International Colloquium on Power-Frequency Magnetic Fields,
Sarajevo 3rd-4th June, 2009
Tutorial on
MITIGATION TECHNIQUES
OF POWER FREQUENCY MAGNETIC FIELDS
ORIGINATED FROM ELECTRIC POWER SYSTEMS
Programme
1
Ener Salinas
General principles - Methods of assessment Strategies
2
Pedro L. Cruz
Romero
Conductor management - Compensation
- Mitigation for T&D lines (EHV, HV, MV, LV)
3
Jean
Hoeffelman
Shielding by metallic materials - Power cables
4
Ener Salinas
Substations - Examples
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
1
About the working group C4.204
• CIGRÉ Working Group formed in 2001
• Motivation: Concerns from customers,
utilities and researchers in relation to some
alleged health risks (in particular childhood
leukaemia) of long-term exposure to power
frequency magnetic fields
• Initial aim: To collect discuss and
synthesise the available technical data
referring to different existing techniques to
mitigate extremely low frequency (ELF)
magnetic fields
• Final form: A published Technical
Brochure (TB 373)
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
2
1.1 General Principles
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
3
Sources of power-frequency magnetic fields (PFMFs)
The flow of electrical
energy from the
generation plant to the
customer.
Along the way there
are different types of
sources of powerfrequency magnetic
fields
The PFMFs sources
and techniques can be
classified according
to their origin:
•Power lines
•Underground cables
•Complex sources
(e.g. substations)
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
4
Difference between Electric and Magnetic fields
ELECTRIC
FIELD
MAGNETIC
FIELD
Effect on humans
•The electric field E does not penetrate
the house
•The magnetic field B penetrates the
house easily
•As the field reaches the walls, the
electric charges (generated as a
consequence of this field) are diverted
to earth and recombined
•Only certain materials with specific
geometries or dedicated circuits could
oppose to this action
•Even in the case of lightning, the
lightning rods connected to ground
will do this diversion successfully
•The purpose of designing mitigation
techniques is to find out what are the
most appropriate materials, geometries
or circuits that achieve this action
effectively
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
5
Interaction of AC magnetic fields with materials
AC Source
(a)
Ferromagnetic enclosure
(b)
“Concentration”
Region of
interest
Region of
interest
Coil
“Deviation”
Ferromagnetic
Plate
“Rejection”
Magnetic fields can
have different
interactions with
different materials
The geometry
and the field
incidence are
also important!
(c)
Pure conductive
Plate
(d)
Region of
interest
Region of
interest
Some important design parameters:
Skin depth  
1
 f
B P 
Shielding
SF P   0
Factor
B s P 
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
6
1.2 Methods of assessment of the mitigation
techniques
Analytical
Biot-Savart formula
Numerical
At power frequency we use the quasistatic approximation, i.e. displacement
currents are neglected
Experimental
Shielding experiments with busbars
and conductors at normal scale
Small scale
experiment of a
3-phase
underground
cable
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
7
1.3 Some strategies for mitigation
A relevant factor regarding
the technique to use is the
choice of the location i.e.
where it is to be applied.
In other words apply it to
the source or to the area of
interest?
This is not an easy
question since the
definition of the area of
interest is not always
unambiguous.
As a general rule, it may
seem natural to think that it
will be more cost-effective
to mitigate at the source
than at the area of interest.
However, the choice can be
different. For example in
some cases where the
source is rather large (e.g.
long busbars); or if the
purpose is to mitigate the
field in a small region.
The green outlines are symbolic representations – not necessarily
metal plates – they could indicate a loop, an active device, or any
other mitigation action within that region.
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
8
International Colloquium on Power-Frequency Magnetic Fields,
Sarajevo 3rd-4th June, 2009
Tutorial on
MITIGATION TECHNIQUES
OF POWER FREQUENCY MAGNETIC FIELDS
ORIGINATED FROM ELECTRIC POWER SYSTEMS
Programme
1
Ener Salinas
General principles - Methods of assessment Strategies
2
Pedro L. Cruz
Romero
Conductor management - Compensation
- Mitigation for T&D lines (EHV, HV, MV, LV)
3
Jean
Hoeffelman
Shielding by metallic materials - Power cables
4
Ener Salinas
Substations - Examples
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
9
2.1 Conductor management
Applied mostly to linear sources: overhead lines, underground cables, busbars, etc.
Original configuration
Layout
Compaction
Change of geometry of
conductors keeping the
same phase-to-phase
clearance
Keeping the same geometry
reduce the phase-to-phase
clearance
Balanced
system !!
Contour curves values in T
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
10
2.1 Conductor management
Phase splitting
Single-phase line
Current dipole
1
B 2
r
r : Distance to centre of dipole
Current quadrupole
Faster
reduction with
distance to
source !!
1
B 3
r
r : Distance to centre of quadrupole
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
11
2.1 Conductor management
Phase splitting
Three-phase line
Two split phases
1
B 3
r
Three split phases
No great
improvement !!
1
B 3
r
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
12
2.1 Conductor management
Phase cancelation
Multi-circuit line
Super bundle
1
B 2
r
Low-reactance
Both circuits should be equally loaded
Changes in protection relays could be needed
Changes in corona performance in overhead circuits
1
B 3
r
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
13
2.2 Compensation
Passive compensation
Location close to the source
of a loop or coil.
Magnetic field generated by
the
coil
that
partially
compensates the original
field.
Induced current in the loop
due to the flux linkage.
Increase of effectiveness:
insertion of capacitor to
compensate the inductance
of the loop.
Design parameters:
• Shape of the coil
• Location of the coil
• Electrical parameters
of the conductor
Not complete
compensation !!
• Number of coils
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
14
2.2 Compensation
Passive compensation
Single-phase line
Loop
With capacitor
Contour curves values in T
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
15
2.2 Compensation
Active compensation
Current in the loop generated
by an external power source.
Current control in amplitude
and phase.
More sophisticated equipment
is required.
Costly and less reliable than the
passive loop.
Higher flexibility in the location
of the loop. Possibility of
locating it far from the source.
Not complete
compensation !!
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
16
2.3 Mitigation for T&D overhead lines
Techniques
Increasing the
height of masts
Conductor
management
Compensation
• Low-medium cost
• Medium-high cost
• Medium-high cost
• Low-medium reduction factor
• Low-medium-high reduction factor
• Medium-high reduction factor
Shielding factor =
reduction factor !!
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
17
2.3 Mitigation for T&D overhead lines
EHV and HV Power lines
Increasing the height of masts
25
H=11.34 m
Reduction restricted to
underneath the line.
RF 
Bnon mitigated
Bmitigated
4
20
H=14 m
H=16 m
15
H
H=18 m
V = 380 kV
I = 1500 A
H=20 m
10
H=22 m
H=24 m
rms
Reduction factor at x=0
Induzione magnetica
a 1above
m dal suolo
- Beff [T]
- [µT]
B
1m
ground
H=12 m
5
0
-100
-50
0
Distanza
dall'asse
[m]
Distance from della
line linea
centre
50
100
[m]
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
18
2.3 Mitigation for T&D overhead lines
EHV and HV Power lines
Conductor management: changing the geometry of conductors
380 kV
Low reduction factor close to
the line
RF  2
Low reduction factor far from
the line
RF  1.4
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
19
2.3 Mitigation for T&D overhead lines
EHV and HV Power lines
Conductor management: compaction
Medium reduction factor
115 kV
RF  4
• Lower visual impact
• Reduction of line surge impedance
• Difficult
to
maintenance
perform
live-line
• EHV line: increase of corona effect
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
20
2.3 Mitigation for T&D overhead lines
EHV and HV Power lines
Conductor management: phase cancellation
Low reduction factor close to
the line
380 kV
1500 A
RF  2
Medium reduction factor far
from the line
RF  3
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
21
2.3 Mitigation for T&D overhead lines
EHV and HV Power lines
Conductor management: phase splitting
Medium reduction factor close
to the line
380 kV
1500 A
RF  5
High reduction factor far from
the line
RF  6
Star line: complete
reduction at 35 m !!
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
22
2.3 Mitigation for T&D overhead lines
EHV and HV Power lines
Passive compensation
Low reduction factor close to
the line
RF  2
Medium reduction factor far
from the line at one side
RF  4
High reduction factor far from
the line at the other side
RF  8
Capacitor: nonsymmetrical reduction!!
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
23
2.3 Mitigation for T&D overhead lines
EHV and HV Power lines
Effect on other electrical parameters
Magnetic
Field
Electric
Field
Audible
Noise
Radio
Interference
Unbalance
Height increase



(1)
=
Layout


=


Compaction





Vertical super-bundle low-reactance



(2)

Phase splitting





Passive/active loop


=
=
=
Method
(1)
Starting from certain distance (about 50 m) the effect is the opposite
(2)
It rises lightly from about 30 m off
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
24
2.3 Mitigation for T&D overhead lines
MV and LV Power lines
Differences with EHV and HV lines
•
Variation of current along the feeder
•
Different distribution systems  different presence of zero sequence current
–
3-wire 3-phase
–
4-wire 3-phase
–
5-wire 3-phase
–
2-wire
–
1-phase
•
Lower voltages  use of covered and insulated conductors
•
Shorter phase-phase clearance  Field mitigation only of interest near the line 
more effectiveness in raising the poles.
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
25
2.3 Mitigation for T&D overhead lines
MV and LV Power lines
Mitigation technique
Reduction
level (%)
Installation
cost
Global performance
over conventional
Effect of unbalanced
current
Small compaction
25-45
Low
Lower
Low
Crossarms  armless
 60
Low/medium
Lower
Medium
Tree wires
 60
Medium
Higher
Medium
Spacer cable
 80
High
Higher
High
Aerial Boundle Cable
100
Very high
Higher
High
Underground line
 90
Very high
Higher
High
Phase split
70-80
Medium
Lower
High
Increase clearance to
ground
25-60
Low/medium
Lower
Low
Compensation loop
35
Medium
Lower
Medium
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
26
International Colloquium on Power-Frequency Magnetic Fields,
Sarajevo 3rd-4th June, 2009
Tutorial on
MITIGATION TECHNIQUES
OF POWER FREQUENCY MAGNETIC FIELDS
ORIGINATED FROM ELECTRIC POWER SYSTEMS
Programme
1
Ener Salinas
General principles - Methods of assessment Strategies
2
Pedro L. Cruz
Romero
Conductor management - Compensation
- Mitigation for T&D lines (EHV, HV, MV, LV)
3
Jean
Hoeffelman
Shielding by metallic materials - Power cables
4
Ener Salinas
Substations - Examples
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
27
3 Shielding by metallic materials
Two types of shielding materials
Magnetostatic shielding
Flux-shunting mechanism
Shielding by eddy currents
Induced currents mecanism
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
28
3.1 (pure) ferromagnetic shielding
Initial Relative
Permeability
r,ini
Maximum
Relative
Permeability
r,max
Iron, 99.8% pure
150
5000
Steel, 0.9% C
50
100
300 - 400
2000
Ultra Low Carbon Steel (ULC)
250
1100
Hot rolled Ultra Low Carbon Steel (HR ULC)
250
2000 to 5000
Material
Low Carbon Steel (LCS)
Silicon steel (Si 3%) - Grain oriented (GO)
78 Permalloy (μ-material)
40,000
8,000
100,000
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
29
Relative Permeability
3.1 (pure) ferromagnetic shielding
10000
Oriented silicon-iron alloy
(Rolling direction)
Nonoriented
silicon-iron alloy
1000
Oriented silicon-iron alloy
(Transversal direction)
low carbon steel
100
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40
Magnetic flux density [T]
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
30
3.1 (pure) ferromagnetic shielding
Htengential continuous
Bnormal continuous
To be efficient at distance a ferromagnetic shield needs to be
closed !
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
31
3.1 (pure) ferromagnetic shielding
To be efficient a ferromagnetic shield needs to encompass completely the source.
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
32
3.1 (pure) ferromagnetic shielding
Closed shield
cylinder
sphere
3
shielding factor
10
/0=10000
/0=1000
2
10
/0=100
1
10
/0=10
0
10
-4
10
-3
10
-2
10
-1
10
0
10
1
10
thickness to inner diameter ratio
Closed ferromagnetic shields can have a very high efficiency
mainly when they are not too large with respect to their thickness.
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
33
3.1 (pure) ferromagnetic shielding
Open shield
5.0
L = 1 m, d = 0.2 m,  = 1 mm
r = 100
r = 500
r = 1000
r = 10000
shielding factor
4.5
4.0
3.5
3.0
2.5
2.0
1.5
(a)
1.0 (b)
0.2
(c)
0.4
0.6
0.8
1.0
1.2
1.4
distance y (m)
At distances higher than the shield width, the shielding efficiency is virtually zero.
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
34
3.2 (pure) conductive shielding
Closed shield

a
SF ~    a
Good shielding materials need to
have a high conductivity () like
copper or aluminium
Contrary to what happens with the pure ferromagnetic shielding, the shielding
factor (SF) increases with the shape of the shield.
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
35
3.2 (pure) conductive shielding
Open shield
L = 1 m, d = 0.2 m,  = 10 mm
 = 1 MS/m
 = 5 MS/m
 = 10 MS/m
 = 50 MS/m
25
shielding factor
20
15
10
5
0
0.2
(a)
0.4
(b)
0.6
0.8
1.0
(c)
distance y (m)
1.2
1.4
Even at distances higher than the shield width, the shielding efficiency remains
important.
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
36
3.3 actual shielding materials
Metal
Conductivity in MS/m
Copper
59
Aluminium
36
Iron
10
Steel
6
GO steel
2
Permalloy
1.8
In ferromagnetic materials the conductivity plays also an important part in the
shielding efficiency.
Sometimes multilayer shield involving both high permeability material and good
conductive metals are applied.
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
37
3.4 Underground cables
How to mitigate the fields ?
 Acting on laying geometry and laying depth

Introducing passive loops

Allowing currents to flow in the metallic sheaths

Shielding by conductive metallic materials

Shielding by ferromagnetric metallic materials
Independently from the shielding efficiency of each of the above
solutions, the best solution strongly depends on whether the
intervention must be carried out on an existing cable
already in operation or on a new cable still to be laid down.
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
38
3.4 Underground cables
Passive loop
5
Cavo/sez. trincea
Configurazione in piano ( =100 mm)
Posa senza loop di compensazione
Induzione magnetica a 1 m dal suolo - B eff [µT]
x
Con loop di compensazione L = 500 m (I
Loop = 77
A)
h calc. = 1 m
2
1.6 m
1
1
2
0.25 m
0.25 m
C
L
V = 132 kV
I = 250 A
0.5
0.2
0.1
Con loop di compensazione L = 500 m e con
condensatore di ottimazione (I Loop = 134 A; C1 = 13 mF)
0.02
0
5
10
Distanza dal centro linea [m]
15
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
39
3.4 Underground cables
Passive loops (joint chamber)
Double loop : SF  2
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
40
3.4 Underground cables
Closed ferromagnetic shielding
150
0.2
I = 3000 A
100
x
h mis. = 0 m
I = 3000 A
B rms, 1 m above ground - [µT]
I = 1500 A
B rms, 1 m above ground - [µT]
x
h mis. = 0 m
50
I = 750 A
30
I = 375 A
I = 250 A
20
10
5
3
2
scherm o: L = 66 m;
I = 1500 A
pp =1m
0.1
CL
pp =1m
CL
I = 750 A
I = 250 ÷ 3000 A
 = 406 m m; s = 10 mm
I = 250 ÷ 3000 A
0.05
I = 375 A
0.03
I = 250 A
0.02
1
0.5
0.3
0
0.5
1
1.5
2
2.5
3
3.5 0.014
Distance from line centre [m]
0
4.5
0.5
5
1
1.5
2
2.5
3
3.5
4
4.5
5
Distance from line centre [m]
Steel tube: SF > 50
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
41
3.4 Underground cables
Closed ferromagnetic shielding
Raceway: SF  20
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
42
3.4 Underground cables
145
Flat conductive shielding
0.3
d
25 25
100
Copper plane shield (flat formation): SF > 7
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
43
3.4 Underground cables
Flat conductive shielding
In order to be effective, the
shielding plates have to be
welded together
Aluminium may also be used
but is less effective
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
44
3.4 Underground cables
Open conductive shielding
100
150
welding
bridge
80
20
25
20
80
Aluminium H shield (flat formation): SF > 7
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
45
3.4 Underground cables
Open conductive shielding
bridge
15
0
welding
32
62
20
60
Aluminium square shield (trefoil formation): SF > 7
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
46
3.4 Underground cables
Synthesis
Passive loops
Open
ferromagnetic
shielding
Closed
ferromagnetic
shielding
Conductive
shielding
Shielding
factor SF
1.5 to 4
(flat formation)
depends on
distance to
shield !
> 15
>7
Losses
low
low
low to medium
medium
Corrosion risk
/
needs
protection
needs
protection
Cu: OK
Al: depends
on soil pH
Costs
low
medium
high
Cu: high
Al: medium
Maintenance
easy
rather easy
variable
rather easy
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
47
International Colloquium on Power-Frequency Magnetic Fields,
Sarajevo 3rd-4th June, 2009
Tutorial on
MITIGATION TECHNIQUES
OF POWER FREQUENCY MAGNETIC FIELDS
ORIGINATED FROM ELECTRIC POWER SYSTEMS
Programme
1
Ener Salinas
General principles - Methods of assessment Strategies
2
Pedro L. Cruz
Romero
Conductor management - Compensation
- Mitigation for T&D lines (EHV, HV, MV, LV)
3
Jean
Hoeffelman
Shielding by metallic materials - Power cables
4
Ener Salinas
Substations - Examples
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
48
4.
Substations
LV SUBSTATIONS
The main characteristics of these sources, and the ones that
differentiate them from power lines and underground cables, are:
•
•
•
Complexity
Local concentration
Proximity
The list of possible sources contributing to the emitted PFMF is:
•
•
•
•
•
•
Busbars
Transformers
Low-voltage cables
Low-voltage connections
High-voltage cables
Neutral/stray currents
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
49
Typical LV in-house substation located in the cellar of a building
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
50
Mitigation of PFMFs from busbars
Busbars can have different shapes. Yet,
longitudinal profiles are often common
and it can be sometimes a reasonable
approximation when designing
geometries and selecting shielding
material
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
51
More elaborated shielding designs for busbars
(a)
(b)
(c)
(d)
Combination of 2 passive
shields and one active
loop
Busbars
Averaged shielding factors <SF> in front
of the second shield
 <SF> = 2 when cancellation loops are used alone
Windows and
apertures
Narrow gaps
 <SF> = 4 when the 1010-steel is used alone
 <SF> = 6 when the Al shield is used alone
 <SF> = 9 when aluminium and 1010-steel are used
 <SF> larger than 20 when aluminium, 1010-steel and
loops are used
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
52
Magnetic field from transformers-1
Because of the core and
cover, transformers (by
themselves) emit almost
no magnetic field
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
53
Connections from the LV side
The responsible
for field
emissions
nearby
transformers are
often the
connections
from the
secondary side
A possible
mitigation
technique is to
optimize phase
mixing
Before
phase
managemen
t
After phase
managemen
t
R ST
R ST
T
R S
Mixing phases
R
S
T
R
S
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
T
54
Field mitigation techniques for MV/LV substations
Source
Strategy
Technique
Method
Short busbars
(residential)
Mitigation at the source
•Conductive shielding
(e.g. aluminium)
•Passive compensation
-3D-FEM or Integral
methods
-Lab experiments
Long busbars
(industrial)
Mitigation at the source
may not be cost efficient.
Thus mitigation at the
affected area may be
needed
•Conductive or
ferromagnetic shielding
•Active compensation
-2D-Numerical methods
-Analytical
Transformers
Mitigation at the source,
by optimizing the
connections at the
secondary side
•Phase cancellation
•Distance management
-3D-Numerical
-Experiments with the
relevant components
(connections at the LV
side)
Mitigation at the source
•Shielding with metal
plates
•Passive compensation
with loops
-Analytical
-2D-FEM
Cables
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
55
Mitigation of PFMFs from HV/MV substations
•In HV substations, the
highest magnetic
fields are also
registered at the
secondary side
•However these are
located mainly
between the
substation limits
•Some emission over
the 1-microtesla level
can be registered
outside the substation
boundaries
•A possible mitigation
technique is distance
management, i.e.
moving the affected
area or extending the
fence some metres.
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
56
Examples of
Implementation of
Mitigation Techniques
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
57
Example 1: Ferromagnetic pipes in Genoa
•The three cables are enclosed inside
a ferromagnetic tubular section, which
acts as a shield trapping the magnetic
flux
•The material used is low carbon steel,
with an external diameter of 508 mm
and a thickness of 9.5 mm
•2 km of circuit of 150 kV 1x1000 mm2
XLPE cable were shielded with this
technology
•Field at 1m above the ground < 0.2 μT
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
58
Example 2 Passive lops in Vienna
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
59
Example 3: High Magnetic Coupling
Different designs
Source
cables
Shielding
Cables
Magnetic
core
Shielding
cables
o'
Shielding
cables
Source
cables
Magnetic
core
Windings
Source
cables
Windings
o
Jointing zone
S2
S1
o
S3
y
Section S1 and S3
z
Section S2
x
x=0m
Results
x=10m
x=20m
x=30m
i=50 cm
d=11.8 cm
(HV cable
1600 mm2)
SF = 7.3
Source only
Configuration 1
SF = 88.4
Configuration 2
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
60
Example 4: Castiglione Project, a case of active shielding of a
HV overhead line in Italy
The scope of this project was the reduction of the magnetic field - in an area of children activity - to values
below 0.2 μT as requested by the local administration.
Before mitigation
After mitigation operations
After works,
inactivated
screen
Before works
After works,
activated
screen
Cabin containing loop feeding devices
Regulated current
generator
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
61
Example 5: Shielding of busbars in a secondary substation
Results
After implementation of the
two separated shielding
plates (back of the
switchboard and ceiling)
The maximum value of the
magnetic field in the area of
interest was 0.4 μT
The average value of the
magnetic field was 0.2 μT
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
62
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
63