Page 999 - Industrial Power Engineering and Applications Handbook

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conductor and the enclosure for an IPB. The final sizes or the enclosure, or both, and/or augment the heat
attained can then be laboratory tested to establish their dissipation through forced cooling to meet thermal
accuracy. requirements. The theoretical design is then put to
laboratory tests to establish its accuracy.
Current rating vanes with the surface area of a conductor
31.12 Determining the section and its thickness (annulus). In our sample calculation, to
and size of conductor and establish the basic parameters of the conductor and the
enclosure, we have considered the current density for
enclosure both as 400 A/inch2.
The outdoor part of the enclosure has to perform more
An isolated phase bus is also a tailored system to suit a onerous duties as it has to withstand weather conditions
particular layout. Size and area of cross-section will vary and also absorb solar radiation. It has also to dissipate
with the manufacturer. The insulators also play an important the heat of the conductor in addition to its own. It is
role in determining the diameter of the enclosure and its therefore possible that the surface area of enclosure so
thickness as well as the diameter and thickness of the chosen may have to he increased, and this will be revealed
conductor. The manufacturer of an IPB may have the during thermal calculations which are carried out to check
insulators made to specifications (for voltage, height, its suitability.
clearance and creepage distances) or make a choice from With this assumption, the basic cross-section of the
what is available in the market and compromise on the conductor and the enclosure can be chosen. It is then
enclosure’s diameter. To standardize on the size of conductor counterchecked whether the size so chosen is adequate
and enclosure for such systems is not normal practice. A to reach a thermal stability. When desired, the t can be
manufacturer, however, can do so for his adopted design suitably modified to reach thermal equilibrium. The sizes
practices and standardize on some vital parameters such can he optimi7ed by plotting a few theoretical graphs:
as the type of section (round, hexagonal or octagonal),
depending upon thecurrentrating as well as the availability
of the extruded aluminium sections produced by indi- Losses versus t (Figure 31.12) and
genous manufacturers, design of insulators, area of cross- Cost versus t (Figure 3 1.13) to optimize t and hence,
section and the system of cooling, etc. Generally, natural the cost of the conductor and the enclosure. For this t,
cooling and hollow cylindrical sections are preferred. the diameter may be modified to arrive at a more
In smaller bus systems, say, up to 3200 A, standard economical design. A higher t will mean a lower
data for sizes of busbars and their current ratings have diameter and vice versa, and may be modified to satisfy
been long established, as noted in Section 30.2 and Tables the conductor’s current-carrying and the enclosure’s
30.4 and 30.5 for different sections and cross-sectional heat-dissipating requirements. Since the size of a hollow
areas. These data can be used in the design of a particular conductor, for large to very large current ratings, is
bus system. But an TPB has to be specially designed. not standardized the cylindrical diameter (6) and the
Below we provide brief guidelines for designing such a wall thickness (t) can be varied, depending upon the
system from fundamentals, i.e. by applying the theory of rating, the extruded sections available and the cooling
thermal equilibrium between the heat generated by the system adopted by the manufacturer. The exact d and
system (conductor and the enclosure) and that dissipated t is then established by trial, and optimized as noted
by the conductor and the enclosure surfaces. above. Figures 3 1.12 and 3 1.13 suggest that for a more
The amount of heat generated will decide the size and economical design, the thickness must be less than the
shape of the main current-carrying conductor, the rating depth of penetration (6,). In enclosures a more
and size of the enclosure and the provision of external economical design is achieved by keeping t in the
cooling, if necessary. As noted earlier, forced cooling, if vicinity of 50-60% of 4. This may mean some field
it can economize on the cost of the bus system, can be in the space, but its severity is already mitigated by
used. The one factor against the forced cooling is the arresting most of it by the enclosure. Leading
need for piping network, additional equipment and its manufacturers have established their own data and
regular upkeep. Breakdown will not result in a total programmed them on computers for routine reference
shutdown but a reduced rating of the bus system and an and designing a bus system. These data are then checked
underutilization of the generating unit. For larger ratings, for their accuracy by conducting heat run tests on sample
however, above 25 000 A, forced cooling may become lengths (Section 32.3.4). After a few initial designs it
mandatory to dissipate the great heat generated in the is possible to optimize and predefine the vital parameters
enclosure. In such cases the size of the enclosure which for a particular rating.
is restricted by the size of the generator terminals may
prove insufficient to dissipate the heat generated. The exercise below is purely theoretical, based on the
We have estimated the likely heat that may be generated information and the data available on the subject, with a
by a particular size of conductor and enclosure for a view to providing the basic guidelines to a practising
certain current rating and then have counterchecked engineer to design an IPB system.
whether the conductor and the enclosure so chosen can Since ANSI-C-37.23 is available in the FPS system,
dissipate this heat by radiation and natural convection, we have adopted this for ease of corroboration. Also,
and reach a state of thermal stability within permissible since continuous enclosures are more often used for their
limits or we may have to increase the size of the conductor obvious advantages, we have also considered them in