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DEVELOPMENT OF “PRE-STRETCH” TYPE UP-RATING CONDUCTOR TO REALISE COST REDUCTIONS
H. ISHIHARA*, H. OKADA, A. SHINODA K. NAGANO, H. KUBOKAWA, S. TERADA
Chubu Electric Power Co., Inc. J-Power Systems Corp.
(Japan)
Summary The raising of environmental consciousness and expanded urbanization make it increasingly difficult to obtain
right-of-way for new routes and rebuild existing steel towers. For these reasons, gap type thermo-resistant aluminum alloy conductor steel reinforced (GTACSR), and extra-thermo-resistant aluminum alloy conductor invar reinforced (XTACIR), which do not increase sag much even if they carry a high current, have been used as a way to increase the capacity of transmission lines without rebuilding steel towers in Japan up to the present. However, GTACSR has a disadvantage that it requires much time and labor for sagging because of its complex structure, and XTACIR has a disadvantage of high material cost because it uses an invar alloy core that performs little thermal expansion. Therefore, the authors have developed, for the first time in the world, a pre-stretch type up-rating conductor. 1. INTRODUCTION
Up-rating and up-grading overhead lines has been an important point for new technology development of transmission line design. Therefore, up-rating conductors has been actively discussed in WG12 of CIGRE SCB2.
The background of rising environmental consciousness and expanded urbanization make it increasingly difficult to obtain rights-of-way for new routes and rebuild existing steel towers. For these reasons, GTACSR, XTACIR, and loose type thermo-resistant aluminum alloy conductor steel reinforced (LTACSR), which do not increase sag so much even if they carry a high current, have been used as a way to increase the capacity of transmission lines without rebuilding steel towers in Japan up to the present.
However, GTACSR has a disadvantage that it requires much time and labor for sagging because of its complex structure, and XTACIR has a disadvantage of high material cost because it uses an invar alloy core that has little thermal expansion. LTACSR has a disadvantage of difficulty in manufacturing and quality control, and declining fatigue strength due to nicking of the aluminum layer after the stranding process.
Therefore, the authors have developed, for the first time in the world, a pre-stretch type up-rating conductor (PS conductor [1]).
This paper describes a performance evaluation of the PS conductor developed by a newly designed manufacturing method, an examination of the accessories and the construction method, and applications for this conductor on actual transmission lines. Potential problems with this newly developed conductor were discussed, that is, the stress shared by the aluminum cladding of the steel core and the amount of displacement between the aluminum and the steel core in stringing the conductor.
21, rue d'Artois, F-75008 Parishttp://www.cigre.org © CIGRÉ
Session 2004B2-315
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2. CHARACTERISTICS OF THE PS CONDUCTOR
The structure and specification examples of the PS conductor, thermo-resistant aluminum alloy conductor steel reinforced (TACSR), aluminum conductor steel reinforced (ACSR) and conventional low-sag up-rating conductors are shown in Table 1. With the gap type conductor, a segment structure is used for the inner layer of the aluminum layer to provide a gap with the steel core, by which the tensile force is born by the steel core alone. By adopting this structure, conductor sag can be reduced. However, special tools and construction methods are required for sagging the conductor to the steel tower.
The steel core of the invar core type conductor uses invar material with a small linear expansion coefficient to reduce the sag, but since the invar material is expensive, the cost of the conductor is high.
The most distinctive characteristic of the PS conductor is that both low sag and low cost are achieved by improving the manufacturing methods (stranding process) of conventional up-rating conductor (TACSR) without special structures and materials. More specifically, in this new manufacturing method, an aluminum layer is stranded with a stretched steel core and then tension is released with the ends of the stranded conductor fixed, making the aluminum layer loose. This technique allows the steel core alone to share the tension, keeping the conventional conductor structure intact. Therefore, the same accessories and installation methods as those of standard conductors (ACSR) can be applied to the PS conductor. It is possible to manufacture conductors that suit the user's specifications because the sag characteristics of the PS conductor are determined by the manufacturing temperature and tension in stretching the steel core. Furthermore, PS conductors do not decrease their fatigue strength as happens with LTACSR.
Though the sag of the PS conductor is a little smaller than that of standard conductors of the same size, it can increase transmission capacity by up to 1.6 times. If the aluminum layer that carries current is made of super-thermo-resistant aluminum alloy conductor, the capacity can be increased by up to about 1.9 times.
Table 1 Structures and Specifications of Conductors (Conductor Size Equivalent to 240 mm2� Conventional low-sag up-rating conductors
Type of conductor PS conductor (PSTACSR/AC)
Thermo-resistant aluminum alloy
conductor steel reinforced
� TACSR�
Aluminum conductor
steel reinforced (ACSR)
Gap type thermo-resistant aluminum alloy
conductor steel reinforced � GTACSR�
Extra-thermo- resistant aluminum
alloy conductor invar reinforced
� XTACIR�
Structure
Outside diameter (mm) 22.4 22.4 22.4 23.1 21.2
Weight (kg/m) 1.073 1.073 1.110 1.187 1.184
Sag*1 (m) 8.3 10.5 8.9 9.1 8.9
Difference in sag compared with ACSR (m) -0.6 +1.6 � +0.2 0.0
Transmission capacity*2
� MW� 121 121 77 124 155
Capacity comparison with ACSR 1.6 1.6 1.0 1.6 2.0
*1 The conductors were assessed at the following conductor temperatures on the assumption that the maximum tension in use is 35.5 kN and the span between steel towers is 300 meters: ACSR (90°C),PSTACSR/AC, TACSR and GTACSR (150°C), and XTACIR (230°C) *2 The transmission voltage was calculated at 77 kV
Gap & Anticorrosive
Thermo-resistant aluminum alloy
Steel core (Pre-stretched)
Thermo-resistant aluminum alloy
Steel core
Aluminum
Steel core
Thermo-resistant aluminum alloy
Steel core
Extra-thermo-resistant aluminum alloy
Steel core(invar alloy wire)
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3. PERFORMANCE EVALUATION OF THE CONDUCTOR 3.1. Load – elongation characteristics of conductor The PS conductor has some looseness in the aluminum layer. If a tensile force is applied to the conductor at constant temperature, the load is initially born by the steel core alone, and when it exceeds a certain load level, the load begins to be shared by the aluminum layer and the steel core. This load level is called the knee point, and the larger it becomes, the more sag is reduced. This conductor is basically designed to up-rate existing ACSR transmission lines, by setting the knee point load at 25.5 kN so that the sag will be equal to or less than that of ACSR (wire size: 240 mm2). To confirm the knee point load, we measured the load-elongation characteristics of this conductor with a 100 m span. An example of the measurement is shown in Fig. 1. As shown in Fig. 1, the knee point confirmed by the load at which the aluminum layer begins to share is roughly 25.6 kN, and this agrees with the design value. We calculated the elastic coefficient from Fig. 1. The elastic coefficient is the same as that of the steel core (aluminum clad steel wire with 14% conductivity) up to a load level of 25.6 kN, and above this level, it changes into the composite elastic coefficient of the steel core and aluminum layer. From these results, we confirmed that the load is shared in accordance with the design.
Fig. 1 Stress-strain properties of PS conductors Fig. 2 Thermal expansion properties of PS conductor 3.2. Thermal elongation properties
For the PS conductor, when the temperature is changed from low to high with a constant tensile force, the aluminum layer and the steel core initially share the load. When a certain temperature is exceeded, the load is born only by the steel core. This temperature is called the knee point temperature, which varies with the conductor installation conditions. For example, the knee point temperature is -34.2°C when the span is 500 m and the maximum working tensile force is 35.5 kN. We measured the thermal expansion properties by varying the conductor temperature from -30°C to 180°C (maximum working temperature of the conductor) by installing this conductor on a 10 m span using the calculated tension of 17.4 kN at 20°C. The results are shown in Fig. 2. No knee point was observed, as shown here, within the measured temperature range. However, calculating the thermal expansion coefficient from the slope of this graph, it corresponds with a steel core (14 AC wire) design value of 12.0 × 10-6/°C. From this result, we confirmed that load sharing is performed in accordance with our design even for the thermal expansion properties.
3.3. Sag properties
We conducted a sag test using a test line (span 120 m, height difference 0 m), and verified the sag property of the PS conductor. This test was conducted by heating the conductor with an electric current. The test results are shown in Fig. 3. We confirmed that the sag property of the PS conductor corresponds with the calculated values, and that the conductor has the specified performance. In Fig. 3, the sag property (calculated value) of the standard conductor (ACSR 240 mm2) is also indicated. The sag of the PS conductor is 0.7 m smaller than that of the standard conductor at a continuous allowable temperature (150°C), and this indicates that the sag can be reduced using a PS conductor.
Under a strong wind condition, the steel core is stretched by the wind load, and the steel core and aluminum Fig. 3 Sag properties of PS conductors
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layer become the same length. Because of this, the aluminum layer and the steel core also share the load when the wind is strong. In this case, the sag of this conductor under a lateral swinging condition is roughly equal to that of the thermal-resistant aluminum alloy conductor aluminum-clad steel reinforced (TACSR/AC) which is used conventionally used as an up-rating conductor.
3.4. Vibration fatigue characteristic
For the PS conductor, tension is always born by the steel core, and it is therefore necessary to examine the vibration fatigue characteristics of the aluminum cladding of the steel core (14 AC wire) in stead of the aluminum layer around the steel core. The stress born by the aluminum cladding was calculated as high as 190 MPa at regular tension (approximately 20% of the minimum tensile strength of the conductor). The allowable stress of aluminum for vibration fatigue is generally 6.2 MPa (0.01% as an allowable strain). The vibration fatigue stress for the shared stress of 190 MPa is calculated by equation (1) as 0.05 MPa, and the safety factor for the allowable stress of aluminum is calculated as 0.008, which is nearly zero, and this cannot secure the safety ratio of 3 to 4 as required in the design.
[Calculation of fatigue stress] σ′ = σ0 × (� - σb/σa� (1)
σ′: Fatigue stress with tensile load (MPa) σ0: Fatigue stress without tensile load (Aluminum clad portion of steel core: 38.25 MPa) σb: Shared stress of steel core aluminum cladding (190 MPa) σa: Tensile strength of steel core aluminum cladding (actual measurement: 190.25 MPa)
On the other hand, it was reported that the stress of aluminum cladding of the steel core relaxes because of creeping effect of aluminum cladding, if tensile force was applied to the steel core[2]. However, no actual measurement of such stress relaxation was reported.
We therefore carried out stress measurement by X-ray diffraction and creep measurement of the steel core (both aluminum cladding and the steel), thereby verified the stress relaxation characteristics of aluminum cladding of the steel core.
3.4.1. Stress relaxation mechanism of steel core clad part
A schematic diagram of stress relaxation in the aluminum cladding of the steel core is shown in Fig. 4. When tension T is applied to composite wire such as steel core with aluminum cladding, elastic elongation occurs at once, and then creep is generated in the aluminum cladding and steel corresponding to their share of the stress. The aluminum has a smaller elastic coefficient than steel, therefore the aluminum cladding would have larger elongation than steel. However, since the aluminum cladding is in tight contact with the steel, it receives a compressive force from the steel with a larger elastic coefficient, and this results in relaxation of stress in the aluminum cladding.
By taking the creep speed of the aluminum cladding and steel of the steel core as Ψa(t) and Ψs(t) , the elastic coefficient of the aluminum cladding and the steel as Ea and Es , the section area of aluminum cladding and the steel as Aa and As, and the shared stress of the aluminum cladding as σa, then σa can be represented by equation (2).
σa(t ) = { } ⎥⎦
⎤⎢⎣
⎡−−
+ ∫t
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Fig. 4 Schematic diagram of stress relaxation
In it ia l sta te
After sta r t ing the creep
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Tension T
Tension T
Compressive force wh ich a luminum cladding receives from steel
Creep elonga t ion of a luminum cladding
Creep elonga t ion of steel
Elast ic elongat ion
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3.4.2. Stress measurement in AC wire aluminum cladding by X-ray diffraction To confirm the stress relaxation phenomenon in aluminum cladding, we measured the shared stress in the aluminum cladding by X-ray diffraction method. This X-ray diffraction method requires that there are a plenty of crystal grains in the X-ray radiated area, and also that the orientation of the crystal grains is random. However, since the aluminum cladding of the AC wire has changed into a processed structure in which the orientation of the crystal plane is not random, we conducted heat treatment under the condition of 300°C × 2 hr in order to remove the influence of the processing. The shared stress in the aluminum cladding when the tensile force is applied to the steel core would be approximately 190 MPa on calculation. The measurement results of the shared stress in the steel core aluminum cladding are shown in Fig. 5. Primarily, the shared stress of 190 MPa should be observed immediately after applying the tensile load. However, since the shared stress in the aluminum cladding is quickly relaxed, the shared stress of approximately 40 MPa is observed as the initial shared stress in Fig. 5. The shared stress in the aluminum cladding was relaxed down to a constant level of approximately 20 MPa in two to three hours. By this result, we confirmed that the shared stress will be quickly relaxed after applying the tensile load.
3.4.3. Stress evaluation of steel core aluminum cladding by creep measurement
By expanding equation (2), we obtain equation (3). Using equation (3), we can calculate the shared stress in the aluminum cladding by measuring the creep ε a and ε s in aluminum cladding and steel, respectively.� Ψ(t)=dε(t)/dt�
σa(t) = { }[ ])t(s)t(aEsAsTEsAsEaAa
Ea εε −−+
(3)
We attached strain gauge to the aluminum cladding and steel of the steel core (processed material) used in X-ray measurement, and measured the creep. The shared stress of the aluminum cladding and steel was set as 190 MPa and 573 MPa, respectively.
(a) Creep of heat treated material (b) Stress value Fig. 6 Analysis results of stress relaxation by strain gauge
The measurement of the creep in aluminum cladding and steel are shown in Fig. 6(a). Fig. 6(b) shows the
shared stress calculation for aluminum cladding by substituting the approximate expression of creep into equation (3). As shown, the shared stress of aluminum cladding lowered to 20 MPa in approximately two hours, and it corresponds with the actual measurement by the X-ray diffraction method. After three hours, the stress curve shows a gradual decrease, and actual stress may remain at a constant level, as indicated by actual measurements in Fig. 6(b). We also show the shared stress in aluminum cladding calculated for non-heat treated material by assuming for the use of an actual transmission line. The time required for stress relaxation for heat treated material was a little shorter than non-heat treated material. This is probably the effect of softening aluminum due to heat treatment.
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3.5. Corrosion test Compared to the standard conductor, rain water and seawater salt grains easily penetrate into the PS conductor,
and thus might accelerate corrosion. We therefore conducted a corrosion test simulating actual conditions, and verified the corrosion characteristics by relative comparison with the standard conductor.
We investigated previous papers regarding the method of corrosion test, and conducted the test under the conditions as listed in Table 2. For the salt water spray condition, we adopted the general test method of the Japanese Industrial Standards (JIS Z 2371).
Also, to simulate the actual transmission line, we applied an electric current through the conductor from the transformer for heating. The test configuration is shown in Fig.7. Further, we covered the conductor with a test gauge, and sprayed salt water on the conductor inside the gauge. In this case, regular conductor tension was applied. We set the conductor temperature to 80°C, at which aluminum wire and aluminum clad of the steel core are likely to corrode.[3] For the salt water spray cycle, we conducted a preliminary test with the PS conductor and a standard conductor and determined the spray condition as spraying for 18 minutes + drying for 42 minutes. This condition aimed at providing maximum chlorine ion deposits to the inside of the wire in a short time. We previously confirmed that the inside of conductor, which was subject to the condition, dries in 30 to 40 minutes, hence it was still wet inside the conductor.
Under the test conditions, we conducted the corrosion test and evaluated the corrosion characteristics through relative comparison with the standard conductor. We selected three types of conductors; PS conductor (PSTACSR/AC 240 mm2), standard conductor (TACSR/AC 240 mm2), and AC stranded wire 55 mm2 (14% conductivity). Evaluation parameters consist of the amount of chlorine ions per unit area deposited on the surface of the aluminum wire and steel wire and the depth of corrosion. The five test cycles were 300, 500, 1000, 1500 and 2500 cycles.
The amount of chlorine ions per unit area deposited to the inner aluminum wire layer and steel wire of each conductor is shown in Fig. 8.
As shown in Fig. 8, the amount of chlorine ions increases for both the PS conductor and the standard conductor as the number of cycles increases, and the amounts deposited are almost similar. No excess intrusion of corrosion agent into the inner layer of the PS conductor due to loosening was observed.
The spacing ∆D between outermost aluminum wires of the PS conductor was calculated as 70µm. There is no difference in the shape with the standard conductor. Therefore, there may be no influence on the intrusion and release cycle of the corrosion agent into the inside of the conductor.
Table 2 Test conditions Condition
Item Spraying Drying
Ambient temperature (°C) 20 ± 2 Salt water concentration wt (%) 5 ± 0.1 -
Salt water pH 6.5 – 7.2 - Spray pressure (MPa) 0.07 – 0.17 -
Walt water temperature (°C) 20 ± 2 - Conductor temperature 80°C
Conductor tension 15% of minimum tensile load
Fig. 8 Chlorine ion adhesion characteristics
0.0
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2.0
0 500 1,000 1,500 2,000 2,500 3,000
No. of cycles (times)
Chl
orin
e io
n ad
hesi
on a
mou
nt (
µg/c
m2
)
PSconductor inner layer aluminumStandard conductor inner layer aluminumPS conductor steel layerStandard conductor steel layer14 AC stranded wire outer layer
Fig. 7 The condition of corrosion test
PS conductor
Standard conductor
Transfer
Test gauge
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4. EXAMINATION OF ACCESSORIES AND INSTALLATION METHOD 4.1. Examination of accessories 4.1.1. Dead-end clamp
We conducted a line grasping test by installing the PS conductor to a single wedge type strain clamp to confirm whether an ordinary clamp (for TACSR/AC) could be used. In the result, the line grasping force was more than 90% of the minimum tensile strength, and this proved that an ordinary one can be used.
Since this conductor has a loose structure, it is necessary to confirm whether the steel core slips off when an excessive load is applied to the conductor. Hence we performed an impulse load test as shown in Fig. 9 and measured the amount of slide of the steel core at the clamp after the load was applied 10 times.
Based on this test, we confirmed that the steel core alone will not slide off although the aluminum wire would become extended due to the nicking effect at the clamped section as the number impulse load applications increased.
4.1.2 Suspension clamp
We conducted the wire grasping test with the free center type suspension clamp designed for standard conductors. The test results showed that, although the grasping force lowers a little due to loosening, this type of clamp can be used provided that the maximum working tension of the existing transmission line is at the standard level. 4.2. Examination of installation method 4.2.1 Stringing test
Since the PS conductor has a loose aluminum layer, there is a concern that slippage of the aluminum layer might occur during the stringing procedure. To confirm whether slippage occurs between the aluminum layer and the steel core, we performed a stringing test using conventional tools and methods as shown in Fig. 10.
We measured slippage by measuring relative displacement between the magnetized point of the steel core and the marked point of the aluminum layer at four places, that is, before and after passing block and before and after passing block .
The test showed no slippage between the aluminum layer and the steel core, and we confirmed that passing over a block will not erase the effect of looseness. We also conducted the stringing test with 240 mm2 size PS conductor under the conditions shown in Table 3. After that, we measured the outside diameter, pitch and nicking ratio. There was no change in the outside diameter and pitch after the conductor passed over the block. After the test, we disassembled the conductor, and examined the nicking ratio. The maximum value was 6.2%, and was lower than the allowable nicking ratio of 10%, and it became clear that the conductor would cause no problems in practical use.
The reason why slippage between aluminum layer and steel core did not occur might be as follows: As shown in Fig. 11, the excess length of the aluminum strand in the roller contact area is absorbed by the strand before the block, and this causes the aluminum layer to rise (Fig. 11 (a) ). After the conductor passes over the block, the aluminum layer returns to its original state, and the stringing process continues (Fig. 11 (b) ). This results in no slippage between the aluminum layer and the steel core.
Table 3 Stringing test conditions of PS conductors Roller diameter Φ 300 mm Horizontal angle 30° Bearing angle 60°
Stringing tension 20% UTS No. of times of passage 10
Fig. 10 Method of stringing test
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(a) Before passing over the block (b) After passing over the block Fig. 11 Behavior of PS conductor passing before and after stringing equipment
4.2.2 Come-along test To check whether conventional stringing tools can be used we conducted a wire seizing test and sagging test
using conventional come-alongs. The results showed that, although the seizing force decreases due to loosening, sagging is possible if one or two ordinary come-alongs are used in series. 5. APPLICATION IN SERVICE LINES
The difference in sag between each type of conventional conductor and the PS conductors is shown Fig. 12. Although the sag of the PS conductor is a little smaller than that of standard conductors of the same size, it can increase the transmission capacity by up to 1.6 times. If the aluminum layer that carries current is made of a super-thermo-resistant aluminum alloy conductor, the capacity can be increased by up to about 1.9 times. Therefore, the total cost, including construction cost, can be significantly reduced, compared with the conventional case where GTACSR and XTACIR are used or steel towers are rebuilt (cost can be reduced by about 84% compared with rebuilding steel towers).
The low-sag property of the PS conductor can bring not only cost reductions in up-rating work but also has the following advantages: (1) Rebuilding the steel towers at the present sites is unnecessary if high-strength PS conductors are adopted to strengthen existing transmission lines (conductor up-sizing), such as at crossing railways and highways; (2) Sag reduction makes it possible to raise the height of the conductor by merely replacing the conductors of the existing transmission lines; and (3) The use of PS conductors for newly constructed transmission lines makes it possible to lower the height of their steel towers.
So far, the PS conductors have been introduced on thirty-four lines of four electric power companies in Japan. We are planning to promote the application of the PS conductors to increase the capacity and to strengthen existing transmission lines less expensively. 6. CONCLUSION
(1) By adopting a unique manufacturing method, we developed the PS conductor by making the aluminum layer loose as specified in the design for the first time in the world. We also confirmed that it is possible to have the load be born only by the AC steel core with a conventional structure.
(2) We measured the thermal expansion properties by varying the conductor temperature from -30°C to 180°C. Consequently, the measured value of the thermal expansion coefficient proved to conform with the steel core design value, and all tension of this conductor is born by only the AC steel core. We confirmed that the sag property of the PS conductor corresponds with the calculated values.
(3) It was confirmed by creep investigation and the vibration test that no cracks and damage occurred due to vibration.
(4) We conducted a corrosion test of the PS conductor and the standard conductor simulating actual conditions. The amount of chlorine ions for both PS conductors and standard conductors was almost similar, and no excess intrusion of corrosion agent into the inner layer of the PS conductor due to loosening was observed.
(5) The same accessories and installation methods as those for standard conductors (ACSR) can be used with the PS conductor.
(6) Using the PS conductor, the total construction cost can be significantly reduced, compared with the conventional up-rating conductors such as GTACSR and XTACIR. This conductor is a very effective
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Aluminum st rand
Steel core
Block
Excess length in st rand in roller con tact ing a rea is absorbed. Aluminum st rand
Steel core
Block
Fig.12 Difference in sag between PS conductor(150�) and other conductors (size: 240 mm2)
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0150 200 250 300 350
Œ aŠ Ô’ ·(m)
��
‘¼“dü‚Æ‚Ì’o“x·(m)
Difference in sag compared with GTACSR (150°C)
Difference in sag compared with TACSR (150°C)
Span (m)
Difference in sag compared with ACSR (90°C)
Difference in sag compared with XTACIR (230°C)
Diff
eren
ce in
sag
com
pare
d w
ith
oth
er c
ondu
ctor
s (m
)
E-mail:[email protected]
9
way to increase the capacity of existing transmission lines. More applications of the PS conductors are expected to increase the capacity and strengthen the existing transmission lines less expensively.
REFERENCES
� 1� Ishihara et al.: Development of New Type Low Sag Conductor Increased in Capacity", T.IEE Japan, vol.122-B, No.12, pp.1458-1463 2002
� 2� Hoshino: Characteristics of Conductor “AS-170” for Long Span Power Transmission, Hitachi Review, May 1962 � 3� Kobayashi et al.: The influence for Al corrosion behavior in aqueous solution under 100 degrees. The discussion of
corrosion and anti-corrosion. The association of corrosion and anti-corrosion , P356� 359,1986
