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Harmonics

With the exception of the incandescent light bulb, every load today creates harmonics. Unfortunately, these loads vary with respect to their amount of harmonic content and response to problems caused by harmonics.

Harmonics: It surfaced as a buzzword in the early 1980s, making many people reconsider the effectiveness of their building's wiring system. Yet, many still view the concept as a relatively new phenomenon. However, harmonics have been around since well before the early '80s: The associated problems existed in the electrical world way back when transistor tubes were first used in the 1930s. Aside from grounding, many deem harmonics as one of the greatest concerns for the power quality industry today. In this issue, we'll discuss the fundamentals of harmonics and the problems it can cause within the premises wiring system.

What is harmonics? We define harmonics as voltages or currents at frequencies that are a multiple of the fundamental frequency. In most systems, the fundamental frequency is 60 Hz. Therefore, harmonic order is 120 Hz, 180 Hz, 240 Hz and so on. (For European countries with 50 Hz systems, the harmonic order is 100 Hz, 150 Hz, 200 Hz, etc.)

We usually specify these orders by their harmonic number or multiple of the fundamental frequency. For example, a harmonic with a frequency of 180 Hz is known as the third harmonic (60x3 = 180). In this case, for every cycle of the fundamental waveform, there are three complete cycles of the harmonic waveforms. The even multiples of the fundamental frequency are known as even-order harmonics while the odd multiples are known as the odd-order harmonics.

How do we create harmonics? Up until 1980, all loads were known as linear. This means if the voltage input to a piece of equipment is a sine wave, the resultant current waveform generated by the load is also a sine wave, as seen in Fig. 1 (in the original text).

In 1981, manufacturers of electronic hardware converted to an efficient type of internal power supply known as a switch-mode power supply (SMPS). The SMPS converts the applied voltage sine wave to a distorted current waveform that resembles alternating current pulses, as seen in Fig. 2 (in the original text). Obviously, the load doesn't exhibit a constant impedance throughout the applied AC voltage waveform.

Most utilization equipment today creates harmonics. In all likelihood, if a device converts AC power to DC power (or vice versa) as part of its steady-state operation, it's considered a harmonic current-generating device. These include uninterruptible power supplies, copiers, PCs, etc.

What are the effects of harmonics? The biggest problem with harmonics is voltage waveform distortion. You can calculate a relationship between the fundamental and distorted waveforms by finding the square root of the sum of the squares of all harmonics generated by a single load, and then dividing this number by the nominal 60 Hz waveform value. You do this by a mathematical calculation known as a Fast Fourier Transform (FFT) theorem. (FFT is beyond the scope of this article. IEEE's Standard Dictionary of Electrical and Electronic Terms gives a definition of Fourier series.) This calculation method determines the total harmonic distortion (THD) contained within a nonlinear current or voltage waveform.

Triplen harmonics. Electronic equipment generates more than one harmonic frequency. For example, computers generate 3rd, 9th, and 15th harmonics. These are known as triplen harmonics. They are of a greater concern to engineers and building designers because they do more than distort voltage waveforms. They can overheat the building wiring, cause nuisance tripping, overheat transformer units, and cause random end-user equipment failure.

Circuit overloading. Harmonics can cause overloading of conductors and transformers and overheating of utilization equipment, such as motors. Triplen harmonics can especially caus e overheating of neutral conductors on 3-phase, 4-wire systems. While the fundamental frequency and even harmonics cancel out in the neutral conductor, odd-order harmonics are additive. Even in a balanced load condition, neutral currents can reach magnitudes as high as 1.73 times the average phase current.

This additional loading creates more heat, which breaks down the insulation of the neutral conductor. In some cases, it can break down the insulation between windings of a transformer. In both cases, the result is a fire hazard. But, you can diminish this potential damage by using sound wiring practices.

When most electrical engineers design the building's wiring, they usually leave the sizing of the neutral conductor to the dictates of NEC. In most cases, the installed neutral is the same size as the phase conductors. However, the Notes to the Ampacity Tables (in NEC Art. 310) instruct you to consider the neutral conductor as a current-carrying conductor if electronic equipment or electronic ballasts are used at the site. This correlates into the neutral conductors being sized larger than they would be with conventional wiring means.

To be on the safe side, more engineers are doubling the size of the neutral conductor for feeder circuits to panelboards and branch circuit partition wiring to handle the additive harmonic currents.

 

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