This project belongs to the field of electrical engineering. Multi-voltage-level multi-circuit transmission technology on the same tower has the unique advantage of saving the land area occupied by transmission channels and is currently the mainstream transmission mode. Affected by factors such as line length and difficulty in transposition, a large number of multi-circuit lines on the same tower with no or insufficient transposition exist. Non-transposition of the line will inevitably lead to unbalanced electrical parameters, which will have a non-negligible impact on line protection. Relying on the science and technology project of the State Grid Corporation of China and the science and technology project of the East China Branch of the State Grid Corporation of China, the project team conducted research on the operating characteristics and key protection technologies of multi-voltage-level and multi-circuit non-transposed lines on the same tower, and analyzed the non-transposed transmission lines on the same tower under various operating conditions. The root cause of zero-sequence unbalanced current and its impact mechanism on zero-sequence current protection and zero-sequence directional protection under different operating modes and different fault forms, proposed solutions to key problems of relay protection for the first time. The implementation and application of the project ensured the safe and stable operation of East China Power Grid and assisted the economic and social development of the Yangtze River Delta region.  The main innovations of this project are as follows: (1) Based on the pass-through type and circulation-type zero-sequence networks in which longitudinal and horizontal zero-sequence power supplies are alternately superimposed when multiple un-shifted lines on the same tower fail, it reveals the mechanism that the line does not transfer and the fault generates circulation-current type zero-sequence current causes the protection malfunction of the line on the same tower and the longitudinal zero-sequence voltage causes the protection malfunction of the zero-sequence direction of the non-faulted line.  (2) A line protection technology based on comprehensive directional elements and zero-sequence directional elements based on negative sequence components, positive sequence components, and fault components is proposed, which realizes accurate identification of forward and reverse faults on lines on the same tower and solves the problem of misoperation of zero-sequence directional protection on multiple voltage and non-transposition lines on the same tower.  (3) Aiming at the problem that zero-sequence current protection on lines on the same tower causes maloperation of zero-sequence current protection due to external faults, a zero-sequence inverse time-limit protection algorithm based on current compensation of adjacent lines on the same tower is proposed, which significantly improves the adaptability of zero-sequence current protection to multi-voltage and non-shifted lines on the same tower.  (4) Proposed primary operation response measures such as line current limiting and busbar operation in separate rows, and proposed secondary operation response measures such as optimization of zero-sequence current protection setting value, optimization of distance protection setting value, and optimization of interphase distance for three-phase faults, which solved the problem of reliability protection for non-shifted lines on the same tower.  Through the research of this project, 4 invention patents have been authorized, 2 invention patents have been accepted, 7 papers have been published (5 EI searches), and 1 monograph has been published. The evaluation opinions show that the research results obtained by the project have generally reached the international advanced level. After the implementation of the project, a total of 9 important hidden dangers were avoided from 2016 to 2018, reducing economic losses by more than 45 million yuan. The project results have been successfully applied in the Qinshan Nuclear Power Transmission Project of East China Power Grid, the Power Grid Project around Caojing, the Suzhou-Huxi Project and the Kongdian-Huaihong Project, with remarkable results. The evaluation opinions show that the research results can be extended to the dispatching operation and power grid planning departments of various power grid companies across the country, and have promotion and application value.