For feeders of moderate ratings, say up to 600/800A, cables are preferred, while for higher ratings (above 1000A) the preference is to opt for solid conductors (LT bus systems) on the grounds of safety, reliability, maintenance, cost, appearance and ease of handling. For larger ratings, more cables may become unwieldy and difficult to maintain and may present problems in locating faults. The solid connections extended from the supply side to the receiving end through busbars are called bus-ducts. These bus bars are housed in a sheet metal enclosure.
The major concern will be dealing with large currents rather than voltages. Large Currents are more difficult to handle than voltages due to mutual induction between the conductors and also between the conductor and enclosure. This article elaborates on the types of metal enclosed bus systems and their design parameters to select the correct size of the conductor sections and the bus enclosures for a required current rating and system voltage.
Types of Metal-Enclosed Bus Systems
A bus system can be one of the following types, depending upon its application:
-Rising mains (vertical bus systems)
-Overhead bus (horizontal bus system)
Non-segregated phase bus system
In this construction, all the bus phases are housed in one metallic enclosure with adequate spacing between them and also with the enclosure but without any barriers between the phases as shown in figure-1. Being vivid, it is most widely used methodology for all types of LT systems.
Segregated phase bus system
In this construction all the phases are housed in one metallic enclosure as earlier but with a metallic barrier between each phase as shown in figure-2. The metallic barriers provide the required magnetic shielding and isolate the busbars magnetically from each other.
The enclosure can be of MS or Aluminium alloy and the barriers chosen can be of same metal as the enclosure. The purpose of providing a metallic barrier is not only to shroud the phases against short circuits but also to reduce the effect of proximity of one phase on the other by arresting the electric field produced by the current carrying conductors within the barrier itself. It now operates like an enclosure with an interleaving arrangement balancing the fields produced by the conductors to a substantial extent and allowing only a moderate field in the space.
These are generally used for higher ratings 3000A and above on all voltage systems. These unlike the former, are preferred on a HT system.
Isolated Phase Bus (IPB) system
Used for very large ratings 10,000A and above. In this construction the conductors of each phase are housed in a separate non-magnetic metallic enclosure to isolate them completely from each other with the following advantages.
-It eliminates phase to phase faults.
-It minimizes the proximity effects between the main current carrying conductors of the adjacent phases to almost zero due to magnetic shielding.
-The bus system is easy to handle, flex and install.
Rising mains (Vertical bus system)
Used in vertical formation to supply individual floors of a high- rise building. It rises from the bottom of the building and runs to the top floor. To reduce the cost, the ratings may be in a decreasing order after every three or four floors, as after every floor the load of that floor will be reduced.
Overhead bus (horizontal bus system)
Unlike a high riser, now the overhead bus system runs horizontally, below the ceiling at an appropriate height, to distribute power to light and small load points. In an overhead busbar system, the power can be tapped from any number of points to supply the load points just below it through a plug-in box analogous to that used on a rising mains.
Design Parameters and Service Conditions for a Metal Enclosed Bus System
A bus system would be assigned the following ratings
-Rated insulation level
-Power frequency voltage withstand
-Impulse voltage withstand
-Continuous maximum rating
-Rated short time current rating
-Rated momentary peak value of the fault current
-Duration of the fault
The purpose is to determine the minimum size of current carrying conductors and decide on the mounting arrangement.
A short circuit results in an excessive current due to low impedance of the faulty circuit between the source of supply and the fault. This excessive current results in excessive heat in the current carrying conductors, which thus generates electromagnetic effects and electro-dynamic forces of attraction and repulsion between the conductors and their mounting structure. These forces are distributed uniformly over the length of conductors.
The effect of a short circuit henceforth requires these two factors (thermal effects and electro-dynamic forces) to be considered while designing the size of the current carrying conductors and their mounting structure, which includes mechanical supports, type of insulators and type of hardware, in addition to the longitudinal distance between the supports and the gap between phase to phase conductors.
With normal interrupting devices the fault current lasts for up to 1 sec. This time is too short to allow heat dissipation from the conductor through radiation or convection. The total heat generated on a fault will thus be dissipated by the conductor itself. The size of the conductor therefore should be such that its temperature rise during a fault will maintain its end temperature below the level where the metal of the conductor will start to soften. Aluminium, the most widely used metal for power cables, overhead transmission and distribution lines or the LT and HT switchgear assembly and bus duct applications, starts softening at a temperature of around 180-200 deg. C.
As a rule, on a fault, a safe temperature rise of 100 deg.C above the allowable end temperature of 85 deg.C or 90 deg.C of the conductor during normal service i.e., up to 185 deg.C-190 deg.C during fault condition is considered secure and taken as the basis to determine the size of the conductor.
The welded portion such as at the flexible joints, should also be safe up to this temperature and should not be used for this purpose where brass soldering is preferred.
To determine the minimum size of conductor for a required level Isc to account for the thermal effects alone the following formula is used to determine the minimum size of conductor for any fault level.
qt = temperature rise in 0C
Isc = symmetrical fault current in A
A = cross sectional area of the conductor (mm2)
a20= temperature coefficient of resistance at 20 0C
0.00403 for pure Aluminium
0.00363 for Aluminium alloys
0.00393 for pure copper
q = operating temperature of the conductor at which the fault occurs in 0C
K = 1.166 for Aluminium and 0.52 for copper
t = duration of fault (in seconds)
Example-1: To Determine the minimum size of conductor for a fault level of 50kA for one second for an Aluminium conductor, assuming the temperature rise to be 100 deg.C and the initial temperature of the conductor at the instant of fault 85 deg.C, the cross section of the conductor would be
100 = (1.166/100) * (50000/A)2. (1+0.00403*85) *1
By solving A = 625.6 mm2 for pure Aluminium
= 617.6 mm2 for alloys of Aluminium
= 416 mm2 for pure copper
The short circuit current is generally asymmetrical and contains a DC component. The DC component, although it lasts for only three or four cycles, creates a sub transient condition and causes excessive electro-dynamic forces between the current carrying conductors. The mounting structure, busbar supports and the fasteners are subjected to these electrodynamic forces. Although this force is only momentary, it may cause permanent damage to the components and must be considered when designing the current carrying system and its mounting structure. The maximum force in flat busbars may be expressed by
Fm = Estimated maximum dynamic force that may develop in a single or three phase system on a fault
ISC = rms value of the symmetrical fault current in amperes
k = space factor, which is 1 for circular conductors.
For rectangular conductors, it can be found from space factor graph (figure-3) corresponding to (S-a)/(a+b)
S = centre spacing between two phases in mm
a = space occupied by the conductors of one phase in mm
b = width of the conductors in mm
-Size of the enclosure
-Skin and proximity effect
For higher ambient temperatures, current capacity should be suitably reduced to maintain the same end temperature during continuous operation (derating). The end temperature for Aluminium is considered safe at 85-90 deg.C, at which the metal does not deteriorate or change its mechanical strength over a long period of operation. Table-1 lists the permissible operating temperatures of the various parts of a bus system. Table-2 lists the de-rating factors for a higher ambient temperature or a lower temperature rise for the same end temperature of 850C or 900C respectively.
Size of enclosure
The enclosure of the bus system provides the cooling surface for heat dissipation. Its size has an important bearing on the temperature rise of conductors and thus to affect their current carrying capacity. The enclosure effect and the ventilating conditions of the surroundings in which the enclosure is installed should thus be considered when designing a bus system. The ratio of the area of the current carrying conductors to the cross sectional area of the enclosure will provide the basis to determine the heat dissipation effect. Table-3 suggests the scenario of the approximate dissipation factors that can be considered as likely de-ratings for a bus system under different conditions. (To be continued)…
The author is from the Engineering Services Division, Indira Gandhi Centre for Atomic Research, Kalpakkam, India