12.1.1 Introduction

Interference between overhead lines, communication circuits and pipelines is caused by asymmetrical currents, which may be due to short-circuits, asymmetrical operation or asymmetrical design of equipment, especially asymmetrical outline of overhead line towers with respect to pipelines and communication circuits. This interference is based on inductive, ohmic and capacitive coupling between the short-circuit path (e.g., overhead line) and the circuit affected by interference (e.g., pipeline). Normal operating currents, respectively voltages, cause magnetic as well as electric fields which are asymmetrical in the vicinity of overhead lines which may cause interference problems in the long-time range.

Short-circuit currents on overhead transmission lines and cables and short-circuit currents through earth cause interference in the short-time range only when the short- circuit is switched off after some seconds by the power system protection. Interference problems may arise in cable and overhead line systems to a different extent depending on the handling of the system neutral. The induced voltage in pipelines and communication circuits may endanger technical installations and safety of workers when defined limits will be exceeded. Within the context of this book, only the short-time interference, in the following called interference, is dealt with. Table 12.1 outlines the needs for the analysis of interference problems.