A crowd gathered, including some VIPs, as the huge engine rumbled to life, flashed its field, and connected to the bus flawlessly. Everything looked perfect. Operators advanced the throttle to apply a little load. Everything remained perfect except neutral current, it was quite low, but 180 hz not 60 hz. Three times operating frequency, third harmonic.
A little more load, a little more neutral current still 180 hz. With each small load increase came an increasingly larger increase in 180 hz neutral current. Before 50% load we had enough 180 hz neutral current to overload our little transformer. And that current didn't go beyond the transformer. Bingo. So we stopped the test before the transformer got wrecked.
Engineering designed a simple wiring change to block the third harmonic current.
The reason this error slipped through design is this. It is common practice to connect a three-phase generators and transformers in this configuration, wye-wye-delta. The delta connected transformer's secondary absorbs the harmonic distortion from the generator, which is typically just a few percent. And that's good because it cleans up the waveform. This particular generator had about 5% distortion, not bad at all, but the generator and transformer are usually the same size. Ours weren't.
By looking at our electrical wiring drawings you wouldn't notice our size difference, both are about an inch tall on paper. But 5% of our generator amounts to several hundred percent of our poor little transformer, and the better transformer, with its lower impedance, were even less able to resist the harmonic current. And that's why it burned up so much quicker.
ó James D. Hardy is a hands-on engineer with 30 years I&C experience in oil/gas and nuclear power plants, electronics, process computers, simulators, sales, technical writing, and public speaking.
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