DC fault protection in HVDC system and the impact on AC frequency response

Student thesis: Doctoral Thesis

Abstract

The integration of renewable energy generations requires the transmission of bulky power over long distance and in many applications, HVDC transmission systems become a more preferable choice compared to conventional HVAC systems. Thus, the reduction of conventional synchronous generators in the system coupled with the increase of DC connections reduces system inertia and weakens the effect of primary frequency regulation leading to larger frequency derivation in the event of large transients. This thesis investigates frequency support strategies utilizing the flexible power flow controllability of MMC-HVDC. Different control strategies for providing frequency support in a 2-terminal and 3-terminal HVDC systems are studied in detail.For HVDC systems, DC protection strategy can significantly impact on the duration of power outage in the event of a DC fault. The current definition of maximum loss-of-infeed for an AC network does not consider the duration of the power outage, and the impacts of DC fault protection arrangements which result in different speed of power restoration, on the system frequency of connected AC network, have not been properly understood. Different DC protection arrangements using DC switches, fast and slow DC circuit breakers on frequency response of the connected AC networks are investigated. A 3-terminal meshed HVDC system is studied to demonstrate system behaviour during DC faults. It is found that both the amount of power loss and outage duration affect system frequency response, and thus the two need to be considered simultaneously when determining the maximum 'loss-of-infeed' for future AC-DC hybrid power grids.The full-bridge submodule based MMCs have DC fault blocking and active fault current control capability. An energy based virtual damping control for FB-MMC is proposed to rapidly de-energise large meshed DC network in the event of a DC fault by quickly absorbing energy from the DC network through the FB-MMCs. The proposed method regulates the DC terminal current of the FB-MMC to follow the DC voltage to behave as a virtual damping resistor to quickly suppress the potential circulating DC fault currents. This enables fast fault isolation using DC switches and thereby fast fault recovery after fault isolation. The fault isolation time is significantly reduced from around 50ms with the existing FB-MMC fault control method to around 15 ms. The validity of the proposed control is verified in a three-terminal meshed DC network.
Date of Award30 Jul 2020
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorLie Xu (Supervisor) & Neville McNeill (Supervisor)

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