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Asst.Professor,
EEE Department, SSTC, Chhattisgarh, India, e-mail id: [email protected]

 

Abstract

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In India, ACSR and AAAC are most commonly used conductors for
transmission of power through overhead lines for transmission.To meet theincreased
load demand either we have to construct the new UHV or EHV T/L for transmission
of bulk power through long distances. Uprating of transmission lines i.e.
modifications in the existing transmission line to enable increased current
flow limits.Making a new transmission lines also have few constraints:

ROW constraints (Lack of
availability of corridors for construction of new transmission lines due to High
Population Density, Forest conservation) and Time constraints (due to shorter time schedules, the construction
of transmission lines projects need to match with generation projects).

 

 

Keywords: Ampacity, Uprating, HTLS,INVAR.

 

Introduction

A rapid increase in electric power consumption is
witnessed which results the increase in demand of the uninterrupted power
supply. The new generation units are being built with increased installed
capacity, but theexisting transmission lines are reaching their critical limits
of ampacity and there is shortage of corridorsparticularly in dense populated
cities to construct the new overhead lines specially in country like india.Most
of the times it becomes impossible to obtain a right of way for the new
transmission lines and hence present circumstances demands the use of available
lines with cheaper solution than going in for an underground transmission (1 Dae-Dong Leea, 2011)

 

The
constructions of new line have several disadvantagesIn addition, there is a
large limitation ofconstruction space, ROW issues and construction costs are
very high when rebuildingthe towers, hence the best suited  method is to increase the operating
temperature by adopting heat resistant aluminum alloy conductors .The purpose
of developing a new type ACSR conductor was to double the current capacity by restringing
conductors on existing steel towers Thus itis unnecessary to either rebuild or
construct tower with longer in height, the steel towers to enlargethe
capabilities of overhead transmission lines. New HTLS requires
lowerconstruction costs, has a shorter construction period and does not
needlarger towers, larger conductors, or bundled conductors. The structure,
fittings, and construction methods of HTLS are designed to be the same as those
of ordinary ACSR conductor  (2S. Sakabe
N. Mori, 1981).

 

During last few
decades the world is going through a phase of rapid industrialization. A the
time the electrification  in developing
countries in being carried out at a high 
rate and due this cumulative effect the power demand  is increasing day by day. In response  government and private projects fare involved
to increase the power generation, subsequently the transmission  and distribution of increased required power
is becoming a great challenging for the utilities in terms of cost and
capacity, where the existing lines have reached their maximum limits. Hence on
the solution to build new lines parallel with existing one but this is not an
economical solution. One of most cost effective solution is to adopting high
temperature low sag (HTLS) conductor for transmission and distribution.These
are different from the conventional ACSR conductor in terms of material but
same in size. It can  carry  approximately 2 to 2.5 times the current that
of conventional ACSR conductors of same size and can withstand higher
temperature (>200 °C),due to high current carrying capability the elongation
of conductor is less, so the sag is very less. One of the major advantages of
HTLS over conventional ACSR conductor is to re-conductoring  the existing 
double circuit line with HTLS without disturbing the another circuit.The
possibility of replacing conventional overhead conductors with new generations high
performance conductor is called high-temperature low-sag (HTLS) conductors, it
is attractive choice particularly in those corridors which are thermally limited
and it can operate upto temperatures as high as 210 C, almost doubling the
ampacity of existing ACSR conductors.

 (3
Chatterjee, 2016)3,  (4 Antonio
Gómez Expósito, 2007)4.

 

 Characteristics of  HTLS (High Temperature Low Sag Conductors):

 

G(Z)TACSR (Gap Thermal Alloy Conductor Steel Reinforced),

ZTACIR (Thermal Alloy Conductor Conductor Invar Reinforced)

ACSS (Aluminum Conductor Steel Supported)

 

Here G refers to Gap between steel and aluminum conductor and Z refers
to trapezoidal same of aluminum coundctor. HTLS conductors are similar to
conventional ACSR conductor in terms of electrical conductivity and geometrically.
The main difference is that it offers the low coefficient of thermal expansion
and as results of this HTLS can operate at a higher temperature with an
increased CCC (current carrying capacity) with maintaining same sag that of
traditional ACSR conductors.

 

A)                 
G(Z)TACSR

 

In G (Z) TACSR type conductor is known as Thermal Resistance Aluminum
Alloy conductor Steel Reinforcement as shown in Figure 1, where inner core is
composed of galvanized steel and outer layers are composed of thermal resistant
aluminum conductor. A small gap is maintained between the steel core and the
innermost aluminum layer,and the gap is filled with heat-resistant grease to
reduce friction between the steel core and the aluminum layer and to prevent ingress
of waterand hence its improves corrosion resistance. (5 G.Filippone, 2014)

 

Figure1. Crossectional
view of G(Z)TACSR Conductor

 

 

 

B. ZTACIR

Super thermal alloy (STAL) is made from Al-Zr
(Aluminum Zirconium) alloy. The conductor comprises of an inner core of Aluminum
clad Invar (36%Ni in steel) and outer layer are made of STAL wires..

 

Here the Figure 2 shows the cross sectional
view of (Z) TACIR conductor. (5 G.Filippone, 2014)

Figure2. Cross
sectional view of ZTACIR Conductor

 

Instead of using conventional steel in conventional ACSR conductor, in
ZTACIR conductor, INVAR is used which is made of an alloy of steel and 36%
nickel and as a results the coefficient of expansion practically becomelinear
and it is invariable with application of heatand that’s why the name was givenas
INVAR.

 

Super thermal alloy contains Zr which deposits over the grain boundary of
Aluminium, thus increasing the recrystalisation temperature of Aluminium which
enables STAL tooperate at high temperature without any loss in strength.

 

C.                 
ACSS

ACSS is known as Aluminum Conductor Steel supported as shown in Figure.3.
(5 G.Filippone, 2014)

 

In ACSS the core is made of round steel and aluminum strands are made of
trapezoidal shape.The steel wires may either galvanized wires or aluminum clad
(aluminum coating). In ACSS conductors the aluminum wires can be the standardround
strand or it may be trapezoidal aluminum strand.

 

Figure.3. Cross
section of ACSS Trapezoidal Conductor

 

IN HTLS
conductor the main modification is done on aluminum strands which are
completely annealed wires and steel core which is made of INVAR strand and
conductivity of core is enhanced by 14%, where in ACSR conductor the
conductivity of core is almost zero. During stringing when tension is applied onthe
HTLS conductors, the permanent elongation takes place quickly in aluminum wires,
since the core is made of INVAR strands, where the coefficient of linear
expansion is invariable with temperature and as a results the sag of the
conductor will be greatly reduced. In operating conditions, the coefficient of expansion
of conductors is close to the value provided by the steel core, in the order of
(10to 13×10-6°C),which is quite low as compared to conventional ACSR
conductors i.e. order of (18 to 22×10-6°C) and results of this reduction
in overall sag and therefore an increase in the ground clearance.

Table1 Conductor Parameters

 

Description

Different Conductor

ACSR

G(Z)TACSR

ZTACIR

ACSS

Area (mm2)

307.7

308.4

306.9

307.7

Rated Ultimate Tensile Strength ( kgf)

9945

10960

10065

9900

DC Resistance at 20 °C(ohm/km)

0.108

0.110

0.1106

0.107

Weight (gm/km)

1067

1097

1082

1067

Coefficient of linear expansion (10-6/°C)

18.8

11.4

16.3

11.5

 

Here Table 1
shows the parameters of different type of HTLS conductor having approximately
same cross sectional area. (5
G.Filippone, 2014)

 

The conventional ACSR and AAAC are designed to operate
continuously at temperature of 85°C and 95 °C respectively. High Temperature Low Sag (HTLS)
conductors are designed tooperate continuously at temperature of at least 180 °C. Some HTLS conductors can beoperated as high as 240 °C. The new material used in HTLS conductor differs from
conventionalsteel reinforced ACSR.

 

The new material includes INVAR steel (Fe-Ni alloy),
temperatureresistant Aluminum-Zirconium (Al-Zr) alloys, annealed aluminum, high
strength steel and both metal & polymer composites. A conductor in general
is a simple combination of core and aluminumand aluminum alloy. HTLS conductor
is stranded with combination of

aluminum alloy wires for better conductivity and
reinforced by steel core.

 

 

 

 

 

 

Figure 4 Different scheme of uprating of Transmission
line

 

 

Here figure 4 indicates the two different way of upraitng of transmission
line,firstly the uprating can be done by constructing a new transmisison line with
traditional ACSR conductor,by extra HV lines or with bundling of transmission
line or making bigger size conductor diameters.Secondan way to change the
conductor with advanced material by increaing therir thermal raing. (Recommendation, 6)

 

Different type of HTLS conductors are
ZTACIR (with INVAR steel core), GZTACSR (with specified gap between steel core
and inner layer of aluminum wires), ZTACSR (with steelcore), ACSS (with steel
core)

The TACSR, GZTACSR, ACSS and ACCR are
available with both round wire and trapezoidal Al-Zr alloy wires in the
outermost layer. ACCC uses only trapezoidal annealed aluminum wires. GZTACSR,
commonly known as Gap type conductor, the Gap is filled with heat resistant
grease (filler material) to prevent water ingress and improves the corrosive resistance,
such type of conductors are mainly required in coastal areas.

 

Advantages of HTLS
over Conventional ACSR conductor is as shown in Table.2

                                                                                Table 2

Build a new
Line

Replacement of  old conductor with HTLS conductor

Build period

Transmission line approx.30 km

18 month

6 month

Construction
cost

Preliminary work

Required

Not req.

Cost of right of way

High cost

No cost

Tower foundation

Required

Not req.

Conductor cost

required

2 to 3 times the ACSR

Stringing cost

Required

Required

 

Here Table 2  shows the comparative analysis of construction
of new transmission line and replacement of old conductor with new HTLS
conductor. (upgrading., 7)

 

 

Materials
used

STAL wire containing Zr(Zirconium)element which has
highly improved annealing property, without loss of tensile strength..HTLS using Al clad
invar has low thermal coefficient of expansion (approx 1/3 rd) of steel at Temp
210°C.

                                   

 

Figure5. Annealing characteristic of STAL wires                           Figure 6.Coefficient
of thermal expansion

 

 

The ordinary hard drawn aluminum wires used in
conventional ACSR, start losing tensile strength at 90°C and therefore it
is not suitable for long term use at temperature above this. Al-Zr aluminum
alloys wires have the same conductivity and same tensile strength as ordinary EC
Grade aluminum wire but it can operate at higher temperature range upto 150 to 200°C.

 

In India since lastfew years, the need for use of
HTLSconductors in some corridors has been felt. The power flow in those
corridors has increasedand congestion has been reduced by using such
conductors. Such conductor would be requiredwhere the power transfer over the
line is constrained due to consideration of thermal loading.

InIntra-state transmission system, requirement of such
conductor is expected at 220kV, 132kV and66kV level. The requirement of such
conductor may not be much in ISTS, which is dominatedby 400kV and 765kV
network. In case of ISTS lines, the HT/ HTLS conductor would be a
goodsubstitute to Quad bundle ACSR and AAAC conductor, particularly at 400kV
level when linelength is short.Therefore the HTLS conductor can
be considered for reconductoring of existing lines and can also beused in new
lines. The cost of such conductor is about 2 to 3 times the cost of
conventional ACSR conductor. (8) Draft
guidelines for HTLS Conductors CEA, 20168)

 

 

 

 

 

Methodology:

Power Line System –Computer Aided Design and Drafting (PLS-CADD)is the most powerful and comprehensive program/tools for the
structural and geometric design of overhead lines. It covers all environment
aspects of transmission line design, including terrain modelling, route
selection, manual or automatic minimum cost spotting, sag-tension, clearance
and strength checks, plan & profile drafting and much more.

For new transmission
lines projects PLS-CADD will significantly increase capabilities and
productivity of line.

reconductoring of existing lines and can also beused
in new lines. The cost of such conductor is about 2 to 3 times the cost of
conventionalACSR conductor.

By placing six nos of 400 KV tower structure in plain terrain
placed approx 400meter apart in by using PLS CADD Tools as shown in Fig.7

 

Fig.7
Model of 400KV Transmission line using PLS Cadd

 

 

 

 

Case Studies:

Case 1 :
Comparison when maintaining same Current and their operating temperature:

                                                            Table 3

Description 

MOOSE (ACSR)

ACCC-Moose (HTLS)

Calculations are carried out at
temp  degree

85

76.70

Current to be maintained:

902

902

AC Resistance (ohms/km)

0.0687

0.0532

Line losses in kW/ckt

168

130

Power Factor

0.85

0.85

Power Transferred in MW/ckt

531

531

Price Loss (in Lacs Rs/KW)

256

198

 

 

Fig. 8 Comparison
chart of ACSR Moose and ACCC Moose (HTLS) conductor when operated at same
current rating

 

 

 

 

 

 

 

Conclusion of Case 1: The maximum operating
temperature of ACSR Moose conductor is 850C and maximum current
carrying capacity is 902 Amps in specified working condition, therefore the
comparison is done at 902 Amps between ACCC-Moose (a type of HTLS
conductor)conductor and  ACSR conductor
and all the calculation is done based this ampere rating.

ACSR
Moose conductor reaches 902 Amps at 850 C (operated at maximum
operating temperature level) and while ACCC-M achieved this current rating at
reduced temperature level i.e. 76.70 C (well below the maximum
operating temperature level i.e. 1800C ).

 

For
ACSR conductor the ac resistance is 0.687 ohm/kms whereas for ACCC-M conductor
the ac resistance is only 0.0532 ohm/kms which is quite lower as compared to
ACSR Moose conductor and as a results of this the line losses will be lower
side i.e. 130 kw/ckt  which is approximately  % lower than the ACSR Moose conductor.

 

For
ACCC-Moose the price losses will be only 198 (Lacs/kw/ckt) as compared to 256
(Lacs/kw/ckt) of  ACSR Moose conductor.

 

Case-2 : Comparison when
maintaining maximum Current in Amp at maximum continuous operating temperature:

Table 4

 Description 

MOOSE (ACSR)

ACCC-M (HTLS)

Calculations are carried out at
temp degree

85

180.00

Current to be maintained:

902

1960

AC Resistance (ohms/km)

0.0687

0.0706

Line losses in kW/ckt

168

814

Power Factor

0.85

0.85

Power Transferred in MW/ckt

531

1154

Price Loss (in Lacs Rs/KW)

256

1242

 

Fig.
9 Comparison chart of ACSR Moose and ACCC Mumbai (HTLS) conductor when operated
at maximum current and maximum rating

Conclusion of Case 2: In this case both the
conductors are operated at their maximum operating temperature and maximum
current carrying capacity. The maximum operating temperature of ACSR Moose
conductor is 850C and maximum current carrying capacity is 902 Amps
in specified working condition, whereas the maximum operating temperature of
ACCC-M conductor is 1800C and maximum current carrying capacity is
1860 Amps and all the comparisons were done based their maximum operating
levels.

•     ACSR Moose conductor is
limited to operate upto 850 C maximum while ACCC-M can be operated
upto much higher temperature level i.e. 1800 C .

•     The power transfer
capability of ACCC-M is 1154 (MW/Circuit) almost doubled the power transfer
capability of ACSR conductor which is 531(MW/), it means by for transferring
same amount of  power by using ACSR
conductor ,we have to construct the another transmission line, that will become
another time consuming and costly project.

 

Fig. 10 Current V/s Temperature curve for ACSR and HTLS
conductor

 

Here
Fig. 10 represents the Current V/s Temperature curve, curve shows that
ACCC-Moose (HTLS ) can be easily operated upto 200 0C, but the
maximum operating temperature of ACSR Moose conductor is 85 0C only
(Thermal limit).

 

v  Note: For comparison purpose only, upto 200 0C the
report were calculated for  the ACSR
conductor, otherwise the ACSR conductor can’t be operated above 85 0C
.

 

 

Fig. 11 Resistance V/s Temperature curve for ACSR and
HTLS conductor

 

Here
Fig.11 represents  the Resistance V/s
Temperature curve, curve shows that ACCC-Moose (HTLS Conductor) can be
easily  operated upto 200 0C  with minimum resistance but the maximum
operating temperature of ACSR Moose conductor is 85 0C only (Thermal
limit) and because of  less resistance as
compared to ACSR conductor, HTLS conductor offers less (I2R loss).

 

 

Fig. 12 Temperature V/s Power Transfer Capability curve

 

Here
Fig. 12represents the Temperature V/s Power Transfer Capability curve, which
shows that ACCC-Moose (HTLS Conductor) can be easily operated up to 200 0C
with better power transfer capability as compared to ACSR Moose conductor.

 

Fig. 13 Temperature V/s Power Loss curve

 

Here
Fig. 13 represents the Temperature V/s Power Loss curve, curve shows that
ACCC-Moose (HTLS Conductor) can be easily operated upto 200 0C with
better power transfer capability as compared to traditional ACSR Moose
conductor.

 

 

 

 

 

 

 

Conclusions

In present scenario the major
difficulties of construction of new transmission line is to get right of way
(ROW) approval from public and local administrations. For this squeezing more
power into existing corridors in becoming quite crucial and for which the HTLS
constitutes a attractive and cheaper solution. These conductors being capable
of working at over 200 °C,with
double the ampacity as compared to conventional ACSR conductor with maintaining
approximately same sag or ground 
clearance.

In growing congestion in existing corridor of
transmission and distribution network, the enhancement of power flow per unit
(or meter) of Right of Way and reduction in losses under normal as well as under
emergency condition is highly recommended. High Temperature Low Sag (HTLS)
conductors should be considered in those corridors where the power transfer
over the line is constrained due to consideration of thermal loading of
conductor. In Intra-state transmission system, requirement of such conductor is
expected at 220kV, 132kV and 66kV level. In case of ISTS
(Inter State Transmission System) the HTLS conductor would be a excellent substitute
to Quad bundle ACSR and dual HTLS conductor, particularly at 400kV level when
line lengths were short.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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