Don Bunnag
M. Eng. Electrical engineering, Kasetsart University
Senior Engineer Provincial Electricity Authority(PEA)

What have we got from transmission line?
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• Electric fields
Magnetic fields
Corona effects
Audible noise(AN)
Electromagnetic interference(EMI)
Health effect, shock hazard
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• (Power System) Electric and Magnetic Fields
Measurement of Electric and Magnetic Fields
High-Voltage Corona Effects
Power Line Electromagnetic interference(EMI)
Safety and EMF exposure limits
Field Management and Mitigating
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David W. Fugate,
Ph.D.
The Pennsylvania
State University http://www.electri
c-research.com/ dfugate@electriresearch.com
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Electric fields from charge
Electric fields from
power line
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• Electric fields occur only with overhead lines
– Easily shielded by nearly all materials
– Exist when overhead line is energized, even with no load
– Magnitude depends on conductor separation, height, and line voltage
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Magnetic fields
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• Magnetic fields
– Produced only when line carries current
– Difficult to shield
– Magnitude depends on conductor separation, height, and all current including load currents
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Transmission line
Substation
Transformer vault
Main switching for large building (e.g. 4000 ampere) electric service
Page 9 https://www.systronemv.de/en/services/46-isolinienberechnungen-en.html
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http://thefragmentationparadox.blogspot.com/2014/03/electromagnetic-fields-emf-in-high_16.html
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Magnetic fields add or subtract with earth’s geomagnetic field depending on current direction
Do not induce currents except in moving objects
Page 12 https://en.wikipedia.org/wiki/Highvoltage_direct_current
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• Magnetic fields evaluated around perimeter fence, and along profiles moving away from the station.
Typically fields dominated by fields from overhead and underground lines entering and exiting station.
Should design layout of large equipment to keep it away from the perimeter fence, especially sensitive areas.
Page 13 https://www.systronemv.de/en/services/isolinienberech nungen-en/130-isolinienberechnungen-en.html
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Measurement of Electric and Magnetic Fields
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• Electric and magnetic field sensors
Field characteristics
Page 14 https://www.protelturkey.com/en/test-measuring-instrumentssoftware/electromagnetic-fields-emr-measurement/
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Electric field strength E
– kilovolts/meter (kV/m)
Magnetic flux density B “magnetic field”
– milligauss (mG) or microtesla ( μ T)
1 mG = 0.1 μ T or 10 mG = 1 μ T
10,000 G = 1 tesla (T)
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• Free body meter
– Two conducting plates
– Electric field induces charge
– AC field cause charge to
oscillate between plates
– Current I = dQ/dt
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Hall effect sensor (AC and DC)
– Measures voltage across thin layer of semiconductor, prependicular to B field
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Fluxgate sensor (AC and DC)
– Ferromagnetic cylinder with two coils that uses saturation to measure field strength
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• IEEE Std. 644-1994 “IEEE Standard Procedures for
Measurement of Power Frequency Electric and Magnetic
Fields From AC Power Lines”
• Cigre 375, “Technical guide for measurement of low frequency electric and magnetic fields near overhead power lines”
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1 meter above ground level
Sensor generally in vertical position
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Nearly all objects change the electric field
Person standing beneath an overhead line focuses the electric field
Need to measure unperturbed electric field, without any objects near the sensor
Distance from sensor to operator > 2.5m
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10 kV/m
110 kV/m
80 kV/m
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38 kV/m
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1 meter above ground level
For single-axis sensor read maximum value
For three-axis sensor read rms value
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What is corona?
How weather affects corona
Audible noise (AN)
Electromagnetic noise (radio noise, RN)
Mitigating corona effects
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Corona discharge
– Corona is local ionization, breakdown of air due to strong electric fields
– Occurs at location of strong electric field, typically at phase conductors
– Results in small energy loss
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Water droplets cause enhancement of electric field at surface of phase conductors
Corona on AC lines strongest during wet weather
– Rain
– Snow, ice
– Fog, mist
Corona on DC lines strongest during fair weather
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https://www.electricalidea.com/2016/12/09/what-iscorona-effect/
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Corona discharges on high-voltage power lines cause AN
Each discharge contributes to a minute change to the local sound pressure level
Many sources along phase conductors add to produce sound pressure level above ambient
(background)
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Generally, AN levels become significant for lines at 345 kV or above
At high voltages only the corona on conductors is responsible for observed AN levels
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Qi Li, Member, IEEE , Simon M. Rowland, Fellow, IEEE , Iain
Dupere, and Roger Shuttleworth
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Corona-produced electromagnetic noise can cause interference (EMI, RI, TVI) if level is sufficiently high
Corona EM noise is NOT constant
Number of corona sources and their intensity depends on line design and weather factors
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Goal is to reduce electric field intensity very close to the conductions
This field is influenced by:
– Voltage
– Size of conductors, phase spacing
– Line configuration
Page 35 https://en.wikipedia.org/wiki/Corona_ring 29/12/62

“Radio Noise Design Guide for High-Voltage
Transmission Lines”, IEEE TRANSACTIONS ON POWER
APPARATUS AND SYSTEMS, VOL. PAS-
90, NO. 2, MARCH/APRIL 1971, IEEE Radio Noise
Subcommittee Report - Working Group No. 3
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Power Line Electromagnetic Interference (EMI)
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EMI is the general label for condition when electromagnetic fields from an external source cause interruption, degradation, or failure of an instrument or system.
Most often refers to interference
caused by high frequency
electromagnetic fields
– Radio frequency interference (RI or RFI)
– Television interference (TVI)
Page 37 https://slideplayer.com/slide/10414960/
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Two types of fields from AC power lines:
1) Power-frequency electric and magnetic fields (EMFs)
 Electric fields rarely cause interference because electric fields are easily shielded
 Magnetic fields cause interference with sensitive instrumentation
2) Electromagnetic noise, or radio noise (RN)
 Corona
 Gap-sparking
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Electromagnetic Noise: Gap Sparking
Gap sparking was responsible for most power line interference complaints (especially interference with old analog television broadcasts—not so much with digital broadcast)
Occurs at tiny electrical separations
that develop between mechanically
connected metal parts
Page 39 https://calsignsolutions.com/todo_electrical_ transformer_damage.php
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ELF Magnetic Field Interference
Three general types of facilities may have instrumentation susceptible to power-frequency magnetic fields
1) University and research laboratories
 Chemistry, physics, biology, engineering, nano-fab
2) Medical facilities and hospitals
 MRI, PET, CT scanners
3) High-technology research and manufacturing
 Semiconductor production, clean rooms
 Nano-fabrication, materials characterization
 Pharmaceutical research and development
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EX. Magneto resource imaging (MRI)
(Interference limit 1-3 mG/0.1-0.3uT)
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Biological effects
– Direct effects result from direct interaction of fields with the body, biological tissue
– Indirect effects involve interactions with an object at a different electric potential from the body, i.e., shock hazard
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Three Direct Coupling Mechanisms of EMF
Capacitive Coupling: Low-frequency electric fields produce flow of current, resulting in surface charges on the body.
Inductive Coupling: Low-frequency magnetic fields induce electric field, driving circulating currents in conductive tissue.
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Human Exposure to Electromagnetic Fields: From
Extremely Low Frequency (ELF) to Radiofrequency
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Human Body Absorption of EM Energy
• Electromagnetic Absorption: Exposure to electromagnetic fields at frequencies above about 100 kHz can lead to significant absorption of energy and temperature increases
• 100 kHz to 20 MHz: Frequency dependent absorption in the trunk, neck, legs.
• 20 MHz to 300 MHz: Relatively high absorption can occur in the whole body,
• 300 MHz to several GHz: Significant local, nonuniform absorption occurs.
Page 44 Human Exposure to Electromagnetic Fields: From
Extremely Low Frequency (ELF) to Radiofrequency
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• Stimulation of peripheral nerves
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Stimulation of muscles
Shocks and burns caused by touch
• Elevated tissue temperatures,
resulting from absorption of energy
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Indirect Coupling Mechanisms Induction from Transmission Line EMF
Electric fields induce voltage and currents in conductive objects
– Spark discharge occurs when a person comes in contact with a conductive object charged by an electric field
Magnetic field can induce current in objects in parallel with power lines
– Long fences, Pipelines
Coupling of EMF to medical devices worn by, or implanted in, an individual
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Induction on Vehicles Beneath Transmission Line
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Exposure Standards and Guidelines
• International Council on Non-Ionizing Radiation
Protection (ICNIRP), endorsed by World Health
Organization (WHO)
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Difficulties in Establishing Guidelines
There are many different published results in scientific literature, with a wide range of conclusions
Groups of scientists do comprehensive reviews of applicable literature. Can the results be duplicated reliably?
Also, it is difficult to determine if laboratory study results can be extrapolated: Are the results of cellular and animal studies applicable to humans?
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Two Basic Types of EMF Biological Studies
Laboratory study
– Experiments controlled by researcher looking for measureable, repeatable EMF effects
– In vitro: cellular studies, independent of living organism
– In vivo: plant or animal study
Epidemiological study
– Statistical study of disease
in population as a function
of exposure
– Shows association, but not
necessarily causation
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Occupational vs.
General Public
In an occupational setting
– adults are exposed to "known" conditions,
– trained to be aware of risks,
– and trained to take precautions.
General public involves
– people of all ages with varying health status,
– who may be unaware of exposure to EMF,
– and cannot reasonably be expected to take precautions with respect to EMF.
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“Guidelines for Limiting Exposure to Time-Varying
Electric and Magnetic Fields (1Hz-100kHz)”
Objective is to establish guidelines for limiting EMF exposure that will provide protection against known adverse health effects.
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Reference Level, General Public
@ 50 Hz 5 kV/m 200 uT (2,000 mG)
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@ 50 Hz 10 kV/m 1,000 uT (10,000 mG)
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Contact Current Reference Levels
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Limits on Right-of-Way (ROW)
A number of states have electric and magnetic field limits
– Electric field limits on ROW to limit shock hazard
• Typical limits are 7-8 kV/m on ROW
– Magnetic field limits on ROW or at edge of ROW have been specified with no basis except that they are comparable to existing magnetic field conditions for operating transmission lines http://152.87.4.98/power/righto fway/wire_border_zones.htm
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Limits on Right-of-Way (ROW)
Example Electric Field Magnetic Field in R.O.W. Edge R.O.W. On R.O.W. Edge R.O.W.
EGAT (500 kV) 15 kV/m 2 kV/m - 150 mG
Florida (230 kV) 8 kV/m 2 kV/m 150 mG
New York (125kV) 11.8 kV/m 1.6 kV/m - 200 mG
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Field Management and Mitigating Approaches
Layout, location of sources
– Using distance to best advantage
Line configuration
– Compaction, delta arrangement, load flow oppsite
– Transposition, Split-phase, twisted
– active or passive cancellation
Shielding
– Vault shielding, walls
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Reversed Phase
Circuit I load 1500 A
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Increase Distance/Line Compaction
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Increase Distance
Line Compaction
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Compaction Example
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Load Flow Opposite
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Passive Cancellation B-Field
Time varying magnetic field from transmission line induces the current in the loop oppose the magnetic field generated from transmission line
40-60% reduction
Multiple grounded point
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Passive Loop Cancellation
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Passive Cancellation Loop
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Electric Field Shielding
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Two basic shielding mechanism for power frequency magnetic fields
Induced currents (eddy current)
– Time-varying magnetic flux induces rotational electric field, currents flow in conducting materials, opposite field produced.
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Two basic shielding mechanism for power frequency magnetic fields
Flux shunting (high permeability shielding)
– Induced magnetization produces canceling field
– More commonly explained that shield provides a low reactance path for flux
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Shielding Example: Main
Switchboard Room

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Basic steel
Transformer steel, silicon steel
Nickel alloys
Copper, aluminum
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Magnetic fields are the most controversial aspect of power system field effects, and therefore require the most attention.
AC magnetic field interference is an issue only with very sensitive instrumentation, typically in hospitals, universities or research facilities.
Corona effects can be mitigated with good design.
Electric fields can easily shield by metal object
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Email: don.b@pea.mail.go.th
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