Page 909 - Industrial Power Engineering and Applications Handbook

28/860 Industrial Power Engineering and Applications Handbook

Figure 28.2(a) Low rating LT bus duct

Non-magnetic or Barriers (same metal Clamps (same metal
magnetic enclosure as the enclosure) as the enclosure)

Figure 28.2(b) A segregated phase bus system

melallic barriers provide the required magnetic shielding unbalance between the three phases as discussed in Section
and isolate the busbars magnetically from each other, 28.8.2 and enhance the electrodynamic forces that may
rather like an isolated phase bus system (IPB). For more lead to a phase-to-phase fault at higher rated currents.
details see Section 3 1.2. The enclosure can be of MS or
aluminium and the barriers can be of the same metal as Application
the enclosure. The purpose of providing a metallic barrier
is not only to shroud the phases against short-circuits but They are generally used for higher ratings, 2000 A and
also to reduce the effect of proximity of one phase on the above, on all voltage systems. They are, however, preferred
other by arresting the electric field produced by the current- on an HT rather than an LT system for reasons of cost,
carrying conductors within the barrier itself. It now such as between a unit auxiliary transformer (UAT) and
operates like an enclosure with an interleaving its switchgears and a station transformer and its switch-
arrangement (Section 28.8.4) balancing the fields produced gears as in a power-generating station and shown in
by the conductors to a great extent and also allowing Figure 13.21.
only a moderate field in the space, as in an IPB system
(Section 31.2). The enclosure losses with such an Nute
arrangement may fall in the range of 6M5% of conductors For such ratings, enclosure of non-magnetic material alone is
recommended due to high iron losses in a magnetic material.
in case and 30-35% in aluminium enclosures for all
voltage systems 3.3-1 1 kV and current ratings above Nominal current ratings
3000A and up to 6000A or so. Only aluminium enclosures
should be preferred to minimize losses and enclosure These will depend upon the application. The preferred
heating. The effect of proximity is now almost nullified ratings may follow series R-10 of IEC 60059, as described
as is an imbalance in the phase reactances. An unbalance in Section 13.4.1(4). They may increase to 6000 A or so
in the reactance is otherwise responsible for a voltage depending upon the application.