1876-6102 © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of ICAER 2015doi: 10.1016/j.egypro.2016.11.183

Energy Procedia 90 ( 2016 ) 179 – 184

ScienceDirect

5th International Conference on Advances in Energy Research, ICAER 2015, 15-17 December 2015, Mumbai, India

Analysis of High Temperature Low Sag Conductors used for High Voltage Transmission

Subba Reddy B* and Diptendu Chatterjee High Voltage Laboratory, Dept. of Electrical Engg, Indian Institute of Science, Bangalore-560012, India

Abstract

Presently there is a continuous demand for the electric power consumption across the globe, however with the existing distribution lines are reaching critical limits of ampacity and sag, it has become difficult in finding corridors to construct new overhead lines in many industrialized countries including India. Replacing the existing ACSR conductors with high temperature high current low sag (HTLS) conductors almost of the same diameter is one of the recent methods. The present work a parametric study is conducted for steady state surface temperature, thermal time constant, change of emissivity, absorptivity etc for various ACSR and HTLS conductors using the developed computer code which is in accordance with IEEE Std.738. Some experimental study is also conducted and the results obtained are presented. © 2016 The Authors.Published by Elsevier Ltd. Peer-review under responsibility ofthe organizing committee of ICAER 2015.

Keywords: HTLS conductors; ampacity;ACSR; low sag;Simulation; Experimentation

1. Introduction

The increase in power requirement is becoming a great challenge for the utilities in terms of cost and capacity where the existing lines have reached their maximum limit. One of the solutions is the installation of a parallel structure like the existing towers, but this is not an economical solution. The other way to find a cost-effective and more viable solution is in adopting high temperature low sag (HTLS) conductors for distribution systems [1]. These conductors are different from conventional conductors in terms of material or structure or both.

_____________ * Corresponding author. Tel.: +91-080-22932550; fax: +91-080-22932550. E-mail address: reddy@ee.iisc.ernet.in

Available online at www.sciencedirect.com

© 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).Peer-review under responsibility of the organizing committee of ICAER 2015

180 B. Subba Reddy and Diptendu Chatterjee / Energy Procedia 90 ( 2016 ) 179 – 184

The significance of HTLS conductors is they can carry 2.5 times the current that of the conventional ACSR

conductors of same size and can withstand higher temperature (>200°C) with less sag. One of the several advantages of HTLS over conventional ACSR is by re-conductoring an existing line with

HTLS conductor the power delivery capacity can be increased. But an HTLS conductor for long transmission is not recommended as it will cause higher voltage drop and power loss due to high current. So increasing voltage level will be wise. Several HTLS projects are being planned and implemented throughout the world including India [1, 2].

The study of HTLS conductor was first initiated by Douglass [3] explaining the practical applications used for Connecticut Light and Power Company. Later Alwar et al [4] have discussed about conventional ACSR conductors and the composite core conductors for low sag at high temperature. IEEE Standard 738 [5] explains several factors that affect the temperature of bare overhead conductor. The equations to find the current temperature relationship are given in this standard. Several researchers [6-9] discussed about the emissivity, radial temperature distribution, corrosion and effective radial thermal conductivity in bare solid and stranded conductors. Ravi Gorur [10] characterised the composite cores for HTLS conductors and studied surface temperature vs. time curve, core temperature with current, with emissivity, absorptivity, thermal conductivity etc in accordance with IEEE Std 738[5]. Further Harvey and others [11-15] studied temperature creep and sag-tension performance of HTLS conductors. Recent IEEE Standard 1283[16] gives the guidelines for determining the effects of high temperature operation on conductors, connectors and accessories. It describes possible adverse impact on operating overhead transmission line at high temperatures. Gerald et al [17] discussed about how HTLS conductors can be a solution to the ever increasing power demand. A technical report [18] describes the structure and properties of aluminium conductor composite reinforced (ACCR) conductors. Researchers [19-24] have used different models for calculation of various parameters for HTLS conductors. Recently several planned projects [25] using HTLS conductors are being implemented in the country. Hence this work was initiated with the view that the data obtained will be useful for further implementation of projects as well as in enhancing the current literature.

2. Simulation Study

In the present work, simulation studies are carried based on IEEE-738 Standard [5]. The study consist of a developed Matlab code to simulate: Surface temperature variation with time for a given current level, variation of surface temperature with different parameters like ambient temperature, absorptivity and emissivity of the conductor material, variation of temperature along the radius of the conductor etc. Separately (i) a graphic user interface (GUI) is developed for use in optimal design of different transmission and distribution accessories to be used for HTLS conductors which simulates temperature variation with current and different parameters also (ii) Simulation of magnetic field near the conductor due to increased current in case of HTLS conductors is attempted.

The technical details of various types of HTLS and ACSR conductors used for the present work are given in table 1 below:

Table. 1. Specification of conductors used for simulation Details ACSR HTLS1 HTLS2 HTLS3 HTLS4

Overall Dia (mm) 28.12 28.14 28.62 28.118 31.77

Resistance per length at 25deg C (ohm/km)

0.0728 0.0554 0.0674 0.0702

0.0431

Resistance per length at 75deg C (ohm/km)

0.0869 0.0662 0.0741 0.0843 0.0511

Heat Capacity per length (W-sec /m-C)

1309 756 1177 1296 1495

Following assumptions were made for the estimation of current and temperature: Ambient Temperature=40 degree centigrade; Velocity of wind=.61 meter/sec; Absorptivity=.5; Angle of the flow of wind with conductor axis=90 degree; Emissivity=.5; Day number of the year=161; Altitude=0 meters; Latitude=43; Azimuth of line=90 degree; Time of the day=11 a.m.

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3. Simulation Results

The equations specified in [5] and the assumptions made, the thermal response and parametric variation of temperature is presented. The thermal response of one ACSR and different HTLS conductors at 1750 Ampere is given in Fig.1. Variation of steady state surface temperature of ACSR and different HTLS conductors with emissivity and ambient temperature at 1000 Ampere are presented in Fig.2. The variation of steady state surface temperature with both emissivity and absorptivity at 1000 Ampere for ACSR and HTLS conductor is given in Fig. 3. For the desired surface temperature it is possible to get the operating point and fix the emissivity and absorptivity value of the conductor to get the desired temperature. From the results it is seen HTLS conductors perform better over ACSR.

0 0.5 1 1.5 2 2.5 340

60

80

100

120

140

160

180

200

time(minutes)

Sur

face

Tem

pera

ture

(in

deg

C)

ACSR

HTLS-1

HTLS-2HTLS-3

HTLS-4

20 25 30 35 40 45 50 55

50

60

70

80

90

100

110

120

Ambient Temperature (in deg C)

Ste

ady

Sta

te T

empe

ratu

re (

in d

eg C

)

ACSR

HTLS-1

HTLS-2HTLS-3

HTLS-4

Fig. 1. Thermal time response for a step current of 1750 A Fig. 2. Variation of Steady State Surface Temperature of with Ambient Temperature at 1000A

0 0.2 0.4 0.6 0.8 1 0

0.5

180

90

100

110

120

130

140

absorptivityemissivity

Stea

dy S

tate

Tem

pera

ture

(in

deg

C)

85

90

95

100

105

110

115

120

125

130

135

0 0.2 0.4 0.6 0.8 1 0

0.5

170

80

90

100

110

120

absorptivityemissivity

Ste

ady

Sta

te T

empe

ratu

re (i

n de

g C

)

75

80

85

90

95

100

105

110

115

(a) ACSR (b) HTLS

Fig. 3. Variation of Steady State Surface Temperature with emissivity and absorptivity at 1000A

A Matlab based graphical user interface (GUI) has also been developed where in the environmental conditions, conductor dimensions, accessories material properties etc, given as input to get the optimal dimension of the accessories, and the time response of the conductor temperature.

4. Estimation of magnetic Fields

The HTLS conductors operate at a higher current level, hence produce a proportionally higher magnetic field. A 3D magnetic field simulation is carried out using a commercially available FEM software COMSOL Multiphysics®[26]. The magnetic field near the region of the conductor has been estimated with the distance. Both the cross-sectional plot and 3-D plot of the magnetic field for single and double conductor setup are presented.

182 B. Subba Reddy and Diptendu Chatterjee / Energy Procedia 90 ( 2016 ) 179 – 184

International guidelines [27] for the permissible magnetic field through human body both for continuous and discontinuous application help in deciding the approximate height of the conductors from ground level. Simulation of magnetic fields in this work includes contour and 3-D magnitude plot of the magnetic field in the cross sectional surface of single and double line. For all the cases, current through all the conductors is assumed to flow 1750 Ampere and the region of interest is up to 6 meters in all the direction perpendicular to the axis of the lines. Magnetic field contours for single and for the double transmission conductors are estimated Fig.4 shows for double line. Similarly the magnitude of the magnetic field in cross-sectional surface of the conductor for double lines in 3-D are shown Fig. 5 respectively. It is seen nearer to the conductor the magnetic fields are very high and reduce with the distance.

Fig.4. Magnetic field contours due to double lines Fig.5. Magnitude of magnetic field in the cross sectional surface each carrying 1750 Ampere of double lines each carrying 1750 Ampere

5. Experimentation: Results and Discussions

The experimental arrangement is shown in Fig.6, consists of a specially fabricated towers of height 1.5 meters having a span length of 6.5meters with a provision for conductor tension. A specially fabricated high current source of 6kVA, 2000A is used for the experiments. Two connecting leads of 25mm x40mm rectangular cross-section aluminium busbars of length 3.5meters (approx) are used. For temperature measurement non contact type laser instrument and a testo make thermal imager model 875-II were employed. Various samples of ACSR Conductors: Bersimis, Zebra, Moose along with HTLS Conductors: GTZ ACSR GAP Conductor, INVAR Moose, ACSS Curlew etc were used for the experiments. Also HTLS conductor accessories like Mid-Span compression Joint, End Joint, Repair Sleeve, T- Connectors etc, were evaluated. Two types of experimentation (short term and long term) were carried out on all the types of HTLS conductors and accessories.

Fig.6. Experimental arrangement (ACSR & HTLS) Fig.7. Typical measurement using thermal imager

For short term experimentation the conductor is connected in between the two towers with span of 6.5 meters and the end terminations suitably connected with the bus-bars to the high current generator to provide a closed path (Fig.6). For ACSR conductors, the input current is varied from 0 to 600 Amps in steps 0f 100 Amps after every 5 minutes. The temperature is recorded at different points on the conductor, busbars, end terminations etc. For temperature measurement a thermal imager Testo-875II model was used along with non contact laser based

B. Subba Reddy and Diptendu Chatterjee / Energy Procedia 90 ( 2016 ) 179 – 184 183

instrument. A typical measurement carried out on one of the sample using thermal imager is shown in Fig.7. From the measurements it was observed the temperature was higher mainly at the connecting joints. In case of HTLS conductors input current is varied from 0 to 1000 Amps in steps of 100 Amps after a gap of 5 minutes and the values of temperature are recorded with the laser and also thermal imager. Similarly for the experimentation on conductor accessories, the experimental setup remains same except the accessories are connected suitably between the towers using conductor joints and appropriate sleeves.

The long term experimentation was planned to obtain the thermal time constant for different ACSR and HTLS conductors using the same electrical connections as used in short term experiments. For ACSR conductors a step input current of 400 Amps is applied and the surface temperature of the conductor is measured at every 10 seconds up to 5 minutes and the temperature variation with time is obtained. Then the applied current is reduced and switched off and allowed to cool down for 30 minutes. The experiment is repeated for 500 Amps and 600 Amps respectively and the values of temperature variation are obtained. In case of HTLS conductors a step current input of 400 Amps is applied and the surface temperature of the conductor is measured at every 10 seconds till 5 minutes and temperature variation with time is obtained. Then the system is cooled down for 30 minutes and the experiment is repeated for 500Amps, 600 Amps and 1000 Amps respectively. Similar experiments were carried out for various accessories and the values obtained are reported. Experiments for HTLS were limited to 1100A as it was seen that temperature was high near the connecting joints. The results obtained are analyzed and presented individually. Variation of steady state surface temperature of different ACSR and HTLS conductors with currents is presented in Fig.8. and variation of steady state surface temperature for different HTLS conductors accessories with current is shown in Fig.9.

(a) ACSR (b) HTLS

Fig.8. Variation of steady state surface temperature for different ACSR/HTLS Conductors with Currents y p

Fig.9. Variation of steady state surface temperature of different HTLS accessories with current

6. Conclusions

In the present work effort has been made to study and compare the performance of different types of HTLS and ACSR conductors. A new experimental facility was established for the investigations.

It was seen for the application of same current, the steady state surface temperature of the HTLS conductor is lesser than that of the ACSR conductor of similar rating. The thermal time constant is low for HTLS conductors in comparison to the ACSR conductors of similar rating as it depends on resistivity, radial thermal conductivity and shape/surface of the conductor.

The difference in average temperature between core and strand is lower in case of HTLS conductors. It is not more than 2 degrees for application of 1750 Amps, while for the same in case of ACSR conductor it is nearly about

184 B. Subba Reddy and Diptendu Chatterjee / Energy Procedia 90 ( 2016 ) 179 – 184

10 degrees. With the change in emissivity, absorptivity and ambient temperature the change in steady state surface

temperature of HTLS conductors are similar to that of the ACSR conductors. With increased emissivity, surface temperature of the conductor decreases and with increased absorptivity,

surface temperature of the conductor increases. The accessories subjected to same current level acquire less temperature than the conductor. Magnetic field near the vicinity is similar for both ACSR and HTLS conductors, but only the magnitude

proportionally increases because of the higher current in case of HTLS conductors.

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