Magnetic flux density in the apartment
Transformer substations located close to the living areas could induce high long-term exposures to extremely low frequency magnetic fields of nearby inhabitants. Some of the researches have shown increased risk to childhood leukemia associated with the long term exposure to the elevated levels of magnetic fields.
With the increasing public interest, prudent avoidance and good practice it is becoming more and more important to minimize the exposure of the nearby inhabitants.
The most cost efficient way of achieving this goal is to include the magnetic field mitigation in the early stage of the planning and construction/reconstruction process of each transformer substation.
Using numerical modeling we studied different variants of reconstruction of a typical transformer substation located in the basement of a block of flats with a living room located right above the transformer. To obtain a comprehensive snapshot of current situation before the reconstruction we performed detailed spot and 24-hour measurements of magnetic flux densities.
After the reconstruction, numerical results were evaluated by the comparison of the numerical results with the results of the measurements after the reconstruction and numerically predicted 10-fold reduction factor of the highest values of the magnetic field was confirmed.
- Materials and methods
- Discussion and conclusions
International Agency for Research on Cancer included extremely low frequency (ELF) magnetic field among the possibly carcinogenic factors for humans .
This decision is based on the results of two studies [2, 3], which showed that elevated 24 hours averaged values of ELF magnetic field (<0.3-0.4 μT) do increase the risk of childhood leukemia but the biological mechanisms of this risk remain un-known [1, 4, 5, 6].
Minimization of the magnetic field is possible either by moving all the parts with high currents away from the apartments, by changing the geometry of the conductors, especially the low voltage (LV) busbar, by replacing components of the TS (with components with lower emissions) and finally it is possible to shield the nearby apartments with materials with either high permeability or high conductivity.
Materials and methods
Typical TS with nominal power 630 kVA, nominal voltage 10/0.4 kV is located in the basement of the residential apartment building. In the first room there is transformer (Figure 1) whereas in the second one both 20 kV and low voltage switchgear is located.
We were contacted by the owner of the apartment above the TS to estimate the field levels inside the apartment. First measurements shoved magnetic flux density up to values of 15 μT inside the apartment above the TS.
Magnetic field is linearly correlated to the actual current load, but this can vary during the day depending on the present use.
To obtain the detailed snapshot of the magnetic field in the apartment above the TS it is therefore not enough to make only spot measurements, but also 24-hour measurements to evaluate the time variability of the magnetic field and determine worst case condition.
For spot measurements we have used Wandel & Goltermann EM field analyzer EFA-3 with the B field probe. For 24-hour measurements we have used automatic measurement station PMM 8055 which measures the magnetic flux density continuously 24 hours per day.
It consists of measurement probe for ELF magnetic flux density HP-051, control unit with the GSM modem to send the measurements from the measurement station to the server connected to the internet, housing with solar cells and accumulator. After the data are automatically transferred to the server, they could be viewed by everyone through an internet application.
According to the Slovenian legislation and the international standards (IEC 61786) the magnetic flux density is measured at the height of 1 or 1.5 m above the ground. But in the apartment, it is not uncommon that the children have their beds on the floor or do they play on the floor and with measurements 1 m above the ground the exposure would be greatly underestimated.
Therefore all the measurements – spot and continuous 24-hour were taken at the height of 0.2 m above the ground.
We used program package Narda EFC-400EP for numerical modeling of the magnetic flux density in the vicinity of the TS. It is based on segmentation method where each conductor is presented with finite segments.
Corresponding material and electromagnetic characteristics are assigned to all the segments and the resulting magnetic field is the sum of the contributions of all the segments.
Spot measurements were carried out on the 22 of October 2008 between the 10.30 and 11.30. Measurements were taken on 15 locations inside the TS as well as on 17 locations in the apartment above the TS. Measurement results in the apartment together with the locations are shown in Table 1. Based on the data from electric distribution company the value of the current in the LV busbar during measurements was ≈100 A.
Table 1 – Measured values of magnetic flux density in apartment above TS
Continuous 24-hour measurements were carried out first time between the beginning of December 2007 and the end of January 2008 and second time between the beginning of spot measurements (22 October 2008) and 6 December 2008.
The highest measured magnetic flux density was 15.6 μT with the highest 24-hour average of 9.4 μT. Based on the results of continuous measurements we estimated the real worst case load of the TS.
To verify our numerical model of the TS we compared measured and calculated results under the same load conditions at the same locations (Table 2) and it could be seen that both values agree well. Small differences are possible due to different factors (simplification of the model, changes of the load of the TS…).
We made separate calculations for different parts of TS to evaluate which parts are the most critical and identified LV busbar, which is fixed on the ceiling of the TS and therefore the distance between the LV busbar and the floor of the above apartment is only 0.5 m. Other important sources are also switchgear and transformer.
Based on this findings we numerically analyzed the re-construction of the TS with following modifications:
- Removing LV busbar, new LV busbar located under the floor of the TS;
- Change of the switchgear, since it is already old, with a new, which should be as low as possible;
- Proper design and realizations of all the busbars in the TS (all cables close together and arranged in the triangle, as short busbars as possible, especially parts of them which are higher than the floor of the TS).
- We propose transformer to remain, since it was replaced few years ago.
Table 2 – Comparison of measured and calculated magnetic flux density inside the apartment above the TS
From the results in Table 3 and Figure 2 to 5 we can see that the reconstruction will lower the maximum value of the magnetic flux density for about 10 times.
Magnetic flux density before and after reconstruction
Results of the numerical calculation of the magnetic flux density in the apartment at the height of 0.2 m above the floor before (left) and after the reconstruction (right). The current in the LV busbar is 100 A.
Results of the numerical calculation of the magnetic flux density in the apartment at the height of 0.2 m above the floor before (left) and after the reconstruction (right). The current in the LV busbar is 909 A.
MAGNETIC FIELD OF TRANSFORMER SUBSTATIONS
Results of the numerical calculation of the magnetic flux density in vertical plane through LV busbar before (left) and after the reconstruction (right). The current in the LV busbar is 100 A.
Results of the numerical calculation of the magnetic flux density in vertical plane through LV busbar before (left) and after the reconstruction (right). The current in the LV busbar is 909 A.
To verify the findings of the numerical calculations measured the magnetic flux density after the reconstruction on the same locations as before the reconstruction (Figure 2). During the measurements after the reconstruction, the current load was slightly higher current: before it was about 100 A, after it was about 160 A.
In Table 4 where the values of the magnetic flux density are given for the measurements before the reconstruction (second column) and after it (third column). The ratio between both measurements showing the reduction of the magnetic field in the apartment above TS, given in the fourth column, therefore slightly underestimate the effectiveness of the reconstruction, as the current load was 1.6 times higher during the measurements after the reconstruction.
Table 4 – Measured magnetic flux density in the apartment above the TS
We analyzed the magnetic field in the apartment above the TS before the reconstruction. With numerical calculations the reconstruction was optimized in terms of cost effectiveness and the goal to reduce the levels of the magnetic field in the apartment above the TS.
To further lower the magnetic field in the apartment it will be necessary to either change the transformer with a one with reduced magnetic field emissions or to use shielding materials for ELF magnetic field. Shielding materials are either good conductors (aluminium or copper plates) or have high permeability.
With shielding materials it is possible to reduce the magnetic field for a factor of 5 or even more, but these solutions are associated with much higher costs compared to the proposed one.
Authors: BLAŽ VALIČ and PETER GAJŠEK (INSTITUTE OF NON-IONIZING RADIATION, POHORSKEGA BATALJONA 215, 1000 LJUBLJANA, SLOVENIA)
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