Case Study Overview: Warranty-Protected Failure Caught Just in Time

When a Near-New Transformer Develops Thermal Runaway: The Warranty Claim That Online DGA Made Ironclad

A catastrophic failure in a brand-new transformer is every utility's nightmare—especially when warranty coverage depends on proving the root cause was a manufacturing defect rather than operational issues. This case study documents how continuous monitoring not only prevented complete asset loss but also provided the unequivocal evidence needed for successful warranty remediation.

The Scenario:

A 1,100 MVA, 345 kV generator step-up transformer had operated for less than two years when rate-of-change alarms triggered on August 28, 2016. The transformer had experienced early cooling issues—a backwards-running pump discovered through DGA correlation with cooling bank operation. After that correction, gas levels remained elevated but stable for 21 months.

The Critical Detection:

On August 28th, all combustible gases began climbing steadily. By midnight, the escalation was unmistakable despite not triggering the Buchholz relay. Every diagnostic method—Duval Triangle, Duval Pentagon, Rogers Ratios, IEEE standards—converged on thermal fault between 300-700°C with paper involvement. Even reducing the load failed to stop gas generation. The transformer was de-energized on August 30th for emergency inspection.

The Root Cause:

Physical inspection revealed severely damaged crimped connections between LV cables and bushing terminals. Detailed failure analysis determined that high eddy losses in the crimps, combined with poor oil circulation and uneven current distribution, created working temperatures exceeding 300°C—exactly matching DGA predictions. The localized overheating initiated a degenerative thermal runaway that carbonized the surrounding paper insulation.

The Warranty Impact:

The hour-by-hour DGA documentation provided irrefutable proof that the fault originated from a manufacturing defect in the crimped connections, not operational misuse. This evidence streamlined warranty claims, enabling full remediation at the manufacturer's expense.

Download to see the dramatic gas progression curves, comprehensive diagnostic analysis, and inspection photos documenting the failed connection.