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Project: T144700 #50/121
 

A telecommunication line parallel to an overhead or underground power distribution line is subject to electromagnetic induction from the power system under both normal loading and fault conditions. In order to minimize the possible hazards and damage caused by the electromagnetic coupling from the power system, bonding of an overhead telecommunication messenger or underground communication cable sheath to the electric utility’s neutral is common practice.

This project studied the electromagnetic coupling between the telecommunication and power distribution lines. The report explains how the bonding of the telecommunication sheaths to the electric utility’s neutral impacts the electromagnetic coupling, and examines the rationale for the bonding interval of 300m required in some standards. The major work in this study involved CDEGS software modelling to assess the impact of the bonding interval on the induced voltages and currents on telecommunication sheaths.

Four different scenarios were analyzed. The first two scenarios involved telecommunication lines parallel to 12.5kV and 25kV overhead distribution feeders. The latter two scenarios considered telecommunication lines buried parallel to 12.5kV and 25kV underground power cables. In each scenario the impact of the bonding interval was considered under normal loading conditions, and line-line-line and line-ground fault conditions.

Complete results of the induced voltages and currents on the telecommunication system for a range of loading and fault conditions are provided in the report. Charts summarizing the trends for these voltages and currents as the distance between bonds was varied, are also provided. The model results illustrated that the most appropriate bonding interval should be determined by a consideration of the following: the power system Ground Potential Rise (GPR), the Ground Potential Difference (GPD) between connections to any two points along the length of the neutralor communication grounding systems, the continuous voltage induced on the communication messenger\sheath, and the currents that can flow in the communication messenger\sheath under normal load and fault conditions. Evaluation of the bonding interval can be accomplished by calculating the levels of the voltage and currents mentioned above and ensuring that they do not exceed relevant limiting values, such as: 3000V as a limit for GPR voltages on communication sheaths; GPD limits, where applicable (a 300V peak limit based on a shock hazard threshold was used as an example in this report); and a 50V continuous limit for voltage induced on the communication sheath.

The following are the key findings of this work. Grounding the communication messenger\sheath for a communication cable running in parallel with distribution power lines is necessary to limit electromagnetic induced voltages. Under L-N fault conditions, a GPR distributes along the power system neutral. The maximum GPR is dependent upon the grounding resistance of the multi grounded neutral. This maximum GPR dominates the overvoltage that is imposed on the telecommunication sheath that is bonded to the neutral. The maximum GPD between the power system neutral and telecommunication messenger\sheath at the same location is greatly affected by the bonding interval. With a 300m bonding interval, the voltages on the communication cable sheath were maintained below the accepted limits for the cases in this study where multiple 25Ω neutral ground resistances were assumed. Further investigations/simulations are required to study the bonding interval impacts on distribution and telecommunication designs in joint use installations that are more complex than those addressed in this report (i.e. varying pole ground rod resistances and multiple grounding sources of the power system).

Keywords:

Bonding, Grounding, Telecommunication, Ground Potential Rise, Overhead and underground distribution, Bonding interval, Electromagnetic coupling