Nonlinear Stability Identification and Mitigation in Large Offshore Wind Power Plants

Sujay Ghosh*

*Corresponding author for this work

Research output: Book/ReportPh.D. thesis

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Abstract

The integration of renewable energy sources, such as wind and solar power, into electrical grids introduces a significant departure from conventional power grid characteristics. The widespread adoption of power electronic-based converters as grid interface units for these renewable energy sources presents a fundamental shift from traditional fossil fuel-based thermal power plants. This transition reduces grid inertia, thereby amplifying challenges associated with the proliferation of grid-connected power converters. Traditionally, small-signal/linear stability has been used to assess the stability of a wind power plant (WPP) inter-connection. It is reported that linear analysis performs very well in the high-frequency converter range. But when it comes to the low-frequency range, there is a considerable deviation compared to the actual model behaviour. To this end, appropriate large-disturbance/nonlinear stability assessment methodologies are required to access stability. However, nonlinear systems lack a universal solution and frequently require time domain simulations for their solutions. While analytical methods are proposed in the literature to improve the computational burden, they can be overly conservative.

Furthermore for assessment of nonlinear stability, it becomes crucial to establish precise models for WPPs. Nonetheless, actual electromagnetic transient (EMT) models are frequently unavailable, black-boxed, or computationally intensive for comprehensive modelling. To this end, appropriate simplified reduced order models (ROMs) are required to access the nonlinear stability. Existing ROMs, while beneficial, often prove overly simplified, lacking a systematic approach. Furthermore, a unified methodology for both modelling and assessing the nonlinear stability of multi-converter systems is not clearly established in the literature. Bridging these gaps is essential for advancing our understanding and analysis of nonlinear stability in complex power systems. The project, therefore, aims to provide nonlinear stability analysis methods review, instability identification and quantification solutions, as well as mitigation measures to improve offshore WPP robustness to maximise the installed capacity and distance from shore.

In this comprehensive research, the primary focus revolves around the development and robust validation of ROMs tailored explicitly for wind turbines (WTs) and WPPs. The objective is to address the intricate challenges posed by nonlinear stability analysis, especially under demanding grid conditions, such as severe low-voltage grid faults. These ROMs, extensively validated through exhaustive simulations in PSCAD, capture the dynamic behaviour of WTs during grid disturbances with high accuracy. The research covers both balanced and unbalanced grid disturbances, empowering the models to reflect the intricate dynamics of WTs across diverse and challenging scenarios. The scope extends beyond individual turbines, encompassing entire WPP incorporating STATCOMs. Employing single-port and multi-port frequency scan techniques provides invaluable insights into the dynamic behaviour of these systems.

This work further explores various methodologies for assessing nonlinear stability. Time-domain simulations offer flexibility in evaluating system responses under diverse conditions but can be computationally intensive. Phase portrait analysis provides insightful graphical representations of system trajectories. Analytical methods, including Eigenvalue Analysis, Energy Area Criterion (EAC), and Lyapunov functions, offer powerful means to assess stability, each with its unique advantages and limitations. An innovative hybrid approach that combines time-domain simulations with reverse-time trajectory simulations to estimate the Time-Limited Region of Attraction (TLRoA) is introduced. This approach efficiently estimates RoAs in complex systems, while minimising the need for extensive time-domain simulations.

Stability enhancement recommendations occupy a central position in this research, investigating the sensitivity of system parameters and their significant impact on RoA. Grid-code-associated aspects, post-disturbance settling time, active current ramp rates, and grid short circuit ratios significantly affect system stability, offering crucial insights for power system operational considerations. The study highlights the importance of PLL frequency saturation in maintaining system stability and advocates for a balanced selection of PLL controller gains. Overall, this work offers a systematic approach to modelling and nonlinear stability assessment, fortifying the foundation for enhancing WPP stability under varied conditions and parameters.
Original languageEnglish
Place of PublicationKgs. Lyngby, Denmark
PublisherDTU Wind and Energy Systems
Number of pages164
DOIs
Publication statusPublished - 2023

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