Conference-1
Latest Technologies in T & D, Smart Grid, Renewable
Integration and Communication

UPGRADATION OF EXISITING 66 KV TRANSMISSION LINE WITH SUITABLEHTLS CONDUCTOR TECHNOLOGY IN NEPALKARTHIKA PURNIMA.N
Sleepwalkers
India

SYNOPSIS
Utilities in almost all parts of the world face a common obstacle in the construction of new
overhead transmission line - obtaining land for their clearway (ROW). Moreover, due to theever increasing power demand and pressing needs of integrating renewable energy sources
into the grid, increasing the power transfer capacity of existing transmission line, seems tobe the one of the viable solution. The present condition of the existing transmission corridor
may be completely different from the one designed years ago. Hence, the selection ofsuitable HTLS conductor technology considering ruling span and existing tower loadings
may not meet the ground clearance requirements. So, evaluating with respect to present
conditions like actual ground clearance of the complete transmission corridor, current towerloadings, spotting and weather conditions is more important and would bring cost effective
and reliable solution. This paper elaborates the advantages of selecting technology,
considering the present conditions of the transmission corridor over the ruling spanmethodology, in one such case in Nepal. The surveyed data of the existing line has beenused in PLS CADD for the selection of suitable HTLS conductor technology to double thepower evacuation capacity considering optimum cost and reliability.

KEYWORDS
Reconductoring, Uprating, transmission line, High Temperature Low Sag (HTLS) Conductor,ground clearance.

1.

INTRODUCTION

In many countries of the world, new power lines have not been built from many years asobtaining right of way is difficult. Nevertheless, over the same period of the time, the world
experienced an increase in power consumption. But most of the transmission lines arestrung with conventional conductors like ACSR, AAC, AAAC, ACAR etc., and by using thesame lines it is difficult to draw more power. It has thus become necessary to uprate thethermal power transfer capacity. Uprating of overhead line is possible by increasing current
carrying capacity of transmission line, but that may lead to increase in sag value by
considering ruling span with same towers/poles, which may not meet the ground clearanceof existing line. So, evacuating more power with present conditions such as existing groundclearance and towers/poles, there is an optimal technique for Uprating the transmission linewith high temperature low sag conductors (HTLS).
HTLS has been used across the world to uprate the existing transmission line. The
investigation of different techniques to improve the transmission system capacity in power
network shows that HTLS conductor was useful in uprating by increasing current rating.
More than double increase in power capacity was obtained by replacing conventional
conductor with HTLS Conductor, which was evaluated to be the best solution from bothtechnical and economic aspects. This paper elaborates the advantages of selecting suitable
HTLS technology, with respect to the existing tower, weather and ground clearance
conditions of the transmission corridor over the ruling span methodology, in one such case in
Nepal to enhance the power transfer capacity.

2.

HIGH TEMPERATURE LOW SAG CONDUCTORS“High Temperature Conductor” is defined as a conductor that is designed for applications
where continuous operating temperature is higher than the traditional conductors which isabout 750C to 850C.
“Low Sag” is defined on the basis of at that particular operating temperature sag shouldmatches with ‘traditional conductors’ sag value.
HTLS Conductors are made by thermal-resistance aluminium/super thermal-resistancealuminium/annealed aluminium layers stranded around a core of material with low coefficientof thermal expansion.
Note: The maximum permissible duration of emergency operating temperature of HTLS
Conductor shouldn’t be more than 400 hours for total life of the conductor.

2.1. Conductive Layer – Aluminium
Generally, the conventional aluminium and its alloy has limitation of operating temperaturefrom 750C to 1000C throughout its life span. The conventional aluminium and aluminium
alloy (1350,1370,6201,6101, Al59,1120, etc) will lose its strength at higher temperature
because of annealing.

The challenge of increasing the operating temperature of the aluminium and its alloy without
loss of its electrical conductivity was addressed with the following high temperature electricalconductivity aluminium and its alloy:a) High temperature EC grade – 1350 – Annealed Aluminiumb) Thermal resistant aluminium alloy – Al-Zr alloy (Aluminium-Zirconium)
TABLE:1 Comparison of different aluminium alloys
TABLE: 1Properties

Tensile Strength inMPa
Conductivity%IACS
Continuous
OperatingTemperature

ACSR (Harddrawn 1350Al)

Annealed
Aluminium –1350

TAL (ThermalAlloy Al Zr)

STAL (SuperThermal Alloy
Al Zr)

160
61

60
63

160
60

160
60

85250

150

210

2.2. Core – Strength Member
Aluminium has good electrical conductivity and lesser density but the challenge is tensile
strength and endurance limit. So ACSR uses steel core as strength member forreinforcement. Steel has higher strength, lower linear expansion than aluminium but theconstraint is lower electrical conductivity and higher density.
The core material for HTLS conductors should have:• High strength• Less density• Less linear expansion• Good modulus of elasticity
In the past, development in core materials were made to increase the strength of steel or toreduce the linear expansion with alloy steel etc. But in the year 2002, composite materials
were introduced. The composite materials have higher tensile strength, less density, lesslinear expansion but the challenges are compressive strength, modulus of elasticity.
Commercially available core materials are listed below

Steel•••

Ultra High Strength Steel (UHS)
Extra High Strength Steel (EHS)High Strength Steel (HS)Alloy SteelCarbonfibreComposite••High strength Invar
Invar

•

Carbon fibre + Thermosetting
polymer (Epoxy) + Galvanic
protection layer
Carbon fibre + Thermoplastic
polymer + Galvanic protection
layer

••

Metal
MatrixComposite•

Al 2O3 fibre in Aluminium Matrix

TABLE: 2 The list of commercially available HTLS technologies are as follows:
TABLE: 2ACSSAluminium
SupportedConductor

SteelTACSR

Thermal Resistant Aluminium
Alloy
Conductor
SteelReinforced
GTACSR / Gap Type Ultra Thermal
GZTACSR Resistant Aluminium Alloy
Conductor Steel Reinforced
STACIR
Super Thermal Aluminium
Alloy
Conductor
Invar
Reinforced
ACCCAluminium
Conductor
Composite Core
ACFR

Aluminium Conductor Faber
Reinforced

ACCR

Aluminium
Conductor
Composite Reinforced3. NEPAL 66 KV LINE TECHNICAL CONSEQUENCES
3.1 Scope of the Project:
The overhead transmission lines, having voltage level of 66 kv are mostly equipped withACSR 150 mm2 & ACSR 120 mm2 conductors.
To enhance the power transfer capacity, the first solution considered, was to install newoverhead line. For the new transmission lines increasing power transfer capacity is easy, as
it can be performed by increasing portion of aluminium or by increasing the number ofconductors in bundle.
However, this solution has several inconveniencies. First, land saturation that generates
difficulty to get right of way to install a new overhead line. Also, the period of time when theneed 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 andenvironmental impact. These factors make a sector of the society refuse the installation of
new overhead lines.
Taking into account the difficulties to install new line, second solution is to improve thecurrent rating of existing line by replacing traditional conductor with HTLS Conductor.
Reconductoring the line with HTLS conductors without any tower reinforcements with shorttime execution and a reasonable cost. With two times the current carrying capacity, HTLS
Conductors maintaining the existing ground clearances.
3.2 Existing Tower Loadings:
Tower loading is the most vital input for designing any transmission line. The existing towers
in the 66 KV transmission line has been designed with various types of loads. In the load
calculation, wind plays a major role.
In this case the existing component of tower like insulator string, ground wire will rem ain thesame but the existing conductor will be replaced with new HTLS conductor. So, the onlychange in component is overhead conductor.
The existing tower was designed with ruling span and wind conditions depending on theweather condition of the corridors. So, the proposed HTLS conductor should have equal orlesser loading conditions of the existing ACSR conductor under the designed conditions oftower. So, the data of existing tower design plays a vital role in designing a suitable HTLSconductor.

The conductor loading condition:
1) Transverse LoadWind load on conductor
Mechanical tension of conductor2) Vertical load
Loads due to weight of conductor
3) Longitudinal Load
Unbalanced horizontal
load due to mechanicaltension of conductor

4. OUR APPROACH FOR RECONDUCTORING OF EXISTING LINE WITHSUITABLE HTLS TECHNOLOGY

Survey the existing linewith respect to tower
spotting, every 50
meters ground elevationof existing corridor andexisting sag/groundclearance

String the existing
conductor with respect
to the existing groundclearance

Design different HTLS
conductor technology tomatch the existing
ground clearance

5. TECH

Feed the survey
data into PLS
CADDCreate tower profile
similar to theexisting towerheights

Creating existing
conductor wir file

Tower spotting withrespect to theexisting tower
locationsFinalize the suitable
HTLS technology or
any other technology
solutions to meet theneed of the customer

Conclude thetechnology withrespect to terrain
conditions and needof the customer

6. TECHNICAL ANALYSIS
To maintain the line’s safety in operation, for example to conserve the towers and theinsulating strings when using HTLS conductors, the following restrictions have to be fulfilled:
TABLE: 3The HTLS conductor shall meet the following minimum requirements:
Description
Overall diameter of complete
Conductor
Approx. mass of complete
conductor
Direction of lay of outer layer

UnitmmValue
Not exceeding 15.74

Kg/km

Less than or equal to 528-Right Hand

For maintaining the ground clearance of existing line, the HTLS Conductor should meet thefollowing sag tension requirements as specified in table: 4TABLE: 4Sag -Tension Requirements
Description
Tension at everyday condition (28 deg C, no wind)
Tension at 20 deg C, full wind (53 kg/m2)
Tension at minimum temperature -5 deg C, 37.7
% of full wind (20 kg/ m2)
Maximum working stress allowed for conductor

Value
Not exceeding 25% of UTS ofproposed conductor
not exceeding 50% of UTS ofproposed conductor
not exceeding 50% of UTS ofproposed conductor
1323 Kg

Table: 5 shows the comparison of technical characteristics that will have an impact on theexisting tower while replacing with different HTLS technologies.
TABLE: 5ACSR vs HTLS reconductoring Comparison Considering Ruling Span 298 meters
Description

Unit

ACSR

ACSS

STACIR

ACCC

ACFR

Conductor Diameter
DC resistance at 20 deg C
Min UTS of conductor
WeightModulus of Elasticity
Co-efficient of linear expansion
Maximum allowable continuous operating
temperature
Emergency operating temperature
Ampacity at 75 Deg C
AC resistance at 75 Deg CSag at 75 Deg C
Maximum ampacity that can pump per
conductor without exceeding the ACSRSagTemperature at above ampacity

Mm
Ohm/km
KN
Kg/km
Kg/mm2
X 10 -6 /°C

15.74
0.2398
47.6
528
7885
18.33

15.4
0.2465
45.3
525
8134
17.86

15.02
0.2943
51522
8165
14.4

15.65
0.1859
69
470
6779
17.36

15.7
0.1792
46457
6265
18.76

DegC
DegC
A
Ohm/kmMeters75
100
2900.2935
9.15

250
280
283
0.3018
8.28

210
240
256
0.3602
8.52

180
200
325
0.2277
7.22

180
200
335
0.2188
8.16

A

N/A

483

580

707

724

Deg C

N/A

125

195

180

180

Ruling span
Tension 28 Deg C, no windTension at 20 Deg C, Full wind pressure(53 kg/mm2)
Tension at -5 Deg C, wind pressure (20kg/mm2)
Losses at 290 A per CKM(Loss load factor 0.53, circuit - single,Route Length - 1 Km)

MetersKg & %

298
758 & 16298
832 & 18298792 & 15

298
762 & 11298
885 & 19

Kg & %
Kg & %

1323 & 27
1021 & 21

1308 & 28
1021 & 221323 & 25
1015 & 201314 & 19
1021 & 15

1265 & 27
1021 & 22

Kw

39.2

40.4

48.2

30.4

29.3

The calculations of steady-state thermal rating, given a maximum allowable conductor
temperature, weather conditions, and conductor characteristics were performed by thecomputer using PLS CADD based on the IEEE Std 738 - 2006. For steady – state thermal
rating, the following parameters were adopted:-Wind speed: 0.56 m/s;
The angle between wind and conductors: 900;
Emissivity: 0.45;

-solar absorptivity: 0.8;
Air temperature: 400C
Solar radiation:1045 watt/sq.m;
Elevation above sea level:1300 meters;

Current carrying capacity comparison graph at 75 Deg C for Different Conductors

HTLS Conductors Sag comparison at ≥ double current carrying capacity

Graphical representation is based on below assumptions
Ruling span: 298 meters
Everyday temperature: 28 deg C
Operating temperature at double current capacity of each HTLS conductor*The above graphical representation showing the results of sag value for different type of
HTLS technology at after creep condition.

Conclusion:
The replacement of existing conductor should improve the mechanical reliability of the line
conductor. To uprate the 66 KV line in Nepal by using high temperature conductors isconsidered as a feasible desideratum from the technical point of view. All the selected
conductors can be strung to accomplish equal or smaller sag than the initial one, but at
higher temperature.
Finally, from techno – commercial point of view, the detailed analysis of the suitableconductor technology can be considered on the basis of techno commercial evaluation ofdifferent HTLS technology by the customer. From the detailed technical analysis considering
maximum power transfer capacity without violating the ground clearance of existing line, themost attractive solutions are Invar & Composite core technologies. The utility canreconductoring the line either with Invar or composite core conductors if cost will beaccepted.

REFERENCES
Working group of 22.12 CIGRE 207, Thermal Behaviour of Overhead Conductors, Tech.
Brochure 207, 2002
Working Group SC 22-12 Cigre (Chairman R. Stephen). “The thermal behaviour of overhead
conductors Section 1 and 2 Mathematical model for evaluation of conductor temperature in
the steady state and the application there of” (Electra number 144 October 1992 pages 107125).
LFE_CIGRE High Performance Conductors An International viewpoint
Standards:IEEE Std. 738-2006 IEEE Standard for calculating the Current-Temperature relationship ofbare Overhead conductors.