1 Tutorial on MITIGATION TECHNIQUES OF POWER FREQUENCY MAGNETIC FIELDS ORIGINATED FROM ELECTRIC POWER SYSTEMS Programme Tutorial on Magnetic Field Mitigation

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

2Pedro L. Cruz Romero

Conductor management - Compensation - Mitigation for T&D lines (EHV, HV, MV, LV)

3Jean Hoeffelman

Shielding by metallic materials - Power cables

4 Ener Salinas Substations - Examples

2

About the working group C4.204


• CIGRÉ Working Group formed in 2001

• Motivation: Concerns from customers, utilities and researchers in relation to some alleged health risks (in particular childhood leukaemia) of long-term exposure to power frequency magnetic fields

• Initial aim: To collect discuss and synthesise the available technical data referring to different existing techniques to mitigate extremely low frequency (ELF) magnetic fields

• Final form: A published Technical Brochure (TB 373)

3

1.1 General Principles


4

Sources of power-frequency magnetic fields (PFMFs)


The flow of electrical energy from the

generation plant to the customer.

Along the way there are different types of

sources of power-frequency magnetic

fields

The PFMFs sources and techniques can be

classified according to their origin:

•Power lines•Underground cables•Complex sources (e.g. substations)

5

Difference between Electric and Magnetic fields


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


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


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


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



Programme



11 Ener SalinasEner SalinasGeneral principles - Methods of assessment - General principles - Methods of assessment - StrategiesStrategies

2Pedro L. Cruz Romero

Conductor management - Compensation - Mitigation for T&D lines (EHV, HV, MV, LV)

33Jean Jean HoeffelmanHoeffelman

Shielding by metallic materials - Power cablesShielding by metallic materials - Power cables

44 Ener SalinasEner Salinas Substations - ExamplesSubstations - Examples

10

2.1 Conductor managementApplied mostly to linear sources: overhead lines, underground cables, busbars, etc.


Layout Compaction

Change of geometry ofconductors keeping thesame phase-to-phaseclearance

Change of geometry ofconductors keeping thesame phase-to-phaseclearance

Keeping the same geometryreduce the phase-to-phase clearance

Keeping the same geometryreduce the phase-to-phase clearance

Original configuration

Contour curves values in TContour curves values in T

Balanced system !!

11

2.1 Conductor management


Phase splitting

Single-phase line

Current dipole Current quadrupole

2

1

rB 3

1

rB Faster

reduction with distance to source !!

r : Distance to centre of dipole r : Distance to centre of dipole

r : Distance to centre of quadrupoler : Distance to centre of quadrupole

12



Phase splitting

Three-phase line

Two split phases Three split phases

3

1

rB 3

1

rB No great

improvement !!

13



Phase cancelation

Multi-circuit line

Super bundle Low-reactance

2

1

rB 3

1

rB Both circuits should be equally loaded

Changes in protection relays could be needed

Changes in corona performance in overhead circuits

14

2.2 Compensation


Passive compensation

Location close to the source of a loop or coil.

Magnetic field generated by the coil that partially compensates the original field.

Induced current in the loop due to the flux linkage.

Increase of effectiveness: insertion of capacitor to compensate the inductance of the loop.

Location close to the source of a loop or coil.

Magnetic field generated by the coil that partially compensates the original field.

Induced current in the loop due to the flux linkage.

Increase of effectiveness: insertion of capacitor to compensate the inductance of the loop. Design parameters:

• Shape of the coil

• Location of the coil

• Electrical parameters of the conductor

• Number of coils

Design parameters:

• Shape of the coil

• Location of the coil

• Electrical parameters of the conductor

• Number of coils

Not complete compensation !

!

15

2.2 Compensation



Single-phase line

With capacitorWith capacitor

LoopLoop

Contour curves values in TContour curves values in T

16

2.2 Compensation


Active compensation

Current in the loop generated by an external power source.

Current control in amplitude and phase.

More sophisticated equipment is required.

Costly and less reliable than the passive loop.

Higher flexibility in the location of the loop. Possibility of locating it far from the source.

Current in the loop generated by an external power source.

Current control in amplitude and phase.

More sophisticated equipment is required.

Costly and less reliable than the passive loop.

Higher flexibility in the location of the loop. Possibility of locating it far from the source.

Not complete compensation !!

17

2.3 Mitigation for T&D overhead lines


Techniques

Increasing the height of masts

Conductor management

Compensation

• Low-medium cost

• Low-medium reduction factor

• Low-medium cost

• Low-medium reduction factor• Medium-high cost

• Low-medium-high reduction factor

• Medium-high cost

• Low-medium-high reduction factor• Medium-high cost

• Medium-high reduction factor

• Medium-high cost

• Medium-high reduction factor

Shielding factor = reduction factor !!

18



EHV and HV Power lines

Increasing the height of masts

-100 -50 0 50 100

0

5

10

15

20

25

Distanza dall'asse della linea [m]

H=11.34 m

H=12 m

H=14 m

H=16 m

H=18 m

H=20 m

H=22 m

H=24 m

Indu

zion

e m

agne

tica

a 1

m d

al s

uolo

- B

eff

- [µ

T]

V = 380 kVI = 1500 A

H

Distance from line centre [m]

Brm

s 1

m a

bove

gro

und

[T

]

Reduction restricted to underneath the line.

Reduction factor at x=0

Reduction restricted to underneath the line.

Reduction factor at x=0

4

mitigated

mitigatednon

B

BRF

19




Conductor management: changing the geometry of conductors

Low reduction factor far from the line

Low reduction factor far from the line

4.1RF

Low reduction factor close to the line


2RF

380 kV380 kV

20




Conductor management: compaction

Medium reduction factorMedium reduction factor

4RF

• Lower visual impact

• Reduction of line surge impedance

• Difficult to perform live-line maintenance

• EHV line: increase of corona effect

• Lower visual impact

• Reduction of line surge impedance

• Difficult to perform live-line maintenance

• EHV line: increase of corona effect

115 kV115 kV

21




Conductor management: phase cancellation



2RF

Medium reduction factor far from the line

Medium reduction factor far from the line

3RF

380 kV1500 A

380 kV1500 A

22




Conductor management: phase splitting

Medium reduction factor close to the line

Medium reduction factor close to the line

5RF

High reduction factor far from the line

High reduction factor far from the line

6RF

380 kV1500 A

380 kV1500 A

Star line: complete reduction at 35 m !!

23







2RF

High reduction factor far from the line at the other side

High reduction factor far from the line at the other side

8RF

Medium reduction factor far from the line at one side

Medium reduction factor far from the line at one side

4RF

Capacitor: non-symmetrical reduction!!

24




Effect on other electrical parameters

MethodMagnetic

FieldElectric

FieldAudible

NoiseRadio

InterferenceUnbalance

Height increase (1) =

Layout =

Compaction

Vertical super-bundle low-reactance (2)

Phase splitting

Passive/active loop = = =

(1) Starting from certain distance (about 50 m) the effect is the opposite

(2) It rises lightly from about 30 m off

25


• Variation of current along the feeder

• Different distribution systems different presence of zero sequence current

– 3-wire 3-phase

– 4-wire 3-phase

– 5-wire 3-phase

– 2-wire

– 1-phase

• Lower voltages use of covered and insulated conductors

• Shorter phase-phase clearance Field mitigation only of interest near the line

more effectiveness in raising the poles.


MV and LV Power lines

Differences with EHV and HV lines

26



MV and LV Power lines

Mitigation techniqueReduction level (%)

Installationcost

Global performance over conventional

Effect of unbalancedcurrent

Small compaction 25-45 Low Lower Low

Crossarms armless 60 Low/medium Lower Medium

Tree wires 60 Medium Higher Medium

Spacer cable 80 High Higher High

Aerial Boundle Cable 100 Very high Higher High

Underground line 90 Very high Higher High

Phase split 70-80 Medium Lower High

Increase clearance to ground

25-60 Low/medium Lower Low

Compensation loop 35 Medium Lower Medium

27

Tutorial on



Programme




22Pedro L. Cruz Pedro L. Cruz RomeroRomero

Conductor management - Compensation Conductor management - Compensation - Mitigation for T&D lines (EHV, HV, MV, LV)- Mitigation for T&D lines (EHV, HV, MV, LV)

3Jean Hoeffelman

Shielding by metallic materials - Power cables

44 Ener SalinasEner Salinas Substations - ExamplesSubstations - Examples

28

3 Shielding by metallic materials

Two types of shielding materials


Magnetostatic shieldingFlux-shunting mechanism

Shielding by eddy currentsInduced currents mecanism

29

3.1 (pure) ferromagnetic shielding


MaterialInitial Relative Permeability

r,ini

Maximum Relative

Permeability r,max

Iron, 99.8% pure 150 5000

Steel, 0.9% C 50 100

Low Carbon Steel (LCS) 300 - 400 2000

Ultra Low Carbon Steel (ULC) 250 1100

Hot rolled Ultra Low Carbon Steel (HR ULC) 250 2000 to 5000

Silicon steel (Si 3%) - Grain oriented (GO) 40,000

78 Permalloy (μ-material) 8,000 100,000

30



31


Htengential continuous

Bnormal continuous


To be efficient at distance a ferromagnetic shield needs to be closed !

32



To be efficient a ferromagnetic shield needs to encompass completely the source.

33



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



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


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


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


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


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



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



Passive loops (joint chamber)

Double loop : SF 2Double loop : SF 2

41



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]


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]


42




Raceway: SF 20Raceway: SF 20

43



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

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5 5,5 6 6,5 7 7,5 8 8,5 9 9,5 10

Horizontal distance from the line axis (m)

Shie

ldin

g ef

fect

iven

ess

( d

)

d = 5 cm

d = 10 cm

d = 20 cm

2525100

145

0.3d

44



Flat conductive shielding

In order to be effective, the shielding plates have to be welded together

Aluminium may also be used but is less effective

In order to be effective, the shielding plates have to be welded together

Aluminium may also be used but is less effective

45



Open conductive shielding

Aluminium H shield (flat formation): SF > 7 Aluminium H shield (flat formation): SF > 7

bridge welding

100

80

150

20

20

80

25

46



Open conductive shielding

Aluminium square shield (trefoil formation): SF > 7 Aluminium square shield (trefoil formation): SF > 7

bridgewelding

150

32

20

62

60

47



Synthesis

Passive loops Open ferromagnetic shielding


Conductive shielding

Shielding factor SF

1.5 to 4

(flat formation)

depends on distance to shield !

> 15 > 7

Losses low low low to medium medium

Corrosion risk / needs protection

needs protection

Cu: OK

Al: depends on soil pH

Costs low medium high Cu: high

Al: medium

Maintenance easy rather easy variable rather easy

48

Tutorial on



Programme




22Pedro L. Cruz Pedro L. Cruz RomeroRomero

Conductor management - Compensation Conductor management - Compensation - Mitigation for T&D lines (EHV, HV, MV, LV)- Mitigation for T&D lines (EHV, HV, MV, LV)

33Jean Jean HoeffelmanHoeffelman

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


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


51

Mitigation of PFMFs from busbars


Busbars can have different shapes. Yet, longitudinal profiles are often common and it can be sometimes a reasonable approximation when designing geometries and selecting shielding material

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


<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


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


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


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


•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


58

Example 1: Ferromagnetic pipes in Genoa


•The three cables are enclosed inside a ferromagnetic tubular section, which acts as a shield trapping the magnetic flux

•The material used is low carbon steel, with an external diameter of 508 mm and a thickness of 9.5 mm

•2 km of circuit of 150 kV 1x1000 mm2 XLPE cable were shielded with this technology

•Field at 1m above the ground < 0.2 μT

•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


60

Example 3: High Magnetic Coupling


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


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

Cabin containing loop feeding devices Cabin containing loop feeding devices Regulated current generator

Regulated current generator

After works, inactivated screen

After works, inactivated screen Before works

Before works

After works, activated screen

After works, activated screen

62

Example 5: Shielding of busbars in a secondary substation


Results

After implementation of the two separated shielding plates (back of the switchboard and ceiling)

The maximum value of the magnetic field in the area of interest was 0.4 μT

The average value of the magnetic field was 0.2 μT

Results

After implementation of the two separated shielding plates (back of the switchboard and ceiling)

The maximum value of the magnetic field in the area of interest was 0.4 μT

The average value of the magnetic field was 0.2 μT

63Tutorial on Magnetic Field Mitigation Techniques, International Colloquium on ELF EMF, Sarajevo 3rd-4th June, 2009

Documents

1 Tutorial on MITIGATION TECHNIQUES OF POWER FREQUENCY MAGNETIC FIELDS ORIGINATED FROM ELECTRIC POWER SYSTEMS Programme Tutorial on Magnetic Field Mitigation