Abstract
Multi-terminal voltage-sourced converters (VSC) high-voltage direct current (HVDC) transmission system is expected to play a vital role in future power systems. Compared with ac power transmission, dc transmission is more vulnerable to faults due to low dc-side impedances and sensitive power electronics in the converters. Dc protection issues must be tackled before any multi-terminal VSC-HVDC grid can be built. The multi-terminal VSC-HVDC system is studied in detail using switching models for two-level converters, detailed equivalent models for the modular multi-level converters, detailed hybrid circuit breaker switching models and frequency-dependent phase models for dc cables. Using such high-fidelity system models, a systematic study of HVDC fault protection methodologies in more detail than previous studies is conducted. This is the first comprehensive study that includes pre-emptive circuit breaker operation. The results presented in this study underline the benefits of such a detailed treatment of the breaker, and of considering it as part of a fast power electronics system rather than isolated dc equipment. The study identifies the best existing fault detection method and tests it extensively. In order to further improve post-fault system recovery response, which is a key but often neglected part of previous studies, a novel bump-less transfer control has been implemented in the converters.
Highlights
Dc transmission will play an important role in future power transmission networks
Some voltagesourced converters (VSC) topologies are capable of controlling dc-side fault currents, but the technology has not so far been deployed in power transmission networks
If a pole-to-ground fault occurs, the fault current will increase to a high value within a short period of time, which means fast and reliable fault detection and isolation methods are required under such high-voltage direct current (HVDC) scheme
Summary
Dc transmission will play an important role in future power transmission networks. In Europe, the construction of 12,600 km of dc links is planned in the decade [1]. For the meshed dc transmission, protection is an essential area of study, since due to low dc-side impedance, the fault current will rise to a high value within a very short period of time. Leterme et al [16] discuss the influence of the grounding location on the design and protection of the dc system: the grounding location will exert a significant impact on the system's pole-to-ground fault response and the earth current during system's normal operation. Based on [16], if the HVDC grid is configured as the asymmetric monopole, only low impedance grounding could be adopted and the grounding point is at one end of the converter Under such a configuration, if a pole-to-ground fault occurs, the fault current will increase to a high value within a short period of time, which means fast and reliable fault detection and isolation methods are required under such HVDC scheme. The impact pre-emptive control has on the overall protection system has been studied
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