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.
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.
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
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
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
2. Simulation Program Example
Example simulation program command language compiler
input file, demonstrating how the transfer characteristic of a sensor may
be varied automatically.
allows input file parameters to be stepped over a series of optimization
runs and any specific output parameter function to be recorded for each
#Define a three-phase parallel power line system
#Define a driven coil
#Define a sensor
#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
||<< "Sensor transfer Characteristic = " << t<< entl;
||<< "x" << tab << "y" << "tab" << "z"
||<< "RMS" << endl;
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
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
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
UNCOMPENSATED RMS, MAX. POINTBLOCK
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
Figure 10R. Coil geometry for optimized "permanent"
Sensors for Y coil 1 at Y=7.94 and on line through center
Sensor for Y coil 2 & 3 at center coils
Upper Z centers at X-74.5, Z-3 and at Y-13.85 (1st Z
coil)- center of coil
2nd coil Y=29=1.755m from center of coil along Y
3rd coil sensors: y-48.4 - center of coil
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
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
COMPENSATED RMS MAX. POINTBLOCK
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
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
...=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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