1. Design Considerations
Introduction
The word busbar, derived from the Latin word omnibus ('for all'), gives the idea of a universal system of conveyance. In the electrical n, the term bus is ud to describe a junction of circuits, usually in the form of a small number of inputs and many outputs. 'Busbar' describes the form the bus system usually takes, a bar or bars of conducting material.
In any electrical circuit some electrical energy is lost as heat which, if not kept within safe limits, may impair the performance of the system. This energy loss, which also reprents a financial loss over a period of time, is proportional to the effective resistance of the conductor and the square of the current flowing through it. A low resistance therefore means a low loss; a factor of increasing importance as the magnitude of the current increas.
The capacities of modern-day electrical plant and machinery are such that the power handled by their control systems gives ri to very large forces. Busbars, like all the other equipment in the system, have to be able to withstand the forces without damage. It is esntial that the materials ud in their construction should have the best possible mechanical properties and are designed to operate within t
he temperature limits laid down in BS 159, BS EN 60439-
1:1994, or other national or international standards.
A conductor material should therefore have the following properties if it is to be produced efficiently and have low running costs from the point of view of energy consumption and maintenance:
a)Low electrical and thermal resistance
b)High mechanical strength in tension, compression and shear
c)High resistance to fatigue failure
d)Low electrical resistance of surface films
e)Ea of fabrication
f)High resistance to corrosion
g)Competitive first cost and high eventual recovery value
This combination of properties is met best by copper. Aluminium is the main alternative material, but a comparison of the properties of the two metals shows that in nearly all respects copper is the superior material.
狐狸吵架Types of Busbar
Busbars can be sub-divided into the following categories, with individual busbar systems in many cas being constructed from veral different types:
a)Air insulated with open pha conductors
b)Air insulated with gregating barriers between conductors of different phas.
c)Totally enclod but having the construction as tho for (a) and (b)
d)Air insulated where each pha is fully isolated from its adjacent pha(s) by an earthed enclosure. The are usually called 'Isolated Pha 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. The are usually constructed as type (e) but u a gas other than air such as SF6, (sulphur hexafluoride).
g)Totally enclod busbars using compound or oil as the insulation medium.
The type of busbar system lected 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 the factors apply to the design of busbars in electricity generation and industrial process.
Table 1 Comparison of typical design requirements for power generation and industrial process systems Feature Generation Industrial Process
1Voltage drop Normally not important Important
2Temperature ri Usually near to maximum allowable.
Capitalisation becoming important.In many cas low due to optimisation of first cost and running costs.
3Current range Zero to 40 k A a .c . with frequencies of zero
to 400 Hz.
Zero to 200
4Jointing and connections Usually bolted but high current applications
are often fully welded. Joint preparation very
important Usually bolted. Joint preparation very important.
5Cross-ctional area Usually minimum. Somewhat larger if
optimisation is required.Usually larger than minimum required due to optimisation and voltage drop considerations.
6Kelvin's Law Not applied. Other forms of optimisation are
中秋节手抄报图片大全often ud.Applies. Also other forms of optimisation and capitalisation ud
7Construction Up to 36 k V. Individually engineered using
basic designs and concepts.Usually low voltage. Individually engineered. Standard products for low current/voltage applications.
监事会报告8Enclosures Totally enclod with or without ventilation.Usually open. Enclod or
protected by screens when using
standard products.
9Fault capacity Usually large. Designed to meet system
requirement.Usually similar to running current. Standard products to suit system short circuit.
10Pha arrangement Normally 3 pha flat though sometimes
trefoil.Normally flat but transposition ud to improve current distribution on large systems
11Load factor Usually high. Normally 1.0.Usually high but many have widely
varying loads.
小本投资创业项目
12Cost Low when compared with associated plant.Major consideration in many cas.
Particularly when
optimisation/capitalisation is ud.
13Effects of failure Very rious. High energies dissipated into
fault.Limited by low voltage and busbar size.
14Copper type High conductivity.High conductivity.
15Copper shape Usually rectangular. Tubular ud for high current force-cooled.Usually large cross ction rectangular. Tubular ud for some low current high voltage applications and high current force-cooled.
Choice of Busbar Material
At the prent time the only two commercially available materials suitable for conductor purpos are copper and aluminium. The table below gives a comparison of some of their properties. It can be en that for conductivity and
strength, high conductivity copper is superior to aluminium. The only disadvantage of copper is its density; for a given current and temperature ri, an aluminium conductor would be lighter, even though its cross-ction would be larger. In enclod systems however, space considerations are of greater importance than weight. Even in open-air systems the weight of the busbars, which are supported at intervals, is not necessarily the decisive factor.
Table 2 Typical relative properties of copper and aluminium
Copper(CW004A)Aluminium (1350)Units
Electrical conductivity
10161% IACS
(annealed)
1.72
2.83µΩ cm
Electrical resistivity
(annealed)
Temperature coefficient of
0.00390.004/° C
resistance(annealed)
Thermal conductivity at 20°C397230W/mK
Coefficient of expansion17 x 10–623 x 10–6/° C
Tensile strength (annealed)200 – 25050 – 60N/mm2
Tensile strength (half–hard)260 – 30085 – 100N/mm2
0.2% proof stress (annealed)50 – 5520 – 30N/mm2
0.2% proof stress (half–hard)170 – 20060 – 65N/mm2
Elastic modulus116 – 13070kN/mm2
Specific heat385900J/kg K
Density8.91 2.70g/cm3
Melting point1083660°C
The electromagnetic stress t up in the bar are usually more vere than the stress introduced by its weight. In particular, heavy current-carrying equipment necessitates the u of large size conductors, and space considerations may be important. It should be realid that the u of copper at higher operating temperatures than would be permissible for aluminium allows smaller and lighter copper ctions to be ud than would be required at lower temperatures.
The ability of copper to absorb the heavy electromagnetic and thermal stress generated by overload conditions also gives a considerable factor of safety. Other factors, such as the cost of frequent supports for the relatively limp aluminium, and the greater cost of insulation of the larger surface area, must be considered when evaluating the materials.
From published creep data, it can be en that high conductivity aluminium exhibits evidence of significant creep at ambient temperature if heavily stresd. At the same stress, a similar rate of cree
p is only shown by high conductivity copper at a temperature of 150°C, which is above th e usual operating temperature of busbars.
Table 3 Comparison of creep and fatigue properties of high conductivity copper and aluminium
a) Creep properties
Material Testing Temp. °C Min. Creep Rate % per
Stress N/mm2
1000 h
Al (1080) annealed200.02226 *
HC Cu annealed1500.02226 *
Cu-0.086% Ag 50% c.w.1300.004138
Cu-0.086% Ag 50% c.w.2250.02996.
5
* Interpolated from fig.3
b) Fatigue properties
Material Fatigue strength N/mm2No. of cycles x 106
HC Al annealed2050
half-hard (H8)4550
HC Copper annealed62300营救的近义词
half-hard115300
If much higher stress or temperatures are to be allowed for, copper containing small amounts (about 0.1%) of silver can be ud successfully. The creep resistance and softening resistance of copper-silver alloys increa with increasing silver content.
In the conditions in which high conductivity aluminium and copper are ud, either annealed (or as-welded) or half-hard, the fatigue strength of copper is approximately double that of aluminium. This gives a uful rerve of strength against failure initiated by mechanical or thermal cycling.
The greater hardness of copper compared with aluminium gives it better resistance to mechanical damage both during erection and in rvice. It is also less likely to develop problems in clamped joints due to cold metal flow under the prolonged application of a high contact pressure. Its higher modulus of elasticity gives it greater beam stiffness compared with an aluminium conductor of the same dimensions. The temperature variations encountered under rvice conditions require a certain amount of flexibility to be allowed for in the design. The lower coefficient of linear expansion of copper reduces the degree of flexibility required.
Becau copper is less prone to the formation of high resistance surface oxide films than aluminium, good quality mechanical joints are easier to produce in copper conductors. Welded joints are also readily made. Switch contacts and similar parts are nearly always produced from copper or a copper alloy. The u of copper for the busbars to which the parts are connected therefore avoids contacts between dissimilar metals and the inherent jointing and corrosion problems associated with them.
The higher melting point and thermal conductivity of copper reduce the possibility of damage resulting from hot spots or accidental flashovers in rvice. If arcing occurs, copper busbars are less likely to support the arc than aluminium. Table 4 shows that copper can lf-extinguish arcs across s
maller parations, and at higher busbar currents. This lf-extinguishing behaviour is related to the much larger heat input required to vapori copper than aluminium.
Table 4 Self-extinguishing arcs in copper and aluminium busbars
Copper Aluminium
Minimum busbar spacing, mm50100
Maximum current per busbar, A45003220
浏览器快捷键Copper liberates considerably less heat during oxidation than aluminium and is therefore much less likely to sustain combustion in the ca of accidental ignition by an arc. The large amounts of heat liberated by the oxidation of aluminium in this event are sufficient to vapori more metal than was originally oxidid. This vaporid aluminium can itlf rapidly oxidi, thus sustaining the reaction. The excess heat generated in this way heats nearby materials, including the busbar itlf, the air and any supporting fixtures. As the busbar and air temperatures ri, the rates of the vaporisation and oxidation increa, so accelerating the whole process. As the air temperature is incread, the air expands and propels hot oxide particles. The busbar may reach its melting point, further increasin
g the rate of oxidation and providing hot liquid to be propelled, while other materials such as wood panels may be raid to their ignition temperatures. The dangers are obviated by the u of copper busbars.
Finally, copper is an economical conductor material. It gives long and reliable rvice at minimum maintenance costs, and when an installation is eventually replaced the copper will have a high recovery value. Becau of its many advantages, copper is still ud worldwide as an electrical conductor material despite attempts at substitution.
2.Copper for Busbar Purpos
In most countries, coppers of different types for specific applications have been given parate identities. In the United Kingdom this takes the form of an alloy designation number which is ud in all British Standards relevant to copper and its alloys. Copper for electrical purpos is covered by the following British Standards:
培训收获怎么写BS 1432 : 1987(strip with drawn or rolled edges)
BS 1433 : 1970(Rod and bar)
BS 1434 : 1985(Commutator bars)
BS 1977 : 1976(High conductivity tubes)
BS 4109 : 1970(wire for general electrical purpos and for insulated and flexible cords)
金牛女和双鱼男BS 4608 : 1970(Rolled sheet, strip and foil)
(Copies of the are obtainable from the BSI Sales Office. 398 Chiswick High Road, London WS4 4AL.)
To bring the UK in line with current European requirements BS EN standards are being introduced. The European Standards relevant to electrical applications are expected to superde the British Standards in due cour.
The current standards most relevant to busbar applications are BS 1432, BS 1433 and BS 1977 which specify that the end products shall be manufactured from copper complying with the following requirements:
Cu-ETP Electrolytic tough pitch high conductivity copper CW004A (formerly C101)
Cu-FRHC Fire-refined tough pitch high conductivity copper CW005A (formerly C102)
Cu-OF Oxygen-free high conductivity copper CW008A (formerly C103)
European Standards EN1976 and EN1978 have replaced BS 6017:1981. Table 5 shows the European material designations along with International Standards Organisation (ISO) and old British Standard designations.
Table 5 EN, BS and ISO designations for refinery shapes and wrought coppers
Designation
Description ISO cast and wrought European Designation Former UK Designations
Cu-ETP CW004A C101
Electrolytic tough pitch high-
conductivity copper
Cu-FRHC CW005A C102
Fire- refined tough pitch
high-conductivity copper
Oxygen-free high-
Cu-OF CW008A C103
conductivity copper
Copper to be ud for electrical purpos is of high purity becau impurities in copper, together with the changes in micro-structure produced by working, materially affect the mechanical and electrical properties. The degree to which the electrical conductivity is affected by an impurity depends largely on the element prent. For example, the prence of only 0.04% phosphorus reduces the conductivity of high conductivity copper to around 80% IACS. (The approximate effect on conductivity of various impurity elements is shown in Figure 1). The level of total impurities, including oxygen, should therefore be less than 0.1% and copper of this type is known as high conductivity (HC) copper.
Microscopic and analytical controls are applied to ensure a consistent product and in the annealed condition conductivities over 100% IACS are usual. This figure corresponds to the standard resistivity of 0.017241 µΩm t some years ago by the International Electrotechnical Commission.
Figure 1 - Approximate effect of impurity elements on the electrical resistivity of
copper
Types of High Conductivity Copper available
Tough pitch copper,CW004A and CW005A (C101 and C102 )
