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Harmonics

harmonics "distorted" waveform

About[]

Harmonics are distorted signals or wave forms. Harmonics as voltages or currents that at frequencies 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.)

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.

Harmonics can cause overloading of conductors and transformers and overheating of utilization equipment, such as motors. Triplen harmonics can especially cause 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 winding 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.

Nonlinear loads[]

A nonlinear load in a power system is characterized by the introduction of a switching action and consequently current interruptions. This behavior provides current with different components that are multiples of the fundamental frequency of the system. These components are called harmonics. The amplitude and phase angle of a harmonic is dependent on the circuit and on the load it drives. For a fundamental power frequency of 60 Hz, the 2nd harmonic is 120 Hz, the 3rd harmonic is 180 Hz, and so on. The harmonic currents flow toward the power source through the path of least impedance.

Some examples of nonlinear loads that can generate harmonic currents are computers, fax machines, printers, PLCs, refrigerators, TVs and electronic lighting ballasts. Personal computers constitute nonlinear loads since they incorporate switched-mode power supplies. The PC current is mainly dominated by the third and fifth harmonic components. Current harmonics deteriorate the power factor of the system, what is the ratio between the average power of a certain load and the average power calculated for a pure resistive load with equal voltage amplitude. Differently from the reactive power drawn by a linear load, the harmonic currents cannot be corrected by the use of capacitors and inductors, but by the use of Harmonic Mitigating Transformers.

Current distortions can produce voltage distortions. When currents with harmonics flow through electrical generation systems and transmission lines, additional distortions take place because of the impedance of the electrical network.

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