"Protecting Concrete Power Poles from Stray Alternating Current"
Aghajani Koopaee, A. , Golozar M.A., Saatchi, A., Reisi, K., Urgen, M. , Shabani, S.
Abbstract :
Concrete pores can act as capacitors and alternating current (AC) can pass through a capacitor. Thus, AC passes in concrete in two parallel ways—by capacitor and resistor paths. The capacitor paths consist of the pore network and resistor paths consist of the solid phase. If steel bars are embedded in the concrete, then the bars can also act as capacitors for passing AC.1-3 As shown in Figure 1, there are two kinds of capacitors in concrete’s equivalent of an electrical circuit. Therefore, three steel bars embedded in concrete can be used as working, counter, and reference electrodes during an electrochemical impedance spectroscopy (EIS) test. According to Figure 1, the EIS spectrum of concrete consists of two arcs, one that is related to the capacitance of the steel bars and appears in the low-frequency range, and another that is related to the capacitance of the bulk concrete and appears in the high-frequency range.1-3 At Point A in Figure 1, the imaginary part of impedance is very low compared to the real part, so that a higher amount of current can pass through the concrete and a high percentage of it will pass through the capacitance path (pores).1,3-4 So if the AC voltage is high enough and the concrete is saturated with water, then serious nonuniform current distribution forms in concrete. This will lead to thermal and shrinkage stress in concrete and cause the formation of cracks and an increase in the concrete’s permeability.1,3-4 According to our results, the frequency of urban AC power (i.e., 50 Hz) is approximately equal to the frequency of Point A. Under special conditions, including polluted air, rain, and low-quality concrete poles, high-voltage AC can leak from power lines to water-saturated concrete power poles.4-5 Figure 2 shows concrete power poles after experiencing stray AC attack. The concrete surface has many cracks and in some areas the concrete cover has started to fall off. In dry
concrete, AC stray current cannot pass through the capacitor path because there is insufficient liquid in the concrete’s pores. Therefore, impedance of Point A is important only for water-saturated concrete.
In this work, the effects of silica fume, polypropylene (PP) fibers, and water-to-cement (w/c) ratio on the imaginary impedance of Point A in water-saturated concrete samples were studied by an EIS test. This research is a new investigation and we know of no reports in this field. Results of this research are very important in improving the resistance of concrete power poles against high-voltage AC leakage.