Inductive interference

In earlier years, to reach a remote area, where. separate telephone lines had not been laid it was normal practice to rttn them through the same poles as the HT power distribution lines (generally 11-33 kV). This was particularly true of internal communications of the electricity companies for ease of operation and to save costs and time. This commitnication was known as the magneto-telephone system. But the proximity of telephone lines to power lines adversely affected the performance of the telephone lines due to generation of overvoltages (Chapter 20) and eleetrical interferences (conductive and inductive interferences, discussed later) on the telephone lines by the power lines.. Some of these interferenees, particularly system harmonics, had the same frequency as the audio frequency of the telephone lines and alTected their audio quality. 

The running of telephone lines through power lines is long discontinued. They are now run on separate structures within a city and nearby areas at audio frequency (— 0.3-3.4 kHz), and maintain enough distance from HT power distribution lines. They are therefore almost unaffected from such disturbances. Nevertheless, interferences must be kept in mind when installing these lines so that they are out of the inductive interference zone of the power lines. The latest method in the field of communications to avoid disturbances is to use underground optical fibre cables, where possible, as discussed later. Optical fibre cables are totally immune to such disturbances. 

Other than the system harmonics, electrical interferences are also caused by line disturbances, which may be caused by lightning, switching, sparking or a fault. As discussed in Chapter 17, line disturbances occur at very high frequencies but some may coincide with the audio frequency of telephone lines, and cause disturbance in the audio quality of the telephone system. All these disturbances are referred to as inductive interferences. 

We provide more details on inductive interferences in the box below, to make the subject of electrical interference more informative. We also provide a passing reference to communication systems being adopted worldwide. 

All ctfecis caused by electrostatic or electromagnetic inductions are termed Inductive Interferences. With the use t)f glass optical fibre cables in new installaiioiis. this effect is overcome automatically. Optical fibre cables, as discussed later, have no metal content and cany no electrical signals. Therefore the above discussion is more appropriate for existing installations and also to provide a theoretical aspect and moi c clarity on the phenomena of inductive interferences. These can also be applied to other fields rather than communications alone. 

It is also cumbersome to arrange a bus system with phase transposition. This technique has therefore not found many applications in a bus system. It is more useful in dealing with inductive interference in communication lines (Section 23.5.2(C)). 

In this part the author provides all relevant aspects of a reactive control and carries out an exhaustive analysis of a system for the most appropriate control. Harmonic effects and inductive interferences as well as use of filter and blocking circuits are covered. Capacitor switching currents and surges and methods of dealing with these are also described. 

Inductive interference the production of electrical potentials in conductors due to the induction from alternating magnetic fields arising from short-circuit currents or operational currents in high-voltage power lines. 

Inductive interference on pipelines is usually only to be expected with extended and close or parallel routing with three-phase, high-voltage overhead power lines, as well as with conductors and supply lines of ac railways. 

Sakai, H. 1971. Induction Interference and Shielding. Tokyo, Japan Nikkan Kogyo Pub. 

Christoforidis, G. G., D. P. Labridis, and P. S. Dokopoulos. 2005. Hybrid method for calculating the inductive interference caused by faulted power lines to nearby pipelines. IEEE Trans. Power Deliv. 20 (2) 1465-1473. 

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