Unexpected Inrush Current During Generator Energization at Hindalco Mahan Plant before Synchronization

Abstract

In a power plant, the Generator and Generator Transformer are among the most critical and high-value electrical assets. Any unexpected behaviour during their operation or commissioning can raise serious concerns and potential operational risks. Annual overhauling and energising a transformer are regular work in a power plant. An investigation was conducted to identify the root cause of the inrush current and to explore the means to mitigate the problem. The result suggest the use of demagnetising process to be followed DC test conducted on transformer.

Introduction

At Hindalco Mahan Plant, Unit #1 was restarted after a Capital Overhauling. During startup, an unexpected high current (1400 A) appeared briefly when the generator terminal voltage neared 15.75 kV, despite the generator and auxiliary breakers being open. No faults or damage were found, and the unit later synchronized normally.

Various potential causes were investigated, including lightning arrestors, CT terminals, generator circuit breaker, and transformer inrush currents. After expert consultation, it was concluded that the phenomenon was linked to residual magnetism in the transformer cores caused by DC insulation resistance (IR) and polarization index (PI) tests conducted during the overhaul.

DC testing magnetizes the transformer core due to unidirectional current, leaving residual flux that can cause large inrush currents when the transformer is energized. This residual flux can stress equipment and cause false protection trips.

Root cause: The transformer was not demagnetized after DC tests, leading to core saturation and high transient current during energization.

Lessons learned:

• Always perform demagnetization after DC tests like IR or PI.

• Include demagnetization before final energization and box-up.

• Use test equipment with built-in demagnetization functions.

• Make demagnetization mandatory in test and commissioning protocols.

Conclusion

This case underscores how a well-known physical phenomenon — core residual magnetism — can create puzzling and seemingly abnormal behaviour if not anticipated. As the generator voltage approached 15.75 kV: 1) The core experienced flux reversal with every cycle 2) Transformer drew magnetizing inrush current to balance the residual flux 3) Current appeared on the amperemeter, despite no load being connecte 4) Once flux normalized, current dropped back to zero

References

[1] IEEE/ANSI Standards C57.12.00 “IEEE Standard General Requirements for Liquid Immersed [2] Distribution, Power, Regulating Transformers “, 2000. [3] IS 2026: Power Transformers

Copyright

Copyright © 2025 Kumar Gautam. 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.