve the synchronization and DC voltage regulation. Finally, simulations on the delivery of FCR and inertial response are carried out to verify the feasibility and effectiveness of the proposed control.

eess.SY↗

arXiv2024

Methodologies for offshore wind power plants stability analysis

The development of larger Offshore Wind Power Plants (OWPPs) is moving towards multi-vendor setups, ultimately aiming to establish Energy hubs. These structures are characterized by installations from different vendors sharing the same connection or closely interconnected points. Control interactions among Wind Turbine (WT) converters and power systems have been detected, and this critical phenomenon can significantly impact the dynamic stability of the system. Various stability analysis methods have been proposed to analyze the interactions between OWPPs at the Point-of-Connection (PoC) and the power system. However, stability studies rarely consider the complex offshore transmission system behind the PoC. Generally, the overall OWPP is blamed for the instability. However, since it is a complex system, it is important to understand which part of the OWPP behind the PoC is causing the problem or is likely to become unstable under certain conditions. Therefore, this paper provides a detailed overview of the advantages and limitations of the current system screening indexes used to design the OWPP, and the stability analysis methods. Each method is discussed, and the appropriate methods, depending on OWPP structure, are evaluated and discussed. The analysis indicates that a combination of time domain and frequency domain methods is necessary for enhancing the definition of stability boundaries.

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arXiv2020

Closed-Loop Two-Stage Stochastic Optimization of Offshore Wind Farm Collection System

A two-stage stochastic optimization model for the design of the closed-loop cable layout of an Offshore Wind Farm (OWF) is presented. The model consists on a Mixed Integer Linear Program (MILP) with scenario numeration incorporation to account for both wind power and cable failure stochasticity. The objective function supports simultaneous optimization of: (i) Initial investment (network topology and cable sizing), (ii) Total electrical power losses costs, and (iii) Reliability costs due to energy curtailment from cables failures. The mathematical optimization program is embedded in an iterative framework called PCI (Progressive Contingency Incorporation), in order to simplify the full problem while still including its global optimum. The applicability of the method is demonstrated by tackling a real-world instance. Results show the functionality of the tool in quantifying the economic profitability when applying stochastic optimization compared to a deterministic approach, given certain values of failure parameters.

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arXiv2019

Global Optimization of Offshore Wind Farm Collection Systems

A mathematical program for global optimization of the cable layout of Offshore Wind Farms (OWFs) is presented. The model consists on a Mixed Integer Linear Program (MILP). Modern branch-and-cut solvers are able to solve large-scale instances, defined by more than hundred Wind Turbines (WTs), and a reasonable number of Offshore Substations (OSSs). In addition to the MILP model to optimize total cable length or initial investment, a pre-processing strategy is proposed in order to incorporate total electrical power losses into the objective function. High fidelity models are adapted to calculate cables current capacities, spatial currents. The MILP model is embedded in an iterative algorithmic framework, consisting in solving a sequence of problems with increasing size of the search space. The search space is defined as a set of underlying candidate arcs. The applicability of the method is illustrated through 10 case studies of real-world large-scale wind farms. Results show that: (i) feasible points can quickly be obtained in seconds, (ii) points near the global optimum with an imposed maximum tolerance, are calculable in reasonable computational time in the order of hours, and (iii) the proposed method compares favorably against a state-of-the art method available in literature.

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