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Mitigating the effects of low-inertia on HVDC-rich AC grids

Jose, Khadijat 2019. Mitigating the effects of low-inertia on HVDC-rich AC grids. PhD Thesis, Cardiff University.
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The integration of large-scale power from renewable energy sources (RESs) via high voltage direct current (HVDC) transmission will contribute to the achievement of energy targets made by the government of several nations. This will help to reduce greenhouse gas emissions and combat climate change. With variable-speed wind turbines (VSWTs), maximum wind energy can be captured; additionally, multi-terminal HVDC grids (MTDC) can help to connect offshore wind farms (OWFs), solar farms and several countries, thus, aiding cross border trading, balancing services and RES integration. A major component of these technologies is power electronics which makes them not contribute to the system inertia. The research work presented in this thesis is aimed at investigating the effects of reduced system inertia in an ac grid rich in power electronics (i.e. HVDC and VSWTs) and proposing measures to mitigate these effects. The main contributions of this research work are: (1) investigating the effects of large-scale connection of VSWTs to the GB power system, (2) analysis of inertial contribution of VSWTs, (3) coordination of fast frequency support from MTDC grids and (4) experimental validation of frequency control schemes, including a proposed auxiliary dead-band controller (ADC). To investigate the inertial contribution from VSWTs, a test system consisting of a three-machine Great Britain (GB) power system connecting full power converterbased VSWTs was modelled. In this test system, the wind generation capacity was varied and the effect on the system frequency response was studied. It was observed that the system frequency deviation and rate of change of frequency (RoCoF) increased with the wind penetration increase. This study was followed by analysing the VSWT synthetic inertia capability. The temporary overproduction strategy which allows the release of stored kinetic energy during power imbalances was used. HVDC grids may provide fast frequency support to ac grids with the aid of supplementary control algorithms. Three fast frequency control schemes are presented. These supplementary control schemes are fitted with all the converters within a four-terminal HVDC grid connecting an OWF and three onshore ac grids. During periods of frequency support, undesirable power flows and reduced power transfers occurred within the grid. To prevent these issues, an ADC algorithm was proposed. The results show that the ADC improves the performance of the supplementary frequency controllers. An experimental test platform was designed to validate the fast frequency control algorithms and the ADC performance. The three-machine GB power system was implemented in a real-time digital simulator which was connected to a meshed three-terminal HVDC test rig. With this system, the frequency control schemes and ADC were validated.

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Inertial response; Frequency Control; Offshore Wind Farm; HVDC Transmission; Power Electronics; Hardware-in-the-loop simulation.
Date of First Compliant Deposit: 23 May 2019
Last Modified: 23 May 2019 09:48

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