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Open AccessArticle

Improved Control Strategy of MMC–HVDC to Improve Frequency Support of AC System

Power and Energy Laboratory, School of Electrical Engineering, Korea University, Seoul 02841, Korea
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Appl. Sci. 2020, 10(20), 7282; https://doi.org/10.3390/app10207282
Received: 22 September 2020 / Revised: 10 October 2020 / Accepted: 15 October 2020 / Published: 18 October 2020
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. View Full-Text
Keywords: modular multilevel-converter high-voltage direct-current (MMC-HVDC) transmission system; full-scale converter wind turbines (type 4), improved inertia-response control; effective short circuit ratio (ESCR) modular multilevel-converter high-voltage direct-current (MMC-HVDC) transmission system; full-scale converter wind turbines (type 4), improved inertia-response control; effective short circuit ratio (ESCR)
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Zhang, Z.; Lee, J.; Jang, G. Improved Control Strategy of MMC–HVDC to Improve Frequency Support of AC System. Appl. Sci. 2020, 10, 7282.

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