Electric substations produce electric and magnetic fields. In a substation, the strongest fields around the perimeter fence come from the transmission and distribution lines entering and leaving the substation.
The strength of fields from equipment inside the fence decreases rapidly with distance, reaching very low levels at relatively short distances beyond substation fences.
Electric and Magnetic Field Sources in a Substation
Typical sources of electric and magnetic fields in substations include the following:
- Transmission and distribution lines entering and exiting the substation
- Air core reactors
- Switchgear and cabling
- Line traps
- Circuit breakers
- Ground grid
- Battery chargers
Electric fields are present whenever voltage exists on a conductor. Electric fields are not dependent on the current. The magnitude of the electric field is a function of the operating voltage and decreases with the square of the distance from the source.
The strength of an electric field is measured in volts per meter.
The most common unit for this application is kilovolts per meter. The electric field can be easily shielded (the strength can be reduced) by any conducting surface such as trees, fences, walls, buildings, and most structures.
In substations, the electric field is extremely variable due to the screening effect provided by the presence of the grounded steel structures used for electric bus and equipment support. Although the level of the electric fields could reach magnitudes of approximately 13 kV/m in the immediate vicinity of high-voltage apparatus, such as near 500kV circuit beakers, the level of the electric field decreases significantly toward the fence line.
At the fence line, which is at least 6.4 m (21 ft) from the nearest live 500-kV conductor (see the NESC), the level of the electric field approaches zero kV=m. If the incoming or outgoing lines are underground, the level of the electric field at the point of crossing the fence is negligible.
Magnetic fields are present whenever current flows in a conductor, and are not voltage dependent. The level of these fields also decreases with distance from the source but these fields are not easily shielded.
Magnetic fields are measured in Webers per square meter (Tesla) or Maxwells per square centimeter (Gauss).
One Gauss = 10-4 Tesla. The most common unit for this application is milliGauss (10-3 Gauss).
Various factors affect the levels of the fields, including the following:
- Current magnitude
- Phase spacing
- Bus height
- Phase configurations
- Distance from the source
- Phase unbalance (magnitude and angle)
Magnetic fields decrease with increasing distance (r) from the source. The rate is an inverse function and is dependent on the type of source. For point sources such as motors and reactors, the function is l/r2; and for single-phase sources such as neutral or ground conductors the function is l = r.
Besides distance, conductor spacing and phase balance have the largest effect on the magnetic field level because they control the rate at which the field changes.
Magnetic fields can sometimes be shielded by specially engineered enclosures. The application of these shielding techniques in a power system environment is minimal because of the substantial costs involved and the difficulty of obtaining practical designs.
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Reference: Electric Power Engineering Handbook – Leonard L. Grigsby (Get this book from Amazon)