Page 952 - Industrial Power Engineering and Applications Handbook

28/902 Industrial Power Engineering and Applications Handbook
Effect of proximity on the centre phase Y Rdc at 85°C = Rdc20 [1 + az"(62 - e,)] (28.6)
The voltage drop in this phase, assuming X, = 0
azo = temperature coefficient of resistance at
= 2500 x 0.01397 x 50 20°C per "C
1000 RdcZo = d.c. resistance at 20°C
= 1.75 v 13~ operating temperature = 85°C
=
which is only 0.4% of the system voltage. This bus system O1 = since the value of R,, is available at 20°C
will thus provide a near-balanced system. therefore, 01 = 20°C
For the rest of the calculations for the mechanical suitability
of a busbar system the procedure for rectangular sections Proximity effect in terms of busbar reactance
can be followed.
s, = (Sa ' s, ' sy3 (28.7)
List of formulae used S, = effective or geometric mean spacing
Sa, Sb, and S, = spacing between conductors
Short-circuit effects
Use of saturable reactor to balance a large
(1) Thermal effects unbalanced power distribution system
To determine size
l;o
-
e---. (2 - )* . (1 + a,,0). t (28.1)
(28.8)
0, = temperature rise in "C
I,, = symmetrical fault current r.m.s. in Amp Xp = lost reactance of Y phase
A = cross-sectional area of the conductor in mm2 L = inductance of Xp
= temperature coefficient of resistance at 20"C/"C I, = current in R or B phase
6 = operating temperature of the conductor at which IY = current in Y phase
the fault occurs in "C
K = 1.166 for aluminium and 0.52 for copper Reluctance of the magnetic path
t = duration of fault in seconds
or x & = 0.0799 for aluminium for an operating
A (28.9)
temperature at 85°C and end temperature
at 185°C (28.2) I, = length of the air gap in metres
I,, JI = 0.12 for copper for an operating tempera- po = permeability of air (free space) = 4n. H/m
A ture at 85°C and end a temperature at = relative permeability of the silicon steel used for
185°C the laminates in H/m
(28.3) A = area of cross-section of core in square metres
k = total length of the magnetic circuit in metres
(2) Electrodynamic effects
Calculating stresses on a fault
16.1;
x 104 N/m
F,, = k. - (28.4) Bending stress on busbars at section
S F, . I
F, = maximum dynamic force that may develop on a x -x= 12,M.N = kg/cm (28.10)
fault F, = maximum electrodynamic forces acting on each
I,, = r.m.s. value of the symmetrical fault current in support in the event of a fault
Amps 1 = centre distance between two busbar supports
k = space factor M = sectional modulus of each busbar at section
S = centre spacing between two phases in mm x-x
1
Skin effect = -a. b2 in cm3
6
N = number of busbars per phase
Effect on current-carrying capacity
Further reading
I,, = I,, ' (28.5)
1 ERDA, 'Study on feasibility of upgrading the operating
I,, = permissible current capacity of the system temperature of AI busbars without plating'.
Rdc = d.c. resistance 2 Golding, E.W., Electrical Measurements and Measuring
R,, = a.c. resistance Instruments.
Id, = d.c. current 3 Lynthall, R.T., The J & P Switchgear Book. Buttenvorth, London.
4 Thomas, A.G. and Rata, P.J.H., Aluminium Busbar. Hutchinson
Conductor resistance at higher temperature Scientific and Technical for Alcan Industries Ltd.