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Berlin Environmental Atlas

08.05 Electromagnetic fields

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High-voltage Overhead Lines

In the area under study in Buch, there are three high-voltage overhead lines, around which the magnetic flux density was calculated by assuming average currents. The 110 kV supply line is partly underground. We must assume that the electric load changes throughout the day.

The calculations of electric field strength were based on the actual operating voltages at the time of measurement. These were 400 kV for the 380 kV line, 229 kV for the 220 kV line and 110 kV for the 110 kV line (operating voltage can change as a function of load). We must also remember that the sag of the overhead conductors exerts a major influence on field strength at ground level. This is ultimately dependent on the temperature of the cables, which among other things rises with an increase in transmitted wattage or air temperature. To simulate the field, an average sag at an outdoor temperature of +10 °C was assumed (in line with DIN VDE 0210). To demonstrate the influence of cable sag on the magnetic and electrical fields below the power lines, field strengths were calculated for the transverse profiles of three different cable sags (see Tab. 4).

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Tab. 4: Calculated Peak Electric and Magnetic Field Strengths with Sag Variations of ± 1 m (Pylon Field 447-448; Minimum Sag 10.70 m) in the 380 kV/m Overhead Line

Excel
[Table is also available as Excel-File (MS-Excel is required).]

It is apparent that cable sag has a decisive influence on the field strengths at ground level. The smaller the distance to the ground, i.e. the greater the sag on the cable, the greater this effect will be.

To verify the calculations, cross profiles were measured 1 meter above the ground at precisely defined points with known sag while simultaneously recording the conduction current. These measurements show a 95 % agreement with the calculated field strengths (cf. Fig. 7).

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Fig. 7: Measured and Calculated Longitudinal Profile of Magnetic Flux Density 1 m above Ground beneath an 380 kV Overhead Line. The Pylons are Situated at Positions 0 m and 440 m.

These constant changes in cable sag (due to atmospheric temperature and load), and therefore in the distance between the lines of conduction and the ground, are the main reason, together with current flow, why measurements along overhead power lines can only yield momentary field strengths. This means that only measurements taken under specified conditions are of value and can be used as a base for calculations - ideally one cross profile per pylon field. Only if the measurement and calculation tally sufficiently the parameters for calculation can be applied to the whole line.

Underground Cables

The underground section of the 110 kV line was treated by analogy to the overhead line in measuring and calculating field strengths.

In practice it emerged that the field strengths on footpaths and roads created by the underground cable were very low and confined to a narrow space above the trench.

An electric field component does not occur over underground cables, as they are surrounded by an earthed metallic outer casing and lie in soil, which conducts.

Substations

Magnetic flux density was measured within a radius of very few meters around substations. This is sufficient, because at distances above 2-3 m the fields of low-voltage cables are stronger. Standardising time variance was unnecessary, as short-term fluctuations are low. Similarly to overhead lines, the load patterns of substations show slight variation in the course of a day or year. An electric field strength does not occur in the vicinity of substations, as the installation's electric field is shielded by the walls.

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