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National Environmental Standards for Electricity Transmission Activities: technical (4 of 9)

The technical pages provide background and explanations of:

  1. The 'National Grid'
  2. Transmission lines
  3. Transmission operation
  4. Electric and Magnetic Fields (EMF)
  5. EMF and health & regulatory responses
  6. Maintenance and upgrading
  7. Work on support structures
  8. Work on conductors
  9. Ancillary activities

Electric and magnetic fields (EMF)

Electric and magnetic fields are produced by all electrical devices, including domestic appliances, house wiring, street distribution lines and transmission lines.

The short-term levels of electric and magnetic fields, at 'user' distances from domestic appliances, can be comparable to - and in some cases higher than - those present directly below Transpower's transmission lines.

The amount of electric power transmitted on a conductor (wire) at any given time is determined by its voltage and current. Voltage is of the electrical 'pressure' that drives electric charges through a conductor. The flow of electric charge through a conductor is called electric current.

The electric field strength near a transmission line is primarily determined by the line voltage .  As transmission systems are held at a stable voltage, the electric field at any given location around transmission equipment will be largely constant. Magnetic fields, on the other hand, are determined by the current. The current and therefore the magnetic field change in strength over time as the demand for electricity fluctuates.

Magnetic fields are commonly measured in units of micro- tesla (μT) and electric fields in kilovolts per metre (kV/m).

The strengths of both electric and magnetic fields diminish rapidly with distance from their source. Electric fields are easily screened by most materials, and within a home external sources, such as transmission lines, make little or no difference to the electric field levels . Magnetic fields, on the other hand, are not easily screened.

The great majority of the New Zealand National Grid (approximately 12,000 km of lines) transmits electricity as an alternating current (AC) at 50Hz (Hertz); that is, the direction of the current (or flow of electrons) in a conductor oscillates backwards and forwards, making 50 complete cycles every second. The mains electricity delivered to homes and businesses is also at 50Hz. As a result, the associated electric and magnetic fields also change in strength and direction at this frequency. This frequency falls in a range defined as “extremely low frequency” and so the electric and magnetic fields are known as extremely low frequency electric and magnetic fields (ELF EMF).

In High Voltage Direct Current (HVDC) transmission, the flow of electricity through a transmission line conductor is in one direction only. At the time of enactment of the NES for Electricity Transmission, there was only one HVDC line in New Zealand: the Benmore-Haywards HVDC Line, commissioned in 1965. The link includes 572km of overhead transmission line and 40km of undersea cable across Cook Strait. The transmission of electricity as a direct current, as with the HVDC line, results in static electric and magnetic fields which do not alternate in strength or direction. The way static fields interact with humans, and the requirements for modelling, are different from the ELF EMF associated with AC lines. Static fields are described elsewhere in this guidance.

Modelling EMF

The International Commission on Non-Ionising Radiation Protection (ICNIRP) guidelines are not targeted at transmission line design. For example, they do not specify a location relative to the conductors where the Guidelines should be satisfied.

Neither do they consider to what operational situations of transmission lines the guidelines might or might not apply (for example, under fault conditions). The permitted activity standard within the NES does provide a basis for assessing compliance of transmission lines with the ICNIRP guidelines.

Position of prediction

For both electric and magnetic fields, field strengths reduce rapidly with distance from the source. The location relative to the conductors at which the fields are determined is therefore relevant . Modelling convention is to predict exposures beneath the conductors at a height of 1m above the ground level unless exposure is possible at a closer position, such as the upper floor of a multi-storey building. Regulation 10(4) follows this approach. The reference levels of the ICNIRP guidelines apply as a spatially averaged value across the entire human body. So for an adult, a 1m height above ground will provide a reasonable approximation of this value. For a child the approach is conservative.

Screening

Electric fields are effectively screened by grounded objects. Even trees will provide some screening and a reduction in electric field exposure. External sources, such as transmission lines, are virtually irrelevant to the field levels found inside a house . Magnetic fields on the other hand are not easily screened. Regulation 10(5) allows for the screening effects of buildings to be taken into account in the prediction of field exposures.

Conductor position for overhead lines

Besides the direct influence of current and voltage on magnetic and electric field strength respectively, the sag of overhead conductors (wires) also influence field strength. This is because when the line sags, it comes closer to the ground and therefore increases the field levels at the ground. Electric current heats a conductor causing it to expand and sag closer to the ground, increas ing exposure at ground level.

Air temperature, solar radiation, wetness of the conductor and wind will also affect the conductor sag. Regulation 10(6) requires that the modelling predict the highest fields likely under normal operating conditions (under assumptions of conservatively high levels of current, ambient temperature and solar radiation, together with dry conductors and very low or no wind so that there is little or no ambient cooling of conductors). These conditions are also used to ensure that conductors maintain minimum clearance distances from the ground, in line with electrical safety requirements.

The field levels predicted in line with regulation 10 requirements would represent worst case . In reality such exposures would rarely if ever be encountered for the following reasons:

  • They would only be experienced at the position below the line where it sags closest to the ground.
  • The line will rarely, if ever, experience worst-case sag conditions and maximum currents in combination. So even below the line, field levels will normally be lower than the worst-case prediction. Away from the line, EMF levels will be much reduced from the worst-case value.
  • The current or load on a line will grow over time in line with electricity demand grow th. Field levels, especially magnetic fields levels (which are related to the current), might only approach the predicted maximum well into the future, if at all. The magnetic fields are typically much lower than the maximum for the majority of the line's life.
  • Line loads also vary significantly over the typical day with morning and evening peaks. Peaks in magnetic field only occur for short periods of time in line with peak loads. It is also worth noting that loads in summer are typically lower than in winter.

Exclusions

Fault conditions, such as when light ning strikes and short circuits occur, are not required to be considered as they are very short- lived occurrences – regulation 10(9).

Underground cables

No electric field is experienced above an underground cable because of shielding by the grounded cable sheath. Magnetic flux density reflects the current in the cable and reduces rapidly with distance from the source. The conditions about conductor positions which are applied to overhead lines don't apply to underground cables as the conductors are always in the same place, irrespective of electric current and environmental conditions.

Modelling HVDC

The electric field beneath an HVDC line may have contributions from two components, depending on the conditions:

  • a component due to the voltage on the conductors, which can be accurately calculated in a repeatable fashion for a given line conductor configuration
  • a variable component due to the space charge that results from charged ions produced at the surface of the conductors when corona discharge occurs.

Regulation 10(3)(b) requires that the likely space charge contribution is included in the consideration of electric field exposures associated with HVDC lines.

Further information on electric and magnetic fields is available from the National Radiation Laboratory (formerly a unit of Ministry of Health now part of Institute of Environmental Science and Research Ltd - a government owned crown research institute) which produces an information booklet Electric and Magnetic Fields and Your Health, available online.

Transpower produces a range of EMF fact sheets with a focus on EMF as it relates to transmission lines.

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Last updated: 18 April 2012