ELECTRIC AND MAGNETIC FIELD MEASUREMENTS IN AN OUTDOOR ELECTRIC POWER SUBSTATION

Transcription

1 ELECTRIC AND MAGNETIC FIELD MEASUREMENTS IN AN OUTDOOR ELECTRIC POWER SUBSTATION Anastasia S. Safigianni and Christina G. Tsompanidou Democritus University Thrace, Electrical and Computer Engineering Department, GR 67100, Xanthi, Greece ABSTRACT This paper examines the electric and magnetic field in an outdoor electric power substation 150/20 kv. First, the results of previous relevant research studies, as well as the reference levels for safe general public and occupational exposure, given by the International Commission on Non- Ionizing Radiation Protection (ICNIRP), are presented. Next, basic data for the examined substation along with a brief description of the field measurement instruments used are given. The results of the field measurements in the area of the above mentioned substation, accompanied by relevant tables and diagrams follow. Final conclusions concerning safe public and occupational exposure to these fields are set out. KEY WORDS electric and magnetic field measurements, extremely low frequencies, outdoor electric power substation, safe public and occupational exposure. 1. Introduction Though exposure to electromagnetic fields is not a new phenomenon, environmental exposure to man-made electromagnetic fields has been steadily increasing during the 20 th century, as growing demand for electricity, everadvancing technologies and changes in social behaviour have created more and more artificial sources. While the enormous benefits of using electricity in everyday life and health care are unquestioned, during the past 25 years the general public has become increasingly concerned about potential health hazards of exposure to electric and magnetic fields at extremely low frequencies (ELF). Since about 1980, these 50/60 Hz magnetic fields (and their frequency harmonics) have been suspected of causing various types of negative health effects. Internationally accepted scientific groups in [1]-[6] believe that the data are not sufficient to support the conclusion that ELF electric or magnetic fields (EMFs) in the workplace or at home cause cancer or lead to reproductive and developmental abnormalities or to learning and behaviour problems. The most accurate answer to the question do ELF EMFs present a human health hazard? is that numerous studies around the world have failed to produce technically convincing evidence of such a hazard. Although some health effects have been statistically related to ELF EMFs exposure, these effects are poorly understood and may exist only as statistical or scientific errors. The expert working groups prepared a report [7] that evaluates possible health effects from exposure to static and ELF EMFs. The authors of [1], [8], [9] identify gaps in knowledge that require more research to improve health risk assessments, including statistical, epidemiological, experimental, laboratory and other studies. Generally accepted guidelines have been established for safe public and occupational exposure to power-frequency EMFs. The reference levels for general public exposure to 50/60 Hz EMFs are, according to the ICNIRP guidelines, [2]: For the electric field strength, E<5 kv/m For the magnetic field strength, H<80 A/m For the magnetic flux density, B<100 µτ These levels for occupational exposure are: For the electric field strength, E<10 kv/m For the magnetic field strength, H<400 A/m For the magnetic flux density, B<500 µt Various researchers have treated at great length the measurement and model elaboration for the determination of the EMFs generated by transmission lines, particularly by lines that cross residential areas. Other researchers have considered it convenient to characterize the EMFs inside residences. But there is little data available in the literature dealing with EMFs measurements and calculations in power stations and substations of various voltage levels. Specifically in [10], measurements of magnetic field profiles around a specific substation have verified that adding a substation under an existing transmission line does not increase the magnetic field beyond the substation boundary. In [11], magnetic fields in a 187/66-kV substation are calculated for various current conditions and described in terms of contour plots. In [12], the magnetic fields generated by a distribution substation were measured and calculated, based on a computer model that takes into account currents in the grounding systems, distribution feeder neutrals, overhead ground wires and induced currents in equipment structures and ground grid loops. In [13], measurements

2 of the field strength in a combined cycle gas turbine power station are described, for both the steady state situation and the process of starting the generator. The measurement results are compared to the established safe exposure limits. In [14], an integral approach method is used for magnetic field measurements in a 69 kv SF 6 gas insulated substation in Taiwan. Paper [15] shows the results of an initial series of measurements carried out to determine the levels of EMFs in various indoor and outdoor environments (power transmission lines, power cables, substations, domestic electrical equipment) associated with occupational and residential exposure. In [16] and [17], shielding is proposed for an in-house secondary substation in Sweden in order to mitigate the magnetic field. This shielding really reduces the field values compared to the values measured before the shielding installation. We must, however, point out that these values are already far below today s accepted safe exposure limits. Finally, the authors of [18] and [19] report the results of ELF EMFs measurements within several indoor power distribution substations 20/0.4 kv in Greece and an indoor electric power substation 132/11.5 kv in Cyprus respectively. Both these measurements and the elaboration of the relevant results show that the magnitudes of the measured field values are within recognized guidelines. This paper examines the ELF fields at an outdoor electric power substation 150/20 kv in Xanthi, Greece. First, basic data is given for this substation and a brief description of the instruments used to take the measurements is provided. Indicative results of the EMFs measurements in the above-mentioned substation, accompanied by relevant tables and diagrams, follow. Final conclusions concerning safe public and occupational exposure to these fields are set out. 2. Substation description The ELF field measurements have been taken at an outdoor electric power substation 150/20 kv, 6696 square meters in area, situated near a motorway in Xanthi, Greece. There is a ditch preventing access from the motorway to the substation area. Only small industries and commercial enterprises exist in the surrounding area, at an appropriate distance from the substation. A ground plan sketch of the substation area, on a scale of one to five hundred centimeters, is given in Fig.1. Two identical power transformers are installed in the substation area. Each transformer has a nominal power of 40(50) MVA/ ONAN(ONAF), a nominal voltage rate of 150/20 kv and it is fed by one of the circuits of a 150 kv double circuit transmission line entering the substation. Three capacitor banks are also installed in the substation area. Banks C1 and C2 have a nominal power of MVA and bank C3, which is newer than C1 and C2, has a nominal power of 13.3 MVA. The first two banks are connected to the medium voltage buses via coils, having a nominal inductance of 3x0.55 mh and a nominal shortcircuit current of 5 ka. The goal of these coils is the reduction of the voltage rise, when the capacitor banks are connected to the network in order to improve the power factor. Thirteen SF 6 power switches are installed in front of the transformers, having a rated voltage of 170 kv. An equal number of medium voltage lines (20 kv) set off these switches. There is also a supervision room with manually operated control and measurement instruments within the substation area. 3. Electric and magnetic field measurements One of the instruments that can be used for the measurement of low frequency magnetic and electric fields in the frequency range 5 Hz to 30 khz is the EFA-3 analyser. The analyser, constructed by the Wandel & Goltermann Company, supports recording, storage and evaluation functions for sequences of results and can measure electric and magnetic fields. Its characteristics are: isotropic magnetic field probe built-in omni-directional (isotropic) measurements measurement ranges from 5 nt to 10 mt, 0.1 mg to 100 G and 0.1 V/m to 100 kv/m true rms or peak value measurement spectral detection of field components built-in frequency counter preset table audible and visible alarm thresholds user-definable filter frequency timer-controlled recording of results calibrated external electric and magnetic field probes. The measurements were carried out using the three dimensional isotropic probe that ensures omni-directional measurements. The EFA-3 analyzer has the capability of storing 4000 result values. These values can then be output via the built-in printer interface directly to a printer or clearly displayed on a PC fitted with appropriate software. Long-term measurements lasting up to 24 hours can be performed easily using the timer-controlled recording function. During the measurements, the mean load of each one of the two substation transformers was about 32 MVA. First, indicative measurements were carried out in the substation area, in order to decide the closeness of the final measurements. It was decided to take measurements every 6m (horizontally and vertically) in the 150 kv buses area, every 3m in the transformers area, every 2m in the capacitor and the switches areas, every 1m in the supervision room and every 5m in the wider substation area, taking into account the diversification of these measurements. So approximately 300 measurement positions were selected. Three measurements correspond to each position. The first one refers to a head height (1.80 m), the second to a waist height (1 m) and the third 21

3 to the floor surface. All the measurements were carried out at 50 Hz frequency. Table 1 shows the maximum measured magnetic flux density values (rms values in µτ) in the substation area, Fig. 1. By measuring currents and magnetic flux density values in one substation position, every 15 minutes over a 24 hour period, it was found out that the relation between these two quantities is linear. Taking into account the fact that the mean load of the substation transformers was about 32 MVA during the measurement period, its relation to the nominal transformer load (50 MVA) is about Therefore, given in the last column of Table 1, are the measured magnetic flux density values extrapolated to the nominal transformer power. Diagrams are given in Fig. 2 and Fig. 3 showing contour and surface maps for the magnetic flux density at waist height in the substation area and inside the supervision room. The diagrams for head and floor heights are relative. The diagrams have been plotted taking into account the reference levels for safe occupational exposure, because only the electric company technicians can enter the substation area. Access to the public is forbidden. From Table 1 and Fig. 2 it is obvious that the measured and to a much lesser degree, the extrapolated magnetic flux density values exceed safe public and occupational exposure limits in the areas of the capacitor banks C1 and C2 (the values with bold letters in Table 1). These high values were measured near the coils connecting the capacitor banks to the medium voltage buses. The aforementioned excesses are due to the fact that these coils do not have iron core, the lack of which results in great leakage. It must be pointed out that these high values are recorded at close proximity to the cables feeding the coils and they are greatly reduced (at about 50 µt) within a distance only 0.5m from the coils. The measured magnetic flux density values throughout the remaining substation area and in the ring zone are far below the safe public and occupational exposure limits. Apart from the magnetic flux density, the electric field strength was measured in the substation area. The electric field sensor was placed at the measurement positions and connected to the main instrument with a 10 m fiber optic cable. This connection and the distance between the sensor and the main instrument were necessary in order to ensure that the electric field strength would not be perturbed by the presence of persons. The maximum electric field strength value was measured near the high voltage side of the transformer T1. It was equal to 4.3 kv/m and therefore lower than the safe public and occupational exposure limit. Diagrams are given in Fig. 4 showing contour and surface maps for the electric field strength throughout all the substation area. 4. Evaluation of the measurement results and conclusions The measured magnetic flux density values are in their majority far below the reference level for safe public and occupational exposure. No serious differentiation was noted in these values in relation to the body height. In two positions only, near the capacitor banks, the above levels were violated. But it must be noted that firstly, these values greatly decrease with distance and secondly, the positions where these high values were measured, are not occupied by technicians when the capacitors are under voltage. In addition it must be noted that the measured magnetic flux density values are very small in the supervision room, where the supervisor of the substation is working and in the ring zone, where the public has access. The measured electric field strength values are in their entirety below the reference level for safe public and occupational exposure. So we can say that the measured field values are substantially within recognized guidelines, suggesting that these values are not dangerous and, therefore, are no cause for concern among the public or working personnel. References [1] Possible health hazards from exposure to powerfrequency electric and magnetic fields A COMAR Technical Information Statement, IEEE Eng. Med. Biol., 19(1), 2000, [2] International Commission of Non Ionizing Radiation Protection (ICNIRP): Guidelines for limiting exposure to time-varying electric, magnetic and electromagnetic fields (up to 300GHz), Health Physics, 74(4), April 1998, [3] Australian Radiation Protection And Nuclear Safety Agency: Electricity and health. [Online]. Available from: [Last updated June 2003] [4] National Research Council, USA: Possible Health Effects of Exposure to Residential Electric and Magnetic Fields (Nation Academy Press, Washington DC, 1997). [5] National Research Council, USA: Research on Power-Frequency Fields Under the Energy Policy Act of 1992 (Nation Academy Press, Washington DC, 1999). [6] CIGRE Position Statement: Power-Frequency electromagnetic fields and health, on behalf of the technical committee, Electra, 196, June [7] M. H. Repacholi, B. Greenebaum, Interaction of static and extremely low frequency electric and magnetic fields with living systems: health effects and research needs, Bioelectromagnetics, 20(3), 1999, [8] A. W. Preece, J. W. Hand, R. N. Clarke, A. Stewart, Power frequency electromagnetic fields and health. 22

4 Where s the evidence?, Physics in Medicine and Biology, 45(9), 2000, [9] R. T. Hitchcock, R. M. Patterson, Radio-Frequency and ELF electromagnetic energies. A handbook for health professionals (Van Nostrand Reinhold, New York, 1995). [10] W. E. Feero, J. Yontz, J. H. Dunlap, Magnetic fields remote from substations, IEEE Trans. on Power Delivery, 4(3), 1989, [11] L. Hayashi, K. Isaka, Y. Yokoi, Analysis of 60-Hz magnetic fields near ground level in 187-kV switchyard of a 187/66-kV AC substation, IEEE Trans. on Power Delivery, 7(1), 1992, [12] W. K. Daily, F. Dawalibi, Measurements and calculations of electromagnetic fields in electric power substations, IEEE Trans. on Power Delivery, 9(1), 1994, [13] B. Jaekel, Low frequency magnetic fields near energized components of power stations, Proc. of the International Wroclaw Symposium on Electromagnetic Compatibility, Wroclaw, Poland, 1998, [14] Lu Shun-Li, Lin E. Ghin, Ching-Lien, Huang Tsung- Che Lu, Power substation magnetic field measurement using digital signal processing techniques, IEEE Trans. on Power Delivery, 14(4), 1999, [15] A. S. Farag, M. M. Dawoud, T. C. Cheng, J. S. Cheng, Occupational exposure assessment for power frequency electromagnetic fields, Electric Power Systems Research, 48, 1999, [16] E. Salinas, L. Aspemyr, J. Daalder, Y. Hamnerius, J. Luomi, Power frequency magnetic fields from inhouse secondary substations, Proc. of the CIRED-99 Conference on Electricity Distribution, Technical Reports, session 2, Nice, 1999, [17] E. Salinas, Magnetic field management at the transmission and distribution stages, Proc of the Fifth International Power Engineering Conference, Singapore, 2001, [18] A. S. Safigianni, C. G. Tsompanidou, Measurements of electric and magnetic fields due to the operation of indoor power distribution substations, IEEE Trans. on Power Delivery, 20(3), 2005, [19] A. S. Safigianni, A. Kostopoulou, Electric and magnetic field measurements in an indoor electric power substation, Proc of the 4 th Japanese - Mediterranean Workshop on Applied Electromagnetic Engineering for Magnetic, Superconducting and Nano Materials, Cairo, Egypt,. 2005,

5 capacitor bank C3 capacitor bank C2 Transformer T2 supervision room SF 6 switches 150kV buses 150kV double circuit line Transformer T1 N capacitor bank C1 substation entrance Fig. 1 Ground plan sketch of the substation area Table 1 Maximum magnetic flux density values in the substation area Substation areas Maximum measured magnetic flux density values Bmax, (µτ) Magnetic flux density values extrapolated to the nominal power, (µτ) 150 kv buses Transformer T Transformer T Capacitor bank C Capacitor bank C Capacitor bank C SF 6 switches area Supervision room Wider substation area Ring zone

6 Distance y, (m) a. Contour map Distance x, (m) Magnetic flux density B, B, (µτ) (µt) Magnetic flux density B, (µτ) Distance y,(m) Distance x, (m) b. Surface map Fig. 2 Magnetic flux density distribution at waist height in the substation area 25

7 5.00 Distance y, y, (m) (m) a. Contour map Distance x, x, (m) (m) Magnetic flux density Β, (µτ) Magnetic flux density B, (µτ ) Distance x, (m) Magnetic flux density Β, (µτ) Distance y, (m) b. Surface map Fig. 3 Magnetic flux density distribution at waist height inside the supervision room of the substation 26

8 Distance Distance y, (m) y, (m ) a. Contour map Distance x, x, (m) Electric field strength E, (kv/m) Electric field strength E, (kv/m) Electric field strength E, (kv/m ) Distance y, (m) Distance x, (m) b. Surface map Fig. 4 Electric field strength distribution in the substation area 27