Proximal effects

The next important phenomena that the result of supramolecular effect are the concentration and proximity effects concerning the components of analytical reaction, even through they are considerably different in hydrophobicity, charge of the species, complexing or collisional type of interaction. The concentration and proximity effects determine the equilibrium of analytical reaction, the efficiencies of intramolecular or intermolecular electronic energy or electron transfer and as a result the sensitivity of analytical reactions. 

A 16.6 Skin and proximity effects in a multicore cable 16/547 A 16.7 Short-time rating of cables 16/547 A 16.8 Termination of cables 16/548... 

Skin and proximity effects on a current-carrying conductor 28/873... 

In a d.c. system the current distribution through the cross-section of a current-canying conductor is uniform as it consists of only the resistance. In an a.c. system the inductive effect caused by the induced-electric field causes skin and proximity effects. These effects play a complex role in determining the current distribution through the cross-section of a conductor. In an a.c. system, the inductance of a conductor varies with the depth of the conductor due to the skin effect. This inductance is further affected by the presence of another current-carrying conductor in the vicinity (the proximity effect). Thus, the impedance and the current distribution (density) through the cross-section of the conductor vaiy. Both these factors on an a.c. system tend to increase the effective... 

Note Each phase is considered in isolation not inlluenced by proximity effect... 

Figure 28.13(a) Skin effect in isolated rectangular busbars, (neglecting the proximity effect) (Courtesy l ndian Aluminium Co, based Alcon of Canada)... 

A compromise is therefore struck to meet both needs and obtain a more balanced system or other methods adopted, as discussed in Section 28.8.4, to reduce the skin and proximity effects. 

Figure 28.19(a) Reactance of rectangular busbars at 50 Hz on account of proximity effect... 

The proximity effect does not end here. It still has some far-reaching consequences in terms of unequal voltage drops in different phases at the same time. This is more so on large LT current-carrying, non-isolated bus systems of 2000 A and above, resulting in an unbalance in the supply voltage, as discussed below. 

A three-phase system has three current-carrying conductors in close proximity. While the conductors of phases R and B will have an almost identical impedance, with the same skin and the proximity effects, the conductor of phase Kis under the cumulative effect of electric fields... 

Influence of electric fields of conductors R and B is offset in a 3-ip system. The proximity effect in phase Y therefore gets nullified... 

Consider Example 28.6 to determine the content of proximity (i) For reactance on account of the proximity effect, use Figure 28.16 and the graph of Figure 28.24 ... 

Determining the size of reactor Consider a three-phase bus system as shown in Figure 28.27. If and X are the inductive reactances of each phase on account of skin and proximity effects respectively, then the impedances of each of the three phases can be expressed as... 

The values of and ly must be known to determine the value of the reactor, Xp. Otherwise the reactance, X, as determined earlier, to account for the proximity effect may be considered as the lost reactance in the Y phase and for which the reactor may be designed for this phase. 

Drawing inferences from the literature available on the subject (see the further reading at the end of the chapter), based on laboratory tests, practical experience and the field data available, deratings for different configurations are shown in Table 28.7 which should be sufficient to account for the likely proximity effects... 

Table 28.7 Approximate deratings due to proximity effects for different configurations of bus systems...

Industrial Power Engineering and Applications Handbook 28.8.4 Minimizing the proximity effect... 

However, there may not be an appreciable improvement in the proximity effect between each section, unless the transpositions are increased infinitely, as in the case of a stranded three-phase cable which has continuously twisted conductors and represents an ideal transposition. In addition, there is no change in the skin effect. This arrangement therefore has the purpose primarily of achieving an inductively balanced system and hence a balanced sharing of load and equal phase voltages at the far end. 

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