Tài liệu CIRED 2003 - Round Table on Magnetic Field Mitigation Techniques ppt

CIRED 2003
Round Table on Magnetic Field Mitigation Techniques
A. Robert (Chairman), J. Hoeffelman (Coordinator), Belgium
(, )
www.cired-s2.org
CONTENTS
PRESENTATIONS...............................................................................................................................................................1
J. Hoeffelman (Belgium), Introduction.............................................................................................................................1
P. Cruz Romero (Spain), Reduction of Magnetic Fields from Overhead Medium Voltage Lines...................................1
M. Chiampi (Italy), Some considerations about passive shielding...................................................................................7
J. Hoeffelman (Belgium), Shielding of underground power cables, From theory to practical implementation...............8
E. Salinas (GB/SE), Field Mitigation from Secondary Substations................................................................................14
O. Bottauscio (Italy), Experiences in the mitigation of MV/LV substation magnetic field emissions...........................20
R. Conti (Italy), CESI-ENEL Practical Experience in Reducing 50 Hz Magnetic Fields..............................................22
B. Cestnik (Slovenia), Cases from Slovenian practice for reduction of 50 Hz electric and magnetic fields (high
voltage overhead lines and underground cables)............................................................................................................26
M. Tartaglia (Italy), Traction Systems: generated magnetic field and its mitigation.....................................................28
DISCUSSION (summary by J. Hoeffelman)......................................................................................................................30
PRESENTATIONS
J. Hoeffelman (Belgium), Introduction
(ELIA, )
Round Table on
Magnetic Field Mitigation Techniques
Chairman: Alain Robert
Coordinator: Jean Hoeffelman
1. Reduction of magnetic fields from overhead MV lines
– P. Cruz Romero (ES)
2. Some consideration about passive shielding
– M. Chiampi (IT)
3. Shielding of underground power cables
– J. Hoeffelman (BE)

4. Field mitigation from secondary substations
– E. Salinas (SE)
5. Experiences in the mitigation of MV/LV substation magnetic
field emissions
– O. Bottauscio (IT)
6. CESI-ENEL practical experience in reducing 50 Hz Magnetic
fields
– R. Conti (IT)
7. Cases from Slovenian practices for reduction of 50 Hz EMF
– B. Cestnik (SI)
8. Traction systems: generated magnetic field and its mitigation
– M. Tartaglia (IT)
P. Cruz Romero (Spain), Reduction of
Magnetic Fields from Overhead
Medium Voltage Lines
(Universidad de Sevilla, )
Abstract
In this contribution several topics concerning magnetic
fields and overhead medium voltage power lines are
reviewed: simple formulation to assess the magnetic field
(MF) level; characterization of magnetic fields generated
by typical three-phase and one-phase primary distribution
lines, with balanced and unbalanced current; and main
mitigation techniques, analysed in relation with typical
reduction level obtained. Additional data concerning cost
and performance of different solutions are also provided.
Keywords: magnetic field mitigation, primary
distribution, compactness, tree wire, super-bundle, low-
reactance.
Simplified magnetic field calculation

The MF generated by a set of infinitely long, straight
conductors can be formulated by a series decomposition of
the Biot-Savart Law [1]. For points far from the line
(several times the distance between conductors) only the
first non-zero term is needed.
For a single-circuit three-phase line with balanced current
the resultant magnetic field is given by
CIRED 2003 - Round Table on Magnetic Field Mitigation Methods - Thursday 15 May 2003 - updated 21/05/2003 1/31
(1)
where
C : constant that depends on phase configuration
(flat : C = √2 ; regular triangle : C = 1)
d : clearance between adjacent phases
µ
0
: magnetic permeability of vacuum
r : distance from center-of-mass of conductors to
calculation point
I : phase current
For super-bundle double-circuit lines with equal current in
magnitude and phase in each circuit, the formula is the
same, being I the total current of each phase.
For low-reactance lines with equal current in magnitude
and phase the resultant field lays
(2)
where I is the RMS current in each circuit, and s the
distance between both circuits.
For single-phase lines with metallic and ground return the
approximated field is given respectively by
(3)

(4)
From (1..4) relevant conclusions can be deduced:
The MF generated by power lines is proportional to current
and distance between conductors
In case of balanced single-circuit (current dipole) and
super-bundle (SB) double-circuit three-phase lines, as well
as one-phase with metallic return MF decays as 1/r
2
For the low-reactance (LR) configuration MF decays as
1/r
3
For the one-phase case with ground return MF decays as
1/r
Typical MF generated by overhead MV lines
In figure 1 midspan magnetic field profiles generated by
typical primary distribution 3-wire, 3-phase configurations
with geometrical characteristics for 20 kV [2,3] are shown.
It is assumed that the lowest conductor height at midspan is
6 m.
Fig. 1. Magnetic field profiles for different balanced
3-phase MV configurations (units in m)
According to the conclusions previously obtained, the low-
field configurations are the low reactance and the armless
ones. We can also observe that in the conventional
crossarm constructions the better behaviour of delta
configuration is compensated by the higher phase-phase
distance. The height of calculation for this cases and the
rest of simulations is 1 m above ground.
Other types of distribution systems, like unbalanced 4-wire
3-phase [4] and 2-wire 1-phase are also analysed. For the

4-wire 3-phase system several crossarm construction
profiles with different unbalance levels are comparised,
concluding that with no ground return the field increases
with unbalance level, in a higher or lesser extend
depending on relative location of neutral conductor in
relation with phase conductors, and that with ground return
the MF increase is even higher, and growing with ground
current percentage. For the 2-wire one-phase system a
similar behaviour is observed. The negative effect of
ground return current is explained observing eq. (4). An
unbalanced system with ground return can be decomposed
into several current dipoles [5] (MF decay as 1/r
2
) and a
homopolar component (MF decay as 1/r) whose effect will
be dominant at certain distance from the line.
Magnetic field reduction methods
In this section several methods to mitigate MF level from
overhead MV lines [6,7] are reviewed. They can be
classified as follows:
• Methods that try to reduce the load current of the line. If
we can reduce the current, the MF will decrease
proportionately. Some possibilities are the following:
- Increase the voltage level of the line
- Change one-phase lines to three-phase
• Methods that try to compact the line. The aim is reduce
the phase-phase distance.
- Change from crossarm to armless poles construction
- Use covered or insulated cables (overhead or
underground)

- Split the line
• Methods that try to move away the phase conductors
from the interest area . Due to the decay with distance, the
MF influence will be weaker.
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 2/31
- Increase the phase-ground clearance
- Relocate the line
• Methods that try to compensate the power field with a
counteracting external field
- Passive loop. This technique consists on the
installation of a conductor loop near to the line, where a
current is induced. This current creates a field that
partially cancels the original field.

In addition to these general methods, for net current lines
(e.g. multigrounded 4-wire 3-phase) it is needed
simultaneously to take control of the ground return current
levels. Therefore, specific actions must be done in this
sense:
• Balance of phase currents by changing phase
arrangement of loads connected to a 3-phase line [8] or
converting laterals single-phase to 3-phase lines
• Increase of neutral conductor size
• Implementation of 5-wire system instead of 4-wire one
[9]
It is difficult to choose a particular method as the optimum.
The selection of mitigation method is a case-by-case
analysis, where different aspects must be considered:
• New or already existent lines. If a low-field primary
distribution line or set of lines must be projected, methods

that require a global system change could be feasible, like
increase of voltage level, reduction of unbalance, etc.
• Local or whole system reduction. Some methods are
feasible for local application, but extremely costly for a
whole line or network.
• MF reduction level needed
• Cost of reduction method
• Other issues: safety and environmental aspects,
maintenance, reliability, etc.
The presentation is mainly devoted to analyse the more
feasible methods for local applications, although some of
them could be applied for global.
A summary of the methods is shown in table I, where the
typical MF reduction levels at 10 m from the line are
shown. The highest mitigating methods are the ABC
(Aerial Bundle Cable), the underground line and the spacer
cable [10]. Their effectiveness is however conditioned by
the absence of unbalance. Another main drawback of these
methods is the cost. Their use is more feasible when other
issues must be satisfied (reduction of visual impact,
reduction of outages). A method less costly could be the
split of the line, but it is also strongly conditioned by the
unbalanced current. Other set of techniques (use of tree
wires, armless construction and increase of ground
clearance) are less mitigating-effective, but a significant
reduction can be obtained, with the advantage of a lower
cost and an allowed higher unbalance level, specially the
increase to ground clearance. Eventually we can try to
compact the line with no changes in the conductor, like
reducing the span length or replacing string by post

insulators. The reduction obtained is low, about 25-45 %,
and the cost depends mainly of the original span lengths of
the line section to be mitigated.
A last technique, passive loop, has also been considered.
The main drawback is that to obtain a reasonable reduction
of about 35 % it is needed to use for the loop a conductor
of much lower resistance (about 0.12 Ω/km) than
conventional for MV lines, with the additional cost that it
implies.
Table I. Main characteristics of different mitigation techniques
Mitigation
technique
Reduction
level (%)
Installation
cost
Global
performance
over
conventional
Effect of
unbalanced
current
Small
compactness
25-45 Low Lower Low
Crossarms →
armless
∼ 60
Low/medium Lower Medium

Tree wires
∼ 60
Medium Higher Medium
Spacer cable
∼ 80
High Higher High
ABC 100 Very high Higher High
Underground
line
∼ 90
Very high Higher High
Circuit split 70-80 Medium Lower High
Increase
clearance to
ground
25-60 Low/medium Lower Low
Compensation
loop
35 Medium Lower Medium
Conclusions
In this contribution major aspects related with
characterization and mitigation of magnetic fields
generated by medium voltage overhead lines have been
reviewed. The main mitigation techniques have been
analysed, taking into account mitigation effectiveness,
installation cost, global performance (reliability, aesthetic,
maintenance, etc.) and sensibility to unbalanced current. If
a reasonable MF mitigation is the unique objective to
refurbish a section of an existing line the more feasible
methods are the increase of clearance to ground, and the

low-to-medium compactness by discrete reduction of
phase-phase distance, replacement of crossarms by armless
construction or reduction of swinging of string insulators.
References
[1] W.T. Kaune, L.E. Zaffanella, Analysis of Magnetic Fields Produced
Far from Electric Power Lines. IEEE Trans. on Power Delivery. Vol. 7,
No. 4, pp. 2082-2091, Oct. 1993.
[2] W.F. Horton, S. Goldberg, Power Frequency Magnetic Fields and
Public Health. CRC Press, Boca Raton, 1995.
[3] POSTEMEL, S.L., Postes metálicos para líneas eléctricas de alta y
baja tensión, Dic. 1990.
[4] H.L. Willis, Power Distribution Planning Reference Book. Marcel
Dekker, New York, 1997.
[5] P. Pettersson, Simple Method for Characterization of Magnetic Fields
from Balanced Three-phase Systems. Proceedings CIGRÉ Session, 1992,
Paper 36-103.
[6] A.S. Farag, J. Bakhashwain, T.C. Chen, Y. Du, L. Hu, G. Zheng, D.
Penn, J. Thomson, Distribution Lines Electromagnetic Fields: Management
and Design Guidelines. Proceedings CIGRÉ Session, 2000, Paper 36-105.
[7] S. Rodick, P. Musser, Evaluation of Measures and Costs to Mitigate
Magnetic Fields from Transmission and Distribution Lines. 37
th
IEEE Rural
Electric Power Conference, Apr. 1993.
[8] T. Chen, J. Cherng, Optimal Phase Arrangement of Distribution
Transformers Connected to a Primary Feeder for System Unbalance
Improvement and Loss Reduction Using Genetic Algorithm. IEEE
Transactions on Power Systems, Vol.15, No. 3, Aug 2000, pp. 994 -1000.
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 3/31
[9] D. J. Ward, J. F. Buch, T.M. Kulas, W.J. Ros, An Analysis of the Five-

Wire Distribution System. IEEE Transactions on Power Delivery, Vol.18,
No. 1, Jan 2003, pp. 295 -299.
[10]T.J.Orban, Spacer Cable Revisited. Transmission and Distribution
World, Dec. 2002.
Reduction of Magnetic Fields from
Overhead Medium Voltage Lines
Pedro Cruz
Universidad de Sevilla
OBJECTIVES
•Revision of main aspects related with magnetic fields and overhead
medium voltage lines
•
Comparison between different magnetic field mitigation methods
CONTENTS
•Magnetic field generated by OH MV lines
•3-wire 3-phase
•4-wire 3-phase
•2-wire 1-phase
•Magnetic field reduction methods
•Selection of mitigation technique
•Line compactness
•Split of the line
•Increase clearance to ground
•Use of passive cancellation loops
•Effect of unbalanced current
•Summary
MAGNETIC FIELD GENERATED BY OH LINES
MAGNETIC FIELD GENERATED BY OH LINES
MAGNETIC FIELD GENERATED BY OH LINES
MAGNETIC FIELD GENERATED BY OH LINES

3-WIRE, 3-PHASE MV LINES
4-WIRE, 3-PHASE MV LINES
4-WIRE, 3-PHASE MV LINES
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 4/31
4-WIRE, 3-PHASE MV LINES
2-WIRE, 1-PHASE MV LINES
•Existence at primary and secondary distribution system
•p-g clearance (midspan) : 6 m
MAGNETIC FIELD REDUCTION METHODS
MAGNETIC FIELD REDUCTION METHODS
Selection of mitigation technique
•New or already existent project
•MF exposition level allowed
•Cost of reduction
•Local or whole system reduction
•Other issues
–Safety and enviromental aspects
–Reliability
–Insulation and electrical clearance requirements
–Operation
–Maintenance
MAGNETIC FIELD REDUCTION METHODS
Line compactness (balanced current)
•Effectiveness: depends on initial arrangement
•Lesser visual impact
•To keep clearance requirements: often needed to insert a midspan
pole
•Large-scale compacness: reduction of inductance
•Change live-line maintenance practices
•Posibilities of compactness

–Minor changes
–Crossarms -> armless
–Covered wires
–Insulated wires
COMPACTNESS OF THE LINE (BALANCED)
Minor changes
COMPACTNESS OF THE LINE (BALANCED)
Crossarms → Armless
Shorter spans: ~ 50 m
Reduction: ~ 60 %
COMPACTNESS OF THE LINE (BALANCED)
Covered wires
•Avoid Tree treeming
•Reduction of operating costs
•Greater reliability and quality of service (reduction of outages)
•Suitable for complete new lines or upgrade of old ones
•Types
–Tree wire ( PAS, BLX)
–Spacer cable
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 5/31
COMPACTNESS OF THE LINE (BALANCED)
Covered wires
COMPACTNESS OF THE LINE (BALANCED)
Insulated wires
•ABC (aerial bundle cable)
•Dramatic MF decay with distance
UNDERGROUND LINE (BALANCED)
SPLIT OF THE LINE (BALANCED)
•Existing DC Super-Bundle → Low-reactance
•Existing SC lines and mitigation in few spans: special pole SC →

DC
•Conversion of SC pole to DC pole: increase of height/strength
•Need to have equal loading between circuits
INCREASE CLEARANCE TO GROUND (BALANCED)
•Increase poles height
•Installation of new poles at midspan (long spans)
•Reduction effectiveness close to the line
USE OF PASSIVE CANCELLATION LOOP
•More effective in flat configurations (horizontal, vertical)
•Increased reduction with compensation of inductance (capacitor)
•Resistance of the loop conductor: much lower than typical MV
conductor
•Need to reinforce the poles
USE OF PASSIVE CANCELLATION LOOP (BALANCED)
EFFECT OF UNBALANCED CURRENT
•Reduction of mitigation effectiveness
Ground return current = neutral wire current
SUMMARY
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 6/31
M. Chiampi (Italy), Some considerations
about passive shielding
(Politecnico di Torino, )
Some considerations about passive shielding
O. Bottauscio (*), M. Chiampi (*), G. Crotti (°),
A. Manzin (°), M. Zucca (°)
(°) Istituto Elettrotecnico Nazionale Galileo Ferraris, Torino, Italy
(*) Dipartimento di Ingegneria Elettrica Industriale - Politecnico di Torino, Italy
Aim of the presentation
• The presentation is addressed to analyse the shielding capabilities
of different low cost magnetic materials.

• The study has been developed in the Turin Unit by means of both
experiments on a specific test apparatus and numerical
computations using a 2D hybrid FEM/BEM model.
Outline of the presentation
• Magnetic materials for shielding in industrial and civil application
• Influence of shape and building of passive shields
• Experimental and computational results
Magnetic Laminations for shields
• Low-Carbon Steel (Si < 1% wt)
• Lamination thickness: 0.80 mm
• Electrical resistivity: 13.9×10
-8
×Ωm
• Non-Oriented Si – Fe 1.5% wt
• Lamination thickness: 0.50 mm
• Electrical resistivity: 27.9×10
-8
×Ωm
• Grain-Oriented Si – Fe 3.0% wt
• Lamination thickness: 0.30 mm
• Electrical resistivity: 48.0×10
-8
×Ωm
Magnetic Characteristic of the Shielding Materials
Experimental set-up for tests
The set-up is constituted by a 180 cm X 60 cm X 60 cm wooden
frame.
60 cm X 60 cm magnetic sheets can be disposed on the frame.
Two external busbars are supplied by a 50 Hz single-phase system
with currents of some hundred amperes

Shielding configurations
Efficiency of magnetic plane shields
Magnetic flux density in plane shields
r.m.s. values of magnetic flux density in the plane shield:
Lines: computations by a 2D hybrid FEM/BEM model
Points: measures by test coils
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 7/31
Magnetic flux in plane shields
r.m.s. values of magnetic flux in the plane shield computated by a 2D
hybrid FEM/BEM model
Efficiency of magnetic U-shaped shields
Air-gaps in the sheet corners
Effects of air-gaps in sheet corners
U-shaped screen
FEM/BEM model
Efficiency of combined shields
J. Hoeffelman (Belgium), Shielding of
underground power cables, From
theory to practical implementation
(ELIA, )
Summary
This contribution is aimed at presenting the most recent
achievements in shielding techniques for underground
power cables. It is based on the work performed by Cigré-
Cired JTF C4-04-02 and focuses mainly on the use of
aluminium shields, which have been applied on an
important 150 kV link in Belgium.
General field mitigating techniques
The reduction of ELF magnetic fields produced by power
cables can become an important concern due to the fact that

they are sometimes laid very close to inhabited areas.
As for overhead lines, the magnetic field due to
underground cables is inversely proportional to the distance
between conductors. Therefore the easiest mitigation
technique remains, of course, to install the cables in a
trefoil arrangement.
However, when a very high load capacity is required, it is
not always possible to install the cables in trefoil.
Horizontal layouts with distances of several tens of cm
between conductors are sometimes needed.
In that case the magnetic field strength above the
conductors, at ground level, can be higher than that
produced by an equivalent overhead line and can require
some mitigation method.
Figure 1 shows, for both arrangements, and for three
different measurement positions above ground the decrease
of the field with the distance to the axis of the cable layout.
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 8/31
In both cases, the cables are buried at a depth of 120 cm,
have a diameter of 10 cm and are carrying a current of 1
kA. In the horizontal arrangement the distance between
phases is 25 cm.
trifoil arrangement 120-10 cm
0.01
0.10
1.00
10.00
100.00
0 5 10 15 20 25 30
distance to axis (m)

µT
h = 0 m
h = 1.5 m
h = 3 m
horizontal arrangement :120-25 cm
0.01
0.10
1.00
10.00
100.00
0 5 10 15 20 25 30
distance to axis (m)
µT
h = 0 m
h = 1.5 m
h = 3 m
Figure 1: Comparison between trefoil arrangement
and horizontal arrangement
Metallic shielding
When a trefoil arrangement cannot be applied or if a further
field reduction is required, a metallic shielding can reduce
the field at the source.
As stated in [ 0] to [ 0], ferromagnetic material as well as
good conducting material are used.
At low frequencies the physical mechanisms involved by
both materials are completely different:
In the first case (figure 2 b), sometimes called
magnetostatic shielding, the field lines are absorbed by the
low reluctance material, whereas in the second case (figure
2 c) they are repelled thanks to the eddy currents induced in

the material.
Figure 2: Shielding mechanisms: ferromagnetic material
versus conductive material
Shielding by ferromagnetic materials.
Although theoretically more efficient at low frequency than
conductive materials, ferromagnetic materials seem, in
most cases, to be less advantageous. The reasons are the
following :
The effectiveness of conductive shields is more
homogeneous in the space. Ferromagnetic materials are
mainly effective nearby the shield, while conductive
materials are also effective at distance.
Good ferromagnetic materials like permalloy (“Mumetal”)
or transformer laminates are often expensive and highly
sensitive to corrosion. Therefore they need a good
protection coating.
Ferromagnetic materials are more efficient when the
magnetic circuit they offer to the flux lines is closed (no or
few gap). This particular layout is not often practically
achievable unless for shielding short cable lengths.
Hence, the main example where a ferromagnetic shielding
seems to be superior to a conductive one is the steel tube.
In this case a shielding factor up to 50 can be achieved as
shown in figure 3 taken from [ 0].
However, such a tubular shielding has also drawbacks: The
maintenance or the repair of the cables is difficult. The
thermal behaviour of the cables is neither easy to manage,
as the tube needs normally to be filled with concrete.
On the other hand, the installation of a single tube allows a
fast recovering of the trench, the cables being pulled-in by

a single and fast operation afterwards.
Figure 3: Shielding by ferromagnetic material
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 9/31
Shielding by conductive materials.
As far as conductive materials are concerned, two materials
can be considered: copper and aluminium.
Both materials have their own advantages and drawbacks:
Copper has a higher conductivity but also a higher cost
than aluminium. Although copper is easier to weld than
aluminium, modern welding techniques under argon
atmosphere allow assembling aluminium plates on the
yard. Therefore, in the following sections, only shielding
by aluminium plates will be considered.
On the other hand, if some precautions are taken
concerning the neutrality of the soil, corrosion problems
should not arise neither with copper nor with aluminium.
The possible influence of stray currents needs however to
be addressed.
Three main layouts will be taken into consideration: the flat
horizontal shield or plane shield, the U-shaped shield and
the H-shaped shield.
Plane shield
A relatively simple way to mitigate the field produced by a
2 or 3 phases cable system is to install as close as possible
above the cable an horizontal plate.
Shield thickness
2 mm plates give already fair results but the effectiveness
clearly increases with the thickness as far as this latter
remains smaller than the skin effect (about 12 mm for
aluminium and 9 mm for copper).

Shield width
The main problem with plane shields is that the shielding
effectiveness usually strongly decreases with the distance
to the centre of the plate with, as result, that the shielded
field presents two peaks in the vicinity of the edges of the
plate. To avoid this it is necessary to use a plate with a
sufficient width.
Practically it is recommended that the ratio of the shield
width to its distance to the conductors and to the distance
between conductors remains larger than 4. For a maximum
effectiveness the plates need to be as close as possible to
the cables but, if they are too close, the losses due to the
induced eddy currents can become to high. Power capacity,
however, is practically not influenced if the distance
between cable sheets and shielding is not smaller than 5 to
7 cm [ 0].
Shield continuity
For manufacturing reasons, the shield is normally divided
into smaller elements placed near each other with or
without air gaps.
It can been shown [ 0] that the shielding continuity between
the different elements is not absolutely necessary. The
presence of gaps reduces in fact the eddy currents and the
global shield effectiveness, but this effect decreases with
the observation distance. On the contrary, near the
boundaries of the gaps, due to the fact that the eddy
currents are flowing in opposite direction, there is a strong
enhancement of the field that behaves a little bit like as a
compressed fluid leaking through the gaps. A good way to
avoid this enhancement and to approach the theoretical

result achieved with a continuous shield is to use a double
layer of metallic plates, each layer being shifted by half the
length of one plate with respect to the other layer, like the
bricks of a wall. In that case the resulting effectiveness is
close to that of a single continuous shield with the same
global thickness.
It is important to note, here, that the quality of the electrical
contact between layers doesn’t play any part in the
shielding effectiveness.
Performances
Figure 4 shows the comparison between calculation (2 D
FEM-BEM model) and measurements for an aluminium
plate installed at 27 cm
1
above the axis of a three phases
system in flat configuration (distance between phases: 25
cm). The agreement is quite good although the calculation
refers to a 6 mm continue shield (99.5 % aluminium),
whereas the measurements are made on a double layer 3
mm discontinue shield.

Horizontal shielding - Comparison between calculation and

measurement at 1 m above cables axis (73 cm above shield)

0

2

4

6

8

10

12

14

16

50

100

150

200

shielding width (cm)

shielding factor

3 mm
(calc)

6 mm
(calc)

6 mm
(mes)

Figure 4: Shielding by horizontal aluminium plates
U-shaped shield
It has been very often written that a U-shape shield exhibits
better performances than a flat shield. In fact, as shown in [
0], for the same shielding area, it has not really a better
effectiveness than a horizontal plane shield but it doesn’t
necessitate to groove such a large trench as that required
for an horizontal shield of the same total width before
bending.
One problem, however with U-shaped shields is that,
contrary to what happens with plane shields, there is an
absolute need to get a good contact between the vertical
parts of each shielding element (assuming, of course, a non
continuous shield).
1
This corresponds to the typical thickness of the dolomite layer above 2000
mm
2
alu power cables
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 10/31
Figure 5: Shielding by U-shaped conductive plates
Nevertheless with a layout based on 2 mm aluminium
plates of 100 cm (length) x 200 cm (width) bended to
achieve vertical parts of 40 cm and bolded together in the
longitudinal direction, a shielding factor of 4 can be
achieved up to 1.5 m above the shield.

The shielding effectiveness is also less dependent to the
height of measurement than with plane shields.
Another problem with U-shaped shields is the difficulty of
installation.
For cooling reasons, power cables need to be embedded in
a controlled soil (dolomie…) that needs to be tamped. The
presence of a U-shield layout makes the operation very
difficult.
For that reason, instead of using bended plates, it is easier
to use an equivalent layout made of three plates: two
vertical and one horizontal. This layout is known as the H-
layout.
H-layout
In the H layout, two vertical plates are installed in the
trench before to fill it with a first layer of controlled soil.
After the cable laying and the second layer of controlled
soil, the horizontal plates are installed forming with the
vertical plate a H.
Shield continuity
Contrary to what happens with the plane shield, and
likewise the U-shaped shield
2
, a good continuity needs to
be ensured between vertical plates.
Therefore, the electrical circuit formed by the vertical
plates needs to be closed at each extremity of the shielded
area.
It has also been shown experimentally, at least for the flat
cable configuration, that the longitudinal continuity of the
horizontal plates is not very important.

Shield thickness
Calculations show that increasing the shield thickness
above 3 mm does not bring a important improvement in the
shielding effectiveness. On the other hand, for mechanical
and corrosion withstand reasons, it is not safe to use too
thin shields. Hence, the value of 3 mm seems to be a master
choice for this type of aluminium shielding.
2
This longitudinal continuity seems however to be less important for U-
shaped shields because, in each individual element, thanks to the continuity
with the horizontal plate, the circuit is closed.
Shielding effectiveness
Shielding factors up to 10 at 1.5 m above ground have been
calculated with the same 2D model as for the plane shield.
However the continuity problems between elements being
very important, there is real a necessity to make recourse to
a 3D model for taking the discontinuities into
consideration. This 3D model is still under development.
Laboratory results
The measurements results presented on figure 6 have been
achieved with the same cable layout as for the plane shield
(fig 4), i.e. a three phase flat configuration with 25 cm
distance between phases laid 24 cm above the bottom of a
trench of 150 cm depth.
The shield is built with 200 X 80 cm aluminium plates of 3
mm thickness.
Vertical plates are installed at a distance of 100 cm from
each other, whereas the horizontal plates of 80 cm width
are installed 40 cm above the bottom of the trench.
The vertical plates have an overlap of 8 cm and are fixed

together with four M8 bolts.
At both extremities of the shielded area a U-shaped
aluminium cover of the same thickness and width as the
other plates ensures the necessary electrical link between
lateral plates (vertical right and left plate).
On this figure, the important decrease of the shielding
effectiveness with lateral distance is partly due to the fact
that the experimental model, being only 8 m length, gives
rise to important border effects at distance from the axis of
the cables.
The asymmetry in the curves is due to elliptical
polarization of the magnetic field and depends on the
rotation order of the three phases.
H-shaped shielding 100 cm width, 3 mm Al plates of 80 cm X 200 cm,
Cables axis at 24 cm from trench bottom, 25 cm between conductors
Horizontal plates at 16 cm above cables axis
Meas urem ent dis tances from trench bottom
0
2
4
6
8
10
12
14
16
-400 -300 -200 -100 0 100 200 300 400
horizontal distance to cable central conductor (cm)
shielding factor
150 cm

250 cm
300 cm
Figure 6 : Efficiency of a H shaped aluminium shielding
Actual implementation
The H layout described above has been implemented in
Belgium on a new 30 km double circuit 150 kV link
between the nuclear power plant of Tihange and the HV
substation of Avernas.
From the 30 km underground link, 6.5 km are shielded.
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 11/31
The link will be put into service at the end of this year;
hence measurement in situ has not yet been performed.
However, as an assembling technique by welding instead
of bolting has been used, better results than those
extrapolated on basis of the laboratory tests (figure 7) are
expected.
The per km cost increase of the link due this shielding is
estimated to be about 20 %.
This estimation, however, doesn’t take into account the
additional exploitation costs involved by the losses in the
shield.
Field produced by a double circuit 150 kV cables in flat configuration
spaced 25 cm - dis tance between axis of circuits: 2 m, depth: 1.25 m
H-shap ed alum inium shielding - Field at 1.5 m above ground for I = 1300 A
0
5
10
15
20
25

-30 -20 -10 0 10 20 30
Lateral distance (m )
B (µT)
without shielding
with shielding
Figure 7: Expected field in the Tihange-Avernas link
References
[ 0] Transmission Cable Magnetic Field Management
Power Technologies Inc.
EPRI TR-102003 – Project 7898-37 – Final Report June 1993
[ 0] On low frequency shielding of electromagnetic fields
R.G. Olsen
10
th
International Symposium on High Voltage Engineering – Montréal –
1997
[ 0] Geometrical Aspects of Magnetic Shielding at Extremely Low
Frequencies
L. Hasselgren, J. Luomi
IEEE Trans on EMC vol 37, No 3, August 1995
[ 0] Implementation of shielding principles for magnetic field management
of power cables
A.S. Farag et alii
Electric Power System Research 48 (1999) pp 193-209 - Elsevier
[ 0] Shielding Techniques to Reduce Magnetic Fields Associated with
Underground Power Cables
G. Bucea, H. Kent
CIGRE Session 1998, paper 21-201
[ 0] Role of magnetic materials in power frequency shielding: numerical
analysis and experiments

O. Bottaauscio, M. Chiampi, D. Chiarabaglio, F. Fioillo, L. Rocchino, M.
Zucca
IEE Proc. Gener. Transm. Distr., Vol 148, No 2, March 2001
[ 0] Evaluation of different Analytical and Semi-Analytical Methods for the
Design of ELF Magnetic Field Shields
A. Canova, A. Manzin, M. Tartaglia
IEEE Trans. On Industry Applications, vol 38, no 3 May/June 2002
[ 0] A numerical Approach to the Design of Conducting Shields for ELF
Magnetic Field Reduction
O. Bottauscio, D. Chiarabaglio, M. Chiampi, M. Repetto
ETEP vol 12 No 2, March/April 2002
[ 0] Campi ellettrici e magnetici: possibilità offerte dagli elettrodotti in
cavo
A. Bolza, F. Donazzi, P. Maioli – Pirelli Caci e Sistemi 2000
Comments given at CIGRE 2002 Paris: Group 21, PS1, Q 4
[ 0]Techniques for shielding underground power lines to minimize the
exposure to ELF magnetic field in residential areas
A. Cipollone, A. Fabbri, E. Zendri
EMC Europe – Sorrento – Sept 9-13, 2002
Shielding of underground power cables
From theory to practical implementation
Jean Hoeffelman
ELIA
General field mitigation techniques
General field mitigation techniques
Metallic shielding
Shielding by ferromagnetic materials
Mainly effective neer the shield
Expensive (Si steel, permalloy)
Corrosion protection

No gap in magnetic circuit
Steel tube : very efficient
Maintenance, repair
Thermal behaviour of cable
Fast recovering of trench
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 12/31