FMS Publications

Reduction Of ELF Magnetic Fields Emanating From Circuits With Net Current Conditions By Cancellation Techniques

INTRODUCTION:

Elevated levels of power frequency magnetic fields in commercial buildings are often caused by "net-current" conditions present on conduits, bus ducts and other distribution circuits. In multi-grounded, four-wire industrial and commercial building wiring systems, some portion of the neutral current may return to the source transformer via building grounds. In other cases, neutrals of different circuits may be tied together thus allowing current from one neutral to return to the transformer via alternate neutral conductors. In any of these instances, the vector sum of the currents for any given circuit may not add up to zero. When the vector sum of the phase, neutral, and parallel ground wire (if present) for a given circuit does not equal zero, a net-current condition is present. This circuit condition creates net-current magnetic fields that decay at a 1/D rate versus the 1/D2 rate normally associated with magnetic fields emanating from such distribution circuits.

Reconfiguration of conductors is usually not effective at reducing net-current magnetic fields; however some reduction may be possible by arranging conductors in such a way to cause the displacement magnetic field to partially cancel the net-current field. The best mitigation measure for magnetic fields caused by a net-current condition is to correct wiring problems (improper neutral connections, missing neutrals, improper grounding, etc.) so as to minimize or eliminate the alternative current paths that are creating the net-current circuit conditions.

Most often, it is logistically and economically prohibitive to consider reduction of elevated magnetic field from net-current circuits with magnetic field shielding. Elevated magnetic fields from net-current circuits can best be reduced by corrections or changes to wiring systems. However, in some commercial and industrial buildings, such corrections to wiring systems are not practical.

OBJECTIVE:

This study was intended to investigate the effectiveness and commercial viability of using magnetic field cancellation techniques to reduce magnetic fields in commercial buildings with net-current magnetic field conditions present.

The objective of this study was to (1) develop and evaluate magnetic field cancellation schemes, in a laboratory setting, utilizing both passive and active cancellation techniques to effectively reduce magnetic fields produced by net-current circuit conditions, and (2) to investigate the viability and practicality of using such techniques in real world commercial and industrial buildings that have net-current magnetic field conditions present.

METHOD:

Utilizing three 10-foot-high by 10-foot-wide by 10-foot-long non-metallic test structures, a series of single-phase and three-phase conductors in a variety of conduits typical of commercial buildings, were suspended above an x and y measurement grid on the floor of a laboratory to simulate an office beneath the test circuits and conduit(s). Three variable transformers were connected to a 208 volt, three-phase, four-wire commercial electric service. The output of the three variable transformers provided an adjustable 0 to 140 volt source to three loading transformers, each configured for 120 volt input and 5 volt output. The output of each loading transformer was connected directly to one of the phase conductors in the test circuit conduit. The three individual phase conductors plus neutral conductor were connected together utilizing a short length of copper bus located at the exit of the conduit. This test power network thus allowed either three-phase or single-phase current to circulate in values ranging from 100 to 800 amps in the test conductors. Real world circuit imbalance conditions were controlled by different phase current settings and use of different wire sizes for the neutral conductor. Net-current conditions were controlled by connecting a separate external wire loop from the neutral point of the loading transformers to the copper collection bus at the end of the conduit. This external by-pass loop was fastened to the perimeter walls of the laboratory to create distance from the magnetic field measurement grid. An adjustable choke used in the by-pass circuit allowed for further adjustment of circulating current. This arrangement allowed net-current conditions from 1 to 50 amps to be created in the test circuit conduit. Metallic (non-ferromagnetic and ferromagnetic) and non-metallic conduits where included in the test program.

For induced cancellation tests, test "loops" of various conductor sizes (#4/0 to #2/0) and of various lengths were placed both inside and external to the conduit(s). Phase currents were varied to produce a wide range of net-current amplitudes and phase angles. The resulting induced current in the test loop was recorded. Magnetic field values were also taken at 30 inches and 60 inches beneath the test conduit at all measurement grid locations to determine cancellation effectiveness. Both grounded and ungrounded field cancellation loops were tested.

For active cancellation tests, a sensor was placed around the test circuit to monitor net-current values. The output of the sensor fed a custom designed active feedback network which provided precise current and phase angle adjustment to the cancellation signal. The cancellation signal was then fed to a driver circuit connected to an active cancellation loop. A segment of the active cancellation loop was placed immediately adjacent to the length of test circuit conduit. The remainder of the active cancellation loop was placed adjacent to the path of the neutral by-pass circuit on the perimeter walls of the test laboratory. As with the induced cancellation loop tests, the test circuit was then energized with various amounts of net-current. Magnetic field values were documented at 30 inches and 60 inches beneath the test conduit and cancellation loop conductors, at all measurement grid locations, to determine cancellation effectiveness.

Factors evaluated in all of the cancellation scheme tests included: grounding of the cancellation loop, resistance and inductance for the cancellation loop, location of the cancellation loop, effect of conduit material on cancellation loop effectiveness.

RESULTS:

Induced current in a passive conductor loop placed immediately inside the test conduit with the phase and neutral conductors reduced the magnetic field level by approximately 50% in single-phase circuit tests. Although other work on passive loop cancellation of transmission lines has been shown to be successful (see Zafenella, et. al.), the cancellation loop in these experiments was generally less effective when utilized with three-phase circuits. Passive cancellation loops placed exterior to metallic conduits and with non-ferrous conduits such as PVC, generally did not provide meaningful current induction and magnetic field cancellation.

The use of an actively driven cancellation scheme was shown to be highly effective at reducing magnetic field levels from both single-phase and three-phase circuits in metallic conduits. Magnetic field levels created by net-current circuit conditions were reduced by approximately 90% utilizing the active cancellation loop. However, the use of an active cancellation loop proved generally ineffective when placed adjacent to conductors with net-current conditions present and not contained in metallic conduit.

CONCLUSION:

The laboratory test data indicates that in certain instances, it may be possible to effectively reduce ELF magnetic field levels in commercial building distribution circuits with net-current conditions present by the use of an actively driven cancellation loop system. Such cancellation systems are not without risk or complicating side effects which must be addressed in the design stage. As has been shown in other work on this subject, there may be limitations associated with the actual use of such a system in a commercial building caused by the return path of the cancellation signal. This return path creates its own net-current field, without the benefit of cancellation. Further testing is planned to test active cancellation systems in several commercial buildings to test this technology and to develop specifications and methods that will optimize for a range of source types and construction environments.

This paper was presented at the 1998 Annual Review of Bioeffects Research at Tucson, Arizona
and is the result of a cooperative effort between
Field Management Services and Grid Technologies Associates.