2OO5A2A-PAUL9((OIIHAMB KrROAAGT KSPROeINDecember 20-21, 2005, Johor Bahru, Johor, MALAYSIA

Magnetic Field Reduction Principles and Options ofPrimary OverheadDistribution Lines

Intan R. Ibrahim, Ahmed Farag, Ismail Said, Halil Hussain, and Nazaruddin A. Rahman

Electric and Electronic Engineering DepartmentCollege ofEngineering

Universiti Tenaga NasionalKm.7, Jln. Kajang-Puchong

43009, Kajang, Selangor, Malaysia

Abstract - Variables that affect the magnitude of themagnetic fields in distribution line system are themagnitude of the distribution lines' phase currents, thedistance of the distribution lines from the observationpoints, the conductors' configurations, and the groundcurrents due to unbalanced phase current. By using thedelta type and the horizontal type of 33 kV primaryoverhead distribution poles as models, this paperpresents the case studies of the effect ofthat variablesto the primary overhead distnbution lines and proposesthe design options such as increasing the height of thedistnbution lines, increasing the lateral distance fromthe distribution lines, compacting the lines conductors'spacing, minimizing the ground current by balancingthe lines' phase currents, and optimizing phasingarrangement. Another design criteria such as cost,safety, reliability, system stability, etc. also need to beconsidered before the design options recommendedcan be applied.

1. Introduction

All electrical equipments and devices, includinghome appliances, power distribution systems andmany more produce electric and magnetic fields. Anymoving charges will generate magnetic and electricfields. Most recent research only focused on potentialhealth effects of magnetic fields exposure becausesome epidemiological studies have reported anincreased cancer risk associated with estimates ofmagnetic fields exposure [1,2,3,4]. No similarassociations have been reported for electric fields[5,6]. Magnetic fields created by power frequency lineare called power frequency magnetic fields. Powersystem can be broken into three general components,generation, transmission and distribution. Thedistribution system is defined as the system betweenthe transmission and the consumer service points,which tap high voltage power energy from thetransmission system, step down the voltage to the

customer utilization voltage and distribute the powerenergy to the consumers. The distribution system can bedivided into two major components, which are the primarydistribution system and the secondary distnbution system.The primary distribution system is that portion of thedistribution system between the distribution substationsand the distribution transformers, which consists ofprimary feeders. The secondary distribution system isdefined as the portion of the system between the primaryfeeders and the customer's premises [1, 7].

2. Horizontal and Delta Type ofthe PrimaryOverhead Distribution Lines' Poles

There are two types of primaiy distribution linesconstructions, which are the overhead construction and theunderground construction. The overhead constructionconsists of conductors suspended on poles. The conductorsare commonly uninsulated and separated by a distancewide enough to minimize short circuits due to flashover orcontacting each other due to wind. The neutral or groundwire can be located below, above or at the same height asthe phase conductors [7]. In Malaysia, most common typesof the overhead pnimary distribution lines' poles are deltaand horizontal as shown in Figure (1). Meanwhile, theunderground construction consists of single or threeconductor cables, which are laid in a trench or conduit andburied about 36 inches from the ground surface [7]. Thesheath of the cables usually acts as a neutral return whenused in 4-wire systems. The underground construction isshown in Figure (2).

3. Variables that Affect the Magnetic Fields inPrimary Overhead Distribution Lines

Generally, variables that affect the magnetic fields inthe primary overhead distribution lines are (i) themagnitude of phase currents, (ii) the distance of the

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distribution lines from the observation points, (iii) theconductors' configurations and (iv) the groundcurrents resulted in unbalance phase currents [8,9].

(i) The magnitude ofphase currents. The magneticfields are proportional to the magnitude of the phasecurrents.

(ii) The distance of the distribution lines from theobservation points. The magnetic field strength willdiminish with lateral distance from the field's source.Most three-phase distribution lines produce magneticfields that decay as U/r2, where r is the distance fromthe line. The decay rate of llr occurs when the currentson a distribution lines are not balanced and the decayrate of Xr3 occurs when using a low reactanceconfiguration with phase currents of equal magnitudeand reverse phase configuration in double circuits.

(iii) The conductors' configurations. Each phase inthree-phase distribution lines has typically a singleconductor or rarely a bundle of two or moreconductors. The ground level magnetic fields is thesum of all the magnetic fields produced by the cunrentsin all conductors and dependant upon the distancebetween the point of calculation and each currentcarrying conductor.

(iv) The ground currents resulted in unbalance phasecurrents. In the unbalance distribution system, thereturn cufrent will flow back through the neutraland/or through the ground path. As the percentage ofthe unbalance current retuming as ground currentincreases, the higher will be the magnetic fields in thesystem.

4. Magnetic Fields Reduction principles inThe Primary Distribution Lines

By taldng advantage of the characteristics of themagnetic fields in the primary distribution line systemand the variables that affect the magnetic fields in theprimary overhead distribution line, the primaryoverhead distribution line system can be designed tohave reduced im magnetic fields strength. The designoption may include one or combination of thefollowings (i) increasing the distance from the primaryoverhead lines by increasing the pole's height (ii)reducing the space between the conductors bycompacting the conductors (iii) Balancing the phasecurrents by reducing the ground current and (iv)optimizing phasing amngements by manipulatingphase arrangements so that the maximum magneticfields cancellation will be achieved.

5. Case Studies

In these case studies, all the magnetic fields reductionprinciples that have been discussed above are simulatedand studied by using 33 kV delta type and horizontal typeofthe primary overhead distribution poles as models.

(i) Increase the distance from the primary overheaddistribution lines by increasing the pole's height. Figure(3) demonstrates the effect of increasing the distancebetween the conductors and the location of interest. Thistechnique is effective both for horizontal and delta poleswith reduction of the magnetic fields by 37 % for the deltapole and by 32% for the horizontal pole. The peakmagnitude ofthe magnetic fields for this case are shown imTable (1)

(ii) Reduce the conductors' spacing by compacting theconductors. Figure (4) demonstrates effect of compactingthe conductors' spacing for the horizontal and the deltapoles. This technique is effective for the horizontal and thedelta pole with reduction of 18% and 17 % ofthe magneticfields for 20% compaction respectively. Table (2) tabulatesthe peak magnitude of the magnetic fields for theconductors' compaction cases.

(iii) Balance the phase currents by reducing the groundcurrent. For this technique, the effect of unbalance phasecurrents is observed and there are increases of magneticfields when the percentages of unbalance are increased.Figure (5) demonstrates the effect of unbalance phasecurrents for the horizontal and the delta primary overheaddistribution line poles.

Table (3) tabulates the value ofmagnetic fields atROW for the unbalance cases. It is assumed that all 'retuncurrents' are flow back through the neutral. The magneticfields tend to be higher if there are portion of returncurrents are flow back through the ground path.

(iv) Optimize phasing arrangement in the double circuitpoles. For this technique, a new circuit is added to theexisting structure and the conductors ofthe new circuit arearranged so that the maximum magnetic fields cancellationis achieved.

Figure (6) shows the effect of optimized currentphasing arangement and assumes ideal conditions whereboth circuits are carrying the same current in each phaseconductor. The maximum values ofthe magnetic fields forthis case are tabulated in Table (4).

Even though the vertical pole is not inused in Malaysia, it is included in this techniqueto give more alternative method in reducing themagnetic fields.

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5.1 Observation(i) The delta type pole gives lower emission ofmagnetic fields than the horizontal pole. This isbecause the conductors' height above ground level forthe delta pole is higher which is 25.98 feet comparedto 24.80 feet for the horizontal pole. However, if theheight of the delta pole is made similar as thehorizontal pole, the delta pole still produces the loweremission of magnetic fields than the horizontal polebecause of its symmetrical configuration. Theimpedance or resistance of a symmetrical lineconfiguration is lower and better balanced than anunsymmetrical configuration.(ii) The balanced vertical double circuit withoptimized phasing arrangement produces the lowestmagnitude ofthe magnetic field at ROWs and the peakmagnetic field of the pole compared to others becausethe balanced vertical double circuit gives themaximum reduction of the magnetic field. However,the cost for this construction is high.(iii) It is verified that the neutral currents and/or theground currents in the unbalance phase currents of theoverhead distribution system are the majorcontributors to the magnetic fields in the distributionlines system. Therefore, the low cost technique toreduce magnetic field in the distribution lines systemis to ensure that the distribution line system supply thebalanced phase currents.

Table (5) summarizes the different design optionsthat have been proposed with their cost effectiveness.

5.2 RecommendationTo limit the magnetic field exposure below 4 mG

in the existing structure,

(i) The power utility companies should ensure thatthey supply a balanced phase currents and takenecessary actions to eliminate ground curents such asusing a low impedance neutral conductor to force the'flow back' currents to return through the neutral.

For the new construction, it is recommended thatthe overhead distribution line system should beredesigned to limit the EME exposure below 4 mGDesign options may include:-

(i) Using vertical double circuit construction withoptimized phase arrangement (RYB//byr). Thisconfiguration naturally cancels out the magnetic fieldsproduced by each other circuit. However, thistechnique is not used because the cost for thisconstruction is high.(ii) Using the delta type pole for the single circuitconstruction. It should be even better if the deltaconstruction used is increased by 6 feet of the pole's

height, which will result in more reduction in the magneticfield.(iii) Using aerial bundle cable (ABC) construction. Insecondary distribution lines (service lines), ABCconstruction is widely used instead of older bare wiresconfiguration. Nowadays, the usage of ABC has beenextensively used for 33 kV distribution lines. It is knownthat this type of construction produces lower magneticfields due to the small spacing between the conductors. Itis easily installed and maintained. Maybe, in the future,ABCs can also be used in the transmission of the highervoltage power supply such as 132 kV or 275 kV.(iv) By using the principle of "prudence avoidance", it isrecommended that new construction sites should be as faras possible from residential area such that the resultantmagnetic fields at ROWs should be below 4mG. It is alsorecommended that relevant authority should enforce arestriction not to allow any activity within the ROWs orwidened the ROWs so that the exposure of the magneticfields to the public will be minimum.

6. Summary

By using the delta type and the horizontal type of the33 kV primary overhead distribution lines it is shown thatthe overhead primary distribution system lines' magneticfields can be reduced by using a combination of thesetechniques:- (i) increasing the conductors' height aboveground, (ii) compacting the conductors' spacing, (iii)balancing the phase currents, (iv) optimizing phasingarrmgement. The ground currents from unbalance phasecurrents are one of the major contributor ofmagnetic fieldsin distribution system. Therefore for the existingconstruction, balancing the phase currents can reducemagnetic fields. For the new construction, it isrecommended that the overhead distribution line systemshould be redesigned to limit the EMF exposue below 4mG. The design options may include (i) using the deltapole with 6 feet higher (ii) using ABC construction (iii) thenew constuction sites should be as far as possible fromresidential area such that the resultant magnetic fields atROWs should be below 4mG. The relevant authorityshould enforce a restriction not to allow any activity withinthe ROWs or widened the ROWs so that the exposure ofthe magnetic fields to the public will be minimum.

References

[11 Horton W.F., Goldberg S., "Power FrequencyMagnetic Fields and Public Health" CRC Press Inc,Florida, 1995, pp. 2-177.

[21 Intemational Agency for Research on Cancer (IARC),"Monographs on the Evaluation ofCarcinogenic RisksofHumans: Preamble, Lyon, France, 2001.

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[31 World Health Organization (WHO),"Electromagnetic Fields and Public Health:Extremely Low Frequency Fields and Cancer,WHO Fact Sheet #263 October 2001.

[41 WHOAIARC, "Non-Ionizing Radiation, Part 1:Static and Extremely Low-Frequency (ELF)Electric and Magnetic Fields, IARC Monograph80, lARC Press, Lyon, 2002, pp. 429

[51 National Institute of Environmental HealthScience (NIEHS), "Questions and Answers AboutEMF", June 2002. Available at:http://www.niehs.nih.gov/emfrapid booklet/home.htm

[61 NIEHS, "Assessment of Health Effect FromExposure to Power-Line Frequency Electric andMagnetic Field", EMF Working Group Report,Minnesota, 16-24 June 1998. Available athttp://www.niehs.nih.gov/emfrapid/htinl/WGReport/PDF Page.html

[7] EPRI, "Electric Power Distribution Handbook",EPRI PEAC, CRC Press Inc, New York, 2003.

[81 Farag A.S., Bakhaswain J., Cheng T.C., Du Y.,Hu L., Zheng G., Penn D., Thompson, J. 2000."Distribution Lines Electromagnetic Fields:Management and Design Guidelines". ProceedingofCigree Session 2000, pp. 36-105.

[93 Farag, A.S., Ismail S., Halil H., Intan Rahayu I.,Nazaruddin A.R., syed zain, S.A.F., "Review ofPower Line EMF Health Effects and The Role ofElectric Power Utility Companies". Proceeding of3rd TNB Technical Conference 2001.

Table 1: The Effect ofIncreasing the Pole's Height tothe Maximum Magnetic Fields.

Height Maximum magnetic(feet) field (mG)

Horizontal H 32.472H+2 27.547H+6 20.507

Delta H 21.777H+2 18.864

________ H+6 _ 14.556

Table 2: The Effect ofConductors' Compaction to theMaximum Magnetic Fields.

Conductors' Maximum magneticcompaction field (mG)

l_"_ (%/) .. __ __ __

Horizontal 0 32.4725 31.03120 26.529

Delta 0 21.7775 20.85520 18.179

Table 3: The Effect ofUnbalance Phase currents to theMagnetic Fields at the Right ofWays (ROWs) of the

Poles.

Table 4: The Effect ofOptimize Phasing Arrangementto the Maximum Magnetic Fields.

Table 5: The Evaluation ofDifferent Design Optionsand their Cost Effectiveness.

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Unbalance Magetic field atPhase B ROW (mG)

Horizontal Balance 5.620B- 10% 10.428

._B - 20%o 15.672

Delta Balance 5.127B-10% 88250B - 20% 12.833

Phase Maximum magneticconfiguration field (mG)

Horizontal Single circuit 32.472ryb/lRYB 53.121ryb/ABYR 11.823

Delta Single circuit 21.777ryb//RYB 32.952ryb/ABRY 18247

Vertical Single circuit 22.781ryb//RYB 15.851ryb/ABYR 7.385

Design Magnetic Cost CommentsOption Field Increased

Rednction (%/9)Increase the This techniquePole's Height is better suitedHorizontal 37 m 15 to newDelta 33 z 15 constructionConductors' This techniqueCompaction is better suitedHorizontal 18 z -20 to newDelta 17 -20 construcionBalance the This techniquePhase suits forCurrents existing andHorizontal 40 X 5 newDelta 150 # 5 constructions

and veryeffective withminimal costinvolved.

Optimize This techniquePhasing is very effectiveArrangement but the cost isHorizontal 64 90 highDelta 16 90Vertical 86 90 _

R B 3 r ^ tR

R yE B ~ y

f~~

Figure 1: The Delta Type and the Horizontal Type ofPrimary Overhead Distribution Line.

I1

Figure 2: Underground Construction.Figure 6: The Effect ofOptimize Phasing

Arrangements to the Magnetic Field Profiles.

R dD

Figure 3: The Effect ofIncreasing Height oftheconductors above ground to the Magnetic Field

Profiles.

Figure 4: The Effect ofConductors' Compactionto the Magnetic Field Profiles.

Figure 5: The Effect ofcurrent Unbalance to theMagnetic Field Profiles.

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