Measures to Reduce Stray Currents caused by d.c....

© Laboratory for Cathodic Protection and Interference

Technische Akademie Wuppertal e.V.

Measures to Reduce Stray Currents

caused by d.c. Traction Systems

by Ulrich Bette

Light Rail Day 2013, Copenhagen

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Contents

Basic Principles EN 50122-2 Conductance per length Green tracks Interference Conclusion

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Basic Principles

Anodic partial reaction (material removal): 2 Fe -> 2 Fe++ + 4 e-

Cathodic partial reactions Oxygen corrosion: O2 + 2 H2O + 4 e- -> 4 OH-

Hydrogen corrosion: 4 H2O + 4 e- -> 2 H2 + 4 OH-

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Stray Current Corrosion © Laboratory for Cathodic Protection and Interference

Stray current (EN 50122-1):

Part of the current caused by a d.c.-traction system which follows paths other than the return circuit.

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Current entry points: ionic conduction electron conduction transition

O2 + 2 H2O + 4 e- 4 OH-

Current exit points: electronic conduction ionic conduction transition

2 Fe 2 Fe2+ + 4 e-

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Cathodic partial reaction:

O2 + 2 H2O + 4 e- 4 OH-

Anodic partial reaction:

2 Fe 2 Fe2+ + 4 e-

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Faraday‘s law

Basic Principles

m = 𝑀

𝐹 ∙ 𝑧 .

𝐼corr

. 𝑡

where

m is the material loss M is the molar mass, MFe = 55,845 g mol-1 Icorr is the corrosion current t is the time F is the Faraday constant, F = 96 485 As mol-1

z is the valency of the cation, zFe = 2


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Basic Principles


µk= 𝑀

𝐹 ∙ 𝑧

Electrochemical equivalent

mol

Asmol

g

296485

84.55kµ

As

mg28937.0k µ

aA

kg13.9k µ

Faraday‘s law: m = µk . 𝐼corr

. 𝑡

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Experiment on Faraday‘s law:

Basic Principles

dF = 5.0 mm

sS = 1.2 mm

ΡFe = 7.87 kg/dm³


Result:

t = 1.5 h

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Effects of Stray Currents © Laboratory for Cathodic Protection and Interference

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Rail potential along the track

Rectifier substation

Tram

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Potential gradient at an right angle to the track Potential gradient at an right angle to the track: Interference of other installations possible 24


Standards, Directives © Laboratory for Cathodic Protection and Interference

BOStrab: 12/87 German Federal Regulations on the Construction and Operation of Light Rail Transit Systems EN 50122-1:2011 Railway applications - Fixed installations - Electrical safety, earthing and the return circuit - Part 1: Protective provisions against electric shock

EN 50122-2:2010 Railway applications - Fixed installations - Electrical safety, earthing and the return circuit - Part 2: Provisions against the effects of stray currents caused by d.c. traction systems

EN 50162:2004 Protection against corrosion by stray current from direct current systems

VDV 501 Reduction of the Corrosion Danger Due to Stray Currents in Tunnels of DC Traction Systems with Return Current via Running Rails Part 1: 04/93 – Provisions and Bases for Calculation Part 2: 04/93 – Measuring Methods Part 3: 09/95 – Computer Model

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Standards, Directives © Laboratory for Cathodic Protection and Interference

VDV 505: 06/05 Design of and Protective Provisions for DC Rectifier Substations for DC Mass Transit Systems VDV 506: 06/05 Design of and Protective Provisions for Electrical Power Installations in Depots and Workshops for DC Mass Transit Systems VDV 507: 06/05 Design of and Protective Provisions for Electrical Power Installations Along DC Mass Transit Lines VDV 525: 06/12 Overvoltage Protection for Traction Power Supply Systems of DC Urban Rail Systems EN 13146-5:2012 Railway applications - Track - Test methods for fastening systems - Part 5: Determination of electrical resistance EN 13481:2012 Railway applications - Track - Performance requirements for fastening systems Part 2: Concrete sleepers Part 4: Steel sleepers Part 5: Slab track

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EN 50122-2 © Laboratory for Cathodic Protection and Interference

To reduce stray currents, the running rails are to be so insulated against the earth that the stray current leaking relative to the length shall not exceed

Is'= 2.5 mA/m per track

during the operation.

Practical experience during the last 25 years has shown that there is no loss of use of the tracks, inclusive of the rail fastening elements, due to stray current corrosion if this value is not exceeded.

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It is recommended to calculate the mean values of time of the positive rail potential changes before the detailed planning.

The maximum permissible conductance per length can be calculated as follows on the basis of these values and the permissible stray current leaking relative to the length:

RE

''

U

IG

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The conductance per length per track known to date for tramways, whose rail potential changes do not exceed the following values in the positive direction on average, amounts to

≤ +5 V G' ≤ 0.5 S/km for open formation

and to

≤ +1 V G' ≤ 2.5 S/km for closed formation

REU

REU

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Pointer: Running rails laid in an electrically insulating way are not suited as earth electrodes e.g. in connection with lightning protection measures.

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The interconnected reinforcement of these structures, the structure earth connected to these structures and the structure earth of stopping places and rectifier substations have to be separated from the public earth.

Moreover, the reinforcement of tunnels, bridges, viaducts and reinforced track beds has to be so conductively interconnected via metal that the longitudinal voltage drop caused by stray currents does not exceed 0.2 V on average.

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EN 50122-2, Annex A © Laboratory for Cathodic Protection and Interference

Key

1 reference electrode

2 insulating rail joint

Determination of the conductance per length G‘RE for at-grade tracks

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EN 50122-2, Annex A © Laboratory for Cathodic Protection and Interference

Key:

R1 running rail 1

R2 running rail 2

E1 electrode 1 (close to the rail)

E2 electrode 2 (remote electrode)

uRE(t) rail potential in V

u1–2(t) voltage between electrodes E1 and E2 in V

a distance between the outer running rail and the electrode close to the rail in m

b distance between the outer running rail and the remote electrode in m

stg gauge in m

Determination of the local conductance per length G'RE for at-grade tracks

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Stray current transfer ratio

Correlation

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Lab Tests © Laboratory for Cathodic Protection and Interference

Determination of the volume resistivity following IEC 60093 and determination of the achievable conductance per length of chamber profiles:

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Measuring circuit for determination of the electrical resistance Condition: R33 5 k

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Test arrangement

Key 1 spray frame 2 spray nozzles 3 test sleepers 4 wood blocks 5 plastic pads

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Measuring circuit for determination of the electrical resistance Condition: R33 5 k

This test is to ensure that train safety installations function perfectly. It cannot be used to assess stray currents as G‘ 1.2 S/km.

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Conductance per length © Laboratory for Cathodic Protection and Interference

Description Conductance per length G' in S/km per track

from to

At-grade formation

Unbound base layer (mineral mixture), gap filled up with paving blocks 0.3 6.5

Concrete slab/girder, unintended connections between running rails and reinforcement 3.0 12.0

Concrete slab/girder, insulating rail pre-coating and insulating fastening elements, gap filled up with lean concrete or use of concrete chamber stones, mastic asphalt

1.9

5.0

Concrete slab/girder, insulating rail pre-coating and insulating fastening elements, insulating chamber (filling) profiles, insulating track rod coating

0.3

2.0

Concrete slab, insulating rail pre-coating and insulating fastening elements, gap filled up with asphalt 1.0 1.5

Bituminous base layer up to 1 m beside outer rail, insulating rail pre-coating, gap filled up with lean concrete, mastic asphalt

0.3

2.1

Bituminous base layer, chamber stones of concrete, gap filled up with asphalt 0.3 1.2 Ballast bed, concrete sleeper, gap filled up with bituminous base layer 0.6 1.6

Green tracks Ballast bed, concrete sleeper, Geotextil, grass until top of rail 0.7 5.0 Ballast bed, concrete sleeper, chamber filling profiles, Geotextil, grass until top of rail 0.3 1.5 Longitudinal beams of concrete, insulating fastening elements, grass until top of rail 1.0 5.0 Longitudinal beams of concrete, insulating fastening elements, chamber (filling) profiles, rail foot enclosure and insulating track rod coating, grass until top of rail

0.02

0.8

Concrete slab with inlet ducts, running rails with insulating casting mass until top of rail, grass until top of concrete or top of rail

0.002

0.02

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from to

At-grade formation




1.9

5.0


0.3

2.0



0.3

2.1



0.02

0.8


0.002

0.02

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from to

At-grade formation




1.9

5.0


0.3

2.0



0.3

2.1



0.02

0.8


0.002

0.02

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from to

At-grade formation




1.9

5.0


0.3

2.0



0.3

2.1



0.02

0.8


0.002

0.02

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from to

At-grade formation




1.9

5.0


0.3

2.0



0.3

2.1


Green tracks Ballast bed, concrete sleeper, Geotextil, grass until top of rail 0.7 5.0 Ballast bed, concrete sleeper, chamber filling profiles, Geotextil, grass until top of rail 0.3 1.5 Longitudinal beams of concrete, insulating fastening elements, grass until top of rail 1.0 5.0

Longitudinal beams of concrete, insulating fastening elements, chamber (filling) profiles, rail foot enclosure and insulating track rod coating, grass until top of rail

0.02

0.8


0.002

0.02

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from to

At-grade formation




1.9

5.0


0.3

2.0



0.3

2.1



0.02

0.8


0.002

0.02

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EN 50162 Acceptable potential shift for installations made of steel ρ ≤ 15 Ωm ΔU = 20 mV

ρ ≥ 200 Ωm ΔU = 300 mV

15 Ωm < ρ < 200 Ωm m

5,1mV

U

where ρ soil resistivity ΔU positiv potential shift (time average)

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Potential gradient towards earth far away (here: 100 m): (1) where I‘ges is the total stray current leaking relative to the length in mA/m s is the gauge (single-track line) or the distance between track centres (double-track line)

a is the distance between the pipe and the outer running rail

))5.0ln()5.0100(ln('ges

sasρI

U

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Permissible potential shift from steel in the earth (valid for 15 Ωm < ρ < 200 Ωm), see EN 50162: (2)

m5.1

mV

max

ρU

If (1) and (2) are equalised, the following results: (3)

))5.0ln()5.0100(ln('

5.1ges

sasρI

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Equation (3) shows that there is no soil resistivity in the range 15 Ωm < ρ < 200 Ωm. The stray current leaking relative to the length is found by way of conversion: (4) )5.0ln()5.0100ln(

5.1'ges

sasI

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The minimum distance between the pipe and the outer running rail is also found by way of conversion: (5)

ssa gesI

5.0e5.0100'

5.1

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Conclusion © Laboratory for Cathodic Protection and Interference

There is no impermissible interference on third party installations made of steel in earth with 15 Ωm < ρ < 200 Ωm if these installations are at least 1 m away from the running rails (minimum distance according to EN 50122-2), provided the stray current leaking relative to the length is reduced by factor 4 I‘ < 0.625 mA/m per track.

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from to

At-grade formation




1.9

5.0


0.3

2.0



0.3

2.1



0.02

0.8


0.002

0.02

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Conclusion © Laboratory for Cathodic Protection and Interference

Industry offers appropriate systems for insulation of running rails.

The achievable conductance per length mainly depends on the carefulness of the building supervision.

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