Mechanism of pole‐mounted transformer damage by backflow lightning and measures against the damage on low‐voltage distribution line

Abstract

AbstractPole‐mounted transformers are especially vulnerable to lightning damage. The progress of the information society imposes increasingly stringent requirements for the reliability of electric power supply, and this in turn necessitates a reduction in lightning damage to pole‐mounted transformers. Lightning protective devices (surge arresters) are now being installed around the primary bushing of the transformers, which has decreased the number of disconnections around the primary bushing due to lightning. But surge arresters installed on the primary side of the transformer cannot protect it against backflow lightning entering the secondary side of the transformer. The characteristic of transformer damage by backflow lightning is that the electromagnetic force produced by the current flowing into the secondary side deforms the transformer windings. This paper elucidates the mechanism of transformer damage by lightning flowing into the secondary side by comparing actual lightning damage cases with the results of verification tests using a short‐circuit generator. Effective countermeasures against transformer damage by backflow lightning are examined by EMTP calculations, which indicate that neutral grounding on the low‐voltage distribution line is the most effective way of decreasing the current flowing into the transformer. The lower the grounding resistance, the less current flows into the transformer. In addition, decreasing the voltage on the secondary side is important in order to protect the secondary‐side bushing. The calculation results indicate that surge arresters installed around the secondary side of the transformer are effective in decreasing the voltage on the secondary side. © 2010 Wiley Periodicals, Inc. Electr Eng Jpn, 171(2): 1–11, 2010; Published online in Wiley InterScience (www.interscience. wiley.com). DOI 10.1002/eej.20921