Ensuring sensitivity and selectivity of high-speed protection of 10 kV power transmission lines

The paper examines improvements of high-speed protection systems for transmission lines in 6 – 10 kV distribution networks, particularly current cut-offs, focusing on ensuring their sensitivity and selectivity. Traditional methods for setting current cut-offs — based on accounting for the short-circuit current at high-voltage terminals either before the nearest transformer or before the most powerful transformer in the line — are critically analyzed. It is shown that these methods have drawbacks: in the first case, the protected zone length is minimal; in the second case, when the transformer is located at the end of the line, the protection zone is maximal, but when it is connected at the beginning of the line, the zone is minimal, which significantly reduces protection effectiveness. As a solution, a universal setting method is proposed, based on accounting for the three-phase short-circuit current before the transformer located at the end of the protected line. This approach increases the length of the protected zone and enhances protection sensitivity regardless of transformer connection location. Special attention is paid to the practical implementation of the proposed method and its advantages over traditional solutions. Possible cases of coordinating current cut-offs using the proposed setting method with fuses protecting transformers of different ratings are also considered. The research results have significant practical value for designing and operating relay protection in complex network configurations with numerous transformer substations. The proposed solutions enable optimization of high-speed protection systems without compromising selectivity and reliability.