Harmonics in polyphase power systems

Chapter 10.7 Harmonics in polyphase power systems

Lessons In Electric Circuits Volume II – AC Book
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Lessons In Electric Circuits Volume II – AC Book

  • 10.7.HARMONICS IN POLYPHASE POWER SYSTEMS313normal ∆-∆ system. If one of the two transformers were to fail, the load voltage and current woulddefinitely be affected.The following photograph (Figure 322,10.48) shows a bank of step-up transformers at the GrandCoulee hydroelectric dam in Washington state. Several transformers (green in color) may be seenfrom this vantage point, and they are grouped in threes: three transformers per hydroelectric gen-erator, wired together in some form of three-phase configuration. The photograph doesn’t revealthe primary winding connections, but it appears the secondaries are connected in a Y configuration,being that there is only one large high-voltage insulator protruding from each transformer. Thissuggests the other side of each transformer’s secondary winding is at or near ground potential, whichcould only be true in a Y system. The building to the left is the powerhouse, where the generatorsand turbines are housed. On the right, the sloping concrete wall is the downstream face of the dam:Figure 10.48: Step-up transfromer bank at Grand Coulee hydroelectric dam, Washington state,USA.10.7Harmonics in polyphase power systemsIn the chapter on mixed-frequency signals, we explored the concept of harmonics in AC systems:frequencies that are integer multiples of the fundamental source frequency. With AC power systemswhere the source voltage waveform coming from an AC generator (alternator) is supposed to be asingle-frequency sine wave, undistorted, there should be no harmonic content . . . ideally.This would be true were it not for nonlinear components. Nonlinear components draw currentdisproportionately with respect to the source voltage, causing non-sinusoidal current waveforms. Ex-amples of nonlinear components include gas-discharge lamps, semiconductor power-control devices(diodes, transistors, SCRs, TRIACs), transformers (primary winding magnetization current is usu-ally non-sinusoidal due to the B/H saturation curve of the core), and electric motors (again, whenmagnetic fields within the motor’s core operate near saturation levels). Even incandescent lampsgenerate slightly nonsinusoidal currents, as the filament resistance changes throughout the cycle due