EXPOSURE TO ELECTROMAGNETIC FIELDS IN BUILDINGS WITH TRANSFORMER STATIONS AND PREVENTIVE MEASURES

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Nov 16, 2013 (3 years and 6 months ago)

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EXPOSURE TO ELECTROMAGNETIC FIELDS
IN BUILDINGS WITH TRANSFORMER STATIONS
AND PREVENTIVE MEASURES

KRZYSZTOF GRYZ, JOLANTA KARPOWICZ,
PATRYK ZRADZIŃSKI


CENTRAL INSTITUTE FOR LABOUR PROTECTION
– NATIONAL RESEARCH INSTITUTE, WARSZAWA, POLAND




Abstract
Electromagnetic hazards from transformer stations located in dwellings and in buildings with work space are
discussed. Results of investigations (measurements and numerical calculations) of magnetic flux density of
50 Hz in the rooms adjacent to transformer stations are presented. Reduction of the level of magnetic field in the
vicinity of transformer stations is discussed.
Keywords: environmental engineering, transformer stations, magnetic field, electric field, environmental
exposure, reducing magnetic fields.

Introduction
Transformer stations, part of electrical installations supplying various buildings, are sometimes located in the
dwelling buildings and in the buildings with work space. People who work or live in the neighbourhood of
transformer stations tend to be concerned about heir health and safety because of the exposure to electromagnetic
fields produced by transformers. Such a concern might be related to the published hypothesis of adverse health effects
(especially of increased cancer risk) due to the chronic exposure to low-frequency magnetic fields, even relatively
weak (i.e. < 1 µT), present in the vicinity of overhead high voltage transmission lines. On the basis of available limited
evidence from epidemiological and laboratory studies, the International Agency for Research on Cancer (IARC)
classified extremely low frequency (ELF) magnetic fields as possibly carcinogenic to humans (group 2B) [4].
Consequently according to resolutions of the European Parliament and position of the World Health
Organization (WHO), available measures to reduce the level of magnetic field should be undertaken [2, 10].
Some European countries have even undertaken extensive administrative measures to reduce general public
exposure to magnetic field from high voltage (HV) power lines.

Aim of work
The aim of presented work was the assessment of electromagnetic fields produced by transformer stations inside
buildings and of the efficiency of preventive measures to limit the level of electromagnetic hazards.

Material and method
Object of investigations
Presented investigations were done in the transformer stations 15/0.4 kV of power of 160-1000 kVA and in the
adjacent working and living rooms. A transformer station is a part of electrical power system used to distribute
GRYZ K., KARPOWICZ J., ZRADZIŃSKI P.

electrical energy to individual customers. Transformers are supplied by medium voltage (in Poland usually
15 kV/50 Hz) and energy is delivered to final consumers by low voltage (in Poland usually 3-phases 0.4 kV/50 Hz).
In the transformer station the main source of electric field are medium voltage buses connecting the transformer
with the HV part of a power system. This field is shielded by the building walls. Potential hazard from exposure to
electric field might only cover workers present close to medium voltage installation. The main sources of magnetic
field are high current buses and cables connecting transformer with the low voltage end users of electric energy.
Magnetic field is not shielded by walls and might penetrate the rooms next to transformer stations.
Measurements method
Spot measurements of the RMS values of magnetic flux density (B-field) and electric field strength (E-field) were
performed in transformer stations. In rooms adjacent to transformer stations only magnetic field was investigated: spot
measurements of B-field RMS value and many hours’ (even 24-hours) registrations of the B-field RMS variability in
selected locations. Spot measurements in rooms next to transformer stations were done 15 cm from the wall or form
the floor, depending on the localisation of the room against the station (side or above). Maximum in the room space
B-field were sought.
The so-called ELF frequency range (40 Hz - 2 kHz) B-field meters (i.a. Narda EFA-300 and Enertech Consultants
EMDEX II) were used. Current loads of transformers were also monitored by the system’s indicators.
Numerical calculations method
Numerical modelling was done to obtain systematic information on the influence of technical parameters of low
voltage installation on the level of exposure in the investigated rooms. Simulations covered the distribution of
B-field in the surroundings of buses (cables) connecting transformer with a low voltage installation in selected
exposure scenarios. The simulations were performed by OPERA software (Vector Fields, UK). Magnetic field
spatial distribution was calculated by Biot-Savart law’s solver with the use of mesh generated by MODELLER
module.
Simulations were done for various configurations of the transformer connections with a low voltage installation (non-
insulated, separated buses or bunched insulated cables) and for different conditions of a phase current load
(symmetrical or non-symmetrical) and for realistic geometrical parameters.

Results
The measurements were done in the vicinity of over 25 transformers stations. B-field was measured in the rooms
next to transformer stations while current load identified by monitoring was from 10 up to 50% of the nominal
transformers output current. The results ranged from the fraction of microtesla up to 6 µT (fig 1) [3]. Magnetic flux
density inside the stations is significantly higher but in places where workers can be present does not exceed
permissible value for occupational exposure defined by ICNIRP’1998 guidelines (500 µT) [5]. Maximum measured
B-field RMS values for the above mentioned current load conditions reached up to 380 µT. B-field in the investigated
adjacent rooms and stations, estimated for the nominal load of transformers, were: 0.5-27 µT and 95-655 µT
respectively.
The level of B-field in the rooms next to transformer stations depends on the load of a transformer, transformer power
output type (insulated cables or non-insulated buses), and a distance of cables or buses from walls and ceilings of
station, it means the distance from B-field source to the space available for users of adjacent rooms.
Obtained results are similar to the results of investigations done in other European countries [6, 7, 9]. Exposure to
magnetic field of low frequency in the rooms neighbouring to transformer stations is below permissible exposure
level for the general public (according to European recommendation – 100 µT) [1]. Nevertheless such a level of
magnetic field is significantly higher than in rooms in buildings without transformers, where it usually is below
0.2 µT [3].
The data obtained from operators of city electrical system shown a typical range of current loads of transformers: from
50 A (the minimum current for all transformers) up to approx. 460 A (the maximum current in the case of transformers
of 630 kVA nominal power) [3]. The maximum phase current while transformer normal use is usually of (50-70)% of
EXPOSURE TO ELECTROMAGNETIC FIELDS OF TRANSFORMERS ...

nominal output current. Usually the phase load of transformers of 160-630 kVA nominal power is in the range of
50-150 A.

0.1
1
10
160 250 400 630
Power of transformer stations in kVA
Magnetic flux density (microT) .
Minimum
Maximum









Fig. 1. RMS values of magnetic flux density measured in the rooms neighbouring to transformer stations of power
of 160-630 kVA (transformer load 10-50 %)

The differences in current loads result from changes in power consumption depending on the time of the day or the
season of the year, and the number and kind of consumers of electric energy connected to transformer station. At least
a few-fold variations can be observed in a daily pattern of B-field distribution (fig. 2).

a)
0
2
4
6
8
10
12
12:00 06:00 12:00 06:00
Magnetic field density (uT)
Time








0
10
20
30
40
12:00 06:00 12:00 06:00
Magnetic field density (uT)
Time
b)








Fig. 2. An example of daily variability of B-field registered in a low voltage room of a transformer station
located: a) in office building; b) in dwelling building (start: 8 a.m.– stop: 8 a.m. next day)
GRYZ K., KARPOWICZ J., ZRADZIŃSKI P.


The level of B-field inside rooms next to transformer stations is determined by the current load of transformer, but
also by the architecture of transformer stations infrastructure (e.g. the dimensions of a transformer station room
and a low voltage switching-room, the height of a transformer station and the distance of a low voltage
installation from the next working and living rooms), and also a low voltage installation geometrical
configuration and electrical parameters as well. Usually the height of a transformer station is in the range of 3-4
m and the buses or cables of low voltage output are located 0.6-2.0 m under the ceiling of transformer stations.
Sometimes low voltage cables are put directly on the wall, being close to next room.
The kind of transformer power output (cables or buses) significantly influences the level of magnetic field nearby.
The use of insulated cables allows to close cables each other and to increase the efficiency of the phenomenon of
self-compensation of magnetic fields produced by currents of different phases. This phenomenon is stronger
when the distance between cables is smaller and full symmetry of phase current load occurs. The measurements
and calculations showed that self-compensation is the most efficient method to reduce B-field in the
surroundings of electrical installations inside buildings. In the case of transformer stations this method might
give a few fold decrease in B-field in the rooms next to transformers stations (fig. 3, 4).


0 100 200 300 400
cables, current load
symmetry
cables, current load non-
symmetry
Relative magnetic flux density B (%) .







Fig. 3. The example of numerical calculations of relative level of magnetic flux density in the point located in
the distance of 1.5 m from cables of symmetry and non-symmetry (20%) of current load in each phase (relative
value – magnetic flux density in the case of symmetric current level)


0
1
2
3
4
5
6
7
8
9
10
Time (12-hours registration)
Magnetic flux density, μT .
separated buses
bunched insulated cables









Fig. 4. The example of 12-hours registration of B-field RMS value in office room located over the low voltage
buses of transformer station – before (black line) and after (grey line) the changing the buses by the bunched
insulated cables
EXPOSURE TO ELECTROMAGNETIC FIELDS OF TRANSFORMERS ...


Conclusions
Electrical devices used in offices and dwellings are supplied by low voltage 230/400 V. Electric field strength in
the vicinity of low voltage installations decreases quickly with the distance and usually does not exceed 10 V/m
in the distance of 0.5-1 m. Higher electric field from medium voltage installations is shielded by the walls of
buildings. Therefore, from the point of view of biomedical investigations, the transformer stations and supplying
installations in buildings are not important sources of hazards related to E-field. In the space open for the public
electric field from a transformer station is smaller than electric field that typically exists in offices and dwellings
without transformers.
The analysis of electromagnetic hazards in the vicinity of transformer stations located in the buildings should refer
to the assessment of magnetic field, especially in the aspects of results of studies on a possible link between increased
cancer risk and magnetic field, given by IARC [4] and sustained lately by SCENIHR [8]. Many different factors
influence the level of magnetic field in the vicinity of transformer stations (e.g. changes of current load during a day
and a year, distance from a low voltage output being the source, and spatial configuration of installation). In the
aspects of the above mentioned hypothesis of potential adverse health effects caused by exposure to low frequency
magnetic field, available measures to reduce the exposure should be applied according to the so-called precautionary
principle or ALARA (As Low As Reasonably Achievable). The results of both measurements and numerical
simulations showed that the use of bunched cables instead of buses located in greater distance from each other and
balanced load of phases might reduce the exposure to B-field from transformer stations even of an order over 5-fold.

Acknowledgments
Research carried out within the National Programme on Occupational Safety and Health “Improvement of
safety and working conditions” (2008-2010, grant 4.S.38) granted by Ministry of Labour and Social Policy of
Poland (coordinator CIOP-PIB).

References
1. Council of the European Union Recommendation of 12 July 1999 on the limitation of exposure of the general
public to electromagnetic fields (0 Hz to 300 GHz), 1999/519/EC, Official Journal of the European
Communities, L 199/59.
2. European Parliament resolution of 4 September 2008 on the mid-term review of the European Environment
and Health Action Plan 2004-2010, http://www.europarl.europa.eu/sides/getDoc.do?pubRef=-
//EP//TEXT+TA+P6-TA-2008-0410+0+DOC+XML+V0//EN
3. Gryz K., Karpowicz J., Electromagnetic fields in buildings with transformer stations, Biuletyn WAT, vol.
LVIII, no 4, 2000, 125-137. In Polish.
4. IARC Non-ionizing radiation, Part 1: Static and extremely low-frequency (ELF) electric and magnetic fields,
IARC Monographs 80, IARC Press: Lyon, 2002.
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8. Scientific Committee on Emerging and Newly Identified Health Risks SCENIHR, Health Effects of
Exposure to EMF, Opinion adopted at the 28th plenary on 19 January 2009,
http://ec.europa.eu/health/ph_risk/committees/04_scenihr/docs/scenihr_o_022.pdf.
GRYZ K., KARPOWICZ J., ZRADZIŃSKI P.

9. Szabo J., Janossy G., Thuroczy G., Survey of Residential 50 Hz EMF Exposure to Transformer Stations,
Bioelectromagnetics, 28(1), 48-52, 2007.
10. WHO Environmental Health Criteria 238, Extremely Low Frequency Fields (ELF), 2007,
http://www.who.int/peh-emf/publications/elf_ehc/en/index.html
.