1.1. Technology of thermal conductor in power transmission system
In electrical systems, the improvement of the system capacity needs to be always performed to keep up with the development of the additional charge. In the power transmission grid, lines of wire always have to adjust the operation to prevent overload, which is a frequent occurrence, if the overload occurs too long, it may lead to drop in tacks, connections and heat emission, increase the deflection threatening to the system. There are three methods of investment in improving transmission capacity of current lines while maintaining the voltage level as following:

Option 1: Install additional vertical capacitor changing parameters of the line, this must be equipped with additional capacitor along with expensive protective equipments;

Option 2: increase cross-section of conductor;  

Option 3: replace current conductor with new technology conductor that can carry higher load than old one.

About option 2: The increase in cross-section of conductor will make phase-phase distance and phase-ground not to be guaranteed. Moreover, when the load capacity increases, we have to calculate the load capacity of the column in the line system and replace the foundation and column, this costs much and are not feasible.

With option 3: using new technology materials: just replace the wire with the same cross-section but the load carrying capacity increased 1.6 to 2.0 times compared with existing line, no need to replace the available structure (foundation, columns, ..).

It was appeared many manufacturers of this new technology wire long time (1970) ago in the world such as Hitachi (Japan), LS cable (Korea), ZTT (China), Trefinasa (Spain) ... Recently, these products have appeared and been used in Vietnam through representative offices (Hitachi, ZTT offices ...) or official distributors (Trefinasa is distributed by Vietnam Technical Development Limited Company (VTD) ...

II. NEW CONDUCTOR WIRE TECHNOLOGY

There are many kinds of new technology conductor such as High Temperature Conductor (HTC) with high proportion of 99.7% aluminum in the composition of alumina, and other components making up very small and it can operate continuously up to 150OC and load carrying capacity increased by 50% compared with classical conductor (ACSR) with the same cross-section. In this wire, we use temperature resistant aluminum (TAL).

The second is a black wire (Black Conductor) on the surface of each fiber creating conductor, this type of wire is coated with black polyurethane. This type of conductor is able to highly emit heat in the air, when carrying high load, the temperature increases and emits heat to the surroundings, that helps the load capacity increase 70% compared with classical conductor (ACSR).

The third is the conductor using Composite core; The forth is the conductor made from material of conductive ceramic; The fifth is the conductor using nanotechnology ... In these types, type 1 and type 2 and 3 is now commonly used , in which type 1 is more popular and is referred as thermal conductors.

III. INTRODUCTION OF THERMAL CONDUCTOR

1. Classification

Thermal conductors are divided into many types: Wires with gaps: at high temperature, the deflection is just equal to 1/5 in comparison with ACSR; Wires with Invar oxide core: at high temperature, the deflection is only equal to 1/2 compared to ACSR; Wires made up soft aluminum: at high temperature, the deflection is just by half compared to ACSR ....

2. Structure

The structure has often circular fiber texture without gap between the core and the conductive sheath or trapezoidal fiber texture or both circular and trapezoidal fiber with the gap between the core and the sheath.
 
3. Characteristics

When the conductor carries too high load, it will expand the degree of heat emission and the deflection of the wire, which increases the risk of electric discharge. For ACSR, when the temperature reaches 70-90oC, the deflection of the wire is so high that it may cause earth fault. The thermal conductor of some manufacturers when the temperature increases, the deflection of the wire also increases linearly, until the temperature reaches about 100°C, this deflection have tendency of transverse bending and slowly increase to 210oC, then reaches approximately deflection of ACSR at 115oC.

About thermal conductors manufactured by Trefinasa, when the temperature of the conductor reaches 60oC, the deflection of the conductor starts to bend transversely and slowly increase to 210oC, then the deflection approximately reaches the deflection of ACSR conductor at 85oC.

The reason why Trefinasa Thermal conductors have such good deflection is that it has developed production technology of aluminum wire (ACS), used as a thermal conductor. ACS has the following advantages:
  - The tension increases 8 times compared with conventional aluminum wires with the same diameter.

  - Weighing less than 15% compared with steel-coating wire with the same diameter and characteristics.

  - Maintain the mechanical characteristics with the aluminum protection layer.

  - Reduce the electrochemical corrosion.

  - Conduction is 3 times better than steel-coating wire.

  - With high-frequency applications, where the "skin effect" is an important factor, ACS electrical conductivity equal to 100% of aluminum wire because the electric density is concentrated in the outside surface of the conductor.

There are two basic types of thermal conductor manufactured by Trefinasa: heat- insulating conductor TAL and superinsulation conductor ZTACIR. TAL conductors which have heat-resistant aluminum sheath and steel core are capable of withstanding high mechanical and abrasion. This group also contains of GTAL which has gaps between the core and the aluminum layer. Between the two layers is filled by heat-resistant grease. Operating temperature of the TAL reaches 150oC. The biggest difference between ZTACIR and TAL is the replacement of steel core by a new superinsulation material which operating temperature reaches 210oC, high mechanical strength and low degree of thermal expansion, which is Invar oxide, its component contains 36% of Nickel. Beside GTAL, GZTAL also has gaps between core and sheath layer. Core and the conductive layer move to each other freely through the gaps and are supported by heat-resistant grease and high mechanical abrasion.

Compared with ACSR conductor with the same size, most of the parameters of the thermal conductors approximately have the same density / km, mechanical load, resistivity / km ... Etc, only the operating temperature and higher load capacity increases. They are also the outstanding features that help thermal conductor gradually become the best replacement for the current lines.

4. Installation

Because of the different structure from ACSR, the construction and operation requires a new operating sequence which is suitable with the conductor structure such as the core construction paired under mechanical stress, then stripping the aluminum conductor and conducted after pairing, the decision of deflection must follow two steps, the first step was usually taken by 70% of deflection, the second take the deflection as designed to prevent deformation.

To overcome this drawback, at present the company has certainly made improvements in construction methods. In particular, with major projects, Trefinasa commits to provide enclosed machines and essential equipments for the construction of thermal conductor like ACSR conductor.

IV. CONCLUSION

In developing countries as Vietnam, electricity demand is always threatening to their full-load and overload operation of the lines as well as overloaded capacity of substations. The use of heat- insulating conductors and thermal conductors with low deflection, high load line utility for improving and replacing the old lines, which is very suitable and economical. It increases 1.6 to 2.0 times the transmission capacity of the lines, increases the capacity of supplying power to the additional charge. For substations, they also help with the improvement of capacity of the main busbar without replacing the old structure. For a new line or construction, we should also consider the use of technology of new conductor to reduce the load in order to reduce  investment costs of foundation and columns while increase transmission capacity or the power reserve factor for the future.