Assessment of energy storage and dynamic response requirements in modular multilevel converters for frequency response improvement

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Ng, Ashley Wing Yan
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The call for clean energy has driven the power system to become increasingly dominated by renewable energy sources (RESs). This has led to a reduced proportion of power contributions by conventional synchronous generators. As a result, the grid is becoming more prone to frequency excursions from fluctuations between the electrical generation and demand due to the decreasing system inertia. RESs are interfaced to the grid by power electronic converters, such as the modular multilevel converter (MMC). The MMC has an inherent energy storage that can be dynamically controlled. This capability has been used to compensate power imbalances and effectively reduce the magnitude of the consequent frequency excursions. The research in this thesis investigates how different system parameters and operating conditions impact the ability of such energy-based frequency support control to improve the frequency response, through detailed electromagnetic transients (EMT) simulations in PSCAD/EMTDC. One important factor is the response speed of the MMC. A model is proposed to simulate the dynamics of the DC voltages and currents in the MMC, from which a closed-form equation is derived to approximate its settling time. The model is validated by EMT simulations and used to investigate how varying system parameters and operating conditions impact the MMC response speed. It may be used as a design tool in determining operating and sizing requirements for the MMC with frequency support capability.
Modular multilevel converter, Converter energy storage, Frequency response, Response speed