Different categories of harmonic-producing loads are supplied by the electric utilities such as

1. Domestic loads like fluorescent lamps, light dimmers, etc.

2. Ripple control systems for regulating hot-water loads

3. Medium-sized industrial loads like several adjustable speed drives in a cement mill, paper mill, etc.

4. Large loads like high-voltage direct current (HVDC) convertors, aluminum smelters, static var compensators, heavy single-phase ac traction loads for hauling coal trains, etc.

The undesirable effects of the harmonics produced by these loads are listed as follows:1–3
1. Capacitors: These may draw excessive current and prematurely fail from increased dielectric loss and heating. Also, under resonance conditions, considerably higher voltages and currents can be observed than would be the case without resonance.

IEEE Std 18-19924 gives limits on voltage, current, and reactive power for capacitor banks based on their ratings. These can be used to determine the maximum allowable harmonic levels.

2. Power Cables: In systems with resonant conditions, cables may be subjected to voltage stress and corona, which can lead to dielectric (insulation) failure. Further harmonic currents can cause heating.

3. Telephone Interference: Harmonics can interfere with telecommunication systems, especially noise on telephone lines. A “standard” human ear in combination with a telephone set has a sensitivity to audio frequencies that peaks at about 1 kHz.

Two systems with slightly different weighting systems are used to obtain a reasonable indication of the interference from each harmonic. The two systems are:

C-message weighting by Bell Telephone system (BTS) and Edison Electric Institute used in the United States and Canada

Psophometric weighting by the International Consultative Commission on Telephone and Telegraph System

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