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Increasing of 110 kV Power Transmission Line Capacity by Using GTACSR/GZTACSR Conductors

Borut Zemljarič

IBE Consulting Engineers Hajdrihova 4, Ljubljana

E-mail: borut.zemljaric@ibe.si,

Franc Jakl University of Maribor,

Faculty of Electrical Engineering, Computer Sciences and Information Technology Smetanova 17, 2000 Maribor E-mail: franc.jakl@uni-mb.si

Abstract – As a consequence of increasing power consumption, new power generator facilities connected to the transmission grid, increasing standards for reliability and quality of electric power delivery, owners of transmission power lines are forced to increase power capacity of particular overhead transmission lines in the grid. That can be made by using “gap type” conductors. Because of their unique construction and mechanical material properties, significant increase of power capacity can be reached without tower line modification.

Povečanje prenosne moči 110 kV daljnovoda z uporabo GTACSR/GZTACSR vodnikov

Povzetek – Povečevanje porabe električne energije, vključevanje novih elektroenergetskih objektov v prenosno daljnovodno omrežje, zviševanje norm zanesljivosti dobave in kakovosti električne energije zahtevajo od upravljavcev prenosnega in distribucijskega omrežja povečevanje zmogljivosti obstoječega daljnovodnega omrežja oziroma njegovih delov. Prenosno moč daljnovoda je mogoče povečati z zamenjavo obstoječih vodnikov z novimi vodniki “z režo”, ki s svojimi konstrukcijsko mehanskimi lastnostmi omogočajo visoke prenosne kapacitete pri visokih temperaturah ob zelo ugodnih povesnih karakteristikah, ki ne zahtevajo poseganja v nosilne elemente daljnovoda – stebre.

I. INTRODUCTION

High voltage overhead transmission lines provide a reliable and economic manner to transmit large amounts of electric power. As demands for electricity grows, there is a need for additional transmission lines or uprating existing ones to maintain or improve reliability of power delivery.

Today, as the construction of new lines, rebuilding and upgrading of existing electric transmission lines become increasingly difficult, new approaches are being devoted to increase the current capacity by using new type of conductors.

The paper presents a possible approach to increase the power capacity of existing 110 kV transmission lines with “Gap” conductors. The conductors were developed in Japan in late 1960s. Gap type conductor is a high temperature, low sag conductor with a nominal power capacity which is up to 2 times higher

than the capacity of the standard ACRS conductor. Gap conductors allow to increase the power capacity of the line without tower strengthening. This is a very good solution in cases where reliability N-1, N-2 must be met.

The first part of the paper deals with the construction of the gap type conductor. It is followed by the presen-tation of increasing 110 kV one circuit transmission line capacity equipped with standard ACSR 240/40 by replacing them with GTACSR/GZTACSR 240. Calculation of conductor’s ampacity, sags and power losses are presented. Increased currents have an influence on the electromagnetic field under the transmission line, for which the basic calculations are presented.

The intent is not to provide a basis for setting policy but to provide insight into an uprating method in order to facilitate thinking about a good alternative of increasing power transmission line capacity.

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II. GAP TYPE CONDUCTORS

The conductors known as “Gap type” consist of an extra high strength steel core surrounded by stranded layers of aluminum alloy. The inner alloy is made from trapezoidal wires to form a tube. The inner diameter of this layer is larger then outer steel core diameter. Thus a gap is formed, which is filled with high temperature silicone grease. Figure 1 shows gap type GTACSR conductor.

Conductors GTACSR use heat resistant aluminum alloy (TAl) and can withstand up to 150°C and have the same mechanical properties as hard drawn aluminum. Conductors GZTACSR use super heat-resistant aluminum alloy (ZTAl) and they withstand up to 210°C and have the same mechanical properties as hard drawn aluminum. TAl and ZTAl have the same mechanical and electrical properties except for the allowable temperature.

Fig 1. Conductor’s cross-section type GTACSR

Above sagging temperature all tension is born by the

steel core. Because the thermal coefficient of steel is one half of that for aluminum, the thermal elongation characteristic of GTACSR/GZTACSR is smaller than that of conventional ACSR, better temperature – sag increase characteristic is achieved.

III. GAP CONDUCTOR CURRENT

AMPACITY AND SAG

Gap conductors GTACSR/GZTACSR are designed to withstand high temperatures at higher conductor ampacity than the standard ACSR conductor. To determine the highest temperature of conductor, it is important to determine the required power capacity. When transmission line uprating is done without tower modification then usually cross-section and diameter of existing conductor is not exceeded.

In the paper we deal with the standard ACSR 240/40 conductor and gap conductors GTACSR 240 and GZTACSR 240. Their basic mechanical properties are shown in Table 1. “Black” conductor properties are also included in the table. Black conductor BTACSR/ACS 240/40 is a high temperature (up to 150°C) conductor with basic of ACS core.

Current carrying ampacity curves are presented in Figure 2. Curve calculations are made for 35°C ambient temperature, wind velocity 0.6 m and emission coefficient 0.9. At standard temperature range up to 80°C conductor’s ampacities are almost equal.

TABLE 1 CUNDUCTORS PARAMETERS

ACSR 240/40

GTACSR 240

GZTACSR 240

BTACSR/ACS 240/40

Diameter (mm)

21.8 20.6 20.6 21.8

Cross-section (mm2)

282.5 279.6 279.6 282.5

Weight (kg/m)

0.985 0.951 0.951 0.925

Temperature (°C)

80 150 210 150

0

200

400

600

800

1000

1200

1400

50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210

Conductor temperature (°C)

Cur

rent

car

ring

capa

city

(A)

ACSR 240/40 GTACSR 240 GZTACSR 240

Fig 2. Conductor’s current carrying capacity With raising temperature up to 150°C for GTACSR the current ampacity is 998 A and temperature up to 210°C for GZTACSR ampacity reach 1238 A. Maximal conductor ampacity reach the ratio of 1.7 to 2.1 according to standard ACSR value at 80°C. For comparison the ratio of BTACSR/ACS conductor is practically equal to GTACSR.

TABLE II MAXIMAL CUNDUCTOR AMPACITY

Conductor Temperature (°C)

Ampacity (A)

Ratio

ACSR 240/40 80 586 100 BTACSR/ACS 240/40 150 1095 187

GTACSR 240 150 998 170 GZTACSR 240 210 1238 211

Power capacity based on previously mentioned

conductor ampacity shows figure 3. At maximum conductor temperature (210°C) single circuit power line can transmit approximately 240 MVA. Higher temperature means higher value of conductor sag, so in case of existing power line the balance between maximum operating temperature and conductor sag need to be found.

Operating at high temperature increases conductor resistance and through this power losses increase. As figure 4 shows the ratio between losses when operating up to 80°C and operating up to 210°C can reach 6. These conductors can be used effectively to assure enough power capacity in the operational security cases N-1 or N-2.

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0

50

100

150

200

250

50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210

Conductor temperature (°C)

Pow

er c

apac

ity (M

VA)

ACSR 240/40 GTACSR 240 GZTACSR 240

Fig 3. 110 kV single circuit power line capacity

7107207307407507607707807907

1007

50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200 210

Conductor temperature (°C)

Pow

er lo

sses

(kW

/km

)

ACSR 240/40 GTACSR 240 GZTACSR 240

Fig 4. Conductor power losses

The temperature sag properties were calculated for conductors and conditions given below:

• Span length 300 m, • Conductors ACSR 240/40m, BTACSR/ACS

240/40 and GTACSR/GZTACSR 240, • Taken boundary conductors tension at -5°C

and 8.4 N/m additional load is 24 kN • Taken turning temperature for

GTACSR/GZTACSR is 25°C and tension 12.8 kN and

• No cooling effect of natural convection was taken in sag calculations.

Figure 5 shows calculated temperature sag properties.

7,58

8,59

9,510

10,511

11,512

12,5

0 20 40 60 80 100 120 140 160 180 200

Conductor temperature (°C)

Con

duct

or s

ag (m

)

ACSR 240/40 BTACSR/ACS 240/40 GTACSR/GZTACSR 240

Fig 5. Conductor’s temperature sag curve

If the temperature of the ACSR conductor is 40°C, the temperature of the GTACSR/GZTACSR conductor can be 80°C with clearances still within original design value. A re-assessment of clearances under specific line usually shows that condition temperature can be uprated. Some sections can be subject to minor retensioning. If a temperature of 80°C for the ACSR conductor is allowed, the gap conductor equivalent temperature is 160°C. Another advantage of gap conductors can be achieved by taking into account the cooling effect and analysis of internal tensioning reserves in existing steel towers. Tension difference analysis shows that every 1kN difference in tension means around 0.5 m difference in sag.

III. ELECTRIC AND MAGNETIC FIELDS

Increasing current ampacity has influence on the magnetic field above the ground level while electric field stays unchanged. As we mentioned before the goal of uprating transmission line with gap conductors is increa-sing the power capacity without tower modification. This means that clearances to the obstacles and ground stay practically unchanged.

In this chapter we present the study case of uprating a 110 kV single circuit transmission line with tower dimension shown in figure 6.

Fig 6. Single circuit 110 kV line tower head

Calculations of electric and magnetic field are made under the transmission line 1 m above ground and 9 m mean height conductors above ground. Precaution values according to Slovenian Law [5, 6] in case that the existing line is for low- frequency 50 Hz electric field 1.8 kV/m and for low-frequency 50 Hz magnetic field 15 µT . Figure 7 presents the electric field.

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-20 -15 -10 -5 0 5 10 15 20D istance f rom tow er ax is [m]

Elec

tric

field

[kV

/m]

.4

.6

.8

1

1.2

1.4

1.6 110 kV

Fig 7. Electric field

The increased magnetic field is shown in figure 8. At

power capacity 50 MVA magnetic field in line axis is 5 µT, at 100 MVA is 10.5 µT, at 150 MVA is 16 µT and at 200 MVA it reaches approximately 21 µT.

-20 -15 -10 -5 0 5 10 15 20D istance f rom tow er ax is [m]

Mag

netic

fie

ld [

mic

roT]

4

6

8

10

12

14

16

18

20

22262 A (50 MVA)525 A (100 MVA)788 A (150 MVA)1050 A (200 MVA)

Fig 8. Magnetic field

Comparing calculated values with required yields to

conclusion that uprating is possible.

IV. CONCLUSION

We present an electrical conductor that allows the current capacity of an overhed wire conductor to be doubled over that of convential ACSR conductor. That can be done by restringing the conductors on existing towers.

This conductors can be used effectively to assure enough increase power capacity in the cases N-1, N-2 operational security in areas where rebuilding the steel towers is connected with right of way problems.

Gap conductors GTACSR/GZTACSR are designed to withstand high temperatures at higher conductor ampacity than the standard ACSR conductor. Because of unique construction, sags at high temperatures are much lower than in standard ACSR conductors.

Increased current ampacity has influence on the magnetic field above the ground level while electric field stays unchanged. Comparing calculated values with required yields to conclusion that uprating is possible.

REFERENCES [1] A.J. Peterson, S. Hoffmann, “Transmission Line

Conductor Design Comes of Age”, Transmission and Distribution World, pp. 62-68, June 2003

[2] M.J. Tunstal, S.P. Hoffmann, N.S. Derbyshire,

M.J. Pyke, “Maximising the Ratings of National Grid’s Existing Transmission Lines Using High Temperature, Low Sag Conductors”, CIGRE, Paper 22-202, Paris, 2000

[3] F. Jakl, K. Bakič, “A New Generation of

Overhead Line Conductors”, Paper No. 2-4 on “Vidmar's Day”, Ljubljana, 1998

[4] “Conductors for uprating of overhead lines”,

ELECTRA, No 213, April 2004 [5] Ordinance on Electromagnetic Radiation in the

Natural and Living Environment, Official Gazette of the Republic of Slovenia, No. 70-3819/96

[6] Regulatory on the First Measurements and

Operational Monitoring for the Non-Ionizing Radiation Sources and Conditions for its Performance, Official Gazette of the Republic of Slovenia, No. 70-3819/96

[7] “Low Sag Up-rating Conductor for Up-rating

ACSR 240/40”, JTD 80-0142, J-Power Systems Corporation, August 2002