1 Tutorial on MITIGATION TECHNIQUES OF POWER FREQUENCY MAGNETIC FIELDS ORIGINATED FROM ELECTRIC POWER SYSTEMS Programme Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3 rd -4 th June, 2009 International Colloquium on Power-Frequency Magnetic Fields, Sarajevo 3 rd -4 th June, 2009 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
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1
Tutorial on
MITIGATION TECHNIQUESOF POWER FREQUENCY MAGNETIC FIELDS
ORIGINATED FROM ELECTRIC POWER SYSTEMS
Programme
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
International Colloquium on Power-Frequency Magnetic Fields, Sarajevo 3rd-4th June, 2009
1 Ener SalinasGeneral principles - Methods of assessment - Strategies
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
• 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)
3
1.1 General Principles
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
4
Sources of power-frequency magnetic fields (PFMFs)
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
MAGNETIC FIELD
ELECTRIC FIELD
Effect on humansEffect on humans
•The electric field E does not penetrate the house
•As the field reaches the walls, the electric charges (generated as a consequence of this field) are diverted to earth and recombined
•Even in the case of lightning, the lightning rods connected to ground will do this diversion successfully
•The electric field E does not penetrate the house
•As the field reaches the walls, the electric charges (generated as a consequence of this field) are diverted to earth and recombined
•Even in the case of lightning, the lightning rods connected to ground will do this diversion successfully
•The magnetic field B penetrates the house easily
•Only certain materials with specific geometries or dedicated circuits could oppose to this action
•The purpose of designing mitigation techniques is to find out what are the most appropriate materials, geometries or circuits that achieve this action effectively
•The magnetic field B penetrates the house easily
•Only certain materials with specific geometries or dedicated circuits could oppose to this action
•The purpose of designing mitigation techniques is to find out what are the most appropriate materials, geometries or circuits that achieve this action effectively
6
Interaction of AC magnetic fields with materials
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
FerromagneticPlate
Region of interest
Region of interest
Ferromagnetic enclosure
Region of interest
Coil
AC Source
(a) (b)
(c) Pure conductivePlate
Region of interest
(d)
Magnetic fields can have different
interactions with
different materials
“Deviation”
“Rejection”
“Concentration”
f
1
Some important design parameters:
PB
PBPSF
s
0Skin depth Shielding Factor
The geometry and the field incidence are
also important!
The geometry and the field incidence are
also important!
7
1.2 Methods of assessment of the mitigation techniques
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
Numerical
Biot-Savart formula
Analytical
Experimental
Small scaleexperiment of a
3-phase underground
cable
Shielding experiments with busbars and conductors at normal scale
At power frequency we use the quasi-static approximation, i.e. displacement currents are neglected
At power frequency we use the quasi-static approximation, i.e. displacement currents are neglected
8
1.3 Some strategies for mitigation
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
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?
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.
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?
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.
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.
This is not an easy question since the definition of the area of interest is not always unambiguous.
This is not an easy question since the definition of the area of interest is not always unambiguous.
9
Tutorial on
MITIGATION TECHNIQUESOF POWER FREQUENCY MAGNETIC FIELDS
ORIGINATED FROM ELECTRIC POWER SYSTEMS
Programme
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
International Colloquium on Power-Frequency Magnetic Fields, Sarajevo 3rd-4th June, 2009
11 Ener SalinasEner SalinasGeneral principles - Methods of assessment - General principles - Methods of assessment - StrategiesStrategies
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
31
3.1 (pure) ferromagnetic shielding
Htengential continuous
Bnormal continuous
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
To be efficient at distance a ferromagnetic shield needs to be closed !
32
3.1 (pure) ferromagnetic shielding
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
To be efficient a ferromagnetic shield needs to encompass completely the source.
33
3.1 (pure) ferromagnetic shielding
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
Closed ferromagnetic shields can have a very high efficiency
mainly when they are not too large with respect to their thickness.
Closed shieldClosed shield
(c) (a) (b)
34
3.1 (pure) ferromagnetic shielding
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
At distances higher than the shield width, the shielding efficiency is virtually zero.
0.2 0.4 0.6 0.8 1.0 1.2 1.41.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
shie
ldin
g f
act
or
distance y (m)
L = 1 m, d = 0.2 m, = 1 mm
r = 100
r = 500
r = 1000
r = 10000
Open shieldOpen shield
35
3.2 (pure) conductive shielding
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
aa
SF ~ aSF ~ a
Closed shieldClosed shield
Contrary to what happens with the pure ferromagnetic shielding, the shielding factor (SF) increases with the shape of the shield.
Contrary to what happens with the pure ferromagnetic shielding, the shielding factor (SF) increases with the shape of the shield.
Good shielding materials need to have a high conductivity () like copper or aluminium
Good shielding materials need to have a high conductivity () like copper or aluminium
(c) (a) (b)
36
3.2 (pure) conductive shielding
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
Even at distances higher than the shield width, the shielding efficiency remains important.
0.2 0.4 0.6 0.8 1.0 1.2 1.40
5
10
15
20
25
shie
ldin
g f
act
or
distance y (m)
L = 1 m, d = 0.2 m, = 10 mm = 1 MS/m = 5 MS/m = 10 MS/m = 50 MS/m
Open shieldOpen shield
37
3.3 actual shielding materials
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
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.
Metal Conductivity in MS/m
Copper 59
Aluminium 36
Iron 10
Steel 6
GO steel 2
Permalloy 1.8
38
3.4 Underground cables
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
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.
How to mitigate the fields ?How to mitigate the fields ?
39
3.4 Underground cables
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
0 5 10 150.02
0.1
0.2
0.5
1
2
5
Distanza dal centro linea [m]
Ind
uzi
on
e m
agn
etic
a a
1 m
dal
su
olo
- B
eff
[µT
]Con loop di compensazione L = 500 m (I Loop = 77 A)
Posa senza loop di compensazione
Con loop di compensazione L = 500 m e con condensatore di ottimazione (I Loop = 134 A; C1 = 13 mF)
CL
1.6 m
0.25 m
x
h calc. = 1 m
V = 132 kVI = 250 A
Cavo/sez. trinceaConfigurazione in piano (=100 mm)
0.25 m
1 2
Passive loop
40
3.4 Underground cables
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
Passive loops (joint chamber)
Double loop : SF 2Double loop : SF 2
41
3.4 Underground cables
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
Closed ferromagnetic shielding
Steel tube: SF > 50Steel tube: SF > 50
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 50.3
0.5
1
2
3
5
10
20
30
50
100
150I = 3000 A
I = 1500 A
I = 250 A
I = 750 A
I = 375 A
CL
pp =1m
x
h mis. = 0 m
I = 250 ÷ 3000 A
B r
ms
, 1 m
abo
ve g
roun
d -
[µT]
Distance from line centre [m]
0.01
0.02
0.03
0.05
0.1
0.2
I = 3000 A
I = 1500 A
I = 250 A
I = 750 A
I = 375 A
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
CL
pp =1m
x
h mis. = 0 m
I = 250 ÷ 3000 A
scherm o: L = 66 m; = 406 m m; s = 10 mm
B r
ms
, 1 m
abo
ve g
roun
d -
[µT]
Distance from line centre [m]
42
3.4 Underground cables
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
Closed ferromagnetic shielding
Raceway: SF 20Raceway: SF 20
43
3.4 Underground cables
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
Flat conductive shielding
Copper plane shield (flat formation): SF > 7 Copper plane shield (flat formation): SF > 7
Effectiveness of the shieldings calculated at 1 m above the ground
Shielding by metallic materials - Power cablesShielding by metallic materials - Power cables
4 Ener Salinas Substations - Examples
49
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
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
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
50
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
51
Mitigation of PFMFs from busbars
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
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
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
52
More elaborated shielding designs for busbars
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
<SF> = 2 when cancellation loops are used alone
<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
<SF> = 2 when cancellation loops are used alone
<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
(a) (b)
(c) (d)
Windows and apertures
Windows and apertures
Narrow gapsNarrow gaps
Combination of 2 passive shields and one active
loop
Combination of 2 passive shields and one active
loop
Averaged shielding factors <SF> in front of the second shield
Averaged shielding factors <SF> in front of the second shield
BusbarsBusbars
53
Magnetic field from transformers-1
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
Because of the core and cover, transformers (by themselves) emit almost no magnetic field
Because of the core and cover, transformers (by themselves) emit almost no magnetic field
A possible mitigation technique is to optimize phase mixing
A possible mitigation technique is to optimize phase mixing
54
Connections from the LV side
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
R S T R S T
Before phase management
After phase management
R S T R ST
R ST
Mixing phases
The responsible for field emissions nearby transformers are often the connections from the secondary side
The responsible for field emissions nearby transformers are often the connections from the secondary side
55
Field mitigation techniques for MV/LV substations
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
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)
Cables Mitigation at the source
•Shielding with metal plates•Passive compensation with loops
-Analytical-2D-FEM
56
Mitigation of PFMFs from HV/MV substations
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
•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.
•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.
57
Examples of Implementation of
Mitigation Techniques
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
58
Example 1: Ferromagnetic pipes in Genoa
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
•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
•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
59
Example 2 Passive lops in Vienna
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
60
Example 3: High Magnetic Coupling
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
Different designsDifferent designs
Configuration 1Configuration 1
ResultsResults
ShieldingCables
Sourcecables
Magnetic core
Sourcecables
Shieldingcables
o'
o
Windings
Shieldingcables
o
Sourcecables
Magnetic core
Windings
Section S1 and S3 Section S2
d=11.8 cm
(HV cable1600 mm2)
i=50 cm
Section S1 and S3 Section S2
d=11.8 cm
(HV cable1600 mm2)
i=50 cm
Jointing zone
S1S2 S3
x
y
z
x=0m x=10m x=20m x=30m
Jointing zone
S1S2 S3
x
y
z
x=0m x=10m x=20m x=30m
Configuration 2Configuration 2Source only
Source only
SF = 88.4SF = 88.4
SF = 7.3SF = 7.3
61
Example 4: Castiglione Project, a case of active shielding of a HV overhead line in Italy
Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009
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.
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 mitigationBefore mitigation
After mitigation operationsAfter mitigation operations