EMF Analysis


Uniform procedures for the measurement of the electric and magnetic field near substations operating at the power frequencies (50Hz), overhead and underground power lines have been established, based on the IEEE standards in order to standardize the measured data and provide credible measurements. Accurate predicting of the main sources of electric and magnetic fields was performed, identifying the locations with the highest electromagnetic field intensity levels. Mesh measurements, by discretising the measured area into a grid of resolution 0.5m were performed, recording the electric and magnetic field strengths at three height levels of 0.30m, 1m and 1.75m above ground. The mesh grid sampling method of discretising the investigated area into a mesh is a particularly useful technique for obtaining a representative picture of the measured fields.

Substations

Electric and magnetic fields present within substations were measured very carefully as they provide more complex and more difficult to measure components than the fields near power lines.

Within substations, the concentration of electric and magnetic field sources arises, due to the many interconnecting points in the distribution lines. It is therefore important to identify the most prevalent sources of electric and magnetic fields in substations which mainly include: high voltage and low voltage lines, transformer, underground cabling entering / exiting the facility, ring main unit, low voltage pillar and capacitor banks. Before performing the measurements a preliminary survey is carried out, so as to characterize the spatial distribution of the magnetic field, identifying the locations with the highest field strength levels.

A floor plan of the substation is created and the area is discretized into a mesh of resolution 0.5m. Spot / mesh measurements of the resultant magnetic field or maximum magnetic field may then be performed always recording the electricity usage levels, i.e. the load currents at the moment of measurement.

 

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Three-axis probe instrumentation such as the EFA-300 is used to measure and record the resultant magnetic field as a function of time and frequency. Power frequency and its harmonics should be noted by using field meters connected to spectrum analyzers or by using meters that have a sufficiently high sampling rate to perform Fourier analyses on the logged values. The frequency range of the instrumentation should be adequate enough for obtaining the power frequency and its harmonics fields up to the 7th harmonic. The measurement uncertainty of the instrumentation should be less than 10 %. The magnetic field strength of substations is recommended to be measured at three level heights of 0.30m, 1m and 1,75m above ground level. These heights represent the height levels of foot, waist and head of an average person. Subsequently the Electric field is measured using the electric field strength meter with the operator standing at least 2.5m away from the apparatus at the time of measurement. In all cases appropriate sketches of the areas where measurements were performed should be provided. The images below show how a substation's premises are mapped (first image) and how the contour of the measured electric and magnetic fields' intensity appears.

High voltage overhead power lines

The measurement of the magnitudes of the electric and magnetic fields intensity of overhead power lines is based on the actual operating voltages and currents, at the time of measurement, and both these values are a function of the load at the time of the measurements. It must also be noted that the sagging of the overhead conductors exerts a major influence on field strength at ground level. Ultimately, this is dependent on the temperature of the cables, which among other things rises with an increase in transmitted wattage or air temperature.

By using the isotropic magnetic field probe the quantity of the RMS magnetic field strength can be measured and is used to characterize the field at the position of measurement. The measurement of the magnetic field is not impaired by the operator being close to the probe. Non-permanent objects containing magnetic materials or nonmagnetic conductors should be at least three times the largest dimension of the object away from the point of measurement in order to take an unperturbed field value. For the measurement of the electric field, the distance between the electric field strength meter and the operator should be at least 2.5m, as this is a distance that reduces significantly the proximity effect (called shading E-field) of a grounded 1.8 m tall observer to between -1.5% and -3%.

The elliptic plane produced due to the power lines operating in ideal conditions (horizontal power lines and a flat ground surface), is perpendicular to the direction of the conductors. This is the case under actual power lines when no nearby objects are present and the terrain is not very rough. Measurements are performed in the plane of the ellipse, with the field meter line being parallel to the conductors. The lateral profile of the magnetic and electric field strength at several points of interest should be measured at selected intervals in a direction normal to the line at 1m intervals. These measurements should begin from the center line, in the area of interest, and exceed to a horizontal distance of at least 10 m beyond each side of the outside conductor.

During the field study, background information such as environmental conditions (temperature, humidity, and ground cover), transmission line parameters (line voltages and currents, conductor geometry, measurement locations) and instrumentation used should be reported.

High voltage underground power lines

The procedure of measuring the electric and magnetic fields of underground power cables is by analogy similar to the overhead power line case.

After the appropriate survey is undertaken so as to characterize the spatial distribution of the fields, measurements should be taken exactly above the conductor at a height level of 0,30m while other readings at heights 1m and 1,75m, as mentioned before, can also be taken. The fields are also measured at intervals of 1m in both sides away from the conductor for a distance as specified.

Procedures and measurements as indicated for high voltage overhead lines should be followed carefully for underground cable measurements too.

EMF measurements in Cyprus

An amount of EMF measurements is being undertaken throughout Cyprus and the results are added periodically on this web site. It is our intention to carry out an exhaustive number of measurements throughout the duration of the project in areas where there is concern by the public in order to better inform the people regarding the exposure levels in the vicinity of high and medium voltage assets. Please select your area of interest from the map above to be taken to the page that contains the available reports for the measured assets in that area. More reports will be added soon..