Investigation of a neutral reactor burn in a 500 kV transmission system

The electrical and magnetic coupling are two of many issues to be looked at when planning high voltage double circuit overhead transmission lines. Besides the ordinary concerns such as transmission line design, setting its protection system and others, attention must be paid in the electromagnetic interaction between circuits of double circuit lines, especially if non-conventional schemes are adopted. This is relevant because depending on the circuits characteristics it may result in serious overvoltage problems. However the electromagnetic coupling is well known and can be easily found in the literature, this paper brings out a real case which shows how this problem can lead to equipment damage. To figure out the reasons behind this event, an investigation was performed after a Brazilian utility had reported a neutral reactor burn in a 500 kV transmission system with not many clues about what had triggered it. The investigation started from gathering up all documentation and event reports available, ending with simulations that brought out the technical explanation required by the Brazilian National System Operator (Brazilian ISO).