AC Stray Current and Environmental Effects on Concrete Power poles

The purpose of this article is study of effects of high voltage AC stray current (380 VAC) and environmental effects on durability of concrete power poles. In Iran, annually spend large costs for replacing them with new concrete poles. In this study, the degradation effects and mechanisms of 380 VAC stray current on water saturated concrete were investigated via field and laboratory tests. They included electrical and electrochemical tests and microscopic inspections. Based on the results of tests both environment and AC stray current reduce durability of the concrete power poles. Formation of galvanic macro cell, carbonation, vibration of concrete pole due to wind force and weight of cable, diffusion of corrosive ions from soil to concrete power pole and its accumulation in near ground level are some of environmental effects on degradation of concrete power poles. In addition pollution of air can form deposit layer on insulations of concrete power pole. In raining condition, the layer absorbs water and surface resistance of insulators reduces considerably. So high voltage AC current can creep in surface of the insulator and can exchange between two other phases and ground. Generally grounding system cant act immediately or have not suitable low electrical resistance. Meanwhile if concrete of power pole has low quality then it absorbs high content of rain and its resistivity decreases considerably. Thus some parts of fault current can pass through water saturated concrete power pole. In an special location around Isfahan with polluted air condition, in raining condition current creep through the surface of some insulators occur and immediately many cracks forms on concrete cover of power pole and in some cases concrete cover start to fall without any corrosion in rebar. Based on field study, AC stray current does not occur in concrete power poles which manufactured by centrifugal method and they have very low w/c ratio in their mix design. In this method excess water of concrete mix is removed before staring curing of concrete. Microscopic study of concrete of the molded pole and centrifugal pole showed that the later had much denser structure and for this reason depth of carbonation in the later was much lower than the first. In addition the centrifugal poles have pre-stress condition and it reduce their vibration against wind force and weight of cables. Vibration has destructive effect on concrete cover of concrete poles, special in their ground level because force of vibration concentrates in this location. For study mechanism of degradation of AC stray current, some concrete samples prepared and in water saturated condition subjected to 380 VAC stray current. Based on results of tests, high voltage AC stray current can reduce the concrete pole durability by thermal and shrinkage stress and also by creating vapor pressure within water saturated concrete and transport calcium hydroxide toward cement paste-aggregate interfaces. Application of new technology for reducing water to cement ratio and proper installation and maintenance of the concrete poles considerably increases durability of them against destructive effects of AC stray current.