Harmonics are undesirable sinusoidal voltages or currents whose frequencies are integer multiples of the fundamental supply frequency (usually 50hz or 60hz); for example a 5th harmonic is at 250 Hz (i.e. 5 times fundamental frequency, 7th harmonic is 350 Hz, and so on based on a 50 Hz fundamental).
Harmonic distortion originates due to the non linear characteristics of certain types of electronic loads including :-
• Variable speed AC and DC drives
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Uninterruptible power system systems (UPSs)
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Rectifiers for aluminium smelters, electrolysis, etc.
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Fluorescent lighting ballasts
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Computers
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Fax machines
• TVs
• Battery chargers
• X-ray scanners, body scanners and other medical equipment
The majority of equipment which converts AC power is classed as ‘non linear’ and therefore produces harmonics. Harmonics however, are also produced by generators, motors and power transformers although their magnitude and subsequent effects are usually considered insignificant when compared electronic based non linear loads.
Each individual harmonic current interacts with the system impedance resulting in a voltage drop across the impedance(s) producing resultant voltage harmonic of the same frequency. The individual voltage harmonics sum to produce the ‘total harmonic voltage distortion’ (VTHD). Harmonic recommendation such as the North American IEEE Standard 519 (1992), UK’s Engineering Recommendation G5/4 (2001), South African NRS 048-2 and Australian AS61000 provides guidelines for harmonic current and voltage distortion levels on transmission and distribution systems.
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