Abstract-- The unstoppable rise in electricity demand is

causing the saturation of the power transmission network to such an extent that supply problems are arising in some areas due to overheated conductors sagging above security distance limits.

An approach to solve this problem is the replacement of actual conductors by new conductors with high temperature and low sag characteristics. This type of conductors is able to transmit more power than traditional ACSR conductors, as they can work at higher temperatures while keeping their sag under actual security margins.

However, the higher temperature in the conductors can affect the correct performance of the traditional clamps. To develop an analysis on this subject software tools have been used, consisting on CAD programs for the design and modeling of the clamps and FEM programs for the simulation of the models. This paper shows the results of the study describing the characteristics of the simulation.

Index Terms— Clamp System, FEM, GTACSR, High Temperature, HTLS, Low-Sag, Simulation.

I. INTRODUCTION HE rise of industrial sector in developed countries has generated an increasing energy demand. On the other

hand, the world population continues expanding and improving its living standards. As a consequence of both effects global demand for electrical energy is also continuously increasing.

A direct consequence is the electrical saturation of transmission lines in general and overhead lines in particular. In overhead lines the problem is particularly worrying. An increase in electrical power in an overhead line causes an increment of its temperature. This produces a higher

This work has been completed by the research team of project UE2000-29

funded by the Basque Government and Industrias Arruti S.A. J. A. Mazón is with Department of Electric Engineering-ESI of Bilbao,

University of the Basque Country, Spain (e-mail: iepmasaj@bi.ehu.es). I. Zamora is with Department of Electric Engineering-ESI of Bilbao,

University of the Basque Country, Spain (e-mail: iepzabei@bi.ehu.es). P. Eguía is with Department of Electric Engineering-ESI of Bilbao,

University of the Basque Country, Spain (e-mail: iepeglop@bi.ehu.es). E. Torres is with Department of Electric Engineering-ESI of Bilbao,

University of the Basque Country, Spain (e-mail: ieptoige@bi.ehu.es). S. Miguélez is with Department of Electric Engineering-ESI of Bilbao,

University of the Basque Country, Spain (e-mail: iebmibas@bi.ehu.es). R. Medina is with Industrias Arruti S.A.- Múgica, Bizkaia, Spain (e-mail:

industrias@arruti.com). J. R. Saenz is with Department of Electric Engineering-ESI of Bilbao,

University of the Basque Country, Spain (e-mail: iepsaruj@bi.ehu.es).

temperature increment in conductor length and, as a result, the increasing sag may eventually overcome the safety limits, as shown in figure 1. This fact makes that traditional ACSR conductors have an operation limit temperature of 80º C.

Fig. 1. Sag and safety limits. T1, T2 and T3 are conductor temperatures (º C).

The first solution to be considered is to install new

overhead lines. But this action has several inconvenients. First, land saturation, that generates an important difficulty to get rights of way to install new overhead lines. Also, the period of time between the first moment the need of a new line is identified, until the line is finally installed can be a decade or longer. Another problem is that a new overhead line produces an increment of visual and environmental impact. These facts make that an important sector of society refuses the installation of new overhead lines.

Taking into account the difficulty to install new overhead lines, the best option is to improve the existing ones. One of the most promising actions is to replace the traditional conductors for new conductors being able at work to higher temperatures without increase the sag. The installation of these conductors, generically called High - Temperature and Low - Sag conductors (HTLS conductors), doesn’t require new rights of way because the operation can be considered as maintenance of the line. Also, these HTLS conductors don’t increase neither visual nor environmental impact because, externally, they are very similar to traditional conductors. The only problem using this kind of conductors is that the high operation temperature may have a negative effect on clamp systems. So, a first step before the installation of HTLS conductors must be to study how the temperature of new conductors can affect to the correct functionality of traditional clamp systems.

Gap - type Conductors: Influence of high temperature in the Compression Clamp Systems

A. J. Mazón; I. Zamora; P. Eguía; E. Torres; S. Miguélez; R. Medina; J. R. Saenz

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II. GENERAL CHARACTERISTICS OF GAP-TYPE CONDUCTORS One of these HTLS conductors is the GTACSR conductor,

also known as gap-type one. The geometrical configuration of these conductors is similar to the one of traditional ones, as shown in figure 2. However, aluminium wires in its internal layer have a trapezoidal cross section. Also, there is a gap between this layer and the steel core layer, filled with grease. This grease reduces friction between both layers and gives GTACSR conductor a high vibration absorption capacity.

Fig. 2. GTACSR and traditional ACSR conductors.

This reduction of friction lets that GTACSR conductors can be tightened only by the steel core, leaving the aluminium layers untightened. This fact means that the gap-type conductor elongation depends only on characteristics of steel core, and therefore, also the sag increment when temperature increaser. Consequently, the electrical power transmitted by overhead lines can be increased using GTACSR conductors without increasing disproportionally the sag. Under normal conditions, GTACSR conductors may operate at 150º C with similar sag to ACSR type. This means that GTACSR conductor is able to drive a 1,6 higher current.

III. DESCRIPTION OF CLAMP SYSTEMS Clamp systems are the elements in an overhead line that

support and keep tightened the conductors. Also, if conductors don’t have an insulating cover, clamp systems must have an insulating element, the insulator string. The primary purpose of the insulator string is to avoid the circulation of electrical current from the conductor to the tower. However, this property depends strongly on the temperature reached by the insulator. In thermal analysis, the insulator is the most delicate element of the clamp system.

In the case of high voltage overhead lines the conductors are always without insulating cover. With ACSR conductors, this fact isn’t a problem, because the maximum conductor temperature is round 80º C. This temperature doesn’t have a negative effect in the insulator. However, with GTACSR conductors it may be a problem. The high GTACSR conductor

temperature (150º C) may negatively affect to the insulators properties. Therefore, it’s important to know the temperature distribution along the clamp system in general, and in the insulator string in particular, when GTACSR conductors are going to be used in overhead lines.

Fig. 3. Simple compression system for simple conductor

There are many combinations of clamp systems: single

chains for single conductors, single chains for double conductors, double chains for double conductors, “V” shaped double chains for triple conductors, etc. Each one of these configurations is in relation with the technical needs. However, the configuration showed in figure 3, simple compression system for simple conductor, is the configuration that generates the highest increase in temperature in the insulator, because this configuration presents the lowest distance between the first insulator and the conductor. Therefore, system showed in figure 3 represents the most restrictive case. This is the reason why this specific configuration of clamp system has been analyzed.

IV. THERMAL ANALYSIS

A. Model The first step before performing the simulation is to create

the 3D model using a CAD software. This process consists in modeling the individual elements taking into account the dimension of the real elements. Once the individual pieces have been modeled, the next step is the assembly to generate the complete system model, as shown in figure 4.

Fig. 4. Compression system’s elements

Finally, the clamp system model is exported to a thermal analysis software. The software selected is a computing tool that calculates the thermal analysis using the Finite Element Method (FEM).

B. The finite element method The FEM is a mathematic method that, basically, consists in

dividing a continuous system into small regions interconnected at points called nodes. The equations that control the complete system control each element too, but the variable to solve, in this case the temperature, is calculated only at nodes. Therefore, a “continuous system” obeying a differential equation with infinite degrees of freedom has changed into an equivalent “discrete system” obeying algebraic equation with a limited degrees of freedom.

The most critical step of the thermal analysis is the meshing, the process to divide the continuous system. More exactly, the total number of elements used to mesh the model directly influences in the precision of the solution and in the time requested by the software to solve the analysis. If a fine meshed is used with a high number of elements, the theoretic solution will be closer to the real solution, but the time requested by the software to solve the analysis will be excessive. However, if coarser meshed is used without many elements, the software solves quickly the analysis, but the theoretic solution and the real solution could be quite different.

In processes like this, coarse meshed is often used in the first simulations to check the behavior of the system, and a fine meshed when a definitive solution is wanted.

C. Conditions of the simulation. Once the model is exported to the thermal analysis software

and the precision of the mesh has been defined the conditions of the simulation must be established. On one hand, the system behavior must be defined with the different contour conditions. This involves assigning different materials to each element in order to establish their physical properties. The pieces in contact with the conductor are made of an aluminum alloy, the insulator is made of glass and the remaining elements are made of steel. The different parts of the insulator are joined with cement.

On the other hand, the different contour conditions that may affect to the system must be determined. There are so many situations that may affect it, so the most unfavorable case has been principally studied, because it has been considered that if the system works correctly in this case it will do it in the remaining cases too. The most unfavorable situation happens when GTACSR conductor reaches its maximum operation temperature, 150º C, with natural convection, which means no wind, and an ambient temperature of 40º C. In addition, the Joule effect has been also considered.

Nevertheless, thermal analysis has been solved with GTACSR and ACSR conductors (with ACSR conductors the maximum operation temperature has been 80º C), and the same contour condition has been considered with the purpose of comparing both results.

D. Results Once the thermal analysis software has finished the

calculation, the distribution temperature shown in figure 5 is obtained. In this figure, the average temperature of each element of the compression system using GTACSR and ACSR conductors is shown.

As said above, the most delicate element of the clamp system is the insulator closest to the conductor. In fact, the most sensitive part of the insulator is the glass dish, whose function is to avoid the circulation of electrical current from the conductor to the tower. This is the element of the clamp system that requires more attention, because its insulating characteristics depend on its temperature. This element reaches its maximum temperature at the contact point between the dish and the insulator’s axis, axis that keeps together the insulator and the kneecap. This contact point has been called Critical Point (C.P.)

Fig. 5. Temperature distribution (º C) in the clamp system

V. LABORATORY TEST Once the theoretic models have been adjusted and safety

limits have been established, laboratory test has been developed in order to validate the simulations.

The purpose of the laboratory test is to check if the high temperature of the conductor can increase the temperature of the insulator’s cement to values higher than its critical temperature, of about 100º C.

A. Description of the laboratory test The test consists in connecting 8 meters of HTLS

conductor, held up by means of a clamp system to the terminals of a current source. The diagram of the laboratory test is shown in Figure 6. The current source supplies enough power to increase the conductor temperature until it reaches the maximum continuous working temperature. Simultaneously, by means of thermocouples, the temperature in different points of the clamp system is measured, as shown in Figure 7. Figure 8 shows measuring points in the compression clamp.

Fig. 6. Thermal sensors in the compresion system

Fig. 7. Thermocouples in the compression clamp

Fig. 8. Measuring points in compression clamp

B. Results of the laboratory test The test conditions were: no sun, no wind, an ambient

temperature of 23º C, and a conductor temperature of 150º C at a point placed 1 meter from the clamp. The results of the laboratory test, and the ones obtained in the simulations, are shown in Table 1. In this case, the average temperature of each element is not shown, but local temperature at particular points.

It can be observed that the conductor temperature decreases in the proximity to the clamp system. This effect is a consequence of that, in the one hand, the electrical resistance

of the clamp is lower than in the conductor (the Joule effect decreases in the clamp) and, in the other hand, the surface available for heat dissipation is higher than in the conductor (increases natural convection).

It has been also observed that the temperature on the limit of the ball socket (F) is about 70º C. It can be assured that this temperature doesn’t affect the correct functionality of insulator.

TABLE I

RESULTS IN COMPRESSION SYSTEM.

Components A B C D E F G H I Simulation temperature

(º C) - 128 112 95 90 67 106 120 131

Lab. test temperature

(º C) 150 132 117 99 93 70 111 126 135

Taking into account the theoretical and test results, it can be

assured that high GTACSR conductor temperature affects neither the correct functionality of the clamp system nor its insulating capacity, because the replacement of traditional conductors only produce an increase 20º C in the Critical Point temperature.

VI. NEW DESIGNS OF COMPRESSION SYSTEMS As it has been checked with the results showed before, the

higher temperature reached in the insulator doesn’t imply a risk in its functionality. However, thermal studies have been developed considering different lengths of the compression clamp system, as it’s shown in Figure 9.

Fig. 9. Extensions in the clamp system

In particular, the following possibilities have been

considered:

•

Compression clamp length (L1) + 15%. • Compression clamp length (L1) + 30%. • Compression clamp length (L1) and derivation clamp

length (L2) + 15%. • Compression clamp length (L1) and derivation clamp

length (L2) + 30%.

The same simulation described in the previous section using

the Finite Elements Method has been developed with each proposed clamp system and GTACSR conductor (150ºC). Taking into account that the most delicate component of the clamp system is the insulator, the temperature at the critical point of the insulator has been also determined for each proposed system, and it has been compared with the solution obtained for the original system. The results of this analysis are shown in Figure 10.

Fig. 10. Critical Point Temperature (º C)

VII. CONCLUSION The present paper has shown the results of the thermal

analysis of conventional compression clamp systems when using GTACSR conductors. The objective of this analysis has been to check the maximum temperature reached by the first insulator when this type of conductor is used in an overhead transmission line.

After the first trial studies we have been able to confirm that compression systems used for stringing ACSR conductors are also valid for GTACSR conductors of equal size, because the insulator never reaches dangerous temperatures.

Nevertheless, new arrangements with longer compression clamps are proposed in order to provide a greater safety margin. In the best case, the simulation of these new clamp

systems by means of the Finite Elements Method has shown a temperature decrease of about 5ºC at the insulator’s critical point.

On the other hand, the comparison between theoretical and test results makes possible state that the models represent the system’s real behaviour with high reliability level, as the difference between both results is only about 5%.

At the present time similar studies are being carried on with other different types of clamp systems. The simulation results obtained from those studies will be also validated with the data obtained from actual laboratory tests.

VIII. ACKNOWLEDGEMENTS The authors wish to express their gratitude to Mr R. Criado

and Mr C. Alonso of the utility Iberdrola S. A. for their collaboration in this project.

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