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T r a n s f o r m e r s
1. Linear loads
Loads where the current waveform conforms to the waveform of the applied voltage. Or loads where a change in current is directly
proportional to a change in applied voltage. For example:
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Resistance heating
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Incandescent lighting
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Water heater
2. Non-linear loads
Loads where the current waveform does not conform to the waveform of the applied voltage. Or loads where a change in current is
not proportional to a change in applied voltage. Examples are:
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Computer power supplies
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Motor drives
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Fluorescent lighting
Non-linear loads produce non-sinusoidal current or voltage waveforms.
3. Sinusoidal current or voltage
This term refers to a periodic waveform that can be expressed as the sine of a linear function of time.
4. Non-linear currents or voltages
A waveform of current or voltage which cannot be expressed as the sine of a linear function of time. A non-linear load would result in
a non-sinusoidal current or voltage.
5. Harmonic
A sinusoidal waveform with a frequency that is an integral multiple of the fundamental 60 Hz frequency.
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60 Hz Fundamental
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120 Hz 2nd Harmonic
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180 Hz 3rd Harmonic
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240 Hz 4th Harmonic
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etc.
Current waveforms from non-linear loads appear distorted because the non-linear waveform is the result of adding harmonic
components to the fundamental current.
6. Triplen harmonics
Odd multiples of the 3rd harmonic (3rd, 9th, 15th, 21st, etc.).
7. Harmonic distortion
Non-linear distortion of a system characterized by the appearance in the output of harmonic currents (voltages) when the input
is sinusoidal.
8. Voltage harmonic distortion (VHD)
Voltage harmonic distortion is distortion caused by harmonic currents flowing through the system impedance. The utility power
system has relatively low system impedance, and the VHD is very low. But, VHD on the distribution power system can be significant
due to its relatively high system impedance.
9. Total harmonic distortion (THD)
The square root of the sum of the squares of all harmonic currents present in the load excluding the 60 Hz fundamental. It is usually
expressed as a percent of the fundamental.
10. Root mean squared current (or voltage) RMS
1: The vector sum of the fundamental current and the total harmonic distortion.
2: Square root of the sum of the squared value of the fundamental current and the squared value of the total harmonic distortion.