MITIGATE THE MAGNETIC FIELD EXPOSURE NEAR

TRANSFORMER SUBSTATIONS

BLA VALI1,2

PETER GAJEK1

INSTITUTE OF NON-IONIZING RADIATION, POHORSKEGA BATALJONA 215, 1000 LJUBLJANA,

SLOVENIA

GENERA, PREVALE 10, 1236 TRZIN

Abstract

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. Based on the results of the numerical

modeling we proposed the optimum and cost effective reconstruction plan. 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.

Introduction

International Agency for Research on Cancer included extremely low frequency (ELF) magnetic field among the

possibly carcinogenic factors for humans [1]. This decision is based on the results of two studies [2, 3], which

showed that elevated 24 hours averaged values of ELF magnetic field (

BLA VALI, PETER GAJEK

Figure 1: Transformer located in the right room of the transformer substation.

Measurements

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.

Numerical calculations

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.

Results

Results of spot measurements

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.

MAGNETIC FIELD OF TRANSFORMER SUBSTATIONS

Table 1: Measured values of magnetic flux density in apartment above TS

Measure distance [m] height [m] B [T]

1 0.2 0.2 0.57

2 0.2 0.2 0.62

3 0.2 0.2 2.54

4 0.2 0.2 3.50

5 0.2 0.2 2.17

6 0.2 0.2 0.98

7 0.5 0.2 7.10

8 1.0 0.2 11.40

9 1.5 0.2 6.95

10 2.0 0.2 4.72

11 3.0 0.2 2.43

12 1.0 0.2 7.53

13 1.0 0.2 7.75

14 1.0 0.2 4.54

15 see Figure on

right

0.2 0.46

16 0.2 0.46

17 0.2 0.26

Results of continuous 24-hour measurements

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 T with the highest 24-hour average of 9.4 T. Based on the results of continuous measurements we estimated the real worst case load of the TS. During spot

measurements the current in the LV busbar was 100 A. A nominal load with the current 909 A represents the worst case condition where such conditions in real situations are very unlikely. Based on the results of

continuous measurements we estimated that in real worst case condition the current in the LV busbar is 200 A.

Numerical modeling

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

Measure Bmeasured [T] Bcalculated [T] Measure Bmeasured [T] Bcalculated [T]

1 0.57 0.521 4 3.50 2.985

2 0.62 0.899 5 2.17 1.416

3 2.54 1.845 6 0.98 0.634

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.

Kitchen and dining room

Passage in front of Bedrooms

17

1615

141312

11

10

9

8

7

653214

Living room Measurementstation

BLA VALI, PETER GAJEK

Table 3: Comparison of the maximum calculated values of the magnetic flux density before the reconstruction

and after it

Current in LV busbar [A] Bbefore [T] Bafter [T]

100 7.5 0.65

200 15 1.5

909 70 8

Figure 2: 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.

Figure 3: 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

Figure 4: 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.

Figure 5: 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.

Spot measurements after the reconstruction

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

Measure Bbefore [T] Bafter [T] Bbefore/Bafter Measure Bbefore [T] Bafter [T] Bbefore/Bafter

1 0.57 0.21 2.7 10 4.72 0.36 13.1

2 0.62 0.29 2.1 11 2.43 0.21 11.6

3 2.54 0.37 6.9 12 7.53 0.35 21.5

4 3.50 0.45 7.8 13 7.75 0.47 16.5

5 2.17 0.60 3.6 14 4.54 0.62 7.3

6 0.98 0.57 1.7 15 0.46 0.12 3.8

7 7.10 0.48 14.8 16 0.46 0.15 3.1

8 11.40 0.40 28.5 17 0.26 0.062 4.2

9 6.95 0.44 15.8

BLA VALI, PETER GAJEK

Discussion and conclusions

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. In spite of the fact that the increase of the costs of the

reconstruction was minimal and the realization of the proposed solution was not problematic, it still reduced the

exposure of the people living in the apartment for a factor of 10.

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.

References

1. IARC. (2002) Non-ionizing radiation, Part 1: Static and extremely low-frequency (ELF) electric and magnetic fields. IARC Monogr Eval Carcinog Risks Hum 80:1395. Lyon, France

2. Ahlbom A, Day N, Feychting M et. al. (2000) A pooled analysis of magnetic fields and childhood leukaemia. Br J Cancer 83:692698

3. Greenland S, Sheppard AR, Kaune WT et. al. (2000) A pooled analysis of magnetic fields, wire codes, and childhood leukemia. Childhood Leukemia-EMF Study Group. Epidemiology 11: 624634

4. Kheifets L, Repacholi M, Saunders R et. al. (2005) The sensitivity of children to electromagnetic fields. Pediatrics 116:303313

5. Swanson J, Kheifets L (2006) Biophysical mechanisms and the weight of evidence for EMF. Radiat Res 165:470478

6. Schuz J (2007) Implications on protection guidelines from epidemiologic studies on magnetic fields and the risk of childhood leukemia. Health Phys 92:642-648

7. Ilonen K, Markkanen A, Mezei G et. al. (2008) Indoor Transformer Stations as Predictors of Residential ELF Magnetic Field Exposure. Bioelectromagnetics: 29: 213-218