Calculating Busbar Size for Temperature Rise in 440V LV Panels

inform me your email address, I will sent you the bus bar design, I cant attached in this forum due to lot of figures, graphs and tables.

Rgds

Sis

Types of Busbar

Busbars can be sub-divided into the following categories, with individual busbar systems in many cases being

constructed from several different types:

a) Air insulated with open phase conductors

b) Air insulated with segregating barriers between conductors of different phases.

c) Totally enclosed but having the construction as those for (a) and (b)

d) Air insulated where each phase is fully isolated from its adjacent phase(s) by an earthed enclosure. These are

usually called 'Isolated Phase Busbars'.

e) Force-cooled busbar systems constructed as (a) to (d) but using air, water, etc. as the cooling medium under

forced conditions (fan, pump, etc.).

f) Gas insulated busbars. These are usually constructed as type (e) but use a gas other than air such as SF6,

(sulphur hexafluoride).

g) Totally enclosed busbars using compound or oil as the insulation medium.

The type of busbar system selected for a specific duty is determined by requirements of voltage, current, frequency,

electrical safety, reliability, short-circuit currents and environmental considerations. Table 1 outlines how these

factors apply to the design of busbars in electricity generation and industrial processes.

Calculation of Current-carrying Capacity

A very approximate method of estimating the current carrying capacity of a copper busbar is to assume a current

density of 2 A/mm2 (1250 A/in2) in still air. This method should only be used to estimate a likely size of busbar, the

final size being chosen after consideration has been given to the calculation methods and experimental results

given in the following sections.

Methods of Heat Loss

The current that will give rise to a particular equilibrium temperature rise in the conductor depends on the balance

between the rate at which heat is produced in the bar, and the rate at which heat is lost from the bar. The heat

generated in a busbar can only be dissipated in the following ways:

(a) Convection

(b) Radiation

(c) Conduction

In most cases convection and radiation heat losses determine the current-carrying capacity of a busbar system.

Conduction can only be used where a known amount of heat can flow into a heat sink outside the busbar system or

where adjacent parts of the system have differing cooling capacities. The proportion of heat loss by convection and

radiation is dependent on the conductor size with the portion attributable to convection being increased for a small

conductor and decreased for larger conductors.

Convection

The heat dissipated per unit area by convection depends on the shape and size of the conductor and its

temperature rise. This value is usually calculated for still air conditions but can be increased greatly if forced air

cooling is permissible. Where outdoor busbar systems are concerned calculations should always be treated as in

still air unless specific information is given to the contrary.

Heat Generated by a Conductor

The rate at which heat is generated per unit length of a conductor carrying a direct current is the product I2R watts,

where I is the current flowing in the conductor and R its resistance per unit length. The value for the resistance can

in the case of d.c. busbar systems be calculated directly from the resistivity of the copper or copper alloy. Where an

a.c. busbar system is concerned, the resistance is increased due to the tendency of the current to flow in the outer

surface of the conductor. The ratio between the a.c. value of resistance and its corresponding d.c. value is called

the skin effect ratio (see Section 4). This value is unity for a d.c. system but increases with the frequency and the

physical size of the conductor for an a.c. current.

Rate of Heat generated in a Conductor,

W/mm = I2 RoS

where I = current in conductor, A

Ro = d.c. resistance per unit length, Ω/mm

S = skin effect ratio

Skin Effect

The apparent resistance of a conductor is always higher for a.c. than for d.c. The alternating magnetic flux created

by an alternating current interacts with the conductor, generating a back e.m.f. which tends to reduce the current in

the conductor. The centre portions of the conductor are affected by the greatest number of lines of force, the

number of line linkages decreasing as the edges are approached. The electromotive force produced in this way by

self-inductance varies both in magnitude and phase through the cross-section of the conductor, being larger in the

centre and smaller towards the outside. The current therefore tends to crowd into those parts of the conductor in

which the opposing e.m.f. is a minimum; that is, into the skin of a circular conductor or the edges of a flat strip,

producing what is known as 'skin' or 'edge' effect. The resulting non-uniform current density has the effect of

increasing the apparent resistance of the conductor and gives rise to increased losses.

The magnitude and importance of the effect increases with the frequency, and the size, shape and thickness of

conductor, but is independent of the magnitude of the current flowing.

It should be noted that as the conductor temperature increases the skin effect decreases giving rise to a lower than

expected a.c. resistance at elevated temperatures. This effect is more marked for a copper conductor than an

aluminium conductor of equal cross-sectional area because of its lower resistivity. The difference is particularly

noticeable in large busbar sections

etc.......................................................................

sorry I cant copy the article to this form... its very dificult....

Rgds

Sis