On Safety Measures in Working with Induced-Voltage Overhead Lines

Abstract

The authors of [1] confirm our standpoint presented in [2] concerning the hazard of induced voltage arising from the ground fault in inducing overhead lines (OL). They agree that it is necessary to determine the maximum ground-fault voltage in a network of inducing OL and reduce the induced voltage to a permissible level using terminal and intermediate OL groundings; they also suggest methods of protection from induced voltage at the repair site. They also question the correctness of the OL inducedvoltage grounding technique recommended in [3] (item 4.3.11), which has been included in the new edition of Safety Regulations [4] (items 4.15.52 – 4.15.54) with the only exception that the maximum induced-voltage potential is taken to be 25 V rather than 42 V. The criticism in [3] seems to be valid to the extent that the Safety Regulations (SR) for electric plants fail to specify requirements to the grounding device at the repair site (for an OL grounded at a single point only). On the other hand, it has been argued repeatedly [2, 5] that the single-point grounding should be installed with allowance for the voltage drop associated with the capacitive component of the ground current. It was further argued that the OL single-point grounding using a ground rod with a length of 0.5 m (the technique accepted by SR, item 3.6.8 [4]) develops a voltage that may be hazardous to human health. We believe that the reasons in [1] are quite interesting and should be taken into account when dealing with 330 kV OLs or higher; however, expanding them on all OLs and OL networks under induced voltage will be problematic. In view of the aforementioned, we reiterate that the OL induced-voltage grounding at a single point (at the repair site) is a simple and sufficiently effective means of protection from the electromagnetic component (including ground-fault situations). Protection of the personnel of electric power plants from the electrostatic component of induced voltage is effected by grounding devices of low resistance (grounding wire for 110 kV OL supports, rods for 220 kV (and higher) steel and concrete OL supports, grounding circuits, and special grounding devices for 6 – 10 kV OLs). The capacitive ground current can roughly be estimated using formulas given in [1] or in [4]; based on these data it is possible to determine the safe ground resistance at the repair site. In compliance with the new SR requirements for which the permissible grounding voltage is 25 V (formerly, 42 V), re-evaluated data in [5] for 110 and 220 V twin OLs can be used for practical purposes: safety conditions are reached using a single grounding with R < 30 U per 60 km for a 110 kV OL and per 30 km for a 220 kV OL. To check for safety, the grounding voltage should necessarily be measured at the repair site; this voltage should not exceed the rated value of 25 V. The technique for measuring the grounding voltage is rather simple, and the necessary measurements can readily be accomplished by the repair team at the working site [6]. The single-point grounding technique for induced-voltage OLs can in principle be applied to overhead lines. The use of this technique prevents the discharge of potential from the substation circuit to the de-energized line (intended for repair) through the grounding in emergency short-circuit situations at the substation or in a connected network (110 or 220 kV). Such situations are not uncommon (at the relatively high failure rate of support porcelain insulators for 110 and 220 kV disconnecting switches or actuation of short circuiters). As required by the SR, to prevent the OL under repair from being accidentally energized (or from the discharge of potential from the substation circuit), the grounding at the substation should be installed before the line-disconnecting switch as viewed from the substation. The single-point grounding (rather than the terminal grounding or on-site grounding) is less effective in the case of a “galvanic effect” produced by the accidental contact with a traversing OL. Such accidental effects are disastrous for any OL grounding scheme; however, accidents of this kind are highly unlikely if the appropriate construction-and-use regulations for electric power plants and electric power lines are strictly observed. It should be noted that in certain situations where the induced voltage is difficult to reduce using conventional grounding schemes (terminal grounding and on-site ground-