Electric and Magnetic Fields of Compact ...

IEEE Transactions on Power Delivery, Vol. 14, No. 1, January 1999

200

lectric and Magnetic Fields of Compact Transmission Lines Miguel O.B.C. Melo Luiz C.A.Fonseca, Eduardo Fontana, Non-Member Non-Member C o m m a Hidro Eletrica do S5o Francisco Rua Delmiro Gouveia, 33 3 Recife -PE - 50761-901 Brazil

UniversidadeFederal de Pernambuco Depto de Eletrbnica e Sistemas Rua Acad2mico H6lio Ramos sln Recife-PE 50740-530 Brazil

Abstract: Experimental and theoretical stukes of the electric and magnetic fields produced by compact transmission lines are descnbed. The lateral and longitudinal field profies at ground level w i t h right of way have been analyzed. The studies include measurements of the profiles of field strength of compact transmission lines as well as an analysis relative to the type of tower, size and type of conductor, and voltage level. Finally a comparison between measured and calculated values are presented Keywords: Compact Transmission Lines, Electric Fields, Magnetic Fields

I. INTRODUCTION

CHESF-Companhia Hidro Elktrica do S5o Francisco, responsible for the electric power generation and transmission in the Northeast of Brazil, plans to expand its transinission system in that regon by installing long 230 kV and 500 kV, transmission lines (TL)[l]. In this regard, feasibility studies have been conducted to determine costs and economical constraints involved in the project. One of the conclusions of these studies is that the use of compact transmission lines could provide several benefits, including, longer htervals between upgrades, reduction of the shunt and series reactive compensations, as well as increase of the transmission capability of existing lines.

S. RNaidu, Member, IEEE Universidade Federal da Paraiiba Laboratorio de AltaTensiio AV. Aprigio Veloso 882 C . Grande-PB 58109-970 Brazil

A new type of compact transmission line known as the High Surge Impedance Transmission Line (HSIL)[2], with a higher level of compactness, has been recently put into operation in a few countries. The HSIL is a new concept of transmission line design because it uses a combination of features that include distance reduction among conductors belonging to Merent phases and increase of both the number and relative distances among sub-conductors of a single phase. In addition, the HSIL uses asymmetricalbundles instead of the symmetrical and circular distribution of subconductors, employed in conventional compact lines. This new geometric configuration equalizes and optimizes the electric field distribution around all subconductors. The HSIL optimization process allows obtaining a substantial reduction of the series inductance as well as a sigriiiicant increase of the shunt capacitance, in h m producing a very high intrinsic transmission line capability. Recent development of the HSIL technology has limited its use to a few countries and therefore, further development and implementation of HSIL towers brings new challenges to experts on transmission line studies and design[3]. The purpose of this work is to analyze the electric and magnetic fields produced by compact lines.

11. STUDIED CASES PE-033-PWRD-0-04-998 A paper recommended and approved by the IEEE Transmission and Distribution Committee of the IEEE Power Engineering Society for publication in the IEEE Transactions on Power Delivery Manuscript submitted September 15, 1997; made available for printing April 24, 1998

CHESF engaged in research and development work to evaluate the performance of HSIL towers for power transmission at 230kV As the studies showed a cost reduction of approximately US$300.00/M\” to US$600.00/MW/km, the company is jointly working with CEPEL (Center for Research in Electrical Energy) and ELETRQBRAS (Brazilian holding company) to implement an HSIL system in the northeast region of Brazil.

0885-8977/99/$10.00 0 1998 IEEE

Authorized licensed use limited to: Universidad Federal de Pernambuco. Downloaded on April 27,2010 at 02:48:08 UTC from IEEE Xplore. Restrictions apply.

20 1 3.0

The specific HSIL design, for operation at 230 kV, that resulted from this joint effort has the configuration illustrated in Fig. 1. It is a single circuit 230kV line and has a bundle of three subconductors per phase ACSR Linnet 336 kcmil and a Surge Impedance Loading (SIL) of 320 MVA CHESF has also developed a new upgrading technique for conventional lines, based upon the HSIL concept, called the Expanded Bundle Transmission Line (EBTL). In this arrangement, steel hangers have been utilized to obtain an adequate increase in the distance between subconductors of the same phase and consequently, to reduce the series reactance. This strategy enables increasing from 20 to 40% of the SIL of conventional lines. This approach has been implemented on a 480 km TL located in the northern region of Brazil. Figure 2 illustrates the conventional tower configuration for double 230 kV circuits that are in operation in that region. It has a bundle of two subconductors per phase ACSR Grosbeak 636 kcmil and a SIL of 406 MVA. Fig3 shows the planned upgrade in tower configuration, for future operation of the system with a single 500kV circuit with a bundle of four subconductors per phase and a SIL of 900 MVA By using intermediate EBTL 2x230kV configuration illustrated in Fig.4, it was allowed four-year postponement of the 500 kV transmission system. It also allowed an increase of the S E . reaching 510 MVA. This leads to a total cost reduction of US$ 10 million in the entire upgrade process due to financial cost reduction and extra capacity gain.

I

6.1

I

I

2.2

,

11.9

I

I

(distance in metas) Fig..2. Conventional transformable double circuit 2x230 kV tower configuration, two subcondudon 636 kanil Grosbeak per phase.

1.6

,

10.3

11.9

I

I

I

10 c

(distance m meters) Fig.3.500 kV tower configuration. four suhductors 636 kcmil Grosbeak per phase

3.0

6.1

2.2

11.9

(distance in metas) Fig 1. HSIL tower configuration. single circuit 230 kV, three subconductors ACSR 336 kcmil per phase (distance in meters) Fig 4. E.xpanded Bundle (EBTL) tower configuration,double Circuit 230kV. two subcondudors 636 kcmil Grosbeak per phase.


202

With the increased use of HSIL towers in transmssion systems it is important to determine the enmronmental impact The electric and magnetic field strength around towers, wires and particularly near the ground level, represent some of the parameters requred for t h s evaluation Because of the subconductors asy”etnca1 geometry associated with the HSIL tower, modlfications in the bundle equivalent radius calculations have to be introduced in the existing computational procedures These modifications also were carried out wthin the C)HESF/CEPEL/ ELETROBRAS joint project

Figure 6 illustrates the electnc field distributio span of the line. The maximum calculated e strength at the tower location is approximately 3 kV/m with a large increase occurring at midspan where the maximum value reaches 11 kV/m. The increase is due to the lower mdspan conductor-to-ground distance relative to that at the tower location EkCb

A. Electric Fields

Electric and magnetic fields are calculated using the traditional equivalent charge method [4]-[6], for the TL geometnes illustrated in Figs 1 through 4 Figure 5 shows the lateral profiles of the ground level electric fields, calculated at midspan for the lines considered in this paper Minimum distances from conductor to ground at midspan are typically. 8 m for the 230kV HSIL and EBTL 2x230kV, 10 m for the 500kV line and 14m for the 2x230 kV. conventional double circuit line It may be observed that the electric field is influenced by the distinct voltage levels and correspondmg conductor-to-ground mnimum distances, as expected It IS also noted that the maximum electnc field strength increases from 2kV/m to 4 5 kV/m when the 230 kV line is converted to the EBTL confgurationThe maximum electric field for the HSIL is 5 kV/m and 11 kV/m for the 500kV converted TL In the edge of the right-of-way (30m) is 2 5kV as illustrated in the Fig 5 , where it IS important to highlight that the usual maxlmum allowed electric field in the edge of the right-of-way is 5 kV/m This value is the same recommended by IRPA for a exposure charactenstics up to 24 h per day [6] 13

Edge of right -of- ’

Electric Field (kV/m)

1

K23OkV

I

4

2

0 -30

-20

-10

0

10

20

30

Lateral coordinate (m) Fig.5. Lateral electric field profiles for the HSIL 230kV, transformable 2x230kV, EBTL 2x230kV and 500kV configurations.

1 1

Fig.6. Surface plot representing the electric field distnbuhon along the span of the 500 kV line

B. Magnetic Fields Figure 7 shows the lateral profile of the magnetic field at midspan. The values shown in this figure refer to the major axis of the field ellipse These fields are strongly dependent on the value of the transmission line current and also on the height of the conductor above ground level [7] It is also noted that the maximum magnetic field increases from 20 pT to 30 pT when the 230 kV line is converted into the EBTL corQuration. These values are still lower than the maximum value of 70 pT, calculated for the 500 kV converted transmission line. It is important to highlight that the usual maximum allowed magnetic field in the edge of the right-of-way at ground level is 100 pT. This value is the same recommended by IRPA for a exposure characteristics up to 24 h per day [61.


203 Magnetic Field (pT)

Electric Field (kV/m)

80

4

,

I

l

I

-30

-20

-10

0

10

20

a

30 I

Lateral coordinate (m) Fig.7. Lateral magnetic field profiles for the HSIL 230kV, transformable 2x230k17,EBTL 2x230kV and 500kV configurations.

-30

-20

10

0

20

30

( d

111. MEASURED VALUES In order to check the theoretical calculations, which take into account the asymmetrical bundle geometry, measurements of the electric and magnetic fields have been obtained for a double circuit TL. It is also expected that the measurements will provide a better understanding of the behavior of the electric and magnetic fields generated by compact lines. They were performed using the equipment FM 130-169 from the Electric Field Measurements Co. and was realized in the Paul0 Afonso / Fortaleza transmission line. Figures 8 and 9 show the measured and calculated fields at midspan as a function of lateral distance. The transmission line under consideration is a 2x230 kV transformable conventional line, with one of the circuit already converted into the EBTL configuration. This mixed configuration is reflected in the asymmetric behavior of the fields. as can been observed in the Figs.8 and 9. These figures also indicate a good agreement between calculated and measured results, thus showing the suitability of the modified simulation method for the determination of the fields of compact lines. It is worth noting from Fig.8 that the measured electric field reach to a value close to zero, 20 m away from the central axis of the line, and the difference with respect to the calculated value may be attributed to the presence of a tree in that position.

-10

Fig.8.Measured and calculated electric field profiles at midspan of a 2x230kV transformable line having one circuit converted into the EBTL configuration.

Magnetic Field (pT) e

01

-30

I

I

-20

-10

I

I

1

I

0

10

20

30

Lateral coordinate (m) Fig.9.Measured and calculated magnetic field profiles at midspan of a 2x230kV transformable having one circuit converted into the EBTL configuration.


204

The results of this work can

slunmarized according to

1-11 is noted that the m ~ electric m field ~ strength increases from 2kVlm to 4.5 kVlm when the 230 kV line is converted into the EBTL c o ~ ~ a t i o nHowever, . these inurn value of 11 kV/m, t r ~ i s ~ s s line. ~ o nIn the HSIL line the ~ f l f i ~ value ~ d is 5 k V h .

2-For the 500kV line field strength at the t0~7e with a large increase ~a~~~ value reaches 11kV/m. magnetic field strength increases from 20 pT to 30 pT when 0 kV t r a n ~ o ~ a bline l e is ~ a ~ These o ~values . are converted into the EB 70 pT, calculated for the lower than the maxi

met~odfor field determination of

The authors a c ~ o w ~ ~Messrs g e F. W a d and S. Gusm20 from CHESF for their relevant participation during mea~ements,and 0.Regis (CHESF) and F. Dart for their contrib~tions and information about lines .

[I] Miguel 0 Melo, A Pessoa, V Quelroga, V Andrade, C Tahan, D Brasil, W Sato, “Viability Stuches of Apphcation of Compact 500 kV Transllllssion Lmes on the CHESF (Brazil) Systems,“ Leningrad S ~ p o s i uon ~ Compact Overhead Lznes,

CIGRE, 1991 [2] G N Alexandrov, “Scienhfic and Engmeenng Pnnciples of Creatmg Compact Lmes wth Increased Natural Capacity,” Leningrad Symposrum on Compacr Overhead Lines, CIGRE, 199 1.

[ 3 ] Oswaldo Regis, M Maia, A Pessoa “UnconvenhonalLmes of Ill& Natural Power Ratmg, An Exercise 111 Prospection 111 69 kV and 138 kV,“ (m Portuguese), Y.ERL;sC. CIGRE-BRAZIL, 1993 [4] Electnc Power Research Institute “Transmisszon Line Reference Book 345 kV and Above‘: Second a h o n . Palo Alto, I982

[5] “Electnc and Magnetic Fields Produced by Transmssion System,“ CIGRE Workzng Group 36 01, Intemahonal Conference on Large High VoltageElectnc Systems, Pans, 1980

[6] “Electric Power Transmission and the Enmonment : Fields, Noise, and Interference,” C E R E Working Group 36.01, Intemational Conference on Large High Voltage Electric Systems,

Pans, 1992. [7] P. S. Mmvada, ”Characterization of Power Frequency Magnehc Fields 111 Different Environments,“IEEE Transachons on Power Delivery, Val 8, No 2, April 1993, pp 598-605

Miguel 0. B. C. Melo was born in Recife, Brazil, in 1953. He received his B.Sc. and MSc. degrees in Electrical Engineering from Federal University of Pernambuco, Brazil in 1976 and 1997 respectively. He is presently enrolled in the doctoral program of the D Engineering Federal University of Paraiba. He joined the Department of Transmission Systeins Studies, CHESF, in 1976, where he is at the present a Senior Engineer. His area of interest includes e l e c ~ o m a ~ e t i c com~tibility, electromagnetictransients and compact transmission lines. LuiZ C.A. Fonseca was born in Recife, Brazil, in 1953. He received his B A . degrees in Electrical Engineering from Federal University of Pernambuco, Brazil in 1977 and completed his postgraduate course 1979 by the Power Technologies, Inc. Mr Fonseca joined CHESF at D e m e n t of Transmission Systems Studies in 1977, where he is at the present a Senior Engineer. His area of interest includes transmission systems studies and power quality. Eduardo Fontana was born in Rio de Janeiro, Brazil, in 1957. He received his B.Sc. degrees in Electrical Engineering in 1980, and M.Sc.. degree in Physics in 1983, both fkom Federal University of Pernmbuco, Brazil. In 1989 he received the Phd degree in Electrical Engineering from Stanford University, CA-USA. His past research activities have included microwave ferrite devices, low temperature magnetic materials, magnetic semiconductors, optics and free-electron lasers. He is presently a Professor at the Electronics and Systems Department, Federal University of Pernambuco. HIS current research activities concern use of surface plasmon spectroscopy in thin-film technology, integrated optics devices, and in the development of optical fiber sensors. SreeramuIu R Naidu received his B.Sc. and M.Sc. degrees from Indian Institute of Technology, Madras, India and Indian Institute of Science, Pangalore, India, in 1966 and 1970 respectively. In 1975 he received his Phd degree by University of Liverpool, United Kingdom. Dr Naidu joint Federal University of Paraiba, Brazil, where he is at the present a Professor of the Department of Electrical Engineering. psis area of interest includes electromagnetic field c o m p ~ ~ t i o and n s electricaltransients computations.
