Page 1000 - Industrial Power Engineering and Applications Handbook

An isolated phase bus system 311945
our sample calculations. For details of non-continuous and enclosure (Yo IACS) 60 (at 20"C, Table 30.1)
encl-osures, reference may be made to the work noted in Grade EIE
IEE Committee (1968). Specific resistance 2.873 (at 20°C,
(P20 in cm) Table 30.1)
= 2.873 x Qcm
31.13 Sample calculations Temperature coefficient of
electrical resistance (a2,, in per "C) 3.96 x
(at 20°C, Table 30.1)
Example 31.1 Effect of solar radiation on the
To apply what we have discussed so far we give below a outdoor part of the bus section ("C) 1 O+
brief outline of a design for an IPB system. The economics of
this design would be a matter of further investigation of its Factor of emissivity, for the purpose of heat dissipation for
performance versus cost. This will require an optimization on the light grey surface of the enclosure noted above (e) 0.65*
the thickness of the enclosure as discussed earlier. Cross-sections of conductor and enclosure are to be the
When optimizing the natural cooling of the enclosure its same for indoor and the outdoor parts. This is normal practice
diameter may have to be increased but this would add to the of all manufacturers to achieve simplicity in design and ease
cost of the IPB, on the one hand and make it too large to of interconnections. We have considered a circular conductor
terminate at the generator end, on the other. The total design (the enclosure is usually circular).
is thus a matter of many permutations and combinations.
Most manufacturers have optimized their designs, based on Establishing the size of the conductor
experience and test results, through complex computer 20 000
programming. In this example we will suggest only a basic Approximate area of cross-section = -
400
approach to establish a design. Its optimization would require
an elaborate cost analysis as noted earlier although its = 50 square inches
authenticity can be verified through laboratory tests. Let us assume the thickness of the conductor to be close to
Assume the following parameters of a turbo-alternator (Table but less than 4, say, around 11 mm, to reduce its diameter
13.8): and hence increase the gap between the enclosure and the
Generator size (MW) 500 at 50 Hz conductor.
Rated p.f. 0.85 :. t = 0.43"
500 - 588
Rated MVA -- and for the conductor's outer diameter d,
0.85
Stator nominal voltage (kV) 21
Stator maximum voltage (kV) 24 Area of hollow pipes = E d,? - 4 dg ]
Stator current (CMR) (A) 16 200 4
Rated current of IPB (CMR) (A) 18 000
IPB to be designed for (A) 20 ooo*
One-second short-time rating (kA) 122
Type of enclosure for IPB Continuous 50 = 4 (dj + d2)(dl - d2)
We limit our discussion to the main length of the generator = rr(d- 0.43) x 0.43
bus. A similar exercise can be carried out to design the neutral
shorting star bus, tap-offs to the two UATs, the delta bus to _._ d = ___ + 0.43
50
interconnect the GT when it is made of three single-phase rt x 0.43
units, tap-offs for CTs, VTs and surge protective equipment = 37.46
and all the auxiliary buses described in Section 31.1.1. Some and the conductor's outer surface area (no() per foot run
of these tap-offs, such as for UATs, which are low-rating bus
sections, can also be made with the construction of segregated A,, = K x 37.46 x 12
phase bus system (Section 28.2.2) to economize on cost. = 141 1.49 square inches
Requirements (U is the length of conductor in inches)
Ambient temperature for enclosure (& in "C) 48 Conductor's inner surface area per foot run
Ambient temperature for conductor (Oat in%), A,, = ~[37.46 - 2 x 0.431 x 12
1
for indoors 80 The same as for
for outdoors 95 the enclosure = 1379.09 square inches
Conductor's maximum allowable temperature (0, in "C) Establishing the size of the enclosure
for indoors 95 Assuming welded
for outdoors 105 i or silver-plated joints As discussed above, that thinner the section, the fewer will
Enclosure's maximum permissible temperature (0, in "C)
for indoors 80
for outdoors 95 +Assuming the approximate absorption coefficient of solar
radiation to be 0.65, for a light-grey external surface, having
Assumptions collected soot and dirt over a period of time, as in ANSI-C-
Conductivity of conductor 37-24 the approximate temperature rise on account of solar
radiation
This is a normal safety margin (= 10%) for such duties. It
accounts for the dip in the grid voltage to which it is connected
which may require the generator to operate at a lower voltage *We have considered the emission of heat, from the surface
but still cope with the full-load (MVA) demand, resulting in a through natural radiation, nearly the same, as its absorption
corresponding rise in its current for a short period. of heat through solar radiation.