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 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 »
The turns ratio test looks like the simplest measurement in transformer work. Apply a small voltage to one winding, measure what comes out the other, divide, compare to nameplate. A handheld instrument does it in seconds and prints a pass/fail. That simplicity is exactly why it gets done badly. People read the ratio number, see it’s within tolerance,… Read More »
A factory acceptance test is the last good chance to catch a problem before a transformer becomes the utility’s problem. Once it leaves the factory, a defect that could have been a warranty conversation in the test bay becomes a field failure, an outage, and a multi-month replacement. The witness in the chair is the line between those… Read More »
The DC hipot test is the most consequential offline test you can run on a stator winding. It applies a high DC voltage — well above operating levels — to the groundwall insulation and holds it there. If the insulation fails during the test, you know before the machine goes back into service. If it passes, you have… Read More »
Most stator failures are not sudden. They are the end point of a process that has been running for months or years — visible if you know what to look for, detectable by testing before the insulation finally fails in service. Understanding the failure mechanism behind a bad test result is the difference between cleaning a winding and… Read More »
Most people who run a Polarization Index test can tell you the formula: 10-minute IR divided by 1-minute IR. Fewer can tell you why that ratio is meaningful, where it stops being meaningful, or what a result of 1.1 on a modern epoxy-mica winding actually implies — compared to the same reading on an asphaltic machine from 1965.… Read More »