适用于新能源经柔直孤岛送出的非注入式单端时域距离保护

doi:10.19595/j.cnki.1000-6753.tces.251544

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Abstract

In renewable energy transmission system via islanded flexible DC, both ends of the 220kV AC transmission lines feature controllable power electronic devices. The short-circuit currents of both ends of the line contain significant harmonic components, which makes it difficult to apply the traditional distance protection. Time-domain distance protection has garnered significant attention due to its immunity to harmonic effects. However, phase differences between currents at both ends of the line, influenced by factors such as line distribution capacitance and feed-in current from the opposite side, compromise the accuracy of traditional time-domain distance protection calculations. Existing improved time-domain distance protection schemes typically employ injection methods. By actively modulating a specific frequency component through the MMC converter, the opposite side system is equivalent to an open circuit, equating double-ended supply systems to single-ended systems to mitigate the impact of current injection from the opposite end. Nevertheless, the implementation of injection-based protection schemes is often constrained by practical engineering limitations. In light of this, this paper proposes a non-injection single-ended time-distance protection for renewable energy transmission system via islanded flexible DC.
Firstly, taking into account the influence of the line distribution capacitance, the line π-equivalent model is used to equivalent fault circuit. Secondly, based on the phase lag or lead relationship between the MMC converter-side current and fault current, equivalent resistance-inductance or resistance-capacitance are introduced to replace the original fault resistance. This accounts for the influence of phase differences between the two sides. Time-domain differential equations are subsequently derived for both phase-to-ground faults and phase-to-phase faults under equivalent resistance-inductance or resistance-capacitance conditions respectively. The intrinsic relationship between these two scenarios is then analyzed, unifying the time-domain differential equation expressions. Subsequently, the fault distance is solved by using the least-squares algorithm, and the protection criterion is constructed. Finally, the feasibility and effectiveness of the proposed scheme are verified by PSCAD/EMTDC simulation platform.
Simulation results show that the proposed time-domain distance protection scheme can be adapted to both phase-to-ground faults and phase-to-phase faults, and has good ability to identify fault resistance. The time-domain distance protection scheme proposed in this paper has good anti-interference performance. Using the time-domain distance protection scheme proposed in this paper to calculate the fault distance can reach stability within 10ms after the fault, and the relative error of the calculated fault distance is less than 5%.
The following conclusions can be drawn: (1) The introduction of the equivalent resistance-inductance or resistance-capacitance instead of the original fault resistance eliminates the influence of the phase difference between the short-circuit current of MMC converter and the current at the fault point on the performance of the time-domain distance protection, and significantly improves the reliability of the time-domain distance protection. (2) By analyzing the intrinsic relationship between the time-domain differential equations represented by equivalent resistance-inductance or resistance-capacitance, the expressions for the time-domain differential equations in both scenarios have been unified. The unified time-domain differential equations can be applied to different fault scenarios. (3) The calculated fault distance according to the proposed time-domain distance protection has a high accuracy, which can be stabilized within 10ms after the fault. The proposed scheme has a good ability to identify fault resistance, and is adaptable to different fault scenarios with good universality.

Key words： Time-domain distance protection equivalent fault resistance line distribution capacitance non-injection single-ended protection renewable energy transmission system via islanded flexible DC

Received: 04 September 2025

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