Thermal Fault Detected at Steel Mill — $2.3M Saved by Early DGA Warning

Background

A major steel mill in Southeast Asia operates a 120 MVA arc furnace transformer — a critical single-point-of-failure asset. Arc furnace transformers endure extreme conditions: frequent overloads, high ambient temperatures (40°C+), heavy harmonic content, and mechanical stress from electrode cycling. A failure of this transformer would halt production for 6-8 months with an estimated loss of $2.3 million in downtime, equipment replacement, and contractual penalties.

In 2022, the plant deployed a PAS DGA DGA-500 hydrogen monitor on this transformer as part of a shift from quarterly laboratory oil sampling to continuous online monitoring.

The Challenge

Arc furnace transformers present unique monitoring challenges. The frequent load cycling (from near-zero to 200% rated load within seconds during electrode strike) causes rapid thermal transients. Traditional quarterly lab DGA sampling — which the plant had relied on for years — was missing these transient events entirely. The plant needed continuous, real-time hydrogen monitoring that could survive the harsh industrial environment.

PAS DGA Solution

A DGA-500 hydrogen monitor was installed via direct valve mount on the transformer oil sampling valve. Installation was completed in under one hour with no outage required. The DGA-500’s Pd alloy thin-film sensor was immersed directly in the transformer oil and connected to the plant’s SCADA system via MODBUS RTU. Alarm thresholds were set at 25 ppm H₂ warning and 50 ppm H₂ critical.

Detection Timeline

The DGA-500 detected a dramatic H₂ increase: from a stable baseline of 8 ppm to 85 ppm in just 48 hours — a 10× increase. The rate of change (38.5 ppm/day) far exceeded the IEEE C57.104 caution threshold of 10 ppm/day for this transformer class.

Diagnostic Analysis

Upon reaching the critical alarm threshold, a follow-up laboratory DGA was immediately performed, confirming the online readings. Applying the Duval Triangle method:

Duval Triangle 1 coordinates: %CH₄ = 29.7%, %C₂H₄ = 69.0%, %C₂H₂ = 1.3% → Zone T2: Thermal fault 300-700°C

This pattern indicated a developing hot spot, most likely involving degraded insulation or a loose connection rather than arcing (C₂H₂ was only marginally elevated). The operations team initiated a controlled shutdown for inspection.

Findings & Resolution

Internal inspection revealed tracking damage on a high-voltage bushing — the early stages of insulation breakdown caused by a loose clamping connection generating localized heating. The bushing was replaced during a 3-day planned maintenance window. Without the early warning, the tracking would have progressed to full bushing flashover within approximately one week, destroying the bushing, potentially damaging the winding, and causing an unplanned outage of 6-8 months.

Outcome & ROI

Key Takeaways

• The rate of change — not absolute concentration — was the critical diagnostic signal. A quarterly lab sample could have missed the transient entirely.
• Hydrogen-only monitoring provided sufficient early warning for this thermal fault. Multi-gas follow-up confirmed the diagnosis but was not needed for initial detection.
• Zero-maintenance operation was essential: the steel mill environment (dust, vibration, heat) would have prevented any regular sensor maintenance schedule.
• The 270:1 ROI demonstrates that DGA monitoring is not a cost — it is the cheapest insurance policy a transformer owner can buy.