Commutation failure prediction and interaction mechanism of Multi-Infeed LCC-HVDC systems under asymmetrical faults

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Chen, Nan ORCID: https://orcid.org/0000-0002-0161-9433 (2022). Commutation failure prediction and interaction mechanism of Multi-Infeed LCC-HVDC systems under asymmetrical faults. University of Birmingham. Ph.D.

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Abstract

Line-Commutated Converter based HVDC (LCC-HVDC) systems have been widely used for long-distance bulk power transmission and will play a vital role in the future decarbonised power systems since renewable energy resources are normally far away from load centres. As a frequent dynamic event in LCC-HVDC inverters, the Commutation Failure (CF) is mostly caused by AC side faults, 70% of which are asymmetrical Single-Line-Ground (SLG) faults, and may cause power transmission cessation, cascading trips, and even blackout. Researchers have studied CF mechanism for decades, but most of them only considered the magnitude change of the faulty phase commutation voltage when predicting CFs under asymmetrical faults. However, this research proves that both the magnitude and phase angles of not only the faulty phase but also the non-faulty phase commutation voltages will also change and become unbalanced under asymmetrical faults, which reveals that previous CF prediction approaches are unrealistic and inaccurate.

By considering these factors, this research proposes mathematical methods for predicting CFs and Continuous CFs (CCFs) in Multi-Infeed LCC-HVDC systems under asymmetrical faults. Then an Immunity Index of the inverter to CCFs (IICCF) based on the proposed CF prediction method is proposed to quantify the safety margin of the inverter in normal operation condition to CCF risks. The Interaction Factor between inverters under asymmetrical faults with CCF risks (IFCCF) based on IICCF is also proposed to quantify the interaction between two inverters that whether CCFs occur in inverter would result in CCFs in its adjacent inverter or not. are analysed in Multi-Infeed LCC-HVDC systems. These mathematical methods can help the power system operators fast and efficiently predict CFs and identify CF risk areas. Furthermore, an extended Phase-Locked Loop (PLL) topology is proposed to suppress CFs in LCC-HVDC systems under asymmetrical faults, considering the impact of the phase angle shifts of the commutation voltages.

The accuracy and effectiveness of the proposed methods and topology are verified by simulations on Real-Time Digital Simulator (RTDS). It needs to be emphasised that RTDS (or any other electromagnetic transient simulation software) is not suitable for practical scenarios due to the large scale and complexity of the actual power system and various types of faults as well as innumerable potential fault points, and consequently it is not capable of fully simulating the actual power grid, and all fault scenarios. In other words, it is essentially impossible to use RTDS to make predictions of CFs in large grids in real world scenarios.

Therefore, the fast, accurate and efficient mathematical CF prevention and mitigation methods reported in the thesis contribute to the predictability and suppression of CFs in LCC-HVDC projects, hence helping the system operators design new LCC-HVDC projects and upgrade existing ones.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):