CAUTION FOR UNDERESTIMATING THE EPK HAZARDS GENERATED ON CORRODED METALLIC PIPELINES FROM DISTRIBUTION POWERLINES WHEN COMPARED TO TRANSMISSION POWER LINES

摘要

Considering single phase-to-phase fault conditions - too often power system / corrosion engineers assume that transmission power lines have the greatest induction effect on metallic pipeline when compared to distribution power lines. In reality the opposite is often valid — distribution powerlines have the highest negative effect on earth potential rise (EPR) hazards. This is largely attributed to the EPR separation distance between the pipeline and the closest earthed part of the power line; and the presence of an overhead earth conductor (OHEC). The former is also dependent on the level of corrosion on the studied pipeline. The paper presents various field study cases of both distribution and transmission power lines in which various overhead earth is considered — unearthed, single OHEC and twin (duplex) OHEC installations are considered. The fault current splitting along the conductive component of the earth fault return current and the inductive component contributing to the shielding effect on the earth fault current is discussed. In this regard a comparison between the 'assumed 30%' stated in AS/NZS 4853:2012 (Table 4.5) and actual calculated values is undertaken. These results are applied to various levels of pipeline coating resistance and soil resistivity. The 30% split in AS4853:2012 specifically relates to the first pass conservative Level 1 assessment which is used in the absence of most field data and only identifies areas of interest for further assessment. The paper conducts what is effectively part of a Level 2 assessment where the actual EPR levels are calculated. Furthermore, the pole/tower footing resistance (PTFR) is a major parameter in determining the fault current split. Reducing the PTFR results in a lower EPR. However, this is relevant to unshielded power lines and is not necessarily relevant to shielded power lines. In the case of shielded power lines a reduced PTFR will increase the fault current flowing via the pole /tower footing and mass of earth return. This may result in an increase in the EPR. The presented case studies indicate this and the sensitivity analysis for varying soil resistivity. All case studies are completed in CDEGS HIFREQ software and are based on powerline voltages of 110 kV transmission and 22 kV distribution.