Electromagnetic Fields in Distribution Feeders and Electrical Substations Analysis: a Study Case in Ecuador John A. Moralesab, Patricia Gavelab and Arturo S. Bretasc Email:
[email protected] [email protected] a Carrera de Ingeniería Mecatrónica, Universidad Politécnica Salesiana, Cuenca, Ecuador. b Carrera de Ingeniería Electrica, Universidad Politécnica Salesiana, Cuenca, Ecuador. c Department of Electrical and Computer Engineering, University of Florida, Gainesville, USA. Abstract—It is well known that human safety regarding electromagnetic contamination due to the presence of electrical devices in residential areas, like transmission or distribution lines, substations and transformers is imperative. These devices contribute to residential or industrial electromagnetic field exposure, which can be correlated with serious drawbacks for the health as childhood leukemia and; cancer. In this context, in order to guarantee people health, it is crucial that utilities and other stakeholders can attend in a suitable way to these worries, being important for such, levels of existing electromagnetic fields in their infrastructure analysis. The main goal of this study is to determine and analyze the EMF exposure levels of four real-life substations and their corresponding distribution feeders (twenty one). With these measurements and estimated ones obtained through the use of commercial packages, comparison with regulatory exposure levels are made. Results are discussed and analyzed. Index Terms— Electromagnetic fields, ionization, exposure levels, non-ionizing radiation.
I. INTRODUCTION
E
LECTROMAGNETIC fields (EMFs) have been presented from the beginning of the humanity. However, the modern use of electricity in all ambiences of the life, differently to EMFs produced by the nature, have generated a new environment [1]. Hence, for many year researches have studied the possibility that EMF could impact or have influence on human health. Some cases have been reported, which have aroused the interest and preoccupation of possible health effects of EMFs [2-4]. In this context, EMF generated not only near houses but also by substations and distribution feeders in residential areas have generated questions and concerns [5-6]. This main concern is due to the generation of electromagnetic fields, especially those fields corresponding to low frequency 50 and 60 Hz, which supply to household electricity denoted as non-occupational EMF [7].
This work was supported by the Carrera de Ingeniería Mecatrónica de la Universidad Politécnica Salesiana and the Department of Electrical and Computer Engineering, University of Florida, Gainesville, USA. J. Morales is with Universidad Politécnica Salesiana, Cuenca, Ecuador,
[email protected]
On the other hand, researches related to EMF exposure and health effects have been investigated and studied in some researches [3-5], [8-9], which have especially focused on the childhood leukemia. [7], for example, presents epidemiological studies about the possible risk of leukemia when EMFS exposure. In some cases, experimental tests with animals are also presented, which have showed possible results of human cancer [8]. A more detailed description about those researches and their possible effect of human health is analyzed and presented in [5]. Currently, utility personnel realize the maintenance of electric networks and substations using the procedure called “Works with Voltage”. Besides that, different methods of EMF mitigation are also used, one of such is the active and passive shield wires utilization based methods. However, this could involve a high economic cost [10-11]. Thus, the objective of this study is to determine through calculations and measurements the EMF exposure levels of actual substations and connecting feeders. Also an objective of this study is to analyze the obtained results and, compared with levels recommended by international norms. II. MATERIALS AND METHODS A. Exposure Level and Case Study System International organizations like ICNIRP, IEEE, Ministry of Environment of Equator and other have each a suggested criterion of exhibition, reference levels useful to practical evaluations of electromagnetic fields, and recommendations applied to values for the occupational and population exhibition [12-14]. In this study, reference levels established by these organizations are used, which are shown in Table 1 and Table 2. As regards the safe threshold values, the environment ministry of Equator indicates the norm corresponding to non-ionizing radiations of electromagnetic fields and minimal requests of safety for exhibition to electrical and magnetic fields of 60 Hz. This norm must be applied to electric substations, structures, towers, poles, cables, potency transformers and other; in order to guarantee the health and safety of the public in general and of workers exposed to radiations non ionizing, which are generated by electrical systems of the national territory. In order to determine EMF values, four substations (three exterior and one subterranean) denoted in this study by
substation A, B, C and D, respectively, and twenty one distribution feeders are available, which are placed in the urban perimeter in south of Equator. These comprise
B1
B4
Table I. Levels for exposures of electric and magnetic fields 60 Hz. Electric Field Magnetic Field Magnetic Intensity Intensity Flux Density
Type of Exposure
(E) (V m -1)
(H) (A m-1)
(B) (µT)
General Audience
4167
67
83
Staff Occupationally Exposed
8333
333
417
Ground
(a)
D6
D5
D4
D1
Table II. Levels for exposure of electric and magnetic fields 60 Hz to high voltage lines measured on the out in the limit of your strip easement. Voltage level
Electric Field Magnetic Intensity Flux Density
Easement Strip Width
(kV)
(E) (V m -1)
(B) (µT)
(m)
230
4167
83
30
138
4167
83
20
69
4167
83
16
volt age levels of 6.3 kV, 22 kV and 69 kV. Further information are presented in Table 3. On the other hand, due to that not only the electrical field but also the magnetic field varies with the position. In this research, in order to determine the spatial change of these fields inside each substation, the mapping method is used. Hence, a total EMF analysis on the substations is developed, including the total length and breadth of the substation, taking measurements principally in their busbars, lines, transformers and connection points. Fig. 1 shows features of some distribution feeders. B. Measurement of EMF As stated previously, by using the mapping method, the procedure IEEE 644 corresponding to Standard Procedure for Measurements of Power Frequency Electric and Magnetic Fields from AC Power Lines, and the regulation expressed by the Department of Environment of Equator [13-14], the measurements of EMF are developed. In this context, measurements in specific points near of the substation as paths and houses considering lateral and longitudinal profile, were realized. Where, in order to assure enough accuracy in the measurement of EMF, a distance corresponding to 1 meter between the ground and the measuring device, must be used. A description more detailed of the procedure used to make the measurements is presented in [13]. Table III. Distribution feeders and substations used. # Substation
Type
# Distribution feeders
Voltage level
A
Interior (Subterranean)
5
22kV/6,3kV
B
Exterior
5
69kV/22kV
C
Exterior
5
69kV/22kV
D
Exterior
6
69kV/22kV
Ground
(b) Fig. 1. Disposition of conductors, a) substation B, b) substation D.
Regarding the measuring device, in this study by using a digital analyzer of electrostress, measurements are developed [15]. This device allows making an evaluation of electric and magnetic fields exhibited, through of a sensor of magnetic field that measures the magnetic flux density in nT, and a sensor of electrical field that measures the electrical field intensity in V/m, respectively. Those values registered are displayed on a screen LCD, where the type and the value of the field, is specified. In addition, in order to make adequate electric field measurements, it is necessary to place both the measuring device and the person who realize the measurements to a reference to ground. The previous is due to that its beginning of functioning is to measure the currents of ground and of the conductive body that is exposed the presence of the electrical field. As regards the magnetic field measurements, the measuring device does not need reference to ground and this takes in consideration that the presence of other persons in the closeness of the device does not affect the measured values. The measurements were realized in every substation following the points of the paths previously established according to their lateral and longitudinal profile. For example in Fig. 2 the points selected of substation A corresponding to the first floor is shown. It is necessary to note that in every point, the magnetic field is measured in three dimensions, i.e. field values in X, Y and Z. Later on, the total magnetic field is calculated as follows: 2
2
B = B X + BY + B Z
2
where, Bx, By and Bz in uT represent the magnetic field in each dimension, respectively.
(1)
V/m
nT
V/m
nT
(a)
(b)
Fig. 2. Field spectrum of first floor of substation A, a) electric filed, b) magnetic field.
On the other hand, in order to make measurements of electrical and magnetic field in substations, it must be realized considering the vertical component and the value rms of the resultant corresponding to electrical and magnetic field in every point. Besides that, in order to cover the totality of the substation, two perpendicular profiles with a separation between 1 and 2 meters among measurements are selected. C. Calculation of EMF As regards to the electric field, distribution feeders and substations structures can be shaped by load lines with a suitable disposition, which can be considered as a problem in two or three dimensions [2]. In order to calculate the electrical field under distribution feeders or substations bars, the images theory considering some idealizations as circular feeders of infinite length can be used [2], [16]. In this context, the field between two spheres is symmetrical with regards to the symmetry plane corresponding to charges. Thus, half spatial field represents an image to speculate of another half. Similar to that any feeder and an infinite conductive plane can be solved like two feeders, the first correspond to the image of the second feeder. The relation matrix between the voltage and charges that linear feeders N take in an EPS is calculated as follows:
⎛ 2h ⋅ ln⎜⎜ i 2πε 0 ⎝ Ri
⎞ ⎟⎟ ⎠
(4)
Pij =
⎛ D' ij ⋅ ln⎜ ⎜D 2πε 0 ⎝ ij
⎞ ⎟ ⎟ ⎠
(5)
1
1
where, Ri represents the conductor ratio i, hi correspond to the conductor height i on the ground (mean value), Dij is the distance between conductors i – j, D'ij represents the distance between the conductor i and the image of conductor j. In case of beam feeders, the calculation is realized by applying a model with an equivalent feeder with radio:
Re q = An
[V ] Nx 1 = [ P ] NxN [ q ] NX 1
(2)
[q] = [C ].[V ] = [ P] −1 .[V ]
(3)
where, [q] and [V] represent column vectors of charges and potentials of feeders, respectively. [P] represents the squared matrix of proper and mutual coefficients corresponding to the potential. It is necessary to note that the squared matrix C is not the feeders capacitance. This variable represents capacity coefficients and is obtained by using coefficients of potential P, which is calculated as follows:
Pii =
nR A
(6)
where, R represents the conductor ratio, A represents the geometric ratio corresponding to the beam feeders, n is the beam feeders numbers. This represents a fictional feeder, which have similar capacitance to the beam feeders with all the nearby feeders. Thus, the electric field is calculated by using the effective distance between the feeder who takes the load qi for unit of length or its image, and the point where it is desired to determine the electrical field value. Finally, by supposing those charge contributions corresponding to components Ex and Ey, the total electric field is calculated as follows [2]: N
Ex = ∑ i =1
N
Ey = ∑ i =1
qi 2πε 0
⎤ ⎡ x − di x − di − ⎢ 2 2 2 2 ⎥ ( x d ) ( y h ) ( x d ) ( y h ) − + − − + + i i i i ⎦ ⎣
(7)
qi 2πε 0
⎡ ⎤ y − hi y + hi − ⎢ 2 2 2 2 ⎥ − + + ( x d ) ( y h ) ( x d ) ( y h ) − + − i i i i ⎣ ⎦
(8)
Table IV. Measured values in substations A.
where, it is supposed that the conductor “i” is localized in the position x = di, y = hi respect to the reference, and N correspond to the total conductors number. As regards the calculation of magnetic field under distribution feeders and substations bars, the currents simulation method is analogous with the charges simulation method for the electrical field, where the field source is the feeder current. The magnetic field components corresponding to each conductor “i” are represented as follows [2]: ⎤ − ( y − yi ) μ I ⎡ y + yi + 2 p B xi = 0 i ⎢ − ⎥ 2 ⋅ π ⎣ ( x − xi ) 2 + ( y − y i ) 2 ( x − xi ) 2 + ( y + y i + 2 p ) 2 ⎦
μ I B yi = 0 i 2⋅π
⎡ ⎤ x − xi x − xi − ⎢ 2 2 2 2 ⎥ ( x x ) ( y y ) ( x x ) ( y y 2 p ) − + − − + + + i i i i ⎣ ⎦
SUBSTATION A (First floor) Measured values
1
μ ⋅σ ⋅ w
= 355.88
ρ f
Total
X
Y
[nT]
1
37
222
164 278.48 1.9
2
33
222
183 289.59
3
34
208
202 291.93 0.6
0.3 0.5
0.6
4
36
200
215 295.84 0.3
0.3 0.5
0.5
5
52
178
173 253.61 0.4
0.8
6
134
179
256 339.90 1.2
1.1 1.4
1.4
7
167
264
555 636.88 2.8
1.3 1.4
2.8
8
54
129
145 201.45 1.1
1.2 1.3
1.3
9
29
104
106 151.30 0.4
0.4 0.4
0.4
10
56
218
107 249.22 0.4
0.7 1.8
1.8
11
55
175
112 214.93 1.8
1.6 3.5
3.5
12
33
200
120 235.56 2.4
2.8 4.7
4.7
13
249
170
134 329.93 0.7
0.8 0.8
0.8
14
407
126
44
428.32 1.4
0.8
1.4
15
405
171
24
440.27
2.5 2.4
(9)
(10)
where, p represents the filmy depth from the field or the distance from the soil surface over the reference plane to the magnetic field. Where, the first and second term represent the contribution of the real and image feeder, respectively. In this context, p is calculated as follows [2]:
p=
Magnetic Field
Point in Fig. 2
Z
Electric Field
Total
Front Side Sky [V/m ]
1
2
1
1.3
0.5
1
1.9 1
1
1
1
2.5
(11)
where, ρ is the ground resistivity. III. RESULTS (a) 60 50
ELECTRIC FIELD, V/M
By using the procedure presented in section 2.B, measurements were conducted at different days and hours, respectively. Thus, in substation A, measurements were conducted at 1 day corresponding to their maximum load (9h-13h). In substations B, C and D, those measurements were conducted at 1 day (10h-18h), 2 days (8h-17h, 14h19h) and 3 days (8h-19h, 8h-9h, 11h-18h), respectively. For example, as regards to the interior substation A composed by five distribution feeders denoted by A1, A2, A3, A4 y A5. Where, their elements are distributed on two floors. The first consist of power transformers and underground lines, while the second consists of breakers, battery bank, etc. The measurements are performed on both floors. Table 4 presents some values of electric and magnetic field, which were measured follow the trajectory presented in Fig. 2. Besides that, Fig. 2 shows the electric and magnetic field spectrum of the first floor of substation A. In addition, Fig. 3 shows those electric and magnetic field values corresponding to the first floor of substation A. In this Figure it is possible to see that the maximum magnetic field value for the first floor localized in the point 24 (3464.10nT), respectively, does not exceed the threshold values (417uT). On the contrary, regarding to the maximum electric field value for the first floor, this value is localized in the point 14 (48.5V/m), respectively. Similar to the previous case, this value does not exceed the threshold values (8333V/m).
48,5 42,1
40
32,8
30,5
30 23,3 20
18,1 13,8
10
0,5
7,5 2,8
0,4
0 1
3
5
7
12,3
12,4
9
8,7 6,3
0,8
18,1
1
1,5 14,9 7 4,7 1,1 0,6 0,5 11 13 15 17 19 21 23 25 27 29 31 33 35
0,7
3,4
# POINTS
(b) Fig. 3. Field total value of second floor of substation A, a) magnetic field, b) electric field.
Moreover, EMF measurement to which the human are daily exposed were also performed, which were registered in the villages and surrounding housing place, where magnetic field maximum values of 3.2uT were recorded. However, this value represent less than 5% of the threshold value for public exposure (83 uT). Regarding to the calculated values, by using the procedure presented in section 2.C, EMF values are estimated. For example, Table 5 presents the characteristics of the 22kV feeders corresponding to the exterior substation B and its 69kV feeder, respectively. On the contrary, Fig. 1.a shows the disposition of the feeders B1 and B4, respectively. In this context, by calculating the potential coefficients matrix and
Table V. Features of feeders B1 and B4. DIMENSIONS AND CURRENTS OF FEEDERS S/E B # FEEDER
B1
B4
Conductors radius (mm)
5.79
5.79
Distance among conductors (phases) (m)
0.92
0.92
Conductors height above the ground (m)
2.6
2.6
Phase current (A)
71
58
capacitance for length unit and thus to calculate the charges, the electric and magnetic field values are calculated. For example in Fig. 4, the electric and magnetic field of the distribution feeder B1 is shown. In this Fig. it is possible to see that the maximum electric field values of the feeder are 700V/m and 1210V/m, respectively. On the other hand, the maximum magnetic field value calculated to 1m from the ground is 10.3µT. From these results, it is clear that those values are smaller than those threshold values (417µT and 83µT for workers and public exposition and 8333V/m). As regards to the other substations, similar analysis was performed. Detailed description is presented in section 4.
(a)
IV. RESULTS ANALYSIS Similar analysis was developed using other distribution feeders and substations. From Table 6 to Table 8, electric and magnetic field values calculated and measured corresponding to other substation are presented. Regarding to the electric field values, their maximum values are registered in the feeder B4, which is due to that the distance between the ground level and the feeder conductors. However, the value registered constitutes 20% of the threshold value. As regards the substation C, their maximum value is registered in the feeder C6 with a value similar to 7.3% of the threshold value. While that in the substation D2, an electric field percentage similar to the 6% of the threshold values, is determined. On the contrary, the maximum magnetic field values are registered in their feeders. However, these values are smaller than 10% of those permissible. Besides that, as regards to the human exposure, which are located outside substations, the maximum electric and magnetic field values recorded were 100V/m and 2.76uT, respectively. Based on the above said, it is clear that these electric and magnetic field values measured are smaller than those values recommended by the ICNIRP and the Ministry of Environment. In addition, From Table 6 to Table 8, it is possible to see calculated electric and magnetic field values. Regarding to the distribution feeders B2, B3 and B5 similar values to those measured values were determined. Regarding to the magnetic field values calculated, a value of 2,15uT is determined. Similar analysis is developed for the other feeders, whose results are presented in previous Tables. Finally, from results it can be verify that these values to which the persons are exposed close to distribution feeders and electrical substations, are smaller than those threshold values.
(b) Fig. 4. Electric field corresponding to the distribution feeder B1, a) electric, b) magnetic. Table VI. Electric and magnetic field corresponding to the substation B. Electric field (V/m)
Magnetic field (uT)
Distribution feeder
Measured
Calculated
Measured
B5
866
905
--
6
B4
1401
1320
3,192
--
B3
1025
970
--
14
B2
1040
980
1,97
2,15
B1
1002
1210
3,25
10,3
Calculated
Table VII. Electric and magnetic field corresponding to the substation C. Electric field (V/m)
Magnetic field (uT)
Distribution feeder
Measured
Calculated
Measured
Calculated
C6
607
600
2,15
3,12
C5
224
239
3,09
4,84
C4
180
184
2,37
2,15
C3
187
186
2,68
1,9
C2
192
200
2,99
2,88
C1
303
307
2,78
3,79
Table VIII. Electric and magnetic field corresponding to the substation D. Electric field (V/m)
Magnetic field (uT)
Distribution feeder
Measured
Calculated
Measured
Calculated
D6
592
500
3,33
3,5
D5
298
267
2,58
2,1
D3
339
275
3,27
3,99
D1
416
492
3,33
3,46
On the other hand, in order to validity those magnetic and electric field values measured and calculated, a comparison with simulated values through the software QuickField 5.4 [17], which is based on finite elements, is developed. After the comparison, it was possible to determine that those values are very similar to the calculated and measured
values [18]. For instance Fig. 5 shows the electric field spectrum corresponding to the substation C.
[4] [5]
[6]
[7] [8]
[9] Fig. 5. Simulated electric field corresponding to the substation C.
V. CONCLUSIONS AND RECOMMENDATIONS Electrical and magnetic fields corresponding to 577 points distributed inside and out of four substations were developed, determining the maximum electrical and magnetic field values to which human are exposed. However, those values determined are smaller than those threshold values suggested by international standards. Therefore, in the study case presented, measurements indicate that level of exposition are under international and national requirements. In this study, it is possible to see that the most intense electromagnetic fields are produced in the input and output of distribution feeders. While, the electromagnetic fields in the output of the substations have low values. By using mathematic procedure, electric and magnetic field values are calculated. Thus, it was possible to note that these maximum values are similar to those measured values. Utilities focused to supply electric energy have to ensure the safe operation of installations, avoiding any health risk, and if any risk is presented, they must take adequate actions. For example, make measurements of electromagnetic fields. VI. ACKNOWLEDGMENT The authors gratefully acknowledge: Carrera de Ingeniería Eléctrica, Universidad Politécnica Salesiana. VII. REFERENCES [1]
[2] [3]
T. Barsam, M. Reza, A. Akbar, Effect of extremely low frequency electromagnetic field exposure on sleep quality in high voltage substations, Irian Journal of Environmental Health Science, pp. 9-15, 2012. N.Morales, Efectos Electromagnéticos y Sobrevoltajes en Sistemas de Transmisión. Seminario dictado en Quito-Ecuador, 22 de Marzo de 1996. http://www.dsalud.com/index.php?pag... sobre las actuaciones de la Fiscalía de Medio Ambiente de Madrid en relación con los centros de transformación de Majadahonda y Móstoles.
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VIII. BIOGRAPHIES John Morales was born in Cuenca, Equator, on January 05, 1985. He graduated as Electrical Engineer (B.E.) at the Universidad Politécnica Salesiana, Cuenca, Equator in 2007. Nowadays, he is Professor at Universidad Politécnica Salesiana, Cuenca, Equator. Furthermore, he is a Ph.D. candidate at the Universidad Nacional de San Juan, San Juan, Argentina, with a scholarship awarded by the German Academic Exchange Service (DAAD). His major interest included power system protection and signal processing. Arturo Suman Bretas was born in Bauru, Sao Paulo, Brazil, on July 5, 1972. Received the Ph.D. degree in electrical engineering from Virginia Polytechnic Institute and State University, Blacksburg, in 2001. Currently, he is a Professor of the Department of Electrical and Computer Engineering, University of Florida, Gainesville, USA. His research interest includes power system protection, control and restoration.
