In the ideal case, alternating voltages and currents are sinusoidal functions having a single frequency f or w. This fundamental frequency, also known as the power frequency, is usually the lowest frequency component in the system.
In reality, there are a number of effects within the power system that may cause the cosine wave to become distorted or “polluted” to some extent. When we say distorted or polluted, we mean that the voltage and current no longer contain just the desired power frequency (50 or 60 Hz).
Any repeating AC waveform, no matter how distorted, may be represented by a combination of waveforms of the fundamental frequency plus one or more harmonics. A harmonic is a frequency which is an integer multiple of the power frequency, (h#, h#integer). In a 60 Hz system, examples of harmonics of the power frequency would be 180 Hz, 300 Hz, 420 Hz, 660 Hz, 780 Hz, etc.
These higher frequencies are called the third, fifth, seventh, eleventh, and thirteenth harmonics of 60 Hz, respectively. The relative magnitude of the fundamental waveform and the number, magnitude, and phase displacement of the harmonic components determine the resultant waveform’s shape.
For instance, Figure 4-23 shows a voltage waveform composed of a 100 V fundamental 60 Hertz waveform and a 20 Volt third harmonic. This waveform contains 100% or one per-unit 60 Hz and a 20% or .02 per-unit third-harmonic component.
The third harmonic crosses the x-axis at the same instant in time as the fundamental waveform. Harmonics can be displaced in time from the fundamental, depending on circuit characteristics. For example, Figure 4-23 shows the 3rd harmonic 180ยบ out of phase with the fundamental.
Under certain circumstances, harmonics can be important and also troublesome. Electronic loads containing power semiconductor devices, which switch on and off to control the flow of energy between the source and load, typically cause power system harmonics.
This switching on and off, hundreds to thousands of times every second, directly modulates the current and corrupts the voltage. In other words, the current modulation causes voltage drops across the impedance of the lines and distribution equipment resulting in the voltage being modulated as well.
Thus, high-frequency harmonic components are injected into the power system. Examples include variable-speed motor drives, electronic lighting ballasts, and electronic equipment power supplies.
As a result, these loads can cause dangerous resonance conditions between the electronic load’s step down transformer and the utility’s power factor correction capacitors.
Harmonics will cause additional heating in wiring and other equipment, and will not be detected by most digital test meters unless they are true rms measuring devices.
In addition, electromechanical meters typically under-register the energy being absorbed by these electronic loads.
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