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Sunday, 28 September 2014

Emerging power conductors for Transmission lines

The rapid increase in electrical load, remote generation and system interconnections have made it necessary to transmit more power over longer distances efficiently and easily to keep pace with the development. The approach includes increasing the power flow on existing lines. 

Conductors used in Power Transmission:

The various conductors available for use in power transmission lines are:

·         ACSR (Aluminum Conductor Steel Reinforced)
·         AAAC (All Aluminum Alloy Conductor)
·         HTLS (High Temperature Low Sag)

ACSR Conductor:

ACSR is the most widely used conductor worldwide for power transmission. These conductors have a certain current carrying capacity because of the temperature limitation (up to 80 oC) and this limitation is partly due to the fact that aluminum anneals at about 100 to 110 oC and thus becoming weaker and less secure. The second reason is that aluminum conductors expand at a greater rate than the steel core provided inside for enhancing the mechanical strength. The aluminum literally pulls on the steel forcing it to elongate and hence increasing the sag at elevated temperatures.

AAAC Conductor:

AAAC conductors are well established in European countries because of their lower losses (or greater efficiency) as compared to ACSR conductors, but they cannot be operated at higher temperatures and are therefore not suitable for countries like India where the ambient temperature reaches nearly 50 oC.

HTLS Conductor:

HTLS conductors are specially introduced to double the current carrying capacity of the existing grid to meet the load demand. Lately the revolutionary Aluminum Conductor Composite Core (ACCC) conductor of the HTLS family was introduced. They have increased the current carrying capacity of existing and new lines and that too at higher efficiency. 

ACCC uses carbon fiber (as replacement of steel) which provides a stronger, lighter and smaller core. This in turn permits a design of conductor that contains 30 % more aluminum material than an equivalent ACSR conductor. Because of the increased conductive material, ACCC conductors are more efficient at all operating temperatures. The ACCC conductors use the annealed HTLS technology and therefore, permit the transmission operator to increase the current carrying capacity while keeping the losses to a minimum.

The low coefficient of thermal expansion of the high strength and light weight composite core material ( of carbon and glass) allowed the ACCC conductor to carry approximately twice the current of a conventional conductor; of course with reduced sag. The added aluminum content and superior conductivity decreased the electrical resistance of the conductor which substantially reduces the line losses ( I2R losses are reduced by nearly 30%) compared to other conductors of same diameter and weight. The additional strength and dimensional stability permits the use of these conductors for greater span length and thereby reducing the capital cost.

ACCC conductors were initially developed to increase the capacity of transmission lines below 400 kV. The additional merits of ACCC conductors can be used to enhance the efficiency, reliability and capacity of Extra High Voltage (EHV) and Ultra High Voltage (UHV) transmission lines
In 2013, the American Electric Power re-conductored it’s heavily loaded 345 kV transmission line in South Texas with ACCC conductors. It was a live-line conductor replacement project, the largest in the live-line conductor replacement category. 

Use of ACCC conductor in India:

In 2012, Torrent Power Ltd., for the first time in India used ACCC conductors of 318 mm2, Lisbon size conductors to double the current capacity of an existing 132 kV line in Ahmedabad, Gujarat. The line earlier used ACSR “panther” conductor and this was a live-line re-conductoring project without disrupting the normal supply. 

Recently in March 2014, the Maharashtra State Electricity Transmission Company Ltd. (MSETCL) up-graded it’s 132 kV line in Nasik city by ACCC, 273.6 mm2 “Casablanca” conductor capable of carrying 1200 A under emergency conditions. In both the projects, the ACCC conductors were supplied by Indian company Sterlite Technologies Ltd.