The triple play is a relatively rare occurrence, with only 733 of them recorded in Major League Baseball’s 147 years. In contrast, triplen harmonics are now a common occurrence in electrical systems, as the percentage of nonlinear loads keeps growing.
Triplen harmonics are multiples of three times the fundamental frequency. For a 60-hertz (Hz) power system, the triplen harmonics include the third harmonic at 180 Hz, the sixth at 360 Hz, the ninth at 540 Hz and so on. These are further classified as odd versus even harmonics, with even harmonics being much less prevalent than odd. Of these, the third harmonic is usually among the most common in power systems, especially where there are significant quantities of single-phase electronic loads, such as printers, desktop and laptop computers, televisions and various IT equipment.
Why are these harmonics special?
What makes these triplen harmonics special in PQ analysis compared to other harmonics? They generate power loss in electromagnetic equipment, resulting in derating motors and transformers. Triplen harmonics pose a unique problem to three-phase, 4-wire wye circuits. The three-phase currents in a wye circuit are intended to cancel out when combined in the neutral or grounded conductor, which is true for the balanced 60 Hz components. However, the triplen harmonics are additive and can produce a waveform nearly twice the other three-phase conductors’ amplitudes. Figure 1 shows what just having a third harmonic value that's 25% of the fundamental’s value can do to the neutral current.
Notice that the third harmonic flattens off the peaks of the sine waves of the phase conductors. This is sometimes referred to as the “Mount St. Helen’s effect.” Having the peak of the voltage waveforms reduced means that there is less charging voltage for the capacitors in the AC-DC converters typical in electronic loads, more accurately known as equipment with rectified input, switch-mode power supplies. Less charge, less ride-through during voltage sags.
Before circuits experienced an increase in neutral current from triplen harmonics, the neutral conductor could be undersized as compared to the phase conductors for balanced systems. Now, it is recommended to be at least the same size, and in some cases, nearly twice the current-carrying capacity of the other phase conductors.
The NFPA Fire Research Foundation sponsored a project to make recommendations to the National Electrical Code-making panels concerning harmonics. This included increasing the conductor size due to the heating from skin and proximity effects from the harmonic, since the larger diameter has lower impedance. When there is elevated current, there will be elevated voltage, since current times impedance in wiring will result in a voltage.
Why are the triplen current harmonics additive in the neutral? Figure 2 demonstrates the concept. If you look at a point on the waveforms of the fundamental of phases A, B and C, and add their values together, the result is zero. The right dotted line shows Phase B at zero, while Phase A is the negative equivalent value of Phase C, resulting in 0.0 when added together in the neutral. However, the third harmonic waveforms—and all triplens—are in phase at all times, rather than being 120 degrees out of phase as the fundamental waveforms are, since 3 x 120 = 360. Between the dotted area, the third harmonic of Phase A looks just like that in Phase B and C. The neutral conductor will then have 3 times as much harmonic current in it than any of the phase conductors have.
The triplen harmonics are also classified as zero sequence components in three-phase power systems. Positive sequence components, such as the fundamental, turn motors in the proper or intended direction. Negative sequence components, such as the 5th harmonic (another one commonly found in today’s electrical systems), try to rotate the motors in the opposite direction. Zero sequence components don’t do any productive work, they just heat things up. Though we have been discussing three-phase wye circuits, the triplen harmonics will circulate in a delta wound transformer, resulting in more heating and derating requirements.
Before there was widespread awareness of neutral current issues flowing through undersized conductors, there were reports of overheated neutrals and even fires resulting from such. Hopefully, the knowledge is the reason for few reports of these lately. It doesn’t mean it can’t still happen. Just like the unfielded triple play—where no fielder touches the batted ball, yet three outs are recorded on the single play—it’s possible, though none have been reported (yet).
About The Author
BINGHAM, a contributing editor for power quality, can be reached at 908.499.5321.