Xem mẫu
- CIRED 2003
Round Table on Magnetic Field Mitigation Techniques
A. Robert (Chairman), J. Hoeffelman (Coordinator), Belgium
(alain.robert@elia.be, jean.hoeffelman@elia.be)
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.....................................................................................6
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).......................................................................................................................31
PRESENTATIONS P. Cruz Romero (Spain), Reduction of
Magnetic Fields from Overhead
J. Hoeffelman (Belgium), Introduction Medium Voltage Lines
(ELIA, jean.hoeffelman@elia.be) (Universidad de Sevilla, plcruz@us.es)
Round Table on Abstract
Magnetic Field Mitigation Techniques
In this contribution several topics concerning magnetic
Chairman: Alain Robert fields and overhead medium voltage power lines are
Coordinator: Jean Hoeffelman
reviewed: simple formulation to assess the magnetic field
1. Reduction of magnetic fields from overhead MV lines (MF) level; characterization of magnetic fields generated by
– P. Cruz Romero (ES) typical three-phase and one-phase primary distribution lines,
2. Some consideration about passive shielding with balanced and unbalanced current; and main mitigation
– M. Chiampi (IT) techniques, analysed in relation with typical reduction level
3. Shielding of underground power cables obtained. Additional data concerning cost and performance
– J. Hoeffelman (BE) of different solutions are also provided.
4. Field mitigation from secondary substations
– E. Salinas (SE) Keywords: magnetic field mitigation, primary
5. Experiences in the mitigation of MV/LV substation magnetic
distribution, compactness, tree wire, super-bundle, low-
field emissions
– O. Bottauscio (IT) reactance.
6. CESI-ENEL practical experience in reducing 50 Hz Magnetic Simplified magnetic field calculation
fields
– R. Conti (IT) The MF generated by a set of infinitely long, straight
7. Cases from Slovenian practices for reduction of 50 Hz EMF conductors can be formulated by a series decomposition of
– B. Cestnik (SI) the Biot-Savart Law [1]. For points far from the line
8. Traction systems: generated magnetic field and its mitigation (several times the distance between conductors) only the
– M. Tartaglia (IT)
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 Fig. 1. Magnetic field profiles for different balanced
3-phase MV configurations (units in m)
magnitude and phase in each circuit, the formula is the
same, being I the total current of each phase.
According to the conclusions previously obtained, the low-
For low-reactance lines with equal current in magnitude and
field configurations are the low reactance and the armless
phase the resultant field lays
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
(2) height of calculation for this cases and the rest of
simulations is 1 m above ground.
where I is the RMS current in each circuit, and s the distance
between both circuits. Other types of distribution systems, like unbalanced 4-wire
3-phase [4] and 2-wire 1-phase are also analysed. For the 4-
For single-phase lines with metallic and ground return the wire 3-phase system several crossarm construction profiles
approximated field is given respectively by 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
(3) 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
(4) observed. The negative effect of ground return current is
explained observing eq. (4). An unbalanced system with
From (1..4) relevant conclusions can be deduced: ground return can be decomposed into several current
dipoles [5] (MF decay as 1/r2 ) and a homopolar component
The MF generated by power lines is proportional to current (MF decay as 1/r) whose effect will be dominant at certain
and distance between conductors distance from the line.
In case of balanced single-circuit (current dipole) and super-
bundle (SB) double-circuit three-phase lines, as well as one- Magnetic field reduction methods
phase with metallic return MF decays as 1/r2
For the low-reactance (LR) configuration MF decays as 1/r3 In this section several methods to mitigate MF level from
For the one-phase case with ground return MF decays as 1/r overhead MV lines [6,7] are reviewed. They can be
classified as follows:
Typical MF generated by overhead MV lines
• Methods that try to reduce the load current of the line. If
In figure 1 midspan magnetic field profiles generated by we can reduce the current, the MF will decrease
typical primary distribution 3-wire, 3-phase configurations proportionately. Some possibilities are the following:
with geometrical characteristics for 20 kV [2,3] are shown. - Increase the voltage level of the line
It is assumed that the lowest conductor height at midspan is - Change one-phase lines to three-phase
6 m. • 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 A last technique, passive loop, has also been considered.
- Relocate the line The main drawback is that to obtain a reasonable reduction
• Methods that try to compensate the power field with a of about 35 % it is needed to use for the loop a conductor of
counteracting external field much lower resistance (about 0.12 Ω/km) than conventional
- Passive loop. This technique consists on the installation for MV lines, with the additional cost that it implies.
of a conductor loop near to the line, where a current is
induced. This current creates a field that partially Table I. Main characteristics of different mitigation techniques
cancels the original field. Global
Effect of
Mitigation Reduction Installation performance
unbalanced
In addition to these general methods, for net current lines technique level (%) cost over
current
conventional
(e.g. multigrounded 4-wire 3-phase) it is needed
simultaneously to take control of the ground return current Small
25-45 Low Lower Low
levels. Therefore, specific actions must be done in this compactness
sense: Crossarms →
∼ 60 Low/medium Lower Medium
armless
• Balance of phase currents by changing phase arrangement Tree wires ∼ 60 Medium Higher Medium
of loads connected to a 3-phase line [8] or converting Spacer cable ∼ 80 High Higher High
laterals single-phase to 3-phase lines ABC 100 Very high Higher High
• Increase of neutral conductor size Underground
∼ 90 Very high Higher High
line
• Implementation of 5-wire system instead of 4-wire one [9] Circuit split 70-80 Medium Lower High
Increase
It is difficult to choose a particular method as the optimum. clearance to25-60 Low/medium Lower Low
The selection of mitigation method is a case-by-case ground
Compensation
analysis, where different aspects must be considered: loop
35 Medium Lower Medium
• New or already existent lines. If a low-field primary
distribution line or set of lines must be projected, methods Conclusions
that require a global system change could be feasible, like
increase of voltage level, reduction of unbalance, etc. In this contribution major aspects related with
• Local or whole system reduction. Some methods are characterization and mitigation of magnetic fields generated
feasible for local application, but extremely costly for a by medium voltage overhead lines have been reviewed. The
whole line or network. main mitigation techniques have been analysed, taking into
• MF reduction level needed account mitigation effectiveness, installation cost, global
• Cost of reduction method performance (reliability, aesthetic, maintenance, etc.) and
• Other issues: safety and environmental aspects, sensibility to unbalanced current. If a reasonable MF
maintenance, reliability, etc. mitigation is the unique objective to refurbish a section of an
existing line the more feasible methods are the increase of
The presentation is mainly devoted to analyse the more clearance to ground, and the low-to-medium compactness by
feasible methods for local applications, although some of discrete reduction of phase-phase distance, replacement of
them could be applied for global. crossarms by armless construction or reduction of swinging
of string insulators.
A summary of the methods is shown in table I, where the
typical MF reduction levels at 10 m from the line are shown. References
The highest mitigating methods are the ABC (Aerial Bundle [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.
Cable), the underground line and the spacer cable [10]. Their 4, pp. 2082-2091, Oct. 1993.
effectiveness is however conditioned by the absence of [2] W.F. Horton, S. Goldberg, Power Frequency Magnetic Fields and
unbalance. Another main drawback of these methods is the Public Health. CRC Press, Boca Raton, 1995.
cost. Their use is more feasible when other issues must be [3] POSTEMEL, S.L., Postes metálicos para líneas eléctricas de alta y
baja tensión, Dic. 1990.
satisfied (reduction of visual impact, reduction of outages). [4] H.L. Willis, Power Distribution Planning Reference Book. Marcel
A method less costly could be the split of the line, but it is Dekker, New York, 1997.
also strongly conditioned by the unbalanced current. Other [5] P. Pettersson, Simple Method for Characterization of Magnetic Fields
from Balanced Three-phase Systems. Proceedings CIGRÉ Session, 1992,
set of techniques (use of tree wires, armless construction and
Paper 36-103.
increase of ground clearance) are less mitigating-effective, [6] A.S. Farag, J. Bakhashwain, T.C. Chen, Y. Du, L. Hu, G. Zheng, D.
but a significant reduction can be obtained, with the Penn, J. Thomson, Distribution Lines Electromagnetic Fields: Management
advantage of a lower cost and an allowed higher unbalance and Design Guidelines. Proceedings CIGRÉ Session, 2000, Paper 36-105.
[7] S. Rodick, P. Musser, Evaluation of Measures and Costs to Mitigate
level, specially the increase to ground clearance. Eventually Magnetic Fields from Transmission and Distribution Lines. 37th IEEE Rural
we can try to compact the line with no changes in the Electric Power Conference, Apr. 1993.
conductor, like reducing the span length or replacing string [8] T. Chen, J. Cherng, Optimal Phase Arrangement of Distribution
by post insulators. The reduction obtained is low, about 25- Transformers Connected to a Primary Feeder for System Unbalance
Improvement and Loss Reduction Using Genetic Algorithm. IEEE
45 %, and the cost depends mainly of the original span Transactions on Power Systems, Vol.15, No. 3, Aug 2000, pp. 994 -1000.
lengths of the line section to be mitigated. [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.
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 3/31
- [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 3-WIRE, 3-PHASE MV LINES
•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
4-WIRE, 3-PHASE MV LINES
MAGNETIC FIELD GENERATED BY OH LINES
4-WIRE, 3-PHASE MV LINES
MAGNETIC FIELD GENERATED BY OH LINES
4-WIRE, 3-PHASE MV LINES
MAGNETIC FIELD GENERATED BY OH LINES
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 4/31
- –Minor changes
–Crossarms -> armless
–Covered wires
–Insulated wires
COMPACTNESS OF THE LINE (BALANCED)
Minor changes
2-WIRE, 1-PHASE MV LINES
•Existence at primary and secondary distribution system
•p-g clearance (midspan) : 6 m
COMPACTNESS OF THE LINE (BALANCED)
Crossarms → Armless
MAGNETIC FIELD REDUCTION METHODS
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
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
COMPACTNESS OF THE LINE (BALANCED)
•Other issues
–Safety and enviromental aspects
Covered wires
–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 COMPACTNESS OF THE LINE (BALANCED)
•Posibilities of compactness
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 5/31
- Insulated wires
USE OF PASSIVE CANCELLATION LOOP (BALANCED)
•ABC (aerial bundle cable)
•Dramatic MF decay with distance
UNDERGROUND LINE (BALANCED)
EFFECT OF UNBALANCED CURRENT
•Reduction of mitigation effectiveness
Ground return current = neutral wire current
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
SUMMARY
INCREASE CLEARANCE TO GROUND (BALANCED)
•Increase poles height
•Installation of new poles at midspan (long spans)
•Reduction effectiveness close to the line
M. Chiampi (Italy), Some considerations about
passive shielding
USE OF PASSIVE CANCELLATION LOOP (Politecnico di Torino, chiampi@polel1.polito.it)
•More effective in flat configurations (horizontal, vertical) Some considerations about passive shielding
•Increased reduction with compensation of inductance (capacitor)
•Resistance of the loop conductor: much lower than typical MV O. Bottauscio (*), M. Chiampi (*), G. Crotti (°),
conductor A. Manzin (°), M. Zucca (°)
•Need to reinforce the poles
(°) 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
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 6/31
- 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 Efficiency of magnetic plane shields
• 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
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
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
Magnetic flux in plane shields
r.m.s. values of magnetic flux in the plane shield computated by a
2D hybrid FEM/BEM model
Shielding configurations
Efficiency of magnetic U-shaped shields
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 7/31
- 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
Air-gaps in the sheet corners 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.
Effects of air-gaps in sheet corners
Figure 1 shows, for both arrangements, and for three
U-shaped screen
FEM/BEM model different measurement positions above ground the decrease
of the field with the distance to the axis of the cable layout.
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
100.00
10.00
h=0m
µT
1.00 h = 1.5 m
h=3m
Efficiency of combined shields
0.10
0.01
0 5 10 15 20 25 30
distance to axis (m)
horizontal arrangement :120-25 cm
100.00
10.00
h=0m
µT
1.00 h = 1.5 m
h=3m
0.10
0.01
J. Hoeffelman (Belgium), Shielding of 0 5 10 15 20
distance to axis (m)
25 30
underground power cables, From
theory to practical implementation
Figure 1: Comparison between trefoil arrangement
(ELIA, jean.hoeffelman@elia.be) and horizontal arrangement
Summary Metallic shielding
This contribution is aimed at presenting the most recent When a trefoil arrangement cannot be applied or if a further
achievements in shielding techniques for underground field reduction is required, a metallic shielding can reduce
power cables. It is based on the work performed by Cigré- the field at the source.
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 8/31
- 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 Figure 3: Shielding by ferromagnetic material
cases, to be less advantageous. The reasons are the following
: Shielding by conductive materials.
The effectiveness of conductive shields is more
homogeneous in the space. Ferromagnetic materials are As far as conductive materials are concerned, two materials
mainly effective nearby the shield, while conductive can be considered: copper and aluminium.
materials are also effective at distance. Both materials have their own advantages and drawbacks:
Good ferromagnetic materials like permalloy (“Mumetal”) Copper has a higher conductivity but also a higher cost than
or transformer laminates are often expensive and highly aluminium. Although copper is easier to weld than
sensitive to corrosion. Therefore they need a good protection aluminium, modern welding techniques under argon
coating. atmosphere allow assembling aluminium plates on the yard.
Ferromagnetic materials are more efficient when the Therefore, in the following sections, only shielding by
magnetic circuit they offer to the flux lines is closed (no or aluminium plates will be considered.
few gap). This particular layout is not often practically On the other hand, if some precautions are taken concerning
achievable unless for shielding short cable lengths. the neutrality of the soil, corrosion problems should not arise
neither with copper nor with aluminium. The possible
Hence, the main example where a ferromagnetic shielding influence of stray currents needs however to be addressed.
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 Three main layouts will be taken into consideration: the flat
shown in figure 3 taken from [ 0]. horizontal shield or plane shield, the U-shaped shield and
the H-shaped shield.
However, such a tubular shielding has also drawbacks: The
maintenance or the repair of the cables is difficult. The Plane shield
thermal behaviour of the cables is neither easy to manage, as
the tube needs normally to be filled with concrete. A relatively simple way to mitigate the field produced by a 2
or 3 phases cable system is to install as close as possible
On the other hand, the installation of a single tube allows a above the cable an horizontal plate.
fast recovering of the trench, the cables being pulled-in by a Shield thickness
single and fast operation afterwards. 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.
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 9/31
- To avoid this it is necessary to use a plate with a sufficient U-shaped shield
width.
Practically it is recommended that the ratio of the shield It has been very often written that a U-shape shield exhibits
width to its distance to the conductors and to the distance better performances than a flat shield. In fact, as shown in
between conductors remains larger than 4. For a maximum [ 0], for the same shielding area, it has not really a better
effectiveness the plates need to be as close as possible to the effectiveness than a horizontal plane shield but it doesn’t
cables but, if they are too close, the losses due to the induced necessitate to groove such a large trench as that required for
eddy currents can become to high. Power capacity, however, an horizontal shield of the same total width before bending.
is practically not influenced if the distance between cable One problem, however with U-shaped shields is that,
sheets and shielding is not smaller than 5 to 7 cm [ 0]. contrary to what happens with plane shields, there is an
Shield continuity absolute need to get a good contact between the vertical
For manufacturing reasons, the shield is normally divided parts of each shielding element (assuming, of course, a non
into smaller elements placed near each other with or without continuous shield).
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 Figure 5: Shielding by U-shaped conductive plates
continuous shield with the same global thickness.
It is important to note, here, that the quality of the electrical Nevertheless with a layout based on 2 mm aluminium plates
contact between layers doesn’t play any part in the shielding of 100 cm (length) x 200 cm (width) bended to achieve
effectiveness. vertical parts of 40 cm and bolded together in the
Performances longitudinal direction, a shielding factor of 4 can be
Figure 4 shows the comparison between calculation (2 D achieved up to 1.5 m above the shield.
FEM-BEM model) and measurements for an aluminium The shielding effectiveness is also less dependent to the
plate installed at 27 cm1 above the axis of a three phases height of measurement than with plane shields.
system in flat configuration (distance between phases: 25
cm). The agreement is quite good although the calculation Another problem with U-shaped shields is the difficulty of
refers to a 6 mm continue shield (99.5 % aluminium), installation.
whereas the measurements are made on a double layer 3 mm For cooling reasons, power cables need to be embedded in a
discontinue shield. controlled soil (dolomie…) that needs to be tamped. The
presence of a U-shield layout makes the operation very
Horizontal shielding - Comparison between calculation and
difficult.
measurement at 1 m above cables axis (73 cm above shield) For that reason, instead of using bended plates, it is easier to
use an equivalent layout made of three plates: two vertical
16 and one horizontal. This layout is known as the H-layout.
14
12
H-layout
shielding factor
10 3 mm
8 (calc)
6 mm
6 (calc)
6 mm In the H layout, two vertical plates are installed in the trench
4 (mes) before to fill it with a first layer of controlled soil. After the
2 cable laying and the second layer of controlled soil, the
0 horizontal plates are installed forming with the vertical plate
50 100 150 200
shielding width (cm)
a H.
Shield continuity
Figure 4: Shielding by horizontal aluminium plates Contrary to what happens with the plane shield, and likewise
the U-shaped shield2, a good continuity needs to be ensured
between vertical plates.
2
This longitudinal continuity seems however to be less important for U-
1
This corresponds to the typical thickness of the dolomite layer above 2000 shaped shields because, in each individual element, thanks to the continuity
mm2 alu power cables with the horizontal plate, the circuit is closed.
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 10/31
- Therefore, the electrical circuit formed by the vertical plates H-shaped shielding 100 cm width, 3 mm Al plates of 80 cm X 200 cm,
needs to be closed at each extremity of the shielded area. Cables axis at 24 cm from trench bottom, 25 cm between conductors
Horizontal plates at 16 cm above cables axis
It has also been shown experimentally, at least for the flat Measurement distances from trench bottom
cable configuration, that the longitudinal continuity of the 16
horizontal plates is not very important. 14
Shield thickness 12
Calculations show that increasing the shield thickness above
shielding factor
10 150 cm
3 mm does not bring a important improvement in the 8 250 cm
shielding effectiveness. On the other hand, for mechanical 6 300 cm
and corrosion withstand reasons, it is not safe to use too thin 4
shields. Hence, the value of 3 mm seems to be a master 2
choice for this type of aluminium shielding. 0
-400 -300 -200 -100 0 100 200 300 400
Shielding effectiveness horizontal distance to cable central conductor (cm)
Shielding factors up to 10 at 1.5 m above ground have been
calculated with the same 2D model as for the plane shield. Figure 6 : Efficiency of a H shaped aluminium shielding
However the continuity problems between elements being
very important, there is real a necessity to make recourse to Actual implementation
a 3D model for taking the discontinuities into consideration.
This 3D model is still under development. The H layout described above has been implemented in
Belgium on a new 30 km double circuit 150 kV link
Laboratory results between the nuclear power plant of Tihange and the HV
The measurements results presented on figure 6 have been substation of Avernas.
achieved with the same cable layout as for the plane shield From the 30 km underground link, 6.5 km are shielded.
(fig 4), i.e. a three phase flat configuration with 25 cm The link will be put into service at the end of this year;
distance between phases laid 24 cm above the bottom of a hence measurement in situ has not yet been performed.
trench of 150 cm depth. However, as an assembling technique by welding instead of
The shield is built with 200 X 80 cm aluminium plates of 3 bolting has been used, better results than those extrapolated
mm thickness. on basis of the laboratory tests (figure 7) are expected.
Vertical plates are installed at a distance of 100 cm from
each other, whereas the horizontal plates of 80 cm width are The per km cost increase of the link due this shielding is
installed 40 cm above the bottom of the trench. estimated to be about 20 %.
The vertical plates have an overlap of 8 cm and are fixed This estimation, however, doesn’t take into account the
together with four M8 bolts. additional exploitation costs involved by the losses in the
At both extremities of the shielded area a U-shaped shield.
aluminium cover of the same thickness and width as the
other plates ensures the necessary electrical link between
Field produced by a double circuit 150 kV cables in flat configuration
lateral plates (vertical right and left plate). spaced 25 cm - distance between axis of circuits: 2 m, depth: 1.25 m
H-shaped aluminium shielding - Field at 1.5 m above ground for I = 1300 A
On this figure, the important decrease of the shielding 25
effectiveness with lateral distance is partly due to the fact
that the experimental model, being only 8 m length, gives 20
rise to important border effects at distance from the axis of 15
the cables. without shielding
B (µT)
with shielding
The asymmetry in the curves is due to elliptical polarization 10
of the magnetic field and depends on the rotation order of 5
the three phases.
0
-30 -20 -10 0 10 20 30
Lateral distance (m )
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
10th International Symposium on High Voltage Engineering – Montréal –
1997
[ 0] Geometrical Aspects of Magnetic Shielding at Extremely Low
Frequencies
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 11/31
- 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 Shielding by ferromagnetic materials
Design of ELF Magnetic Field Shields
A. Canova, A. Manzin, M. Tartaglia Mainly effective neer the shield
IEEE Trans. On Industry Applications, vol 38, no 3 May/June 2002
[ 0] A numerical Approach to the Design of Conducting Shields for ELF Expensive (Si steel, permalloy)
Magnetic Field Reduction Corrosion protection
O. Bottauscio, D. Chiarabaglio, M. Chiampi, M. Repetto No gap in magnetic circuit
ETEP vol 12 No 2, March/April 2002
Steel tube : very efficient
[ 0] Campi ellettrici e magnetici: possibilità offerte dagli elettrodotti in cavo
A. Bolza, F. Donazzi, P. Maioli – Pirelli Caci e Sistemi 2000 Maintenance, repair
Comments given at CIGRE 2002 Paris: Group 21, PS1, Q 4 Thermal behaviour of cable
[ 0] Techniques for shielding underground power lines to minimize the Fast recovering of trench
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
Shielding by conductive materials
Copper Aluminium
Copper more expensive but higher effectiveness
Plates of about 2 to 4 mm
Corrosion OK but AC corrosion ?
Metallic shielding Continuity: not always needed
Welding of aluminium OK
Shapes :
Plane
Ushape
Hshape
Plane shield
Copper Aluminium
Copper more expensive but higher effectiveness
Plates of about 2 to 4 mm
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 12/31
- Corrosion OK but AC corrosion ?
Continuity: not always needed
Welding of aluminium OK
Importance of shield width
Plane shield performances
Hshaped shield (2)
Ushaped shield (1)
Field attenuation more homogenous than with flat shield Practical implementation
Difficulty of installation
Shield continuity 2
sDouble 150 kV link 2000 mm alu
sLength: 30 km
s6.5 km shielded
sPU cost increase: 20 %
Ushaped shield (2)
Currents induced in the shield
(a): real part
(b): imaginary part
Hshaped shield (1)
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 13/31
- effects of these fields. On the other hand PFMFs have the
ability to interfere with electron-beam devices, which
contributes to the awareness as regards EMC regulations.
These two issues have induced efforts to study ways to
mitigate these fields.
Secondary substations represent the final stages of the
electrical delivery system before reaching the customers. In
Sweden and in other European countries it is not unusual,
especially in neighbourhoods with a dense population, to
locate secondary substations inside buildings (e.g. in
cellars). Secondary substations are three-phase systems
composed mainly of: transformers (10 kV/0.4 kV), cables,
and switchboards containing busbars. As the voltage
decreases the current and consequently the magnetic field
increases.
Unlike power lines or underground cables, the magnetic
field originating from a secondary substation is rather
complex as it corresponds to an intricate superposition of
fields from various sources. Thus, in order to find out a cost-
effective way to mitigate this type of field, a variety of
techniques have to be explored.
E. Salinas (GB/SE), Field Mitigation from
Secondary Substations
(South Bank University, London, salinae@sbu.ac.uk and
Chalmers University of Technology, Gothenburg,
ener.salinas@elteknik.chalmers.se) Fig. 1.1 Some typical magnetic field values (in microtesla) from a
secondary substation situated in a cellar of a building.
A usual configuration of major sources and distances is also shown.
Summary
2. Strategy, Techniques and Methods
This work aims to design cost-effective schemes for the
mitigation of magnetic fields from secondary substations
According to the Webster’s Encyclopedic Dictionary,
and integrated solutions for larger systems. The most
strategy is “the art of devising or employing plans or
frequently adopted strategy consists of dealing with the
stratagems to achieve a goal”. In the case of magnetic field
mitigation operation at the origin of the field emission. The
mitigation the goal is to search for cost-effective solutions.
techniques applied are shielding, active compensation, and
Independently of the technique to be used, there are two
phase cancellation. Analytical, numerical (finite-element-
possible strategies to follow. We can choose to apply the
based), and experimental methods are used and improved for
mitigation operation at the victim or to apply it at the
the analysis of the electromagnetic fields. Finally, some
sources. In Fig. 2.1 the field from various sources is
examples of field mitigation in Sweden are presented.
affecting a region (victim area) located at some distance
above the sources. From the point of view of quantity of
1. Introduction
material used to perform the mitigation, applying the
mitigation operation (shielding, passive compensation or
Epidemiological studies of people living nearby sources of
active compensation) to the sources is often more cost-
power frequency (50-60 Hz) magnetic fields (PFMFs) have
effective than applying a similar operation at the victim area.
opened a yet unresolved debate over suspected adverse
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 14/31
- As the field propagates along a distance it becomes weaker.
Accordingly the area to deal with expands with a quadratic
dependence on that distance.
Fig. 2.1 Mitigating the field at the source is in most cases
more cost-effective than mitigate at the victim.
To investigate the field at an affected region it is customary
to measure the field on a certain plane (usually the floor
level) in that region and plot the results in a contour plot. It Fig. 3.1 Some of the complexities inherent to
the use of finite/open shields.
is then natural to ask: is it possible to discriminate in
advance the magnitude of the contribution of each source by
scanning the field values in the affected area? The answer is The field from busbars can be shielded by means of thin
yes. There are two reasons for this: (1) analysis of the field conductive plates located in front of the busbars. The
gradient on the scanned surface could suggest the possible induced eddy currents reduce the magnetic field in the far
source behavior; (2) analysis of the variation of the field zone. Acceptable shielding efficiency is possible even when
values with the distance perpendicular to the scanned the conducting shield is thinner than a skin depth. In cases
surface could provide a 3-dimensional picture of the field when the shield does not form a closed surface, conducting
decay, and subsequent source identification. Moreover, after shields are generally more efficient than ferromagnetic
a simple inspection of the substation itself the problem can shields. This is in apparent contradiction with the concept of
be resolved, and a mitigation technique can be suggested. skin effect, which gives for iron
(σ = 1.07x107⋅ Sm-1, and ∝r = 250) a skin depth
In secondary substations the techniques chosen to mitigate δ = (π f ∝0 ∝r σ)-1/2 ~ 0.14cm,
the magnetic field depend upon the characteristics of the almost an order of magnitude less than for aluminium –
sources. Busbars, transformers and cables are the sources consequently better efficiency would be expected. In fact the
most often encountered in a substation and possible latter is correct right at the other side of the ferromagnetic
techniques to apply are: phase cancellation, conductive and screen, where strong reduction is observed. However, at the
ferromagnetic shielding, passive and active compensation. victim zone (about 4 m over the system) both properties of
The methods used to carry out the mitigation techniques are iron, conductivity and magnetic permeability, manifest their
analytical computations (e.g. using symbolic manipulation antagonism. As the first tries to cancel the field, the other
codes), numerical modeling, e.g. finite element methods tries to attract the field lines. In fact, some of the field lines
(FEM), and experimental setups. are pulled into the region where we want to mitigate the
field. Using double-layer shielding, it was observed that the
3. Busbars order of the arrangement ironaluminum- coil provides a
more effective shielding than the permuted case: aluminum-
Busbars are the most efficient way to transport large iron- coil (Fig. 3.2).
amounts of electrical energy within a reduced space such as
a secondary substation. They are usually made of copper or
aluminium covered by copper and should be able to
withstand current values having several hundreds of
amperes. Depending on their specific design they can have
different lengths, geometrical arrangements, and cross
sections. A typical system of busbars, where a shielding
technique is being applied, is shown in Fig. 3.1. The
complexity of the problem when the arrangement is
modelled by numerical methods, such as finite elements
(FEM), is evident when finite/open shields are utilized.
Fig. 3.2 Double shield arrangement in front of a source, the field resultant
of the permutation Iron-Aluminium is shown.
Eddy currents circulating in a conductive plate due to the
field of busbars give a suggestion about field mitigation by
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 15/31
- trying to imitate the behaviour of these currents. This can be
achieved by passive compensation e.g. adding two
rectangular short-circuited loops in front of the busbars. This
was attempted with a system of square-shaped coils (N = 60
turns) but there was almost no attenuation, presumably the
currents were two small to induce any field-counteracting
effect. When the attempt was made with only N = 1, and
thicker conductor, there was some non-negligible effect,
although this occurred when the loops were placed very Fig. 4.2 Reduction of the magnetic field from a 3-phase transformer by
phase rearrangement at the
close (less than 1 cm) to the busbars. secondary side is a cost-effective mitigation method.
Active compensation was also tested for a system of 2m- 5. Cables
long busbars using an experimental arrangement. Two
rectangular coils were placed in front of the busbars. Each One of the most frequent sources of high magnetic emission
coil is 2m long and has a separation between phases of 0.25 (not only restricted to substations) is the non-cancellation of
m. the feeding current is 1.67 A. The shielding efficiency the field from individual phases in cable bundling. This
was measured at y = 1.5 m (parallel to the plane of the occurs due to poor phase arrangement in the cables. Phase
busbars) as the distance between the coils and busbars varied re-arrangements in cables are often not a complicated task.
between d = 10 and d = 20 cm. The result was a variation The phases are ordered in order to obtain an optimal
between SE = 8.6 dB and SE = 15.4 dB. grouping with a minimum field emission. Fig. 5.1 shows an
example of this procedure. The magnetic field of different
4. Transformers arrangements of three-phase conductors (carrying 100A) is
plotted and compared. The optimal bundling (arrangement
Measurement of the field around transformers usually yields 4) produced a good field reduction in comparison to the
high values, especially if they are not encapsulated. Often arrangement 1. The guidelines to obtain a good phase
these values do not decay with the distance as expected. arrangement are: (a) form bundles with mixed of phases, (b)
Since the leaking field is due to core or coils, they should minimize distances between phases, and (c) twist the cable
decay as ~ r–3. Therefore other must be the cause generating array to diminish longitudinal contributions to the field.
these fields. A substation was study, where measured field Extra considerations have to be taken in order to avoid
values were as high as 22 microtesla, right above the hindrances such as heating and mechanical stresses due to
transformer on the next floor. The transformer was located eddy currents and induced forces.
about 2 m below the measuring points; consequently the
values were too high to be inherently originated from this
source alone. An inspection of the interior of the substation
made clear that the field was due to ill connections and poor
field self-cancellation (Fig. 4.1-a). The solution adopted was
to keep the phases mixed up to the point of connections with
the transformer (Fig. 4.1-b). This solution provides a fairly
good mitigation (17-18 dB), however it still leaves a residual
field of around 3 microtesla. Additional mitigation is
possible and various actions can be recommended, e.g. to
change the paths of the connections to the ground level.
Shielding the transformer with an aluminium metal box,
which takes care of the remaining fields, can also be a
complementary measure.
Fig. 4.1 Mitigation of the field from transformers by rearrangement of
connections at the secondary
side (a) before: equal phases, (b) after: mixed phases.
Fig. 5.1 Different phase arrangement in cables; the field contours are
plotted in the interval [0.1-1] µT.
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 16/31
- 6. Example
Here a study is presented of magnetic fields originating from
a secondary substation placed in the cellar of the
Gothenburg City Library. This public library is located in
the centre of Gothenburg and is surrounded by other public
and urban buildings. About 190 persons work in this
building and it receives around 3,000 visits each day. The
electricity supply to the library and nearby public buildings
consists of a secondary 10/0.4 kV substation (two 800 kVA,
three phase transformers).
Rather high magnetic field values (Fig. 6.1-a) were
registered at the floor above the substation and these field
values propagated over a rather extended area outside the
(a)
substation. Stray currents were discovered to be the cause of
the anomaly. Various mitigation operations were carried out,
taking advantage of renovation procedures. Among them,
phase cancellation via cable management, shielding of
busbars, replacements by low emission transformers, as well
as laminated-ferromagnetic field reducers for cancellation of
stray currents, were used. The final field values (Fig. 6.1-c)
were reduced in average to acceptable sub-microtesla levels.
(b)
(c)
Fig. 6.1 The magnetic field one floor above the library and
its reduction at different stages.
7. Conclusion
Given a source, belonging to a substation, which is
producing relatively high magnetic field values (e.g. in
excess of the microtesla level) on a certain area of interest,
this report has shown various possible ways to mitigate these
values. The emphasis was to use simple and costeffective
methods. For this reason it was preferable to apply the
mitigation to take place at the sources instead of dealing
with shielding of the affected areas.
Various techniques were applied according to the specific
characteristics of each source. Table 7.1 shows the different
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 17/31
- techniques and design methods in order to achieve field •Active compensation
mitigation. The results of this investigation have been •Passive compensation
applied to the renovation and building of secondary •Phase cancelation
substations: Methods
•Analytical computations
Table 7.1 •Numerical simulations (Finite elements)
•Experimental setups
•Optimization methods
Main sources
•Busbars
•Transformers
•Cables
Some of the complexities inherent to the use of
finite/open shields.
Ener Salinas, Anders Bondeson
Summary
•The study aims to develop techniques that can be used for the
reduction of magnetic fields from secondary substations
•
The goals are to develop cost-effective methods for field reduction
and to develop integrated solutions for larger systems FEM formulation for busbars
Pure conductive vs. ferromagnetic shielding
Mitigating the field at the source is often more cost-effective
than mitigating at the victim
Double shield arrangement in front of a source.
The field corresponds to the permutation Iron-Aluminium.
Strategy:
Mitigate the fields at the source instead of at the victim area
Techniques:
•Shielding
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 18/31
- Mitigation of the field from transformers
by rearrangement of connections at the secondary side
(a) before: equal phases, (b) after: mixed phases.
Reduction of the magnetic field from a 3-phase transformer
by phase rearrangement at the secondary side
is a cost-effective mitigation method.
The magnetic field of a shielded and unshielded transformer
at 3m over the floor of the substation.
The shield consists of an aluminium box (5 mm thick); otherwise the
transformers have similar sizes and connections.
The magnetic field one floor above the library
and its reduction at different stages.
Different phase arrangement in cables
The field contours are plotted in the interval [0.1-1] μT
Conclusions
•The mitigation operation was performed at the sources rather than
at the affected areas.
•Magnetic fields and parameters of mitigation techniques were
modelled using modern methods (e.g. Symbolic computation, 3D
finite element codes).
•Magnetic field was mitigated from transformers, busbars and
cables.
•Cost-effective solutions were obtained.
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 19/31
- we intend to obtain:
- Generalized mitigation around the substation
- Mitigation in a specific area
- Possible actions:
O. Bottauscio (Italy), Experiences in the - Optimal layout of the substation (mainly for new installations)
mitigation of MV/LV substation - Design of passive shielding (also for old installations)
magnetic field emissions Model for magnetic field evaluation and reduction
(IEN Galileo Ferraris, botta@ien.it)
• The development of specific numerical tools for field modelling
Experiences in the mitigation of must take into account the features of the problem:
MV/LV substation magnetic field emissions -The domain under study is not limited
-The behaviour of magnetic materials is not nonlinear
O. Bottauscio (*), M. Chiampi (*), G. Crotti (°), -Complex electromagnetic phenomena arise inside shielding
A. Manzin (°), M. Zucca (°) elements
-Shielding elements are thin structures
(°) Istituto Elettrotecnico Nazionale Galileo Ferraris, Torino, Italy - The use of standard Finite Element formulations results to be
(*) Dipartimento di Ingegneria Elettrica Industriale - Politecnico di inefficient
Torino, Italy • A problem sometimes arises in presence of unknown sources
-A preliminary evaluation of their contribution is required
Introduction (inverse problem)
- The presentation will be addressed to present the experiences Mathematical model: 3D hybrid FEM-BEM approach
gained by the Turin group in the mitigation techniques for the
reduction of the magnetic field produced by MV/LV substations. - Shielding elements are thin structures
- The attention will be focused on the design of passive shielding to - The use of standard Finite Element formulations results to be
be adopted in the case of old installations, where the modification inefficient, due to the aspect ratio of the problem
of the layout is not always practicable. - A coupling of separate approaches, addressing a particular
geometrical scale, is required:
Outline of the presentation
- Characteristics of the magnetic field emissions of MV/LV
substations and mitigation strategy
- Shielding efficiency of materials employed for passive shielding
- Modelling aspects (sources and shield structures)
- Application to substations for industrial and civil supply
Characteristics of MV/LV field emissions MV/LV substation for civil supply
- Spatial nonuniformity due to the contribution of different sources Medium voltage: 22 kV;
Low voltage: 400 V;
Power of the transformer: 400 kVA ;
Maximum MV busbar current: 210 A;
MV board insulation: air;
Geometrical dimensions: 3 m x 4 m, h = 2.9 m
- Necessity of a model to evaluate the contributes of the different
sources
Characteristics of MV/LV field emissions
Model of the substation
- Variability due to the substation layout
- Time variability due to the fluctuation of the load
Mitigation solutions
Validation of the mathematical model
- The mitigation strategy is significantly dependent on the final goal
CIRED 2003 - Special Report Session 2 - Power Quality & EMC 20/31
nguon tai.lieu . vn
