This executive summary describes the final report for this project.
Martin-Marietta Energy Systems Contract No. 62X-SV820V
Until the proposed power frequency magnetic field health effects hypotheses are either proved or disproved, no scientific basis for defining safe human exposure thresholds will exist. Long-term planners must nonetheless ask whether it would be technically and economically possible to modify the use of electric power if magnetic fields were ever linked to adverse health.
RAPID Project 8 provides information to help decision makers consider these questions. The project examines field reduction methods for a variety of magnetic field sources, including transmission lines, distribution lines, substations, building wiring, appliances and machinery, and transportation systems. In each category, magnetic field reduction methods are evaluated based on their effectiveness, cost, environmental impact, and safety impact. The report focuses on power frequency magnetic fields because these have garnered the attention of most of the recent health effects research.
Within each magnetic field source category, one or more "problem" sources are identified. These would be exceptionally difficult or expensive to modify into low-field versions if exposure limits were imposed. They include transmission lines operating at voltages of 500 kV or above; unbalanced resultant (zero sequence) current on distribution lines; transmission line substation connections at 500 kV or above; vaults, buses, and feeders in buildings; industrial welding and metal melting processes; and most types of electric railway systems.
A case study approach was used to compare magnetic fields, electric fields, and life cycle costs of various transmission line designs. Both "rural" and "suburban" designs were examined within each of four voltage categories. These included 69 kV, 115 kV, 230 kV, and 345 kV. Rural-only designs were examined at 500 kV and 765 kV.
Several magnetic field reduction methods were considered. These included compaction, phase splitting, higher voltage lines, shielding provided by underground pipe-type cables, and line-side passive cancellation loops.
The analysis showed that transmission line life-cycle costs would increase sharply if magnetic field exposure limits were set at 5 mG or 2 mG for publicly accessible areas. At 69 kV and 115 kV, life cycle costs could increase by as much as 20% to meet a 20 mG standard and could double or triple to meet a 2 mG standard. At 230 kV, costs could increase by as much as 50% for 20 mG and triple or quadruple for a 2 mG limit. Costs for a 345 kV line would triple or quadruple to meet a 20 mG exposure limit. No 500 kV options were identified that could meet a 50 mG or lower exposure limit and no 765 kV options were found that could meet a 100 mG or lower limit on the right of way. A series capacitor compensated cancellation loop might be effective for 500 kV and 765 kV edge of right of way field limits, however.
Underground pipe-type cables provide the lowest transmission line magnetic fields, but are not commercially available for line voltages exceeding 345 kV. Their use would almost certainly be required to meet 2 mG standards. Six-wire and five-wire split-phase lines, the lowest-field overhead conductor designs, could probably meet 5 mG standards at 115 kV and below. The taller towers and shorter spans of the suburban overhead transmission lines studied at 345 kV and below offered much lower peak magnetic and electric fields than their rural counterparts. The effect was less significant outside the right of way.
Unbalanced resultant (zero sequence) currents are usually the most significant magnetic field source outside a transmission line right of way. If low magnetic field levels were mandated, minimizing unbalanced current would be necessary throughout the transmission network. This would entail balancing the line loading at transmission substations, transposing transmission line conductors, and adding low-impedance shield wires to "attract" zero sequence current.
The magnetic fields, electric fields, and life cycle costs of various distribution line designs were also examined during the project. Both "rural" and "suburban" designs were modeled for 7.6 kV single-phase, 13.2 kV three-phase, and 34.5 kV three phase categories. Several magnetic field reduction concepts were evaluated, including compaction, phase splitting, and the use of higher voltage (same load) to reduce current.
For balanced phase current conditions, low-field distribution line life-cycle costs were predicted to increase significantly only for presumed exposure limits of about 5 mG or less. Costs increased as much as 40% for a 2 mG limit at 7.6 kV and 13.2 kV, for which tall compact and split-phase Hendrix cable designs could be used. Life cycle costs for 34.5 kV lines were predicted to increase by 50% to 100% to meet a 2 mG limit, accomplished with a split-phase Hendrix cable design. Heavily loaded distribution lines would have to be shielded, perhaps by underground conduit, to meet a 2 mG limit.
Underground duct and direct burial designs produced the highest magnetic fields at 13.2 kV and 34.5 kV. The underground duct designs nearly triple the baseline design life cycle costs.
Unbalanced resultant (zero sequence) current is often the most significant source of distribution line magnetic fields. If very low magnetic field exposure limits were mandated, control of zero sequence current would be necessary at every point in the distribution network. This significant challenge would require rethinking not only line design methods, but broader network-scale issues such as grounding methods, distribution voltage selection, and transformer sizing.
Most of the magnetic field at a substation perimeter fence is from transmission and distribution lines entering or leaving the facility. The need to build low-field transmission and distribution line segments at the station entrance would heavily influence the feasibility and cost of reducing substation magnetic fields. Field reduction methods and life cycle costs of these line segments would be similar to those listed for transmission and distribution lines. Few, if any, methods are available to allow 500 kV and 765 kV lines to meet exposure limits below 100 mG.
The cost of a "low-field" substation design would also include the cost of expanding the perimeter fence or wall, if needed. More difficult to predict would be the cost of reducing substation worker exposures. Potential methods for reducing worker exposures include shielding, especially with metal-clad switchgear or gas insulated substation buses, and remote operation and maintenance.
Many magnetic field sources are found on the customer side of the electric utility service connection. These include customer-owned power distribution equipment such as transformers, switchgear, buses, feeders, service panels, and general wiring. Grounding methods at and beyond the service connection can also affect magnetic fields if stray return current paths are created. Residential and small commercial environments use mostly single-phase sources. Larger commercial and industrial environments use mostly three-phase sources.
Field reduction methods include rewiring to correct on-premises stray return currents and current loops; installing net current control devices to stop off-premises stray currents; and the use of rigid metal conduit or flat plate shielding for buses, feeders, branch circuits, lighting panels, and transformer vaults.
Only a few sources, such as transformer vaults and heavily loaded buses and feeders, would require attention if a 100 mG exposure limit were specified. At 5 mG or less, all sources would require attention. The greatest cost impacts would occur if vaults, buses, and feeders had to meet a 5 mG or 2 mG exposure criterion. Such installations could at least double in cost. Some office building owners have already spent tens to hundreds of thousands of dollars to reduce computer display interference problems by installing magnetic field shielding.
The primary sources of magnetic fields from end-user appliances are resistive heating elements, motors, transformers and coils, and power cords and wiring. Field reduction methods for these include use of split return or bifiliar heating elements, replacement of inexpensive motors with heavier-duty motors, use of toroidial transformers and coils, installation of shielding for most sources, and conductor compaction for wiring.
The lowest existing magnetic field emission guideline was established for computer video display terminals (VDTs) by the Swedish government in 1991 [Power Frequency Magnetic Fields..., 1995]. That standard, called MPR2, requires VDT magnetic fields to be less than 250 nT (2.5 mG) 50 cm (20 in) from the monitor in the 5 Hz-2 kHz frequency range and less than 25 nT (0.25 mG) in the 2 kHz-400 kHz frequency range. Most new computer monitors are designed to meet the MPR2 standard, since manufacturers have found it possible to meet the standard with little added cost.
No magnetic field guidelines apply to electric blankets, but some manufacturers have altered their designs to reduce magnetic fields. No other low-field appliance examples are known.
The experience of video display manufacturers shows that some appliances and machines can be modified at little cost to meet low magnetic field exposure limits. How far this low cost trend extends to other appliances and machines is unknown, because almost no effort has been expended in this area. Without question, however, some industrial welding and electrically heated metal melting processes would present extraordinary design and cost challenges if low field limits were imposed.
Power frequency magnetic field exposure limits could substantially affect electric railways and other transportation systems. For electric railways, edge-of-right-of-way exposure limits would require changes like those required for transmission and distribution lines. Exposure limits defined for rail passengers would be much more difficult to meet.
Magnetic field reduction methods might include use of DC currents, use of third rail or dual overhead trolley bus type feed systems for lower-speed trains, use of single-ended autotransformer feeds for high speed trains, use of higher voltages, and use of shielding. The uncertain life-cycle costs of these options would have to be weighed against the costs of abandoning electrification in favor of high-speed diesel or turbine motive power.