Examples of Stray Current Interference

Examples of Stray Current Interference

Examples of Stray Current Interference

Stray current interference sources includes Cathodic protection systems, AC or DC powered rail systems, HVDC or AC power lines, and Telluric or geomagnetic currents

Stray Current Interference
Stray currents are currents through electrical paths from external sources which deviate from its intended circuit. Any metallic structure such as a buried pipeline in soil represents a low resistant current path and is thus vulnerable to the effect of stray currents causing either current pick up or discharge or both. This will cause either over protection or under protection or sections undergoing over protection and under protection at different locations.
Harmful effects of Stray Current Interference
Stray current interference is very damaging for steel pipelines and other structures because it involves huge current and a small coating defect or holiday may cause extensive corrosion and metal loss of structure. The corrosion is localized and mostly pitting corrosion and a deep pit is formed due to localized current discharge. The rail transit system or underground mining railroad system pipeline may discharge a large stray current and cause severe corrosion in a small area of holiday. According to Faradays law, Ferrous metals have metal loss of 9.1 Kg/A-year so a large discharging current will destroy the metal in a very short time may be in hours or days depending upon cycle of discharge.
Sources of Stray Currents
  1. DC Current
  • Rail transit system
  • Underground Mining Trolley system
  • DC Welding machines
  • HVDC grounding system
  • HVDC transmission line
  • Cathodically protected structures
       2.  AC Current
  • High Power Transmission System
       3.  Telluric Current

DC Stray current interference due to cathodically protected pipelines

  • The interfered with pipeline (foreign pipeline) is affected by interfering pipeline (protected pipeline) when foreign pipeline either runs parallel to or cross protected pipeline.
  • The foreign pipeline is subjected to anodic interference or cathodic interference or both depending upon the configuration of foreign pipeline whether running parallel to or crossing protected pipeline
  • The foreign pipeline is subjected to anodic interference when it is in the close vicinity of protected pipeline ground bed. The foreign pipeline crosses potential gradient from the ground bed which is positive with respect to remote earth. Here the pipe picks up current from ground bed and becomes cathode and discharges at remote location and becomes anode
  • The foreign pipeline is subjected to cathodic interference when foreign pipeline is in close vicinity of protected pipeline and influenced by voltage gradient of protected pipeline which is negative with respect to remote earth. Current is discharged from foreign pipeline to protected pipeline and becomes anodic while current will be picked up at remote location.
Anodic interference

Cathodic interference
Anodic and cathodic interference occurring together
Anodic and cathodic interference may take place together if foreign pipeline is in close vicinity of anode bed and protected pipeline. It becomes more electronegative at anodic interference and less electronegative at cathodic interference
 
Detection of stray current interference
  • The interference zone due to anodic and cathodic interference can be detected by carrying out close interval potential survey on affected pipeline by interrupting current source of protected pipeline (Interfering pipeline) synchronously.
  • The data recorded by digital voltmeter and data logger in both ON and OFF state (only protected pipeline current source is interrupted) indicates the status of over protection or under protection
  • As per BIS 8062, clause 9.2, The maximum positive potential changes at any part of a secondary structure, resulting from interference, should not exceed 50 mV of potential when current is switched ON from OFF condition for protected pipeline and current is in ON condition for interfered with pipeline. However ifthe structure is receiving cathodic protection and its potential is not more electropositive than -0.85 V then corrosion may not take place
Data of potential measurement at section of stray current interference
Graph of PSP reading of foreign pipeline when protected line is interrupted
Mitigation of Stray current interference
  1. Removing the source or reducing its output
  2. Installing an electrical bond between the interfered with and interfering structure
  3. Providing a metallic shielding parallel to the interfered with structure at the stray current pick up zone
  4. Installing sacrificial anodes at the current discharge locations on the interfered with structure
  5. Applying protective coating on the interfered with structure at current pickup areas or to the interfering structure where it picks up returning stray current
Removing the source or reducing its output
  • Removing the source is difficult proposition as it requires expenditure. A proper coordination between interfered with and interfering structure owner is required to avoid situation of interference.
  • Proximity of anode ground bed of interfering structure to interfered with structure causes pick up of current by interfered with structure. Hence the ground bed distance from interfered with structure should be remote.
  • The current output of interfering structure can also be reduced to minimize interference effect.
  • The crossing of pipeline should also be avoided as even small amount of current picked up by interfered with structure can be discharged from location of crossing.
Installing an electrical bond between the interfered with and interfering structure
  • A variable resistor is installed between interfered with and interfering structure at discharge current location of interfered with structure
  • Interfering current is returned to interfering structure through resistance bond.
  • The selection of proper resistance is important to pass the flow of interfering current and direction of current flow should be from interfered with to interfering structure
  • The resistance is adjusted by measuring potential on interfered with structure and interfering structure at discharge location when current source of interfering structure switched ON from OFF. The potential of interfered with structure should be more electronegative than -0.85 V and potential of the interfering structure should be more electronegative than interfered with structure. In this case current will be flowing from interfered with structure to interfering structure
  • The  direction  of  current  can  be  known  by  measuring  potential between interfered with and interfering structure
Resistance bond between foreign and protected pipeline
Advantages and disadvantages of resistance bond
  • Advantages
  • Relative inexpensive
  • Easily adjustable
  • Current capacity is flexible
  • Disadvantages
  • Resistance bond is vulnerable to AC fault current and may burn unless protective devices are in place.
  • Resistance bond may affect cathodic protection of both the structures as current requirement of one pipeline may affect other pipeline
  • Frequent inspection, monitoring and maintenance is required
  • CP surveys requiring measurement of polarized potential on both the pipelines will require interruption of resistance bond or current sources of both the pipelines
Providing a metallic shielding parallel to the interfered with structure at the stray current pick up zone
The process is simple. A metallic shield is laid just parallel to interfered with structure at current pick up location and connected to negative terminal of the current source. The shield picks up current instead of interfered with structure and returns to interfering structure. The disadvantage is this that interfering structure will face current distribution problem as it will require more current to dispense with bare metallic structure. In such case it may require more current source units.
 
Metallic shielding to control interference
 
Installing sacrificial anodes at the current discharge locations on the interfered with structure
Applying protective coating on the interfered with structure at current pickup areas or to the interfering structure where it picks up returning stray current
The coating is provided at the pick up location of interfered with structure or pick up location of interfering structure as providing coating on discharge location of interfered with structure will cause corrosion failure due to small holiday and high discharge current density
 
Stray current interference due to DC Transit System
The rail at remote from sub-station is positive with respect to remote earth, hence pipeline picks up drainage current from rail and at sub-station, the rails are negative with respect to remote earth, hence current is discharged from pipeline through earth to negative terminal of the sub-station. Corrosion occurs at discharge locations. The current involved is large and unless suitable mitigation measures are taken, pipeline will corrode very fast. The interference can be detected by 24 hour recording.
 
Indication by recording potential and mitigation measures
  • The graph will indicate considerable potential fluctuations during rush hour of morning and evening time and no fluctuation during late night and early hours
  • Considerable long stretch of potential fluctuations and over-protection or under-protection will cause coating disbondment and/or pipeline failures
  • The mitigation measures are as follows:
  1. Electrical isolation of rail and substation.
  2. Electrical bonds
  3. Diodes or reverse current switch
  4. Force drainage bonds along with potential controlled rectifier
  5. Cathodic protection
Mitigation measures
  • Electrical bonds such as variable resistor between pipeline and negative bus of substation together with isolating switches are provided to return the drainage current through resistor. Due to various substations and track, it is possible that reverse current flows to the pipeline through resistance, hence, diodes or reverse current switches are used which allow drainage current to flow in one direction only but blocks current flow in reverse direction. Forced drainage system along with Potential controlled rectifier is also used to maintain potential of structure set at controller. Rectifier is controlling potential in auto mode by controlling current through drainage bond.
Diode station to control drainage current
Drainage bond with potential controlled rectifier
AC Interference
  • The high current flowing in the overhead power transmission lines generate magnetic field which induces an AC voltage in the pipeline and causes AC current to flow. The magnitude of such current depends upon many factors such as distance from transmission lines, power line current, coating condition, soil resistivity etc.
  • AC interference is caused by three mechanisms such as conductive coupling which deals with fault condition, electrostatic or capacitive coupling and electromagnetic or inductive coupling
AC Interference due to transmission line

Conducting coupling
Conductive or resistive coupling occurs when there is power line faults and high discharging currents pass through earth and other underground structures in the earth. The current discharge into the pipeline depends upon magnitude of fault current, coating condition of the pipeline, separation distance between pipeline and faulted structure and soil resistivity. Since current always travels in a low resistance path hence power line structure resistance to earth should be low enough to ground the fault. The high discharge current in the pipeline generates high voltage which poses a shock threat to personnel working on test station and above ground section. Lightening fault or line fault in very high voltage line may cause coating damage or pipeline failure if sufficient protective measures are not taken. The duration of such fault is fraction of second say 0.1 sec before tripping of isolating relay.
 
Mitigation measure
  1. The most effective means of preventing an arc to strike a pipeline is to maintain a minimum safe distance between tower footing and pipeline
               The equation of such separation distance was given by Sunde r = .08 √ If *?for ?= ≤ 100 ?-m
               r = .047 √ If *?for ?= ≥1000 ?-m
               where r = distance between power structure and pipeline(m) If = Fault current in KA, and ?= soil resistivity in ?-m Example :
               Let If = 25 KA and ?= 100 ?-m then r = .08 √ 25 *100 = .08 * 50 = 4 meter
               If ?= 1000 ?-m then r = .047 √ 25 *1000 = .047* 158 = 7.42 m
              Thus safe distance can be calculated by knowing the fault current and soil resistivity or as per directive from power companies.
       2. Good coating on pipeline will have less possibility of effects of fault current as fault current is transmitted to pipeline through coating
       3. If minimum separation distance is not possible then zinc ribbon anode or zinc anodes are installed between tower and pipeline and                       connected to pipeline . The low resistance of zinc ribbon anodes or zinc anodes provide safe path to fault curerent to ground thus saving             pipeline.
 
Shock Hazards
  • The generated high voltage due to conductive coupling poses safety hazard to the pipeline personnel working on pipeline or even standing in the vicinity of charged pipeline in contact with the earth.
  • The maximum current that can be tolerated is a function of body weight and duration of current flow.
  • The equation for maximum current that a human body can tolerate is If = 0.157/ √ ts For 70 Kg body weight, If = .116/ √ ts For 50 Kg weight
                 where, If = Maximum current in Amp, body can sustain and ts is shock duration in seconds
                 For ex if shock duration is 2 second then If for 70 Kg and 50 Kg body weight will be
                .11 A or 110 mA and .08 A or 80 mA
  • Humans face shock hazards due to excessive voltage on pipeline. The common terminology is touch and step voltage which should not exceed maximum tolerable voltage. The voltage depends upon soil resistivity, fault duration and body weight
                The equation is Vstep = (1000 + 6?) * 0.157/√ts for 70 Kg and Vstep = (1000 + 6?) * 0.116/√ts for 50 Kg
                Vtouch = (1000 + 1.5?) * 0.157/√ts for 70Kg and Vtouch = (1000 + 1.5?) * 0.116/√ts for 50Kg
                Touch voltage is voltage between hand and foot and step voltage is voltage between
Touch and step potential
 
Mitigation for reducing shock hazards
  • Examples of step and touch voltage
  • Assuming soil resistivity of 100 ?-m and duration of fault as 0.5 sec, then Vstep for 70 Kg and 50 Kg weight will be 359 V and 265 V and Vtouch voltage will be 258 V and 190 V respectively
  • Mitigation measures for reducing shock hazards
  1. The contact resistance of humans feet with the earth can be increased by placing high resistivity surface material like crushed stone under the feet.
  2. Additionally , gradient control loop which is zinc ribbon is placed between grade and pipeline and connected to the pipeline. This raises the potential of the earth and minimizes potential hazards to the person standing on the above ground installation
  3. It is safer to avoid working on pipeline installations near HT line during rains and cloudy weather when probability of
  4. lightening is more.
Placing Zinc ground mat connected to pipe
Electrostatic (Capacitive) Coupling
  • Electrostatic or capacitive coupling is due to capacitance between power line and pipeline and between pipeline and earth.
  • The induced voltage and current is not large enough to cause shock hazard to humans as capacitance is very small and so capacitive reactance is very large.
  • As pipeline is lying on wooden skids on earth, the electrostatic charge can be grounded through a pipe to earth connection.
  • As pipe is welded and further laid to ditch, it comes in close contact with soil and very little electrostatic charge remains on the pipeline
Capacitive coupling

Electromagnetic (Inductive) Coupling
  • Electromagnetic induction is caused by time-varying magnetic field created by flow of AC current in the power line .
  • When a pipeline runs parallel to the power line of 66 KV and above then due to varying magnetic field voltage and current are induced in the pipeline which is proportional to magnitude of AC current in the conductor and inversely proportional to separation distance between power line conductor and pipeline. As the length of parallelism between pipeline and power line increases, electromagnetic coupling improves.
  • Due to electromagnetic coupling both pipeline integrity and personal safety are affected.
  • The maximum allowable induced AC voltage to which a person should be exposed is 15 V.
  • The voltage peaks at entry and exit of power line corridor
Process of induced AC on buried pipeline

AC Corrosion
 
     •         There is a relationship between AC current density and AC corrosion :
     •        i ac < 20 A/m²                                                                No corrosion
     •        20 A/m² < i ac < 100 A/m²                                            Corrosion unpredictable
     •        i ac > 100 A/m²                                                               Corrosion expected
     •         AC corrosion is most likely to occur at small holiday sizes and corrosion rates increase with decreasing holiday sizes. At holiday size                   of less than 1 cm² , AC corrosion is not indicated.
     •         AC corrosion is most likely to occur in new pipeline with good coating and not visible in old pipeline with poor coating
     •         The AC current density is given by formula : AC current density is a function of induced AC voltage, soil resistivity and holiday                               diameter.
 
AC voltages required to produce 100 A/m²for variety of Holiday sizes & soil Resistivity
At 1 cm² holiday size and soil resistivity of 1000 ?−cm, 5 V is sufficient to cause corrosion
Assessment of AC corrosion
Images of AC corrosion
 
Predictions & Mitigation measures
  • AC current density is calculated at the interference location by measuring soil resistivity, AC voltage and assuming holiday size of 1 cm².
  • If AC current density is more than 20 A/m² then protection measures should be taken.
  • AC voltage should be recorded for 24 hours to measure peak voltage.
  • Safe distance between tower and pipeline should be maintained, at least more than 3 meter.
  • The pipeline is grounded through solid state decoupling device or polarization cell of suitable rating using zinc ribbon anode or pre-packaged zinc anodes to limit induced AC voltage to safe voltage for safety of person and integrity of pipeline.
  • Decoupling device blocks DC current to maintain cathodic protection while passes induced AC current to ground. SSD also protects against fault current or lightening by immediately grounding the fault current
  • The resistance of grounding anode should not be more than 5 ?.
Polarization cell

 

Solid state decoupling device
 
  • Technical specification of SSDC device
  • Threshold Voltage ± 2 V
  • Steady state current rating 45 A
  • Lightening surge current 100 KA at 4 x 10 μseconds
  • Fault current at 50 Hz cycle, 5000 A for 5 KA capacity at 30 cycle
Zinc ribbon anode installation

Telluric current interference
  • Telluric currents are the result of changes in the geomagnetic field on the earth due to sunspot activity or interaction of electromagnetic radiation from the sun with the earths magnetic field. In both cases, due to changes in the magnetic fields related to the pipeline as conductor produces a current that is called telluric current.

Interaction of solar particles with the earths magnetic field
Interference effects
  • The interference effects on pipeline due to telluric current will be potential swing on both negative and positive side depending upon solar activity and tidal waves causing changes in the earths magnetic field.
  • Where pipeline picks up telluric current, it will shift more electronegative and where it is discharged, it will shift in positive direction. Both over protection and under protection will occur causing either coating disbondment or corrosion.
  • The induced voltage will peak due to following factors:
  • Isolation joints
  • Change in direction
  • More pipeline length
  • As coating resistance increases
  • If pipeline is at sea coast
  • Pipeline is near earths magnetic pole
  • When soil has high resistivity

 

Telluric current effect on pipeline
The change in magnetic field is varying with time and accordingly its effects on pipeline, a combination of over protection and under protection.
 
Mitigation measures
  • The mitigation measures for telluric current is to provide safe low resistance path to earth without compromising level of cathodic protection.
  • Distributed sacrificial anode with pre-packaged material is installed to ground the telluric current or zinc grounding cell is installed at different locations to ground the current. Again the resistance of distributed anode should be low as far as possible.
  • Potential controlled TRU is also in operation to control the PSP between specified limit.
  • To eliminate the effect of IR drop due to telluric current, coupon with permanent reference cell or soil tube is installed to measure instant OFF potential without interrupting the TRU.

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