Exceptional High Rise of Temperature in LV Turret Bolt in Generator Transformer

Abstract

Generator transformers are among the most critical and high-value assets in a power plant. Any abnormal behaviour during commissioning or operation poses serious operational and safety risks. This study presents a detailed investigation into an abnormal temperature rise observed in the LV turret bolts of a generator transformer. The root cause analysis identified induced eddy currents and uneven current distribution as the primary contributors. Corrective actions focused on restoring proper electrical continuity and enabling uniform current sharing among turret bolts.

Introduction

At the Hindalco Mahan Plant, a 200 MVA Generating Transformer (GT-2) exhibited apparent smoke near its LV turret on 27 April. Initial inspection revealed that the observed smoke was actually oil vapor caused by transformer oil leaking from the turret gasket and vaporizing upon contact with overheated turret bolts. Thermographic analysis showed bolt temperatures reaching approximately 200°C, well above normal limits and higher than the transformer oil flash point of about 140°C.

A detailed Why-Why Analysis determined that the overheating was caused by induced eddy currents generated by the high LV-side current (around 6000 A). These eddy currents circulated through the turret structure and concentrated in a single bolt that provided the lowest-resistance path to ground. This localized current concentration produced excessive heating.

Several corrective actions were attempted, including bolt replacement, changing bolt materials (MS, GI, and SS), installing copper braid jumpers, and direct grounding. However, these measures failed to resolve the issue because they did not address the root cause.

Further investigation revealed that the turret was designed with 96 bolts intended to share induced currents uniformly. However, paint on the contact surfaces acted as an insulating layer, increasing contact resistance in most bolts and forcing a large portion of the current through one bolt. After removing the paint and cleaning the contact surfaces, electrical continuity was restored, allowing all bolts to share the induced current evenly. This eliminated the localized overheating problem.

Conclusion

The root cause of excessive heating (up to 200°C) in the turret bolt was uneven distribution of induced eddy current, where 1) Other bolts had higher contact resistance 2) Presence of paint layer acted as insulation 3) Only one bolt carried significant induced current After removing the insulating effect and ensuring proper metallic contact across all bolts, the induced current was evenly distributed, thereby eliminating localized overheating.

References

[1] IEEE/ANSI Standards C57.12.00 “IEEE Standard General Requirements for Liquid Immersed Distribution, Power, Regulating Transformers “, 2000. IS 2026: Power Transformers

Copyright

Copyright © 2026 Gautam Kumar. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.