Magnetic Fields of Parallel Transmission Lines

IMPACT OF TOWER STAGGERING ON MAGNETIC FIELD DISTRIBUTIONS UNDER PARALLEL POWER LINES

Shaher A. Mahmoud* A. H. Hamza** Nagat M. K. Abdel-Gawad** Khaled G. Ahmed*

* Egyptian Electricity Holding Company, Cairo, Egypt ** Faculty of Engineering at Shoubra, Cairo, Egypt

ABSTRACT Accordingly, this paper considers the issue of tower staggering impact on the magnetic field distributions The impact of tower staggering on the magnetic field under parallel lines using a three-dimensional distributions under parallel power lines is thoroughly magnetic field computational technique [8]. Typical investigated. The staggering techniques are classified 220, 66 and 11 kV power line configurations in Egypt into two types; artificial staggering applied at parallel are used for the current study. The magnetic field lines with the same span length and natural staggering distributions under parallel 220 kV lines are studied resulted from parallel lines with different span lengths. taking into account the proposed artificial staggering The lateral magnetic field profiles of typical parallel technique (i.e. impose staggering on towers of one of lines for both types of staggering are developed and the parallel lines). As “naturally staggered” lines, the analyzed taken into consideration the effects of magnetic field distributions under parallel 220 & 66 staggering distance, one line disconnection, line kV lines and parallel 66 & 11 kV lines are also spacing, phase arrangement, current direction and line evaluated. A comparative study between the loading. considered cases is also carried out taking into account the relative configurations, line loading and phase 1. INTRODUCTION arrangement as well as the spacing between the parallel lines. The interaction between the power frequency magnetic fields and the living organisms still draw 2. EMPLOYED POWER LINE CONDUCTOR worldwide attention because of the results of a number CONFIGURATIONS of studies which indicated possible harmful effects due to the exposure to these fields. In parallel to the Figures 1a, 1b and 1c present the line conductor laboratory and the epidemiological studies, many configurations and dimensions of the employed 220, researches addressed the evaluation of the 66 and 11 kV lines, respectively. The 220 kV line is magnetically-induced currents in living organisms with a double circuit tower, a line span of 360 m, two [1,2]. subconductors per phase, a spacing between subconductors of 30 cm and a minimum clearance–to- Many studies have been carried out to evaluate the ground of 7 m. The 66 kV line is with a double circuit magnetic fields beneath different power lines which tower, a line span of 250 m, one conductor per phase are considered as major sources of induction [3-6]. and a minimum clearance-to-ground of 6 m. On the However, these lines are dealt with individually, i.e. other hand, the 11 kV line is with a single circuit magnetic fields are evaluated under single power tower, a line span of 80 m, one conductor per phase lines. A recent study was published which investigated and a minimum clearance-to-ground of 6 m. the magnetic field distributions under parallel transmission lines [7]. The results showed that the 8.8m 8.8 m magnetic field profiles under parallel lines were apparently different when compared to those under ( 0o ) ( 0o ) 10.2 m single lines. 10.6m 10.6 m

As a matter of fact, in the normal erection of parallel o ( -120o ) ( -120 ) 9.2 m lines’ towers with the same voltage level and 8 m 8 m configuration (and consequently with the same line o span), the towers of the first line are positioned to face ( 120o ) ( 120 ) the towers of the second line. On the contrary, the erection of parallel power lines’ towers with different voltage levels and configurations (and consequently 15.78 m with different line spans) would result in the displacement (or staggering) of the parallel lines’ towers. Figure 1a: Line conductor configuration of 220 kV line

3.68m 3.68m distances far from the line when compared to the case of a single line. ( 0o ) ( 0o ) ( uT ) o 2.81 m 12 ( -120 ) ( -120o ) 2.81 m o 10 ( 120 ) ( 120o ) d l 8 e i f

c i t 6 e n

11.48 m g a 4 M

2 Line 2 disconnected 0 Figure 1b: Line conductor configuration of 66 kV line -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 CL1 CL2 Distance from centerline1 (m) 0.75m 0.75m Figure 2: Lateral profiles at tower of parallel 220 kV lines ( uT ) o ( 0 ) 30

o o ( -120 ) ( -120 ) 25

10.55 m l e i

9.35 m f

c 15 i t e n

g 10 a M 5 Line 2 disconnected 0 Figure 1c: Line conductor configuration of 11 kV line -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 CL1 CL2 Distance from centerline1 (m) 3. ARTIFICIAL STAGGERING – Figure 3: Lateral profiles at mid span of parallel APPLICATION AT PARALLEL 220 kV LINES 220 kV lines

Each of the parallel 220 kV lines has the same 3.2 Effect of Artificial Staggering configuration of Figure 1a with a full load current per phase of 1200 Ampere. The permissible minimum Unlike the case of normal erection of line towers, and spacing between the nearest conductors of the parallel as sensitivity analysis, it is supposed that the towers of lines is 14 m, according to the Egyptian standards. the second line are staggered (or displaced) from the towers of the first line by a distance which is called 3.1 Magnetic Fields in Case of Normally “staggering distance”, as simulated in Figure 4. Erected Towers To investigate such effect on the magnetic fields In case of erecting parallel transmission lines’ towers beneath the parallel lines, Figure 5 shows the lateral with the same voltage level and configuration (and magnetic field profile at tower of unstaggered line consequently with the same line span), the towers of (line 1) in case of staggering distance equals to half the first line are normally placed to face the towers of span (180 m) compared to the lateral profiles in case the second line. of normally erected towers. It is noticed that higher magnetic field values are found underneath the Figure 2 shows the lateral magnetic field profiles at staggered line conductors (line 2) located at mid span tower of parallel 220 kV lines in case of normal in this case. When compared to the case of normally erection of towers. It is noticed that two peaks exist erected line towers, the impact on the magnetic field near the outermost conductors of the lines. Higher levels underneath the first line (line 1) is limited. magnetic field values are noticed at distances far from the line when compared to the case of a single line. On the other hand, Figure 6 shows the lateral magnetic field profile at tower of unstaggered line (line 1) in On the other hand, and for the same case of normal case of staggering distance equals to quarter span erection of towers, Figure 3 shows the lateral magnetic (90 m) compared to the lateral profiles in case of field profiles at mid span of parallel 220 kV lines. It is normally erected towers. It is noticed that higher noticed that four peaks exist; two within the spacing magnetic field values are found underneath the between the two lines and other two with higher staggered line conductors (line 2). Unlike the above values near the outermost conductors of the lines. case of a staggering distance equals 180 m, the Also, higher magnetic field values are noticed at staggering distance of 90 m would result in reducing the maximum magnetic field underneath the staggered 220 kV line has the same configuration of Figure 1a line conductors (line 2) by about 27 %. with a full load current per phase of 1200 Ampere, Tower while the 66 kV line has the same configuration, as Z Line 1 Line span shown in Figure 1b, with a full load current per phase CL1 of 600 Ampere. The permissible minimum spacing X,Z =(0,0) between the nearest outer conductors of the parallel X Spc Staggering lines is 14 m, according to the Egyptian standards. distance (SD)

Line 2 Figure 7 shows the vertical and horizontal projections CL2 of the parallel 220 and 66 kV lines. It is noticed that X,Z=(Spc,SD) the first tower of the 66 kV line is placed to face the first tower of the 220 kV line. The two lines are with Figure 4: Simulation of line tower staggering of parallel lines different line spans, where the 220 kV line has a line span of 360 m while the 66 kV line has a line span of ( uT ) 30 250 m. Accordingly, a natural line tower staggering Normal Staggered occurs with a staggering distance of 110 m multiplier. (at mid span ) (at tower, line 1) 25 d l 20 4.1.2 Effect of One Line Disconnection e i f

c i t 15 e n

g Figure 8 shows the lateral magnetic field profiles at a 10 M tower of parallel 220 and 66 kV lines taking into 5 Normal account the cases of disconnecting either the 220 kV (at tower ) 0 line or the 66 kV line. It is noticed that the operation -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 of the two lines would result in reducing the maximum CL1 CL2 Distance from centerline1 (m) magnetic field value as well as the values beneath the Figure 5: Lateral profiles of parallel 220 kV adjacent circuits of the lines when compared to the lines in case of artificial staggering case of single 220 kV line. This can be attributed to (Staggering distance = 180 m) the cancellation of magnetic fields among the adjacent innermost conductors. In this case also, higher ( uT ) magnetic field values are found beneath and outside 25 Normal the 66 kV line when compared to the case of single 66 (at mid span ) 20 kV line. ( uT ) 14 d 15 l 66 kV line disconnected e i f

12 c i t

10 d e l

e 10 n i f g

a c i t M Normal 8 5 e

Staggered n 220 and 66 kV

(at tower) g

(at tower, line 1 ) a 6 0 M 220 kV line 4 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 disconnected CL1 CL2 Distance from centerline1 (m) 2 0 Figure 6: Lateral profiles of parallel 220 kV -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 100 lines in case of artificial staggering CL1 (220 kV line) CL2 (66 kV line) (Staggering distance = 90 m) Distance from centerline 1 (m) Figure 8: Lateral profiles at tower of 220 kV line 4. NATURAL STAGGERING for Parallel 220 and 66 kV lines Unlike the above case which considered parallel lines 4.1.3 Effect of Line Spacing of the same voltage level, this case considers parallel lines with different voltage levels; 220 & 66 kV lines Figure 9 shows the lateral magnetic field profiles at and 66 & 11 kV lines. Since these parallel lines have mid span 3 of 220 kV line for parallel 220 and 66 kV different line spans, their towers are naturally lines for different line spacings; minimum spacing of staggered. 14 m and double spacing of 28 m. It is noticed that as the two lines become nearer to each other, the 4.1 Magnetic Fields Under Parallel 220 & 66 kV maximum value of magnetic field in the two lines Lines corridor decreases. On the other hand, the maximum value of magnetic field beneath the 66 kV line 4.1.1 Simulation of Parallel 220 & 66 kV Lines increases and its location is shifted towards the higher voltage line. The parallel 220 and 66 kV lines are considered. The 4.1.4 Effect of Line Loading Figure 11 shows the lateral magnetic field profiles at To consider the variation of load (or current) values tower of parallel 66 and 11 kV lines taking into between the two parallel lines, Figure 10 shows the account the cases of disconnecting either the 66 kV comparative magnetic field profiles at mid span 3 of line or the 11 kV line. It is noticed that the operation 220 kV line for parallel 220 and 66 kV lines for of the two lines would result in slightly reducing the different line loading. It is noticed that reducing the maximum magnetic field value as well as the values in current of the 220 kV line from its full load to half the zone of the two lines corridor when compared to load value (i.e. both lines are with the same current, the case of single 66 kV line. While remarkable 600 Ampere) causes a magnetic field reduction at all reduction of the magnetic field occurs beneath the 11 locations especially under the 220 kV line. The kV line when compared to the case of single 11 kV maximum magnetic field value is reduced by almost line. In general, the effect of 11 kV line on the the same ratio (one half in this case). resultant magnetic field of parallel 66 and 11 kV lines (uT) is very limited. 30 (uT) 7 25 11kV line disconnected 6 Min. line spacing (14 m) 66 kV line disconnected

d 20 l e i 5 F d

c 15 e i i t Double spacing (28 m) 4 F e

n c i g 10 t a e 3 66 and 11 kV Lines n M g

5 a 2 M

0 1 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 10 CL 220 kV CL 66 kV 0 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 Distance from centerline of 220 kV line (m) CL1 (66 kV line) CL2 (11 kV line) Figure 9: Lateral profiles at mid span3 of 220 kV Distance from centerline of 66 kV line (m) line for Parallel 220 and 66 kV lines Figure 11: Lateral profiles at tower of 66 kV line for different line spacings for Parallel 66 and 11 kV lines (uT) 30 4.2.2 Effect of Phase Arrangement 25 d l 20 Figure 12 shows the effect of phase arrangement on e i F

c the magnetic field distribution under the parallel 66 i 15 t e

n and 11 kV lines. The 11 kV line as well as the farthest g

M circuit of the 66 kV line are estimated to have a 5 normal phase arrangement (abc) while the adjacent Line 220 kV half load 0 circuit of the 66 kV line is estimated to have a varying -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 90 10 phase arrangement. Three different phase CL 220 kV CL 66 kV Distance from centerline of 220 kV line (m) arrangements are, thus, considered; (abc, abc/abc), Figure 10: Lateral profiles at mid span3 of 220 kV (abc, bca/abc) and (abc, cab/abc). It is noticed that the line for Parallel 220 and 66 kV lines normal phase arrangement (abc, abc/abc) produces the for different line loading lowest maximum magnetic field value while produces the highest magnetic field values at all locations except those under the 66 kV line.

4.2 Magnetic Fields Under Parallel 66 & 11 kV (uT) Lines 16 14 Another case of parallel lines with different voltage 12

10d levels is the parallel 66 and 11 kV lines. As previously l e i F

shown, the 66 kV line has the same configuration of c i t

Figure 1b with a full load current per phase of 600 n

g (abc, abc/abc) a 4 Ampere, while the 11 kV line has the same M (abc, cab/abc) configuration, as shown in Figure 1c with a full load 2 (abc, bca/abc) current per phase of 200 Ampere. The permissible 0 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 minimum spacing between the nearest outer CL66 kV CL11 kV conductors of the parallel lines is 10 m, according to Distance from centerline of 66 kV line (m) the Egyptian standards. Figure 12: Lateral profiles at mid span3 of 66 kV line for Parallel 66 and 11 kV lines 4.2.1 Effect of One Line Disconnection for different phase arrangements [3] Anis, H., Abd-allah, M. A. and Mahmoud, Sh. A.: 5. CONCLUSION “Comparative Exposure to Magnetic Fields Near High Voltage Transmission Lines”, 6th Middle 1. The artificial staggering of line towers in case of East Power Systems Conference (MEPCON’98), parallel lines affect the magnetic field Mansoura, Egypt, PP. 565-569, Dec. 1998. distribution beneath these lines. The degree of effect depends on the staggering distance. [4] Tsanakas, D., et al. : “Compact and Optimum Phase Conductor Arrangement for the Reduction 2. Reducing the line spacing between parallel lines of Electric and Magnetic Fields of Overhead of different voltage levels would reduce the Lines”, Cigre, Paper 36-103, Paris, 2000. maximum value of magnetic field in the two lines corridor. On the other hand, the maximum [5] Isaka, K., et al. : “Characterization of Electric and value of magnetic field under the lower voltage Magnetic Fields at Ground Level Under EHV line increases and its location is shifted towards Transmission Lines”, 7th International Symposium the higher voltage line. on High Voltage Engineering (ISH-91), Paper 93.01, Dresden, Germany, Aug. 1991. 3. Reducing the loading of the higher voltage line for parallel lines of different voltage levels [6] Melo, M. O., et al.: “Electric and Magnetic Fields would reduce the magnetic field values at all of Compact Transmission Lines”, IEEE Trans. on locations. The maximum value of magnetic field Power Delivery, Vol. 14, No. 1, PP. 200-204, is reduced by almost the same ratio. Jan. 1991.

4. For the studied cases of phase arrangements for [7] Mahmoud, Sh. A., Abd-allah, M. A. and Ahmed, parallel lines of different voltage levels, the Kh. G. : “Magnetic Fields Under Parallel normal phase arrangement produces the lowest Transmission Lines”, 8th Middle East Power maximum magnetic field value while produces Systems Conference (MEPCON’2001), Helwan, the highest magnetic field values at all locations Egypt, PP. 885-890, Dec. 2001. except those under the higher voltage line. [8] Anis, H., Abd-allah, M. A. and Mahmoud, Sh. A.: 6. REFERENCES “Computation of Power Line Magnetic Fields - A Three Dimensional Approach”, 9th International [1] Zemann, E., et al.: “Investigation of Exposition Symposium on High Voltage Engineering (ISH- to Low Frequency Magnetic Fields”, 9th 95), Paper 8333, Graz, Austria, Aug./Sep. 1995. International Symposium on High Voltage Engineering (ISH-95), Paper 8347, Graz, Austria, Address of Author Aug./Sep. 1995. Dr. Shaher Anis Mahmoud [2] Abdallah, M. A, Mahmoud, Sh. A. and Anis, H. I.: Egyptian Electricity Holding Company – 6th Floor “Interaction of Environmental ELF Abbassia, Nasr City, Cairo- Box 11517, Egypt Electromagnetic Fields With Living Bodies”, Electric Machines and Power Systems, 28:301-312, 2000.

Figure 7: Vertical and horizontal projections of parallel 220 and 66 kV lines