Appendix 5-2 Evaluation of Magnetic Fields from the National Grid Canal Street to Salem Harbor 115-kV Underground Circuits The appendix material has been redacted for Critical Energy Infrastructure Information (CEII). Electrical Engineering and Computer Science Practice Evaluation of Magnetic Fields from the National Grid Canal Street to Salem Harbor 115-kV Underground Circuits National Grid has redacted this document for Critical Energy Infrastructure Information (CEII). 9/4/2013 Evaluation of Magnetic Fields from the National Grid Canal Street to Salem Harbor 115-kV Underground Circuits Prepared for Transmission & Distribution Services Burns & McDonnell 9400 Ward Parkway Kansas City, MO 64114 Prepared by Exponent 17000 Science Drive Suite 200 Bowie, MD 20715 September 3, 2013 Exponent, Inc. 0900537.000 M0T0 0913 BC01 September 3, 2013 Contents Page List of Figures ii List of Tables iii Limitations iv Executive Summary v Introduction 1 Calculation Methods 9 Results and Discussion 10 Horizontal Duct Bank 10 Vertical Duct Bank 12 Congress Street Bridge Crossing 13 Manhole Approach and Manhole Entrance 15 Magnetic-field variation with burial depth 18 Passive Loop Configuration 19 Passive Loop Model 20 Passive Loop Results 23 Uncompensated Passive Loops 23 Capacitive-Compensated Passive Loops 25 Summary and Conclusions 28 References 30 i 0900537.000 M0T0 0913 BC01 September 3, 2013 List of Figures Page Figure 1. Map of the proposed route for the reconstruction of the 115-kV underground lines between the Salem Harbor and Canal Street Substations. 2 Figure 2. Horizontal duct bank configuration. 4 Figure 3. Vertical duct bank configuration. 5 Figure 4. Congress Street Bridge crossing configuration. 6 Figure 5. Manhole approach configuration. 7 Figure 6. Manhole entrance configuration. 8 Figure 7. Magnetic field (mG) for the horizontal duct bank configuration. 11 Figure 8. Magnetic field (mG) for the Vertical duct bank configuration. 12 Figure 9. Magnetic field (mG) for the Congress Street Bridge crossing configuration. 14 Figure 10. Aerial and profile views of underground duct bank approaching the manhole sections. 15 Figure 11. Magnetic field (mG) for the manhole approach configuration. 17 Figure 12. Magnetic field (mG) for the manhole entrance configuration. 17 Figure 13. Percentage change in magnetic field (maximum) over the cables as a function of burial depth. 19 Figure 14. Geometry used in the COMSOL simulation to calculate the magnetic fields using passive loops. 22 Figure 15. Transect of the underground duct bank at the manhole entrance including the location of two passive loops each located 9 inches outside of the duct bank. 23 Figure 16. Magnetic field (mG) for the manhole entrance configuration with uncompensated passive loops. 24 Figure 17. Magnetic field (mG) for the manhole entrance configuration with capacitive- compensated passive loops. 26 ii 0900537.000 M0T0 0913 BC01 September 3, 2013 List of Tables Page Table 1. Magnetic field (mG) for the horizontal duct bank configuration 11 Table 2. Magnetic field (mG) for the vertical duct bank configuration 13 Table 3. Magnetic field (mG) for the Congress Street Bridge crossing configuration 14 Table 4. Magnetic field (mG) for the manhole approach configuration 18 Table 5. Magnetic field (mG) at a height of 3 feet above ground for various burial depths for the manhole entrance configuration 18 Table 6. Magnetic field (mG) for the manhole entrance configuration with uncompensated passive loops 24 Table 7. Magnetic field (mG) for the manhole entrance configuration with capacitive- compensated passive loops 27 iii 0900537.000 M0T0 0913 BC01 September 3, 2013 Limitations At the request of Burns & McDonnell, Exponent conducted specific modeling and evaluations of components of the electrical environment of National Grid’s Canal Street to Salem Harbor 115-kV Transmission Line project. This report summarizes work performed to date and presents the findings resulting from that work. In the analysis, we have relied on transmission line design geometry, usage, specifications, and various other types of information provided by the client. We cannot verify the correctness of this input data, and rely on the client for the data’s accuracy. Although Exponent has exercised usual and customary care in the conduct of this analysis, the responsibility for the design and operation of the project remains fully with the client. The scope of work provided by Exponent was to assess the magnetic-field levels associated with various underground transmission line configurations provided by Burns & McDonnell. In some cases this includes effects on magnetic-fields due to the addition of passive loops. Exponent’s scope of work summarized in this report did not include an evaluation of technical or operational issues beyond modeling of magnetic field levels. The findings presented herein are made to a reasonable degree of engineering and scientific certainty. Exponent reserves the right to supplement this report and to expand or modify opinions based on review of additional material as it becomes available, through any additional work, or review of additional work performed by others. The scope of services performed during this investigation may not adequately address the needs of other users of this report, and any re-use of this report or its findings, conclusions, or recommendations presented herein are at the sole risk of the user. The opinions and comments formulated during this assessment are based on observations and information available at the time of the investigation. No guarantee or warranty as to future life or performance of any reviewed condition is expressed or implied. iv 0900537.000 M0T0 0913 BC01 September 3, 2013 Executive Summary National Grid has proposed reconstruction of two existing alternating current 115-kV underground lines (circuits S-145 and T-146) between the Canal Street Substation and Salem Harbor Substation in Salem, Massachusetts. In the proposed design, the circuits will be reconstructed along a new roadway route and placed in a single double-circuit duct bank. The reconstructed circuits between the Canal Street and Salem Harbor Substations will be sources of AC magnetic fields above ground. 1 Along different portions of the route the circuits will be constructed in several different duct bank configurations. The five different duct bank configurations detailed in this report are: horizontal, vertical, manhole approach, manhole entrance, and Congress Street Bridge crossing. In addition, depending on construction conditions, each of these configurations may be constructed at a variety of burial depths. Calculations of all magnetic-field levels were performed at a height of 3 feet above ground and out to a distance of 100 feet on either side of the duct bank in order to characterize magnetic- field levels along the proposed route. 2 Over the majority of the route, the circuits will be installed in the horizontal duct bank configuration, which results in the lowest magnetic-field level for most locations above ground. 3 The maximum magnetic field associated with the horizontal duct bank is 55 milligauss (mG) over the duct bank itself and diminishes to 4 mG at a distance of 25 feet away. The vertical duct bank configuration will be used in locations where the horizontal duct bank configuration is not practical, as well as upon entering and exiting the manholes. Maximum magnetic-field levels directly over the vertical duct bank configuration are 43 mG and diminish to approximately 10 1 Magnetic field calculation results are presented as the resultant of x, y, and z field vectors with root-mean- square (rms) magnitudes. RMS refers to the common mathematical method of defining the effective voltage, current, or field of an AC electrical system, such as the 115-kV transmission line discussed herein. 2 IEEE Std. C95.3.1-2010 and IEEE Std. 0644-1994 specify that magnetic fields should be evaluated at 1 meter (3.28 feet) above ground. Results presented herein, however, are calculated 3 feet above ground for comparison to previous calculations performed by Burns & McDonnell. Magnetic-field levels at 1 meter above ground would be lower in all locations than those presented herein. 3 Directly over the duct bank the magnetic-field levels are higher for the horizontal configuration than for the vertical configuration. However, field levels diminish more rapidly with distance for the horizontal configuration such that the horizontal duct bank configuration results in the lowest magnetic-field level above ground for most locations. v 0900537.000 M0T0 0913 BC01 September 3, 2013 mG at a distance of 25 feet. At the Congress Street Bridge, the circuits will be routed beneath the bridge; they will produce a maximum magnetic-field level above the surface of the bridge of 68 mG directly over the circuits, falling to 4 mG or less at a distance of 25 feet from the centerline. Manholes are a necessary part of the underground cable system for installation, maintenance, and monitoring. Magnetic-field levels are higher at manhole approaches and manhole entrances, but can be mitigated to some extent with the use of passive loops (i.e., de-energized loop of wire). If passive loops are used to lower magnetic-field levels above ground, the highest magnetic-field level above ground at the manhole entrance for an uncompensated passive loop is calculated to be 71 mG (a reduction of approximately 50% from unmitigated levels) and at 25