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Effects of Longitudinal Fault on Pilot Zero-Sequence Directional Component in Four Parallel Transmission Lines Under Different Voltage Levels

When asymmetric longitudinal fault occurs in parallel transmission lines, the shift of load current and mutual inductance may produce large zero-sequence current in the healthy line, leading to the improper operation of pilot zero-sequence directional component with voltage compensation. This paper takes four parallel transmission lines under different voltage levels as research object, analyzing the zero-sequence fault characteristics and pilot zero-sequence directional protection performance in longitudinal fault, considering the influence of the potential transformer position, the electrical connection between different lines, the zero-sequence impedance of systems, and the partial coupling effect. Then the improper operation mechanism of pilot zero-sequence directional protection in the healthy line, which has a strong or weak electrical connection with the faulty line, is presented respectively. Moreover, potential solutions based on negative-sequence components, transmission power and smart substation are proposed.