Public concern over electromagnetic fields (EMF) emitted from power lines has resulted in significant opposition to the construction and upgrading of power trans-mission facilities. Often those opposed to power line installation demand that the load be served with lower voltage and/or underground lines. The problem with this idea is that in typical installations these solutions, while being less aesthetically objectionable, can actually increase EMF field levels. This increase applies to maximum EMF levels as well as EMF levels at the edge of the right-of-way (R/W).
Let's look at a common 138-kV double-circuit steel pole installation and contrast its expected field levels against the levels from alternate installations at 138 kV and 34 kV. Graphic comparisons are presented for each alternative system carrying the same load.
The magnetic field of this baseline double-circuit configuration will be compared to direct-buried, in-conduit and pipe-type underground configurations. In addition, it will be compared to common double-circuit 34-kV construction. This comparison is complicated by the need to compare systems serving the same load and the limited capacity of the 34-kV lines. However, by comparing the baseline to different numbers of 34-kV double-circuit lines, a realistic comparison can be made.
Configurations for Comparison Figure 1 shows the configuration of the baseline 138-kV line and a common R/W width required for this line. The R/W widths shown in these figures will be used later in comparing edge-of-R/W field levels. Figure 2 shows a 138-kV direct-buried configuration with backfill. Figure 3 shows a 138-kV installation in concrete-encased conduit. Figure 4 shows a steel-pipe type 138-kV installation. These are the configurations used in this analysis.
Important to all underground 138-kV installations is the thermal rating. The surrounding medium's ability to dissipate heat is critical to determining the line's ampacity. An underground installation's cost is directly tied to these ampacity limitations and, unfortunately, magnetic field levels. It is beyond the scope of this article to provide detailed analysis of these tradeoffs. However, these are the reasons the 138-kV direct-buried and conduit installations use cross bonding of the cable shields. If this was not the case, the de-rating of the cable would often require multiple cables per phase and would multiply the cost.
In any case, some installations would require two cables per phase to satisfy the load. For simplicity's sake, this article only addresses the one-cable-per-phase case.
Figure 5 shows the double-circuit 34-kV configuration. Magnetic field profiles of multiple 34-kV lines show the lines close together to clarify that multiple locations would be subjected to the resultant field (Fig. 11). In reality, the lines would be some distance apart to get the increased reliability resulting from geographic diversity.
Use Field-Modeling Program Field calculations for overhead lines are made using a computer field-modeling program. This program has been verified against programs from the Electric Power Research Institute (EPRI), Palo Alto, California, U.S., and Southern California Edison, Rosemead, California. Scientists at the Illinois Institute of Technology Research Institute (IITRI) have also evaluated the program and favorably compared its results to measured data. Calculations for underground installations are made using EPRI's Underground Transmission Workstation PCFields module.
Each configuration is evaluated at a system load of 50 MVA. This number approximates the capacity of a 34-kV double-circuit line and allows the use of simple multiples of normalized field levels to be used to approximate field levels from actual loads.
Each profile is shown with minimum and maximum curves. These result from calculating the profile at maximum normal unbalanced conditions with the current distribution in seven arrangements. At each point in the profile the lowest and highest values are graphed. This procedure results in a band that shows the typical range of field levels expected. In effect, it eliminates questions regarding unbalance. The unbalance used in this analysis is + 5% for the 138 kV and + 6 % for 34 kV.
Results The 138-kV Comparisons _ Figure 6 shows the normalized magnetic field profile of the baseline 138-kV configuration. Figures 7, 8 and 9 show the fields resulting from the direct-buried, in-conduit and pipe-type configurations respectively. Table 1 compares the maximum field level and edge-of-R/W field level for each configuration.
As the load varies, the magnetic field level will vary in direct proportion. This variation is true for all configurations. Therefore, the relative magnetic field levels remain as shown regardless of season, time of day or load growth.
As Table 1 shows, the direct-buried configuration creates the highest magnetic field levels. The in-conduit configuration produces slightly lower levels. The standard overhead configuration produces field levels lower than either. The only 138-kV configuration in this analysis with field levels lower than the overhead option is pipe-type. Unfortunately, pipe-type installations are extremely expensive.
The 34-kV Comparisons _ Figure 10 shows the normalized magnetic field profile of the 34-kV double-circuit configuration. In any real installation, there would be three or more 34-kV double-circuit lines to carry the same load as the double-circuit 138-kV line. Figure 11 is an attempt to show the field level comparison for this configuration. Each of the three 34-kV lines in Fig. 11 is carrying one third of the load (16.67 MVA). As can be seen, the fields from the three 34-kV lines are substantial and exist in multiple locations.
Figures 12a-12d show how the field levels compare at different loads. Each curve shows the maximum field levels versus load served for the baseline 138-kV configuration and 1, 2, 3 and 4, 34-kV lines respectively. In each case the 138-kV line serves the entire load, while the same load is divided evenly by the number of 34-kV lines indicated. The fields for the 138-kV line and one 34-kV line are shown in each figure. The bands indicate the range of results expected in each case, given normal variations of phase unbalance.
For example, in Fig. 12c, at 50 MVA load, each of the three 34-kV lines would normally be expected to produce 16 mG to 21 mG maximum magnetic field levels. (This is also the case in Fig. 11). The figure also shows that a single 138-kV overhead line causes less than 10 mG carrying the same load. Therefore, according to the figure, if you were comparing the 138-kV double-circuit overhead option against three 34-kV double-circuit overhead lines, each carrying one third of the load, then each of the 34-kV lines would produce about twice the level of magnetic field as the single 138-kV line.
The purpose of this type of chart is to reduce or eliminate the dependence of magnetic field estimates on load growth projections. Using these charts makes it easy to demonstrate that the fields from the 138-kV line are lower than fields from the 34-kV alternatives. This presentation is more meaningful in this case than a table such as Table 1, because each 34-kV line is carrying a fraction of the load. Tabular presentations tend to precipitate arguments about load variations and forecasts. The graphic presentation eliminates these arguments.
Analysis Public pressure often suggests implementing the option with the lowest magnetic field levels. This would seem to indicate a pipe-type installation. However, cost will always be a concern. Whether the motivation is a regulatory least-cost mandate or the need to keep costs down in a competitive industry, pipe-type construction will not be a realistic solution for most installations anytime soon, because it is usually many times more expensive than the overhead option.
If the cost of a pipe-type line is unrealistic for a given installation, then the overhead option produces the lowest fields. Since overhead installations are usually significantly less expensive than the other 138-kV options, this is the option most commonly recommended.
Some dissenters point out that if you use the same R/W width for direct-buried configurations as for overhead (60 ft or 18.3 m), then the magnetic field levels at the R/W edge are comparable. This can be true. However, the cost of direct-buried lines is significantly higher than overhead lines and would be further increased by the cost of the additional R/W width.
While these configurations take advantage of readily available field-reduction techniques, such as optimizing phasing for maximum cancellation and phase compaction (to the extent practical), many often-discussed methods are not included. As noted in an EPRI report on magnetic field management, "When these techniques are examined using the method-performance matrix, it becomes apparent that, in most cases, some technology needs to be developed and validated before the techniques will be viewed as feasible by the utility industry.
Every effort has been made to avoid the subject of installation costs in this article. The few references made to relative costs are based on actual projects in which the differences were substantial.
If installation costs vary significantly in other regions, for other organizations, or in the near future, these comparison techniques should still be valid for evaluating the relative magnetic field levels as a part of the overall decision matrix.
Conclusions We now have effective graphic techniques for comparing and communicating the magnetic field levels associated with various installation options.
When evaluating the options for the construction of new transmission lines, we have determined that in most cases the 138-kV double-circuit overhead configuration produces lower magnetic field levels than the others, with the exception of installations often costing several times as much.
Editors Note: This article was adapted from the author's presentation to the 1996 American Power Conference, and published in the Proceedings of the American Power Conference, Vol. 58, 1996. TDW
Brian S. Cramer is a technical expert for inductive coordination and electrical effects with ComEd, Chicago, Illinois, U.S., which he joined in 1991. He has the BSEE degree from Lehigh University. His responsibilities include electromagnetic and electrostatic induction, EMF, and other electrical effects from transmission facilities. He is a senior member of IEEE, the Power Engineering Society, the Electromagnetic Compatibility Society, and an associate member of the Association of American Railroads Communication and Signal Division and its Electromagnetic Compatibility Working Group. Cramer is a registered professional engineer in Illinois.