Large-Volume E.L.F. Magnetic Field Compensation [LV-EMFC] Research Project

- Introduction -

        In May, 1994, Linear Research Associates of Trumansburg, NY entered into a research and development contract with regional utility New York State Electric and Gas Corporation to investigate methods for large-volume active-negative-feedback a.c. magnetic field shielding. Research work outlined in the contract specification was based on Linear Research Associates' commercial moderate-volume (to 10 m3) a.c. magnetic field mitigation technology and also on background discussions with the sponsor's transmission & distribution engineering personnel.

        Midpoint goals defined for this project included survey and source/feedback simulation code research tools and survey instrumentation hardware. Project end-point goals included final construction and in situ testing midpoint, simulation work of sufficient quality and quantity to determine the feasibility of cost-effective active shielding for protected volumes in the range of at least 10,000 m3.

        During the period May, 1994 through June, 1995, a specialized magnetic field survey system and proprietary, fully scripted large-volume active-feedback simulation program were created, debugged and tested. Using the new survey instrumentation, Beta Site magnetic field data were collected which could be utilized for optimization runs. In May, 1995, Linear Research Associates demonstrated the apparent feasibility of large-volume active-feedback a.c. magnetic field [ACMF] cancellation over protected volumes up to 26,000 m3 (918,000 ft3).

        In this interim report we describe a generalized LV-EMFC active-negative feedback system and specific Phase I survey and simulation tools developed for research on its practical implementation. This report concludes with an examination of Linear Research Associates' Phase I Beta Site simulation results.

1. Active-Negative-Feedback ACMF Shielding

Proposed Large-Volume [LV] active-negative-feedback shielding technology.

    Negative feedback is a classic engineering principle which can be utilized for a.c. magnetic field [ACMF] reduction. To implement this type of active shielding, a system of sensors, signal processor/amplifiers and driven coils is placed in each axis (one protected axis is depicted below). System operation is based on the physical principle that an axial a.c. magnetic field component, Bi, through the Protected Volume [PV] can be arbitrarily reduced in intensity by applying fields of opposite phase. Such fields are here determined by axial error components, Bi', at a specified number of sample points. At each sampling sensor's location the magnetic field error component will be equal to the difference between the incident (Bi) and compensating fields. Analytically, the a.c. magnetic attenuation at each sensor reduces to a constant factor independent of ambient variation. Compensation is wideband, covering an instantaneous bandwidth which includes the fundamental line frequency and all significant harmonics. Attenuation of the incident field in each axis is gradient-dependent but may be improved as required by subdividing the protected volume into an arbitrary number of sensor-coil cells.

2. Simulation Program Example

Example simulation program command language compiler input file, demonstrating how the transfer characteristic of a sensor may be varied automatically.

        An interpreter allows input file parameters to be stepped over a series of optimization runs and any specific output parameter function to be recorded for each iteration.

#Define a three-phase parallel power line system

SEGMENT  -7.925 0. 14.411 -7.925 10000. 14.411 965.5 2.2943
SEGMENT 0. 0. 14.411 0. 10000. 14.411 965.5 4.18879
SEGMENT 7.925 0. 14.411 7.925 10000. 14.411 965.5 0.
#Define a driven coil
DrivenCoil CoilX-one
 SEGMENT  48.4 4994. 10. 48.4 5006. 10.
 SEGMENT  48.4 5006. 10. 48.4 5006. 0.
 SEGMENT  48.4 5006. 0. 48.4 4994. 0.
 SEGMENT 48.4 4996. 0. 48.4 4994. 10.

#Define a sensor
SENSOR  SensorX-one  48.4 5000. 4. 1. 0. 0.
#Define a new variable which is the transfer characteristic
Variable t = (100.0e6, 200.0e6) %11

#Define a transfer element with variable characteristic "t"
Transfer SensorX-one CoilX-one t
POINTBLOCK  60. 65. 2 4098. 5002. 2 5. 7. 2
    Loop t
       OutputFile << "Sensor transfer Characteristic = " << t<< entl;
       OutputFile << "x" << tab << "y" << "tab" << "z" << "tab"
<< "RMS" << endl;
       Loop x
           Loop y
               Loop Z
                   outputFile << x << tab << y << tab << z << tab << RMS << endl;
      OutputFile << endl;


3. LV-EMFS Site Survey System

   Unique features of this system are its ability to record time-resolved d.c. and a.c. vector field data (while resolving magnitude and relative a.c. phase for each axis), and its intrinsically accurate axial alignment. Shown from left to right are the system 486DX2-40 portable computer, probe instrument, battery charger and UHF-FM remote field reference monitor. For added survey height, extension sections up to 6 meters in length may be fastened onto probe body.

Large-Volume E.L.F. Magnetic Field Compensation Site System [LV-EMFS].
4. LV-EMFS Survey Instrument Diagram
    A.c. magnetic field signals originating at the "reference sensor" are transmitted via UHF-FM link to a receiver within the Probe Instrument. These signals, along with signals from "MAG-03" tri-axial flux gate magnetometer, are processed through the intervening network and are sent via umbilical cable to a PCMCIA analog-to-digital converter card at the 486DX2-40 portable computer.

Block diagrams of remote Field Reference Monitor and Probe Instrument.

5. ACMF Data Collection Using LV-EMFS System
Survey data collection across single-circuit 115 KV transmission line right-of way.

Survey data collection through 115 KV switchyard.

6. 115 KV Right-of-Way Data Record
    Signal of high spectral purity (low harmonic content) proportional to the transmission line currents are evident in the  y, z and R channels. Magnitude of the x channel magnetic field signal is very low due to left-right source symmetry.

Time-resolved 115 KV transmission line right-of-way x, y, z point record [file 43].

7. Fourier-Transformed Data Record
    Spectral analysis is employed by the survey instrument's computer program to extract the value of a fundamental line frequency in the range of 25 to 400 Hz. This capability provides an automatic means of identifying the basic source frequency for calculations such as relative phase.

Frequency-domain transform of 115 KV transmission line right-of-way x, y, z point record [File 43].

8. Survey vs. Simulation Data
Superimposed plots of survey and simulation data for 115 KV transmission line right-of-way.

    Note excellent agreement of magnitude plots, good agreement of phase data and  negligible variance in either data set. Deviation of phase survey data from simulation values over the -20m to +30m interval has been found to agree with ground conductivity effects not modeled in the simulation program.

9. Beta Site Plan

    Overall building dimensions are 42m [128'] x 5m [16']. A.c. magnetic field values ranging between 7 and 14 milligauss, rms have been recorded in rooms which face a nearby, roughly parallel 4-circuit 345 KV right-of-way. At its closest approach, the near edge of the right-of-way lies approximately 14m [46'] from the Site structure.

10. Uncompensated Beta Site ACMF Profile.
Z= 1.0m uncompensated rms magnetic field strength over Beta Site.

    Pointblock magnitudes (i.e., discrete ACMF values on grid of specified resolution) corresponding to the source field are calculated by the simulation program for a best fit to survey data. To generate field magnitudes and relative phases, our simulation model employs an accurate physical description of the 4-circuit transmission line array and its known phasing, and scales the overall line currents as required to best match actual survey data. Uncompensated fields average about 0.61 µT (6.1 mG), rms, over the first 20 m of the building nearest and perpendicular to the source transmission lines.

LV BETA1: Yo @ 2.5m, Z=1.0m


Beta site 1 temporary Simulation 
x= 5.5-145.5, y= -5.5-59.5, z= 1.0 
reo14d1u.out, .inp, prep14du.m 
June 14 1995, DMW

11. Simulation / Optimization Procedure

    Phase I evaluation of simulated active-feedback system for a building such as the Beta Site is multi-step process. First, a detailed magnetic field survey is made throughout a volume which includes the proposed site protected volume [PV]. Next, simulation models are created which produce a close approximation of the survey a.c. magnetic field data. Finally, the active-feedback components (sensors, transfer characteristic and driven coils) are added, along with initial and step parameters, and the optimization iterations are begun. In the proposed Phase II development, a pointblock data integration module permitting direct entry of survey data into the simulator will be added to the simulation code package. 


12. Compensation System Geometry

    For the so-called permanent solution, subject to further optimization studies, several driven coil and sensor placement constraints applicable to the temporary Beta Site installation have been dispensed with. Hallways, for example, are no longer relevant to cable placement, since lower z and y cable segments can be routed through arbitrarily located conduits under the protected structure. Only 9 driven coils are required in the permanent case. The coil locations shown here have been determined by means of optimization scripting in the simulation program.

Figure 10R. Coil geometry for optimized "permanent" installation.

13. Compensated Beta Site ACMF Profile

    A x-y axis slice is taken at Z = 1m, which corresponds to a z-axis region representative of highest human occupancy in the single-story Beta Site building. A.c. magnetic field magnitudes through the PV are significantly lower than in the uncompensated case (Panel 10).

Z= 1.0m compensation rms magnetic field strength over Beta Site with Y0 coil at -2.41m displacement with respect to building wall.
LV BETA1: Yo @ 2.59m, Z=1.0m
Simulation site 1 Permanent installation 
X=5.5-145.5, Y=5.5-59.5, Z=1.0 
ssur 43.inp psur43s.m June 15 1995, RC

14. Compensated vs Uncompensated ACMF Plots

    Z = 1.0m "boresight" linear plot comparing uncompensated magnetic fields rms magnitude with compensated rms magnitude and axial components peak magnitudes for Y0 coil at -2.41m displacement from building.

    Noteworthy are the relatively low average values and peak-to-avarage ratios achieved for both By and Bz components. Both factors are essential indicators of compensation quality. ACMF fields over a large portion of the building interior attenuated to 1mgrms or less.

LV-BETA1: Yo @ 2.59m, Z=1.0
Beta site 1 Permanent Installation X=70, Y=-5.5-59.5, Z=1.0 sys43.Inp pyz43.m  
June 15 1995, RC
    *=rms, 0=rms_u 
    ...=bx, .-.-=by, --=bz

15. Beta Site ACMF Phase Data

    In this plot, survey record Bx,y,z magnitude/phase data have been used to compute relative ACMF phase variation within the site structure. This plot reveals phase shift due to the presence of electrically-conductive structural members throughout the building. Phase variation related to the structural symmetry is plainly visible, with a skew which is due to slight non-parallelism of the building and the transmission-line source right-of-way. Analysis of this survey phase data suggests that compensatory phase shift may be programmed into the LV-EMFC signal processors to increase the active-feedback system field attenuation coefficient in most installations.

Phase data from Data Site survey record.
Bx,y,z PHASE; Z = 0; BETA SITE

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