Case Study: The Silent Threat No Standard DGA Detects

Why Oxygen Monitoring Matters: The Gasket Leak That Could Have Gone Undetected for Years

Most DGA diagnostics focus exclusively on combustible gases—hydrogen, methane, ethylene, and acetylene. But this case study reveals why oxygen measurement is equally critical for protecting transformer assets, even when no thermal or electrical fault exists.

The Mystery:

On November 16, 2017, a 30+ year old nitrogen-blanketed 230kV transformer triggered an alarm that puzzled operators. Oxygen concentration suddenly spiked from a stable 130 ppm to 2,150 ppm over three days before gradually declining to 200 ppm. Every conventional diagnostic method—Duval Triangle, Rogers Ratios, IEEE standards—showed absolutely nothing wrong. No combustible gas generation, no thermal signatures, no electrical faults.

The Investigation:

The isolated oxygen spike with no other gas changes indicated atmospheric air intrusion rather than internal fault activity. Physical inspection revealed a leaky manhole cover gasket combined with nitrogen blanket pressure near atmospheric levels. During thermal cool-down, a partial vacuum developed inside the transformer, literally sucking atmospheric air past the compromised gasket.

The Hidden Danger:

While oxygen accelerates insulation oxidation and reduces paper life, the greater threat is what accompanies atmospheric air moisture. Without oxygen monitoring, this gasket failure could have operated undetected indefinitely, drawing humid air into the transformer with every thermal cycle, systematically degrading the insulation system.

The Broader Application:

The case study includes critical technical notes explaining how identical vacuum conditions occur in conservator (COPS) transformers when breather piping is undersized or restricted, and how degassed oil creates effective vacuum conditions that can breach gaskets, showing no visible oil leaks.

The Bottom Line:

Nitrogen pressure doesn't prevent water ingress—only good gasket seals do. This case demonstrates that comprehensive DGA monitoring, including oxygen measurement, detects asset threats that conventional fault diagnostics completely miss.

Download to understand why oxygen monitoring is essential insurance against silent degradation.