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| Single Terminal Protection Scheme for Flexible DC Transmission Lines Based on Equivalent Fault Section |
| Zheng Tao1, Li Zixiao1, Chen Yunfei1, Song Xiangyan2 |
1. State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources North China Electric Power University Beijing 102206 China;
2. Tangshan Power Supply Company State Grid Jibei Electric Power Company Tangshan 063000 China |
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Abstract Flexible HVDC transmission systems have demonstrated significant advantages in addressing the challenges of large-scale and distant renewable energy integration, representing a key trend in the future transformation and development of power systems. However, flexible HVDC transmission systems comprise numerous power electronic devices, exhibiting characteristics such as low inertia and weak damping, thereby imposing higher demands on the speed of fault isolation and protection. Currently, to minimize the economic investments and enhance the grid dynamic performance, limiting reactors are often centrally installed at the converter station exits, resulting in a lack of clear boundary elements between DC transmission lines. This installation method leads to the possibility that existing protection schemes based on characteristics of line boundary elements may no longer be applicable. On the contrary, traveling-wave-based protection, with its inherent ultra-high speed and independence from line boundary elements, emerges as an effective approach to address the challenges of applying single ended transient protection schemes in flexible HVDC transmission lines under weak boundary conditions or no boundary conditions.
However, existing traveling-wave-based single ended protection schemes often fail to effectively differentiate between faults occurring inside and outside the protected section, leading to undersized protection and difficulty in protecting the entire length of the line. Moreover, traveling-wave-based protection is susceptible to device sampling rates, especially when faults occur at the near end or remote end of the line, resulting in frequent wave reflections between the fault point and the busbar, which may reduce protection reliability.
To address these issues, this paper first analyzes the propagation paths of reverse current waves during faults at different locations along the line, deriving the relationship between the number of same-polarity waves detected at the relay location before the arrival of the first reverse-polarity wave from the fault location. Building upon this analysis, considering the phenomenon of "wave leakage" due to limited sampling rates during near-end or remote-end faults and the influence of line length, the relationship between the number of the same-polarity waves detected at the relay location is further derived. Based on this analysis and introducing the concept of "equivalent fault sections" to enhance the difference between zero mode traveling wave arrival time and line mode traveling wave arrival time between internal faults and external faults, a single ended protection scheme for flexible HVDC transmission lines based on equivalent fault sections is proposed.
Finally, this paper validates the feasibility and effectiveness of the proposed protection scheme through extensive simulation experiments using the PSCAD/EMTDC simulation platform for multi-terminal flexible HVDC transmission systems. Simulation results demonstrate that the proposed scheme reliably addresses the challenge of protecting the entire length of the line, exhibits good transient resistance and noise interference resistance, even under scenarios with weakened or absent boundary elements. Furthermore, by considering the integration of wave polarity and time difference information to mitigate the effects of "wave leakage" due to limited sampling rates during near-end or far-end faults and the influence of line length, the proposed scheme exhibits a certain degree of fault tolerance.
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Received: 17 January 2024
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2025, Vol. 40 
