Product Abstract

Distribution lines constitute one of the major sources of the public's time-weighted average exposure to magnetic fields. This report discusses the three main sources of magnetic fields along with statistically significant data for each source, methods of calculating distribution line magnetic fields, and general guidelines for performing distribution line magnetic field measurements. Technologies for magnetic field exposure reduction are presented and discussed, with emphasis on the most promising techniques.

Background

The general public has become increasingly aware of possible health effects from exposure to electric and magnetic fields (EMF). This has led to controversy, delay, and cost increases in the construction of some utility lines and facilities. EPRI has initiated research to identify field sources, characterize field levels, and provide management options and strategies that could be used to reduce fields and exposures.

Objective

To provide data on typical distribution system current levels, magnetic field levels near the lines, residential exposures to the public, and magnetic field management options.

Approach

Investigators gathered available data on representative current and magnetic field levels. Using a set of simplified expressions for calculating distribution line magnetic fields, they identified the principal parameters governing magnetic field levels and the optimal strategy for magnetic field exposure reduction.

Results

This report provides utility engineers with insight into representative data on magnetic field levels inside residences and in proximity to distribution lines. Typical current levels (average current, current unbalance, and net current) of three-phase distribution lines in the United States are provided for both the fundamental (60 Hz) and harmonic frequencies. The report describes commonly used methods for calculating distribution line magnetic fields based on typical current levels. In addition, it offers guidelines for performing distribution line magnetic field measurements, including spot measurements, lateral profiles, and temporal variations. Finally, it identifies options for managing magnetic fields from distribution lines. Following are the most promising field reduction strategies:

o Balanced primary lines can be reduced by compacting the phase conductors, reducing the line current by increasing voltage, or increasing the distance to the subject by relocating the line or raising the conductors.

o Net current primary line magnetic fields can be reduced by balancing the phase currents, increasing the size of the neutral conductor, isolating the primary and secondary neutrals, or implementing a five-wire system.

o Secondary line magnetic fields can be reduced by using triplex cable in lieu of an open-wire secondary configuration.

o Residential grounding system magnetic fields can be reduced by properly locating the distribution transformer, repairing broken service neutrals, or inserting a net current control device on the service drop.

EPRI Perspective

EPRI's goal is to provide tools, concepts, and guidelines for reducing magnetic fields from distribution lines. Magnetic field characterization and management for distribution lines is more complicated than for transmission lines due to the inherent variability of loads and the relatively high proportion of unbalanced current and harmonic content. Complicating the issue further, distribution lines have gone through a tremendous evolution over the past several decades, yet many of the older distribution line designs are still in operation. While wholesale changes in the design of existing distribution lines may not be practical or necessary, construction of new overhead lines with balanced loads may reduce enhance system protection, and result in low magnetic fields.