Magnetic Field Exposure Assessment of Electric Power Substation in High Rise Building
This study investigated the magnetic field survey of two electric power substations with power capacity of 1500 kVA, 11/0.4 kV which resided in a typical high rise office building in Kuala Lumpur, Malaysia. The purpose of a survey is to examine the exposure levels of magnetic radiation from its electric substation. The method used in this study includes comparative analysis of measured data using EMDEX meter and interfaces with EMCALC software for linear data acquisition. Results of two substations, which located in the basement floor and 15th floors were obtained throughout normal working hours. The measurements were arranged by applying two separate protocol conditions which is near and far fields. The specifications of measuring instruments used in this assessment were also displayed. Final conclusions were made based on the reduction rate calculation as an indicator values to determine the safety levels and comparison to the international standard guidelines.
October 20, 2010; Accepted: January 15, 2011;
Published: February 25, 2011
The issue of ELF magnetic field has been elongated controversial through a
lengthy discussion and review at the international level. Currently, the recommended
international standard limitation or restrictions formed by International Commission
Non Ionizing Radiation Protection (ICNIRP) had suggested 100 μT for public
exposure and 500 μT for occupational exposure (ICNIRP,
1998). Brief observations showed that several countries are still following
the official ICNIRP restrictions. Whilst, others are planning to establish a
stricter limits than the standard practice. In Malaysia, public confidence on
international limits is wedged into reasonable doubt as of numerous health effect
reports emerged in associated journals. One example of recent epidemiological
study in Malaysia given by Rahman et al. (2008)
have concluded that a significant increased risk has been hypothetically linked
with events of childhood acute leukemia on children live within a distance of
less than 200 m. A similar study conducted in Iran by Feizi
and Arabi (2007) is also revealed identical consequences on health problems
which associates distances as its major key parameter. These two studies are
relatively significant with the outcome acquired by Ahlbom
et al. (2000) published in the British Journal of Cancer. However,
the above studies are still inconclusive since it had not used dose exposure
as part of its critical examined parameters. Recent progress on the epidemiological
study conducted by Kroll et al. (2010) had further
improved the previous results by incorporating the magnetic field exposure and
distance parameters as part of its research analysis. The study reveals that
there is a little increment in relative risks for childhood leukemia but statistically,
it is still remain insignificant and consistent with the previous study which
was conducted in the year 2005. Engineering branches had consistently monitored
the dose exposure with the use of measurement survey protocol to those ELF source.
Recent engineering study given by Joseph et al. (2009)
showed that the ELF-EMF research are still remain a relevant subject to be investigated.
Joseph et al. (2009) has performed ELF-EMF magnetic
exposure by executing measurements on large substations 150 and 36/11 kV situated
in prominent urban areas. The outcome shows that the exposure values between
0.051 to 13.17 μT for electric and magnetic fields which approximately
are still within the ICNIRP standard. Similar study had also been conducted
by Safigianni and Tsompanidou (2005, 2009)
which evaluated ELF exposure specifically within the indoor and outdoor electric
power substation rated at 20/0.4 and 150/20 kV. Authors of these papers had
greatly elaborated the electric and magnetic fields exposure survey with special
intention to examine the safety levels by comparing their findings to the international
standard guidelines. Holbert et al. (2009) determine
the magnetic field exposure produced by underground residential distribution
system including exposure from the pad-mount transformer, junction boxes and
service entrance panels. The result concludes that magnetic field is reduced
to less than 0.3 μT at typical distance of 1 m. More comprehensive data
can be viewed given by Farag et al. (1999) where
occupational exposure assessments were conducted for various types of ELF sources
and conditions. These include the exposure values from substations, power lines,
underground cable, manhole, low voltage risers etc. Hamza
et al. (2005) evaluated the magnetic induction inside human at high
voltage substations in Egypt of 220/66 kV open-air substation. It also performs
calculation of induced electric field and current densities to the human body
using approximation to human body parameters such as width and height. Higher
voltages of 380/154 kV substations and power lines in Turkey were studied by
Ozen (2008). Detail measurements were performed at the
switchyard area, control room, incoming and outgoing power lines. Data on exposure
values were analyzed and the final results are satisfying with the ICNIRP values.
Burnett and Yaping (2002), ESAA (1996),
Baishiki and Deno (1987) and Sandstrom
et al. (1993) had indicated problems with ELF exposures from the
electrical installations in high-rise buildings. The paper discusses ELF magnetic
field exposure as a problem source to equipment interference issues prior to
propose mitigation for field reductions. Latest developments of magnetic field
exposure evaluation are still continued until today with various approaches
and interest in characterizing the fields. For example, Proios
et al. (2010) proposed a study on magnetic field exposure near to
the compact kiosk type substation, Ellithy (2010) and
Said et al. (2010) had continued with the standard
methods on magnetic exposure from the 220 kV gas insulated substation and distribution
substation respectively, while Mazzanti (2010) had proposed
with innovative heuristic formulas for predicting magnetic exposure from the
transmission lines. All these studies have proved that the ELF magnetic field
exposures are still valid among the scientific community with curiosity to understand
the relationship between magnetic exposure and human health. In this study examination
of the ELF magnetic field caused by the operation of power substation 11/0.4
kV in high-rise office building located in Kuala Lumpur was carried out. Twenty
multi-storey office building were due to the reports of interference problems
near to the substation. This study of magnetic exposure is crucial because of
its nature which is quite unusual to be conducted on high-rise office building
environment. The average current consumption in the high rise building is relatively
high which approximately in the range of 1000 to 2000 A. Since, distance is
a limiting factor for field reduction in most modern building high exposure
magnetic field is expected to occur anywhere in the office environment. Due
to high demand of administrative and management operations which includes reliable
power supply, another substation scale was built at the upper levels simply
as to ensure stability and as well as mitigating the power losses. In this study,
basic data for the substation is given and a brief description of the instruments
used for the measurements is also provided. The main results of the field measurements
are presented in relevant tables and diagrams. These results are evaluated according
to generally accepted guidelines and final conclusions concerning safe public
and occupational field exposure are set out.
MATERIALS AND METHODS
Substation layout at 15th floor: A typical high-rise office building
in Malaysia normally is equipped with separate rooms which are located at the
upper floor of the buildings. It consists of HT switching room, Transformer
room and LV switching room. Figure 1 shows the area which
covering one part of the whole building on the 15th floor. Above the substation
a computer server room and administrative office department which employs 20
clerical staffs. The transformer room has a dimension of 5 m height clearance
and an area of (5.2x6.7) m2 which accommodates only one power transformer.
It has one opening window suit for ventilation purposes. The operating voltage
for the transformer is 11/0.4 kV with 1500 KVA rated power and was dedicated
to serve the upper level units of various office departments.
||Layout area of the substation located at the 15th floor
The transformer unit is located in an area of (1.8x2.7) m2 with
2.5 m height and is using dry insulation type and the load reading on the low
voltage side is 1200 A. Next to the transformer room is a room occupies by one
unit of High Tension Vacuum Circuit Breaker (HT-VCB) system. The room of an
area of (4.6x4.9) m2 wide and 5 m height clearance is provided with
a single window for air ventilation purposes. The HT-VCB unit is placed at the
center of the room. The circuit breaker systems have no numerical analog or
digital indicator output to be recorded. In-front of the transformer room is
a Low Voltage switching panel room which consists of several switching channels
for controlling transformers, bus couplers and circuit breakers. The room is
(4.0x12.0) m2 wide and same height clearance as previous room. The
system was connected using Triple-pole and Neutral (TPN) bus-bar risers of low
voltage distribution with large multi-core cables.
Substation layout at basement floor: Similar design substation was built
at the basement floor of the building. Figure 2 shows the
area of the basement substation which is divided into transformer room, HT switching
room and LV switching room. The transformer room was occupied by two hermetically
seal power transformers with oil insulation type as indicated in the diagram.
The operating voltages of both transformers are 11/0.4 kV with rated power of
1500 KVA. The transformer unit dimension is (1.4x2.4) m2 with 2 m
height. The room has dimension of (5.0x12.0) m2 and vertical clearance
of 7 m high. In-front of the substation is a walk-way area for public and directly
one floor above the substation is an office area. The HT-VCB room occupies few
panels of circuit breaker systems with room size of (6.0x8.5) m2
and 7 m height clearance. The Low Voltage room consists of few LV switch panels
and occupies the room size of (2.5x12.5) m2 and sharing the same
vertical height clearance as previous room. The current values at the High Voltage
is around 100 A while at the Low Voltage side is Ia = 40 A, Ib
= 20 A, Ic = 30 A. The substation systems were connected using large
multi-core cables through risers which available in each rooms.
In this project, magnetic exposure measurement was undertaken simultaneously
to cater for both substation sites (namely substation located in the basement
and on the 15th floor) which resides in the twenty storey new office building
in Kuala Lumpur. The building is occupied by an institute under the Malaysian,
ministry of education. Concern over the EMF exposure were quite significant
among the staffs in 15th floor in relation to the incidents where health problems
and computer jittering were prevalent.
||Layout area of the substation located at the basement floor
Measurement surveys were scheduled on December 05, 2008 to several points
of measurement locations namely; transformer room (15th floor : 10.09 a.m);
HT switch room (15th floor : 10.18 a.m); LV switch room (15th floor : 10.34
a.m) followed by transformer room (Basement : 10.55 a.m); HT switch room (Basement
: 11.04 a.m); LV switch room (Basement : 11.07 a.m).
Methods: The instruments which used for the low frequency magnetic field
is the EMDEX-II meter. The meter was constructed by Enertech Consultant Company
with technical support by EPRI, US. The meter is a portable device interfaced
with EMCALC software tools as its analyzing processer. The specification of
the meter is shown in Table 1. The EMDEX-II meter is steadily
used for exposure assessment in substation and power lines. It can be easily
to desktop office computer or laptop with EMCALC software using interface cable
or adapter cables. It can store up to 20 distinct data set measurements which
can be collected over a period of many days prior to downloading them from the
battery operated unit. The software is used for data files transfer, storage
and analysis. The EMDEX-II meter is accompanied by several accessories which
will be required before starting the measurement. Two types of measurement modes
can be uploaded in the software which is known as Standard mode or Linear Data
Acquisition (LINDA) mode. The differences between these two modes are Standard
mode is measuring the magnetic field using time based where else for LINDA is
measuring the field against distance. Once the software uploading is completed,
the meter is required to be fitted and connected to the special wheel which
finally becoming a complete set for magnetic field measurement.
||(a) Sample example of magnetic field plotted resultant in
x direction, (b) Sample example of magnetic field plotted resultant in y
direction and (c) Sample example of magnetic field plotted resultant in
|| Comparison of the B resultant taken from various points
The instrument measuring the power frequency magnetic fields use a property
described by Faradays Law in Eq. 1:
Since the meter measures the B field of the 3-axis concurrently, the software
is also capable of producing the resultant field of the three components as
indicated by example in Fig. 3a-c, respectively.
Three phase systems normally produced multiphase fields current. If these currents
are at the power system frequency, the locus of the B vector at any point is
generally an elliptically polarized field. Because single phase meter is not
practical to use for each phase calculation, a three-axis meter is recommended
instead for immediate calculation on the resultant value of the three phase
magnetic field. Determination of the resultant value is given by Eq.
2 (Horton and Goldberg, 1995):
After considering the harmonic effects, the resultant field may be redefined
in terms of the rms values of the fields along the x, y and z axes given in
the Eq. 3:
RESULTS AND DISCUSSION
Results from this measurement survey mainly concentrated on three main areas
in both substations namely; the transformer rooms, HT switch rooms and LV switch
rooms. Figure 4a and b display the resultant
of magnetic field profile for the transformer rooms, while Fig.
5a and b display the resultant of magnetic field profile
for the HT switch rooms followed by Fig. 6a and b
which display the resultant of magnetic field profile for the LV switch rooms,
for both 15th floor and basement floor substations respectively. Results in
the above figures were taken during normal office operation which is between
10:00 a.m. to 12:00 a.m. with 75% of power capacity loading. The above figures
also represent the magnetic exposure for a typical distribution substation which
is less than 20 μT and 80% lower than the ICNIRP standard (ICNIRP,
1998) for public exposures. The field characterizations can be understood
more significantly by employing a set of statistical variables as shown in Table
2. Table 2 confirmed that the magnetic field produced
by the substation in the upper level floor is higher than the one that resided
at the basement floor. Special interests of the statistical variables that is
the mean and median values has to be addressed since these values representing
the total exposure of flux density distributions in the whole substations area.
It is noted that from the table, most of the maximum magnetic field flux density
values recorded at 15th floors are far below the reference level for safe public
and occupational exposure which is from 55.84 to 10.79 μT, with its standard
deviation ranges of 3.08 to 11.10 μT. The results for mean and median values
for the international standards are also relatively consistence with lower limit
ranges from 3.91 to 28.69 and 3.08 to 26.72 μT, respectively. While substation
in basement floor renders mean exposure value ranges from 1.23 to 19.75 μT;
maximum exposure range is between 3.93 to 34.72 μT; median range is between
0.76 to 18.24 μT with its standard deviations between 1.02 to 7.95 μT.
||(a) Magnetic field profiles of transformer room located at
15th and (b) Magnetic field profiles of transformer room located at the
|| Comparison of the B resultant taken from various points
Even in the case of proximity exposure to the equipment as shown in Table
2, the magnetic flux density values are still remaining low and ensure safe
condition for the technicians. In Table 3, the reduction rate
calculation also shows that the magnetic flux density of both substations is
greatly reduced by certain numbers of percentages. The calculation values were
taken from the nearest point of HV equipment and at the point of room perimeter.
From the Table 3, great intention is given to the transformer
room since both transformers are different type design. For example, the transformer
rooms in the 15th floor shows reduction rate of 75.3% while in the basement
floor reduction rate are much higher with 86.5% for transformer 1 and 82.2%
for transformer 2 while other rooms have shown variety of magnetic exposures.
In this case, the reduction rate is used as a variable to monitor and control
the hazardous effects of magnetic field to the environment.
|| Reduction rate by distances
Higher reduction rate means better magnetic exposure and reduce any physical
risks to the other regions. Table 3 had also indicated that
electric substation installed with hermetically seal oil-insulation transformer
producing lower exposure magnetic field in comparison to the substation installed
with cast resin dry-insulation transformer. The results obtained from these
measurement surveys were also correlated and significantly agree with the findings
obtained from Said et al. (2010) and Ellithy
(2010). Said et al. (2010) had reported in
statistics that the mean value for all four substations were in the ranges between
(1.67-4.99 μT); median range between (12.7-1.0 μT); and its maximum
exposure range between (61.6-44.7 μT), except one outliers taken from substation
P.E 8.17 which having 200 μT in one of its measurement points. By looking
into lower resultant values of standard deviations which are ranged between
(5.21-17.8 μT), the measurements conducted by Said
et al. (2010) can be considered as within the acceptable levels.
While Ellithy (2010) in his statistical analysis had
observed the mean values is between (0.39-7.73 μT); maximum exposure is
between (2.14-67.52 μT); and its standard deviation spans well in the range
between (0.32-9.69 μT). This also exhibits example of normal magnetic exposures
for a typically large substation. In terms of distance measurements, relative
results were observed even though the capacity and layouts of substations were
in different scale. For example, Joseph et al. (2009)
had indicated that safety distances for such typically large substation able
to reach 0.4 μT when the average distance is 7.4 m (substation type 1)
and 8.1 m (substation type 2) while Proios et al.
(2010) observed that for a typically compact substation requires distance
clearances of 1.5 m to reach magnetic exposure at 2.1 μT (when door closed)
and 3.0 μT (when door opened). These results are significant with the outcome
shown in Fig. 4a-b, 5a-b
and 6a-b as observed by the author which
is varied within the range between 1.4 m to 2.0 m for low magnetic exposure
in typical distribution substation. Nevertheless, the results are merely contradictive
with the studies conducted by Rahman et al. (2008),
Feizi and Arabi (2007) and Ahlbom
et al. (2000) which observed far distance requirements in determining
safety magnetic exposure. Rahman et al., (2008)
in his result suggests that distance as far (>200 m) is necessary to clarify
as safety zones while Feizi and Arabi (2007) suggest
much farther (>500 m). Ahlbom et al. (2000)
had emphasized for lower magnetic exposure (0.4 μT) which many had considered
as unreasonable case since the suggested values are categorized as background
exposures. Another factor of harmful effect from magnetic exposure is the compatibility
studies as performed by Burnett and Yaping (2002). Magnetic
exposures which are above 1 μT as mentioned by Burnett
and Yaping (2002) served more than human effects mainly because of computer
jittering case in offices. Their exposure value findings are identical with
the third party reports such as Vitale (2008) which also
had indicated the same figures. The said dosimetric value requires more than
95% of reduction rate to arrive at 1 μT which finally commits layout problems
for building designers. In fact, it is purely difficult on normal condition
to obtain such dosimetric values as stated in Table 3 which
refers to highest reduction as much as 86%. Therefore, other forms of mitigation
methods should be sought out in relation with the proposed dosimetric value
for new safety design for substation resided in building.
Magnetic field measurement survey on ELF exposure is considered to be one of
important approaches in assessing the public and occupational safety. While
some studies are suggesting certain effects as lower as 0.4 μT (Ahlbom
et al., 2000) for humans and 1 μT for computer appliances (Burnett
and Yaping, 2002), more data is required in order to further assessing the
risk and its significant effects. This study reports the results of magnetic
field exposure from electric substation which resided within the high-rise office
building having close proximity to the office area. Primarily the measurements
were made to ensure that the magnetic field exposure does not violate the international
standard and guidelines. Secondly to investigate the exposure levels in response
to any problems or effects aroused from this cause to the nearer offices. The
findings obtained from a series of measurements concluded that the measured
field values are within the international acceptable reference values, indicating
that the field is harmless and safe for the working personnel.
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