Page 919 - Industrial Power Engineering and Applications Handbook

Carrying power through metal-enclosed bus systems 28/869

28.4.2 Electrodynamic effects 2 Altitude
3 Atmospheric conditions and
The short-circuit current is generally unsymmetrical and 4 Excessive vibrations and seismic effects
contains a d.c. component, Zdc, as discussed in Section
13.4.1(7). The d.c. component, although it lasts for only
three or four cycles, creates a sub-transient condition 28.5.1 Ambient temperature
and causes excessive electrodynamic forces between the
current-carrying conductors. The mounting structure, The ratings as provided in Tables 30.2, 30.4 and 30.5
and others refer to an ambient temperature, with a peak
busbar supports and the fasteners are subjected to these of 40°C and an average of 35°C over a period of 24
forces. This force is greatest at the instant of fault initiation hours. The end temperature for aluminium is considered
and is represented by the first major loop of the fault safe at 85-90°C, at which the metal does not deteriorate
current, as noted in Table 13.1 1. Although this force is (oxidize) or change its properties (mechanical strength)
only momentary, it may cause permanent damage to these over a long period of operation. Figure 28.9 shows the
components and must be considered when designing the effect of higher operating temperatures on the mechanical
current-carrying system and its mounting structure. The strength of aluminium metal. The oxidation and
maximum force in flat busbars may be expressed by
mechanical strength are two vital factors that need be
16.Z: borne in mind when selecting busbar size to ensure its
F,,, =k.- x 104N/m (28.4) adequacy during long hours of continuous operation. Table
S 28.2 lists the permissible operating temperatures of the
where various parts of a bus system.
F, = estimated maximum dynamic force that For higher ambient temperatures, current capacity
may develop in a single- or a three-phase should be suitably reduced to maintain the same end
system on a fault. This will vary with the temperature during continuous operation. Refer to Tables
number of current-carrying conductors and 28.3(a) and (b), recommending the derating factors for a
their configuration but for ease of applica- higher ambient temperature or a lower temperature rise
tion and for brevity only the maximum for the same end temperature of 85" or 90°C respectively.
force that will develop in any configuration For intermediate ambient temperatures, see Figure 28.10.
is considered in the above equation. It will
make only a marginal difference to the
calculations, but it will be on the safe side. Table 28.2 Operating temperature of a bus system
For more details refer to the further reading
at the end of the chapter. Maximum operating temperature Maximum
Zsc = r.m.s. value of the symmetrical fault current (hot spot) temperature
in amperes limit as in
Factor of IEEE- C-3 7-2P
asymmetry = as in Table 13.11, representing the momen-
tary peak value of the fault current. This Bus conductor with plain connection joints 70°C
factor is considered in the numerical factor Bus conductor with silver plated or 105°C
welded contact surfaces
16 used in the above equation.
k = space factor, which is 1 for circular con- Enclosure
ductors. For rectangular conductors it can Accessible part 80°C
be found from the space factor graph Non-accessible part 110°C
(Figure 28.7) corresponding to Termination at cables with plain connections 70°C
Termination at cables with
silver-surfaced or equivalent connections 85OC
S-a where
a+b aOr as specified by the user.
S = centre spacing between two phases in mm
(Figure 28.8) Note
a = space occupied by the conductors of one For temperatures above 100°C it is recommended to use epoxy
insulatorslsupports, which can continuously operate up to 125°C.
phase in mm, and SMC/FRP (fibreglass reinforced plastic) insulatorslsupports may
b = width of the conductors in mm. not withstand 105'C.
For application of the above equation, refer to Example
28.12. Operating temperatures of bus conductors

28.5 Service conditions Aluminium and copper conductors start oxidizing at about
90°C. The oxides of aluminium (A120) and copper (CuO)
are poor conductors of electricity, They may adversely
The performance of a bus system can be affected by the affect bus conductors, particularly at joints, and reduce
following service conditions: their current-carrying capacity over time, and lead to
their overheating, even to an eventual failure. Universal
1 Ambient temperature practice therefore is to restrict the operating temperature