A transformer dies when its paper dies. The oil can be cleaned, dried, even replaced — the paper can’t. So the real question in condition assessment isn’t “how’s the oil?” It’s “how much life is left in the cellulose?” And that question has a frustrating catch: you can’t measure it directly. The number that answers it is the… Read More »
Of all the tests you can run on transformer oil, this is the one that gets run first. It’s fast, it’s cheap, and a single number in kilovolts tells you whether the oil is doing its main job: holding off voltage without arcing. The breakdown voltage test — BDV — measures the highest voltage the oil can take… Read More »
You measure water in the oil. You care about water in the paper. Those are not the same number, and the gap between them is where most moisture mistakes happen. Karl Fischer gives you a clean figure for the oil — a few parts per million. But the oil holds almost none of the water in a transformer.… Read More »
Water is the quiet enemy of a transformer. You don’t see it, it shows up in single-digit parts per million, and that’s enough to matter. Go from 10 ppm to 30 ppm of water in the oil and the breakdown voltage can drop by half. The water also feeds the slow rot of the paper insulation, which is… Read More »
A NETA-certified test company once failed a 138 kV transformer on polarization index. The IR measurement was healthy. The power factor was fine. The TTR, excitation current, DGA, leakage reactance, and SFRA all came back acceptable. Everything compared cleanly to previous tests. The transformer was returned to service over the contractor’s objection — and continued to run without… Read More »
The on-load tap changer is the only thing inside a power transformer that moves under load. Every other component sits still for forty years. The OLTC operates thousands or tens of thousands of times, each operation involving springs, gears, contacts, transition impedances, and arcing under load — all happening in milliseconds inside an oil compartment that nobody opens… Read More »
A transformer doesn’t usually fail from the inside out. It fails from the bushings. Bushing failures account for a disproportionate share of transformer in-service failures. Industry studies and utility reliability surveys have reported figures of approximately 15% to 40%, varying with transformer population, voltage class, and age profile — and the failures tend to be catastrophic: explosive porcelain… Read More »
A new transformer reads 0.3% power factor on its insulation at commissioning. Five years later, the same transformer reads 0.45%. Both numbers are below the IEEE C57.152 threshold of 0.5% that defines healthy insulation. By the absolute limit, the transformer is fine. It isn’t fine. The insulation has lost a third of its margin in five years. If… Read More »
A transformer winding that has shifted 2 mm inside the tank will pass every conventional electrical test. The turns ratio is unchanged. The winding resistance is unchanged. The insulation resistance is unchanged. The DGA is clean if the movement hasn’t caused arcing yet. Energize the transformer, run it for years, and the shift slowly progresses — until one… Read More »
A winding resistance test sounds like nothing. Inject DC current, measure voltage drop, divide. You get an ohms reading that you write down and move on. Then you find out the reading takes ten minutes to settle on a large power transformer. The number changes if the core wasn’t demagnetized first. The phase-to-phase comparison only works at the… Read More »