Chapter # 1                      

CONDUCTOR MATERIAL                      

1.3   Aluminium

          Aluminium is a highly reactive element and is found in nature as a compound.  The metal was extracted on an Industrial scale only in the late 19^th century. 

          Aluminium is the third most abundant element on earth after Oxygen and Silicon.  It has been alloyed and processed to meet the specific needs of a variety of industries. They are used as architectural sections in building industries or rolled in plates for shipbuilding industries.  The present discussions will be limited to its application in the electrical industry as a current-carrying conductor. 

          The process of extracting Aluminium comprises:

Mining                                  Concentration                                   Extraction

          Aluminium ore is called bauxite and it is mined as hydrated oxides of Aluminium with silica, ferric oxide, and Titanium dioxide as predominant impurities.  The separation of alumina from the ore is achieved by grinding the ore with water and leaching the same with hot Sodium hydroxide.   The produce is then calcined at 1100  (2012 to give pure alumina.

          The process used worldwide for the production of Aluminium from alumina was discovered by Charles Martin Hall in the USA and Paul Heroult in France. 

          Alumina has a very high melting point of approximately 2072 .  It is therefore dissolved in molten Cryolite (Na3AlF6)  and Aluminium is extracted by an electrolytic process where graphite is the anode and the bath is the cathode.  The reaction can be summarized as:

2Al2O3 + 3C → 4Al + 3CO2

          Aluminium extraction is a very energy-consuming process and requires approximately two tons of Alumina and  17 Mwh to produce one ton of Aluminium.

          Pure Aluminium is expensive to manufacture and has very poor mechanical properties.  Commercially viable high purity Aluminium (grade 1350) with temper H 111 has a conductivity of 61% IACS.  Most of the Sheets required for the fabrication of conductor and enclosures for isolated phase bus are rolled from this grade.  Sheets are manufactured by passing an ingot from the furnace, through a series of rollers to reduce the thickness in stages to the desired value.  The sheets can be hot rolled down to a thickness of 5 mm.  Further reduction in thickness is carried out by cold rolling. Cold rolling marginally improves the mechanical properties of the sheet.

          Substitutable alloys for 1350 are 1050, 1100 & 1199 and all have a purity of 99.50 %.

          Controlled quantities of Magnesium and Silicon are added to form Magnesium Silicide alloy and subsequently heat-treated to enhance the mechanical properties of the busbar without excessively sacrificing the conductivity.  Grade 6101 – T6 was developed with a conductivity of 56%.  Higher mechanical properties are obtained with heat treatment and cold working (extrusion).  Aluminium alloys manufactured to grade 6061 – T6 and 6063 are used as busbar material in outdoor switchyards. 

          It is of interest to note that the temperature coefficient of resistance is higher for the grade 1350 than for grade 6101. At an operating temperature of 105 , the current carrying capacity of the alloy bar is only marginally lower (3 – 4%) than that of the electrical grade sheet.

Other elements include Chromium, Copper, Manganese, Zinc, and Titanium

Aluminium Cataloguing is carried out by “The Aluminium Association” which designates the alloy based on composition & heat treatment processing.  (Refer to Table 1.6 and 1.7)

          Aluminium Cataloguing is carried out by “The Aluminium Association” which designates the alloy based on composition & heat treatment processing.  (Refer to Table 1.6 and 1.7)

The heat treatment process comprises annealing & solution heat treatment (quenching and ageing).

          General purpose, commercial-grade Aluminium alloy sheets & extrusions are also good conductors of electricity with conductivity varying from 30% IACS upwards, depending upon the alloy.  Many of them are produced for structural fabrication and may be temporarily used, in case of an emergency, as conductor until a final replacement can be effected.

          In the electrical industry, Aluminium is used as a conductor material in Busbar Systems, busbars in switchgear, switchyards, electrolysis, and metal extraction plants among several other applications.  It is extensively used in cables and overhead lines.  It is also used in foil wound capacitors and cast resin transformers.  Many small distribution transformers are wound with Aluminium conductors.

          The higher electrical resistivity of Aluminium works in its favour since the skin effect and proximity effect are less pronounced when carrying alternating current.  (Refer to Chapter 5)

          For certain applications, listed below, Aluminium is the only preferred material for Busbar Systems. 

·          Conductors of isolated phase bus are fabricated from Aluminium sheets or extruded sections.  Conductors of gas insulated bus are fabricated only from Aluminium extruded sections.  This is due to, efficient use of metal, ease of fabrication, weldability at the works & site, lower weight, and easier handling at the works & site.

·          High current open bus, installed in metal extraction, are made of cast Aluminium slabs that are welded at the site.  Here welding of conductors and laminates is of primary consideration. Aluminium offers better resistance to corrosion than Copper in an open atmosphere.

·          High voltage open bus, installed in switchyards are tubular Aluminium conductors.  More often than not, the size of the conductor is governed by mechanical strength and corona (audible and visual) discharge consideration, rather than limitation imposed by ampacity. Aluminium is far easier to weld which makes it suitable as an ideal conductor for switchyards.  Many of the switchyard clamps and connectors are made suitable for welding on Aluminium tubular busbars.  It has good resistance to corrosion when installed outdoors.