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 »
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 »
A lightning strike on a transmission line lasts microseconds. A power transformer is designed to run for forty years. The lightning impulse test is the bridge between those two facts — it proves, in the factory, that the transformer can survive the microsecond event so it can deliver the four decades of service. The test applies a standardized… Read More »
Every power transformer above 72.5 kV gets an IVPD test before it ships. The test takes more than an hour, costs the manufacturer significant test bay time, and is the final dielectric test in the sequence. Why? Because partial discharge is what kills transformer insulation slowly, and most other tests can’t see it. A lightning impulse test stresses… Read More »
When a power transformer arrives at site, the test report is mostly numbers. LI, LIC, SI, AV, IVW, IVPD, LTAC. Test voltages in kV. PD readings in pC. Clearances in mm. If you didn’t write the spec, you’re reading someone else’s interpretation of a standard you might not have in front of you. That standard, for most of… Read More »
If you’ve been doing motor insulation testing for any length of time, you’ve heard the standard cited a hundred times. “IEEE 43 says the PI minimum is 2.0.” “IEEE 43 requires correction to 40°C.” Most techs and engineers work from the rules without ever opening the document. That works fine until it doesn’t. Until an auditor asks where… Read More »
Every motor failure has a story. Most start the same way — insulation that gave out before the winding did. Heat, vibration, moisture, voltage spikes, chemicals. One or all of them. The question isn’t if insulation degrades. It’s how fast — and what’s driving it. This article covers the five aging stresses, what each one actually does to… 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 »