Transformer Test Suite Builds a Maintenance Baseline

20Transformers Tested

Routine maintenance and a complete diagnostic test program across a pulp mill's fleet of dry-type and liquid-filled transformers — TTR, insulation resistance, power factor (tan delta), and DGA oil analysis establishing a condition baseline to drive maintenance planning and head off unplanned failures.

Baseline Established

The challenge

Power transformers are among the most critical — and most expensive — assets in a pulp mill, with replacement lead times measured in months and a failure capable of idling the whole operation. This mill ran a mixed fleet of 20 dry-type and liquid-filled transformers with no consistent diagnostic history. Routine maintenance was overdue, and without a measured baseline there was no way to tell a healthy unit from one quietly degrading inside.

Our approach

We performed routine maintenance and a full diagnostic battery on every unit, tailored to transformer type. Several tests apply to both dry-type and liquid-filled transformers:

• Turns ratio (TTR) testing across all tap positions to verify winding ratio and detect shorted or open turns and tap-changer faults
• Insulation resistance testing with polarization index (PI) to assess the condition of the winding insulation and reveal moisture or contamination
• Winding resistance measurement to find loose connections, poor contacts, and winding damage
• Power factor / dissipation factor (tan delta) testing to measure dielectric losses and quantify insulation aging — one of the best early indicators of degradation
• Visual inspection, connection torque checks, and a thermographic scan under load

For the liquid-filled units we added oil diagnostics that a dry-type transformer doesn't need:

• DGA (dissolved gas analysis) oil sampling to detect incipient faults — arcing, overheating, or partial discharge — from the gases dissolved in the oil long before they surface any other way
• Oil quality testing — dielectric breakdown voltage, moisture content, and acidity — to gauge the health of the insulating fluid

For the dry-type units, maintenance centred on cleaning accumulated dust and contamination from the windings, close insulation-resistance evaluation, and thermographic inspection — the failure modes that matter most in ventilated, air-insulated designs.

What we found

The program produced a clear, measured snapshot of every transformer in the fleet — separating the units in solid condition from those worth watching. Power factor and PI trends flagged early insulation aging on a couple of units, and DGA results gave the liquid-filled transformers a clean bill or an early warning that no external inspection could have provided. For the first time, the mill had hard numbers instead of assumptions.

Why the Full Battery Matters No single test sees everything. TTR catches winding and tap faults, tan delta and PI track insulation aging, and DGA hears chemical evidence of a fault developing inside the oil. Run together, they cover failure modes that any one test alone would miss.

The outcome

The mill now holds a documented condition baseline for its entire transformer fleet, dry-type and liquid alike. Future routine maintenance measures against it — turning transformer care from reactive guesswork into a planned, data-driven program, with DGA and power-factor trends giving early warning long before an unplanned failure.

Know your transformer fleet's condition

A baseline today is the difference between a planned repair and a six-figure emergency with a long lead time.