Abstract
With the continuous development of power electronics technology, variable-speed offshore wind turbines that penetrated the grid system caused the problem of inertia reduction. This study investigates the frequency stability of synchronous, offshore wind-farm integration through a modular-multilevel-converter high-voltage direct-current (MMC–HVDC) transmission system. When full-scale converter wind turbines (type 4) penetrate the AC grid, the AC system debilitates, and it becomes difficult to maintain the AC system frequency stability. In this paper, we present an improved inertial-response-control method to solve this problem. The mathematical model of the synchronous generator is based on the swing equation and is theoretically derived by establishing a MMC–HVDC. Based on the above model, the inertia constant is analyzed using a model that integrates the MMC–HVDC and offshore synchronous generator. With the new improved control method, a more sensitive and accurate inertia index can be obtained using the formula related to the effective short-circuit ratio of the AC system. Moreover, it is advantageous to provide a more accurate inertial control evaluation for AC systems under various conditions. Furthermore, the impact of the MMC–HVDC on system safety is assessed based on the capacitor time constant. This simulation was implemented using the PSCAD/EMTDC platform.
Highlights
With continuous attention focused on energy issues, the traditional power grid is facing a major challenge of renewable energy generation
An improved inertia control method is presented to evaluate the frequency stability of a hybrid system consisting of a full-scale-converter-wind-turbine (FSCWT) farm connected to an onshore AC system via an MMC-HVDC
It was mentioned that when the type-4 offshore wind turbine penetrates the system, the back-to-back converter relationship cannot provide an effective inertia constant for the system, making it difficult for the AC system to maintain its frequency stability
Summary
With continuous attention focused on energy issues, the traditional power grid is facing a major challenge of renewable energy generation. The authors in [6,7,8,9] presented several different frequency control strategies for the entire power grid, which consisted of offshore wind power plants and line-commutated converter high-voltage direct-current transmission systems (LCC–HVDC). An improved inertia control method is presented to evaluate the frequency stability of a hybrid system consisting of a full-scale-converter-wind-turbine (FSCWT) farm connected to an onshore AC system via an MMC-HVDC. The inertia index is based on the inertia emulation control presented in [16,17] It uses the capacitor of the MMC-HVDC sub-modulation instead of the DC link capacitor to provide the virtual inertia constant without capacity limitation. As the strength of the onshore AC system is different, this index will provide the minimum inertia constant required for system frequency stability This function eliminates the inconvenience caused by other auxiliary measurement methods.
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