Induced voltage on gas pipeline due to alternating current total inteference of faulted overhead transmission line

Abstract

In locations where a buried gas pipeline (PL) shares the same right-of-way with a high voltage overhead transmission line (TL), a relatively higher voltage than normally allowed may be induced in the pipeline due to the alternating current (AC) total interference between the TL and PL. The increase may damage the pipeline coating, connected pipeline equipment, as well as may pose a safety threat to pipeline service personnel. Key questions to be answered are how to evaluate, and minimise the AC total interference made up of inductive and conductive components and their related effects in the event of a power system fault occurring in the TL. This research investigated the pipeline induced voltage behaviour while simultaneously considering the inductive and conductive interferences. Different observation point profiles were considered to obtain various types of induced voltages such as metal ground potential rise (GPR), touch voltage, coating GPR, coating stress and earth surface GPR. A performance comparison between two computational methods, namely electromagnetic field (solutions to Maxwell's equations) and circuit-based (solutions to circuit equivalents of network configuration) approaches were carried out. The TL-PL AC total interference behaviour under various conditions were studied. These included the influence of complex soil structure, soil resistivity, defective pipeline coating, and several other critical parameters. A 30-kIn long, 115 kV TL and a 10 krn long, 24-in PL were used. Results showed that the circuit-based approach performed as good as the field approach (within 5% error). The close agreement between the two approaches shows that the simulation and modelling works carried out in this work are valid. The TL-PL inductive interference increased with the fault current, but decreased with the TL-PL separation distance, the surrounding soil resistivity, and the tower footing resistance. Nevertheless, the conductive interference had to be considered when computing the pipeline induced voltages especially when the soil resistivity was low « 10 Q-m), the fault current was high (> 10 kA), the tower footing resistance was low « 5 Q), and the separation distance between the TL and PL was small « 20 m). In addition, the effect of pipeline coating condition on the induced voltages was dependent on pipeline coating resistivity as well as the soil resistivity. High touch voltage poses threat to human and equipment safety, while high coating stress may accelerate pipeline coating deterioration and corrosion . The results also showed that the variation of the induced voltages in the pipeline buried in complex soil structure depended mainly on the thickness of the first horizontal layer, or the width of the middle vertical layer, or both. The complex soil structure can be replaced with a three-vertical-layer equivalent structure when the width of the middle layer is above 16 krn and the thickness of the first horizontal layer is above 100 m. Adequate soil resistivity measurements must therefore be performed to provide the complete soil resistivity data for the complex as well as non-uniform soil models.