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Effects of HarmonicsThe interfering effects of the harmonics found in power systems depend on both the frequency and the amplitude of these harmonics. A telephone influence factor meter has been devised to measure the tendency of a generator or a power system to produce interference. This tendency to produce interference, due to the harmonics and the resulting irregularities in the power system current or voltage wave shape, is called the telephone influence factor or TIF and is defined29 as "the ratio of the square root of the sum of the squares of the weighted effective values of all the sine-wave components (including in alternating waves both fundamental and harmonics) to the effective value of the wave." The telephone influence factor takes into consideration the characteristics of the receiver and of the ear, and the fact that the induction is roughly proportional to the frequency of the harmonics. As has been mentioned before, a low-frequency power-system harmonic causes very little interference compared to that caused by one in the middle of the voice range. The TIF meter accordingly contains a specially designed weighting network30 having the characteristics shown in Fig. 17. In practice this network is connected to the power source to be studied (through transformers), and the amount of "weighted" current passing through the network, as measured by a thermoammeter in the output, is an indication of the tendency of the power-system current or voltage to produce interference. The TIF of a power-system current or voltage can be calculated from measurements made with a wave analyzer.29
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Last Update: 2011-05-30