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DC Protection of Multi-terminal VSC-HVDC Systems
[Thesis]. Manchester, UK: The University of Manchester; 2016.
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Abstract
Voltage-Sourced Converter High Voltage Direct Current (VSC-HVDC) transmission technology has received great interest and experienced rapid development worldwide because of its compact size, ability to connect two asynchronous AC systems and ability to connect to weak AC grids. It is expected that VSC-HVDC will play a significant role in future power transmission networks. Multi-Terminal Direct Current (MTDC) networks are even being established based on VSC-HVDC and these have great potential to support conventional AC transmission networks. However, such DC networks are vulnerable to any DC side short-circuit fault. DC protection issues must be tackled to enable the development of MTDC networks. This thesis conducts some of the underpinning research for such DC protection studies.As a first step to conduct the protection study, a detailed four-terminal VSC-HVDC system is developed in PSCAD/EMTDC, which consists of both two-level converters and MMC devices. Based on this high fidelity four-terminal system model, a thorough analysis is conducted for the two-level converter and the MMC systems under different fault scenarios. Based on this, a basic understanding of the converter systems’ natural responses to these fault scenarios is obtained. Apart from using a DC circuit breaker to isolate a DC fault, there may be other devices which could potentially be used for DC protection. After the fault analysis, a study is conducted to search for any other DC protection equipment which could help the DC breaker isolate a DC fault. Different types of fault current limiters (FCLs) are reviewed and compared. It is found that the resistive type superconducting FCL (SCFCL) has the potential to be usefully employed for DC protection. Next, a DC fault detection and location strategy study is performed. This thesis conducts a detailed study of different DC fault detection and location strategies using a much higher fidelity model than previous studies. After reviewing different fault detection methodologies, it is found that wavelet transforms presently might be the best option for DC protection. The continuous wavelet transform (CWT) is then extensively tested under different DC faults and transient scenarios to prove its robustness, as this method has not been extensively studied in the previous literature. In the end, by using the CWT and placing the SCFCLs in series with DC circuit breakers, the performance of the SCFCLs under a DC side pole-to-pole fault is examined. This study shows that the SCFCL can help reduce the fault current seen by a DC breaker. In the end, a DC system fault recovery study is performed. Different methods are proposed and studied to examine the impact they have on the converter system’s DC fault recovery process. A novel bump-less control is proposed to help the system achieve a good fault recovery response.