ELECTRICAL INSTALLATIONS SUCH AS POWER LINES, TRANSFORMERS OR DISTRIBUTION EQUIPMENT transmit electromagnetic radiation to the environment. The impact of the emitted nonionizing radiation (NIR) on humans calls for recognition and consideration. Legislators in most European countries introduced exposure limit values (ELV). Switzerland also defined precautionary installation limit values, which will require considerable investment costs for Swiss utilities to comply.

The Zurich power supplier Elektrizitätswerk der Stadt Zürich (ewz) has investigated the problems associated with satisfying the given limit values, particularly with respect to medium-/low-voltage (MV/LV) transformer stations (TS), in that a large proportion of the population live and work in close proximity to ewz's electromagnetic fields.

LEGISLATION AND LIMIT VALUES

The Swiss Ordinance Relating to Protection from Non-Ionizing Radiation (ONIR) became effective in February 2000 and is considered to be among the most stringent regulations of its kind in Europe. The ONIR regulates the limit values of the magnetic and electrical fields in the range of 0 Hz to 300 GHz. The 50-Hz frequency is of interest, and because compliance of the electrical field provokes no technical problem, only the magnetic field — that is the magnetic flux density B (µT) — is of concern. In the ONIR, Switzerland has defined two types of limit values for the 50-Hz magnetic field:

• The internationally recognized ELV of 100 µT, which takes into account the total emission that is present at a particular location. Compliance is mandatory in locations where individuals have access, even for short periods.

• The stricter installation limit value (ILV) of 1 µT places a ceiling on emissions from a single installation. This precautionary principle is in the Environmental Protection Law (USG), on which the ONIR is based and which dictates that exposure must be as low as is technically and operationally possible and economically acceptable, but at least to such an extent that it is neither harmful nor constitutes a nuisance to human beings. These ILVs must be observed in locations of “sensitive use,” which are defined as places where individuals spend prolonged periods of time, such as schools, homes and offices. ILV has to be observed at the rated power of the installation.

Figure 1 shows the meaning of both limit values plus an additional limit value — set five times higher than the ELV — that is valid only for power company staff. A distinction is made between the TS and the adjacent rooms with a general or sensitive use.

Compliance with the ELV should not pose too many problems in Switzerland. Therefore, the stricter ILV applicable in Switzerland has to be addressed, as the owner of the electrical installations has to satisfy this regulation within five years.

TYPES OF TRANSFORMER STATIONS AND REMEDIAL MEASURES

There are three types of transformer stations that have to be considered: new stations; existing stations undergoing renovations where optimization is possible in the planning stage and radiation-reduced equipment can be used; and older stations commissioned before the ONIR went into effect that will continue to operate under their existing configurations.

When selecting the sites for new installations, careful consideration must be taken to ensure that no location of sensitive use lies within proximity of the plant, a condition that can be secured by legal contract. For a new installation that fails to meet these conditions, or when rebuilding existing stations, the following optimizing tools should be considered:

• Optimal arrangement of the electrical equipment (transformer, switchboards)

• Optimal configuration of the cables' phases

• Optimal field arrangements

• The arrangement of the bus bars

• Layout of the earth bus (reduction of the neutral current in the earth system)

• Manufacturer's optimization of their products (switchgear, bus bars). Development work on this has been slow.

Figures 2 and 3 illustrate the possibilities of optimization and the influence of the arrangement of the electrical equipment within TS. This TS was situated in a school and had to be rebuilt because of the children's playground, a location of sensitive use, with the classroom above the TS. After rebuilding, the 1-µT isoline is completely within the TS, both on the side with the children's playground and also above.

A further example shows a reduction in the magnetic field due to optimal field arrangements. In Fig. 4a, the ring current, shown by the dashed line, flows along the entire bus bar; however, in Fig. 4b, the current on the bus bar is reduced and flows partially in the opposite direction. Improved compensation of the feeder magnetic field is obtained, resulting in a reduction of the magnetic flux density (1-µT isoline), shown 0.5 m (1.55 ft) around installation.

For new and rebuilt stations where these optimization measures are insufficient, or for existing stations where the arrangement is fixed, shielding some or all of the equipment must be considered. In these situations, either the electrical components have to be shielded or a surface shielding has to be installed to reduce the magnetic field level.

PLANT SHIELDING MEASUREMENTS

A range of possible shielding arrangements were developed and tested by ewz for the following components:

• MV/LV transformers
• LV switchgear (external and in bus bar proximity)
• Surface shielding on station ceilings and floors
• Cables.

Investigations were performed with materials including steel, µ-metal (mainly Si-Fe or Ni-Fe-alloys), aluminum, and a combination of µ-metal/aluminum. With simple shielding (a single sheet), only the strong field directly by the source is reduced. This proved a suitable method for withholding the exposure limit below 100 µT. To reduce weaker magnetic fields — that is, to establish an installation limit value of 1 µT — a more complex arrangement of shielding and more expensive material (combination of µ-metal and aluminum) was necessary.

• Shielding of MV/LV Transformers

  A transformer shielding box (Fig. 5) was compared with two transformer hoods, which were developed in ewz's EMV laboratory in cooperation with Systron EMV GmbH (Hinwil, Switzerland).

  The field testing of the shielding box confirmed that the expected reduction could not be guaranteed due to unfavorable LV-cable configurations. As a result, a connection shielding hood (CSH), which covered the transformer and the section of the LV cables, was tested prior to the optimized arrangement (Fig. 6). The effectiveness of the CSH led to the construction of the transformer shielding hood (TSH), which can be used alone or in combination with CSH (Fig. 7). The hoods comprise a sandwich construction of aluminum and µ-metal.

  The Eidgenössische Starkstrominspektorat (ESTI), a neutral organization, recorded the measurements under the following technical conditions: rated power = 1000 kVA, secondary current = 1460 A, short-circuit voltage = 4.5% and in short-circuit operation.

  Figure 8 shows the 1-µT isolines of the four shielding variants — CSH, TSH, CSH+TSH and box — in comparison with the unshielded field, no screen (NS). The outermost line, at a height of approximately 5 m (16.4 ft) above the floor of the TS, represents the ILVs without shielding. With the combined screening hoods (CSH+TSH), the magnetic field is reduced to the ILV at about 2.7 m (8.8 ft), roughly half in the vertical direction. Therefore, in most cases, it is possible to keep the 1-µT isoline within the TS.

  Depending upon room height, transformer power rating and the distance to the location with sensitive use, a suitable shielding can be selected.

  Compared with an industrial radiation-reduced transformer, such as 1000 kVA with the 1-µT isoline approximately 2.8 m (9.2 ft) above the floor of the TS, the combined CSH+TSH offers an equivalent solution for existing transformers.

  The accessibility of the hood for fault clearance, maintenance and repair is more favorable than the box, and although the box and CSH +TSH combination costs about the same, the combined hoods are more effective in reducing the magnetic field.

• LV Cable Shielding

  To reduce the magnet field of the LV cables between transformer and LV switchgear, an optimal configuration of the cables' phases must be used. When the routing cannot be changed, such as cable channel below the ceiling, an additional shielding could be necessary. Diverse shielding material was tested with either fully enclosed shielding or a U-shaped shielding over the cables (with and without cable channel).

  The shielding factor is the ratio of the distance of the 1-µT isoline with shielding and without shielding (Table 1).

  The best shielding factor was achieved for fully enclosed, highly permeable material, although a reasonable factor also was achieved with steel or in combination with the perforated steel cable channel.

• Shielding of LV Switchboard

  Table 2 provides information about the shielding arrangements (Fig. 9), and the chosen materials together with the resulting heights of the 1-µT isoline. The shielding was tested under the following technical conditions: secondary current = 1000 A and a 630-kVA transformer in short-circuit operation.

  Partial shielding above and behind the switchboard only reduces the magnetic field directly above to a minimal extent. It does however have a definite reduction for a room above and behind the switchboard, and extending the shielding beyond the extent of the switchboard further reduces the magnetic field. Complete shielding of the LV switchboard would have the greatest shielding effect, except for the fact that access, ventilation and cable openings must be granted.

  The difference in the reduction achieved by the partial and the complete shielding from 2-mm steel and 0.35-mm µ-metal is due to the permeability of the material (initial permeability: steel ≈60 and µ-metal ≈45000) as well as the material thickness. Therefore, an equivalent or better shielding can be achieved with a thinner highly permeable material, although the cost for this is much higher. This flexibility may be advantageous when shielding is required behind existing LV switchboards.

  The permeability of the shielding has a greater influence when it is near the field source. For example, greater shielding results are achieved by shielding on the LV switchboard than by using surface shielding. Results would be even better with shielding placed next to the LV elements (bus bars, switchgear).

• Surface Shielding

  Surface shielding with highly permeable material (passive shielding) effectively diverts the magnetic flux into itself. The magnetic field lines not diverted into the material are drawn toward the shielding, but to a decreasing extent with the distance. Therefore, the shielding must extend to enclose the 1-µT field line. Surface shielding with highly conductive material (active shielding) produces an opposing field due to eddy currents.

The best results were achieved with a combination of permeable and conductive material. The highly conductive material (3-mm Al) and the highly permeable material (0.35-mm µ-metal) alone had approximately the same shielding effect.

MITIGATION MEASURES AND ESTIMATED COSTS

Some 270 of the 860 total TS are critical in the supply area of ewz and have to be renovated. The costs to optimize these stations are estimated to be SFr 13.5 million (US$11 million).

The first shielding measures were undertaken in existing stations, and experience indicates that for new installations and the total rebuilding of existing plants, the ILVs can be adhered to for an additional 10% in costs (subject to the provisions that the ONIR implementing regulation meets with ewz assumptions). However, there are additional capital costs associated with rebuilding a plant before it needs to be. Therefore, the residual value of the plant in these stations must then be added to the rebuilding costs of the new plant. Depending upon the level of renovation required, installing the mitigation measures in an existing station can cost up to SFr 50,000 (US$ 40,600), but optimal restructuring methods can help to lower average costs.

An alternative to shielding the transformer is a reduction of the rated output of the transformers. This solution was selected for a few renovations where a kindergarten was directly above the TS. A smaller, optimized transformer was installed in each case; however, this measure can lead to a decreased security of supply, due to insufficient capacity in the event of network fault switching.

MORE RESEARCH AND DEVELOPMENT

For new and rebuilt transformer stations, the magnetic field limit of 1 µT can be fulfilled in most of the cases by observing some optimizing rules in the project planning phase. The situation becomes more complicated when shielding is necessary. With relatively simple methods, the intense field nearby the source can be reduced, but the less intense, more distant field can only be weakened, but this can involve a more complex arrangement of metal sheets, angles and more expensive materials (µ-metal). Further research and development are expected to determine whether the field reduction factors can be generally used and what will be the influence of different shielding materials on the field components used in distribution transformer stations.

ACKNOWLEDGEMENTS

This article is based on a paper presented by the authors at the CIRED 2003 17th International Conference & Exhibition on Electricity Distribution, and published in the AIM conference proeceedings.

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David Hearn, El. Eng. HTL, studied electrotechnology at the FH Zurich. Since 1999 he has worked for Elektrizitätswerk der Stadt Zürich (ewz), where his responsibilities include special tasks within system design. david.hearn@ewz.ch

Hansruedi Luternauer, El. Eng. HTL, studied electrotechnology at the FH Muttenz. He joined ewz in 1985 and now works in planning and is responsible for the Network Design department. hansruedi.luternauer@ewz.ch

Hans-Heinrich Schiesser, Ph.D., studied geography at the University of Zurich, where he also earned his doctorate in radar meteorology. Since joining ewz in 1999, he has been responsible for special tasks within the range-planning field. hans.schiesser@ewz.ch

Table 1. Shielding factor for LV cable.

Description	Shielding Factor
Fully enclosed
2-mm aluminum L (internal) / 0.35-mm µ-metal (external)	0.30
0.35-mm µ-metal (internal) / 2-mm aluminum (external)	0.40
0.35-mm µ-metal	0.40
2-mm steel	0.50
2-mm steel (perforate)	0.55-0.85
3-mm aluminum	0.95
U-shaped shielding over cables and 2-mm steel (perforated) cable canal below
2-mm aluminum (internal) / 0.35-mm µ-metal (external)	0.55
0.35-mm µ-metal	0.60-0.85
2-mm steel	0.60-0.85
2-mm steel (perforate)	0.55-0.85
2-mm aluminum	0.70
U-shaped shielding over cables
2-mm steel	1.00
0.35-mm µ-metal	1.00

Table 2. Summary of the experimental results with miscellaneous shield.

Description	Shielding Arrangement	Material	1-µT IsolineHeight from Ground
No Shielding	—	—	4.80 m
Shielding on LV switchboard partial shielding	back, above	2-mm steel	4.45 m
	back, above, side	2-mm steel	4.45 m
	back, above	0.35-mm µ-metal	4.20 m
	2x back, above	0.35-mm µ-metal	4.15 m
Shielding on LV switchboard complete shielding	front, back, above	2-mm steel	4.30 m
	front, back, above, side	2-mm steel	4.30 m
	front, back 2x angle, 2x above	0.35-mm µ-metal, above µ-metal and aluminum	3.85 m
	front, 2x back, 2x angle, 3x above	0.35-mm µ-metal, above 2x µ-metal and aluminum	3.20 m
	front (lateral), back(lateral), 2x angle, above	0.35-mm µ-metal	3.05 m
	front, back 5x angle, bottom	0.35-mm µ-metal	2.80 m
	front (lateral), 2x back (lateral), 5x angle, above	0.35-mm µ-metal	2.70 m
	front, 2x back, 5x angle, with insulation	0.35-mm µ-metal	2.95 m
Surface shielding full plate	plate 3.85 m × 2.8 m	2-mm aluminum / 0.35-mm µ-metal	4.15 m
	plate 3.85 m × 2.8 m	0.35-mm µ-metal / 2-mm aluminum	4.15 m
	plate 3.85 m × 2.8 m	3-mm aluminum	4.40 m
	plate 3.85 m × 2.8 m	0.35-mm µ-metal	4.35 m
Surface shielding half plate	plate 2 m × 2.8 m	2-mm aluminum / 0.35-mm µ-metal	4.65 m
	plate 2 m × 2.8 m	0.35-mm µ-metal / 2-mm aluminum	4.65 m
	plate 2 m × 2.8 m	3-mm aluminum	4.80 m
	plate 2 m × 2.8 m	0.35-mm µ-metal	4.70 m
Reinforcement	double grid behind switchboard	steel grid	4.80 m
	double grid on ceiling	steel grid	4.80 m