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Topic Title: Third harmonic on neutral causing communication interference?
Topic Summary: What is the explanation?
Created On: 06 July 2010 03:55 AM
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 06 July 2010 03:55 AM
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Hello, from my latest post on this forum asking about the IBT (Inter Bus Transformer), Link Removed many user said that the current (60Hz or Third Harmonic) on the neutral (Line or earth) can cause communication interference..

From some paper and book that i mention on that topic also said the same. But there is no explanation how the interference happen.

Could anybody tell me the process of the interference? what kind of communication system that will got the interference?

The communication system often have very high frequency in order MHz and GHz. But the power system just have 50/60Hz frequency and the third harmonic is 150/180 Hz only.. how can those current can cause problem on communication system? i need this answer for my studies..

Thanks for your attention..

Adityo Kusumo
 06 July 2010 09:24 AM
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At some point in the communication circuit, you will have to get back to a frequency range we can hear (i.e., between 0-4kHz) using a low pass filter. This is the stage where you will find the harmonics can cause electromagnetic interference.
 12 July 2010 07:52 AM
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from my further search, i found a paper that related to this topic.

"Power line RF Interference -- Sounds, Pattern and Myths"
by Marvin O. Loftness, Life Fellow IEEE
on IEEE Transactions on Power Delivery, Vol. 12, No.2, April 1997

the conclusion is the power-line RFI does not result from harmonics of the power-line frequency. It's from spark generated on the power line (corona)..
 12 July 2010 03:25 PM
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The harmonic currents and voltages associated with nonlinear loads can induce currents and voltages in communication circuits that parallel the power conductors. Harmonic currents induce currents in the two conductors of communication circuits. When these induced currents flow in the communication circuits they produce voltages. If these voltages are not equal (currents in the two conductors are not equal because of spacing or other differences), they will not cancel and the resultant is noise in the circuit. Communication circuits that have twisted pairs, and/or are shielded, minimize the problem. The problem is most likely to appear when distribution lines and telephone circuits share the same pole line. Also Telephone circuits are particularly susceptible to the influence of ground return currents. In rural areas when power or telephone circuits use a ground return, the large inductive loop can have a large influence on the noise produced in telephone circuits. Particular care should be used to minimize nonlinear loads in this case.

For similar discussions you can refer to Transformer Riddle No.5

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