1. Columbia Gas of Ohio/Kentucky Tim Jenkins Corrosion Front Line Leader
AC Interference
Columbia Gas of Ohio/Kentucky
Tim Jenkins
Corrosion Front Line Leader

2. Objectives
Develop a basic understanding of the principles and components of AC
Develop an understanding of the different types and effects of AC Influence
Develop methods of mitigation
Understand safety protocols
Cover AC calculations

3. Basic Electric - AC SINE WAVE Single Phase
Peak of positive side of cycle.
SINE WAVE
AC - alternating current will reverse in polarity 120 times per second. A full cycle is considered one hertz. Typical AC has 60 hz per second.
Half Cycle
You’re typical AC sine wave will switch polarity 120 times a second
Half Cycle
Single Phase
Peak of negative side of cycle

4. Basic Electric - AC SINE WAVE Three Phase
Peak of positive side of cycle.
SINE WAVE
AC – Three Phase, each conductor has the same amount of current and are 120 degrees out of phase
Half Cycle
Half Cycle
Three Phase
Peak of negative side of cycle

5. Fault Currents
If any of the AC waveforms are to get of frequency with each other greater than or less than 120°, then a possible fault current can occur.
Fault currents are large magnitude of current that can occur in brief amount of time (normally in milliseconds, typically .1 second)
Normally electrical towers or structure has grounding and protection devices for this situation that limits the fault current to a brief amount of time

6. Fault Currents
It’s not possible to know when, how or where fault currents will occur, in which makes it difficult to predict the effects of the fault and the mitigation required to protect both the pipeline and personnel
Need to calculate locations of more acceptable for fault currents to occur, such as –
Electrical storms, ice storms, & high winds
Distance from the Power lines
Information provided from the electric company

8. Fault Currents
Even though the fault current is brief, it still presents a danger to personnel and the pipeline
Coating damage can occur
Pipeline failure due to melting or cracking of the pipeline wall
Discuss more in Conductive coupling

9. Three Phase – Three conductors Shielded wires
Counterpoise Lines – Used for the grounding system, normally buried and above connected to each tower to provide grounding

10. Method of measuring AC voltage on Structures
Connect to ground with one lead and measure the AC volts onto the structure with the other lead.
Use an accurate volt meter, set meter on AC volts
Use rubber gloves during measurement and/or
Use a rubber mat for added insulation
High dielectric boots are available as well
Common method, use a copper-copper sulfate half cell with the meter set at AC volts
Must have good soil contact with half cell

11. Effects of AC Influence
Two key factors to consider with AC Influence
Safety
Corrosion

12. Effects of AC Influence
Two key factors to consider with AC Influence
Safety
Corrosion

13. Safety Electrical Shocks Arcing Ignition of volatile liquids
Step voltages
Touch voltages
Arcing
Ignition of volatile liquids

14. Safety Maximum allowable AC voltage = 15 Vac
Based on a typical individual is at 1000 ohms body resistance
And the individual can tolerate up to 15 mA
Ohms law = 15 volts
Anything above 15 Vac, could cause muscular contractions
Prevents the person from letting go

15. Safety Electrical Shock, such as fault currents
Can occur by physical contact or standing in the vicinity of an energize structure in contact with earth
Short time frames of electrical shocks are a concern when currents are above 50mA or greater
Can cause ventricular fibrillation
Certainly occurs at body currents of greater than 100 mA
Death will occur unless De-fibrillation is given

16. Safety Electrical Shock
Fault currents - passes from the structure to ground creating a voltage gradient
Step Voltage –
Is the potential difference between two points on earth’s surface separation by a distance of 1 pace (approx. 1 meter) in the direction of max. potential gradient
Touch Voltage –
Potential difference between the grounded metallic structure and the point of earth’s surface separated by a distance equal to the normal maximum horizontal reach (approx. 1 meter)

17. Safety I (Fault Current) Ouch!!! Potential Touch voltage = 2kV 9 kV
The potential difference of the structure and the point on earth
9 kV
8 kV
7 kV

18. Safety I (Fault Current) Ouch!!! Potential Step voltage = 1kV 9 kV
The potential difference between the two points on the earth where the person is standing.
9 kV
8 kV
7 kV

19. Safety – (Maximum Current Calculation)
Maximum current IB a human body can tolerate depends on shock duration ts (seconds) and body weight
calculated as follows:
IB = 0.157/  ts ( for a 70 kg body)
IB = 0.116/  ts ( for a 50 kg body)

20. Safety – Step and Touch Voltage Calculation
Maximum voltage that human body can tolerate by touch or step –
Step formula -
VStep = ( ) 0.157/  ts ( for a 70 kg body)
VStep = ( ) 0.116/  ts ( for a 50 kg body)
Touch formula -
VTouch = ( ) 0.157/  ts ( for a 70 kg body)
VTouch = ( ) 0.116/  ts ( for a 50 kg body)
Resistance is considering the human body resistance and the contact resistance of the feet and earth (which depends on soil resistivity (m)

21. Safety – Step and Touch Voltage Calculation
Pipe line running parallel to a power line may exhibit 500 volts for a duration of ½ second during line to ground fault
What is the tolerable touch voltage for a 50 kg individual with a soil resistivity of 50 ohms m touching the structure during the fault?

22. Safety – Step and Touch Voltage Calculation
VTouch = ( ) 0.116/  ts - ( for a 50 kg body)
VTouch = ( • 50) 0.116/  (.5) = 176 VAC
Since the possible fault voltage is 500 V then we need to raise the soil resistance
Try 3000 Ω-m of crush stone added to the site
Now the calculation equals to 902 VAC
Which exceeds the maximum pipe to earth voltage of 500 VAC, the pipe is now safe
Voltage gradient mats could provide a higher earth voltage to decrease the potential difference between the hand or feet touching the pipe

23. Safety I (Fault Current) Cool !!! Potential Step voltage = 0 kV 10 kV
Voltage gradient Mat = 10 kV
The potential difference between the two points on the earth where the person is standing.
10 kV
10 kV
7 kV

24. Gradient control mats – Placed at all test station locations in the AC Corridor

26. Zinc Grounding Mat Cut hole for Test station Dimensions = 4’x4’
Wire connected to the zinc ribbon
Cut hole for Test station
Dimensions = 4’x4’
12” crushed gravel
Zinc ribbon
6” Low resistance material – Coke breeze or benonite
6” Low resistance material – Coke breeze or benonite
Note : You can use the native soil, providing soil has good moisture content

27. Zinc Grounding Mat Connected to pipeline in Test station box
Wire connected to the zinc ribbon
Connected to pipeline in Test station box
Copper rods installed to get low resistance with grounding mat

28. Safety (Calculation for Arcing)
One of the greatest concern in dealing with fault currents between a power line structure and the pipeline is whether or not there is enough energy available to create an electric arc through the soil.
Could result in pipeline damage

29. Safety (Calculation for Arcing)
Greatest prevention of Arcing with fault currents is to maintain safe distance between power lines and the pipeline
One must obtain information from the electric company or producer such as
fault currents maximum measurements
Need to find soil resistivity in area
Perform sufficient amount of testing samples in order to accurate obtain average

30. Safety - (Calculation for Arcing)
One Safe distance calculation by Sunde - for prevention of arcing
Distance r (m) over which an arc could occur, based on soil resistivity  in (Ω-m) and fault magnitude If (kA).

31. Safety - (Calculation for Arcing)
R(m) = 0.08
Use this formula with lower resistivity
R(m) = If •  ( = > 1000-m)
Use this formula with extremely high resistivity
R(m) = Distance measured in meters
If = Magnitude of fault current
 = Soil resistivity measured in meters
If •  ( = < 100-m)

32. Safety - (Calculation for Arcing)
For an example,
Soil  = 6700 ohms-cm = 67 ohms -m
Fault Current If =17.9 kA
Use formula
R(m) = 0.08
R(m) = 87.6 meters
If •  ( = < 100-m)

33. Lightening
Pipeline
90 Meters
Fault Currents

34. Lightening
90 Meters
Pipeline
Fault Currents
Zinc Ribbon

35. Safety - (Calculation for Arcing)
If safe distance can not be obtain,
Screening electrodes between the pipeline and towers maybe used to intercept the fault currents
Such as zinc ribbon, or banks of sacrificial anodes

36. Lightening
Fault Currents
Pipeline

37. Lightening
Fault Currents
Zinc
Pipeline
Zinc Ribbon

38. Effects of AC Influence
Two key factors to consider with AC Influence
Safety
Corrosion

39. AC Corrosion on Pipelines
AC influence can cause corrosion to take place on coated steel pipe line
Study performed in Germany, recently in the 1990’s, had determined that corrosion occurs at specific AC current density -
(>100 A/m²) = Corrosion will result
(20 A/m² A/m²) = Corrosion is unpredictable
(< 20 A/m²) = Corrosion will not result

40. AC Corrosion on Pipelines
There has been documented cases of pipe to soil potentials being above VCSE with pH samples at 11, indicating pipe being cathodically protected, but corrosion was found due to AC current density in the range of 800 A/m²
Pipe must be mitigated by dropping the AC voltage with the use of grounding devices such as zinc ribbon, copper wire, etc..

41. AC Calculation for Current Density
Calculation to determine AC current density -
Iac = 8◦Vac/ ••d
Iac = Current density
 = soil resistivity in meters
d = holiday area in cm’s

42. AC Calculation for Current Density
Calculation to determine AC current density -
Iac = 8Vac/ ••d
Resistance and area of holiday will be the key factors in determining the AC current density
For an example – 1cm² holiday found with 5 Vac in a soil resistivity of 10 Ohms m (1000 ohms CM)
= 127 amp/m² ((( Corrosive)))
But below the 15 Vac

43. Documented cases of AC Corrosion Found -
Pipe to soil potential readings were above –1.0v CSE DC
Pipe met DOT criteria for CP – above V CSE

44. Documented cases of AC Corrosion Found -

46. AC Stray Current – Interference Methods
Electromagnetic Coupling –
Inductive
Electrostatic Coupling –
Capacitive
Conductive Coupling -
Direct path

47. AC Stray Current – Interference Methods
Electromagnetic Coupling –
Inductive
Electrostatic Coupling –
Capacitive
Conductive Coupling
Direct path

48. Electromagnetic Coupling – Inductive
Works in the same capacity of a inductive pipeline locator –
Induces an audio signal onto the buried pipeline
Or in the same capacity of a transformer
Primary coils inducing current by a electromagnetic field to the secondary windings

49. Electromagnetic Coupling - Inductive
Primary characteristics include:
Medium to High Voltages
High induced current levels

50. Electromagnetic Coupling - Inductive
The level of interference decreases with increasing separation of conductors
The strength of the magnetic flux is in direct proportional to the current magnitude and inversely proportional to the distance of the conductor
Induction effects experienced during power line faults can be a hazard to personnel
Normally peaks at the point of entry of AC corridor and at the point of exit

51. Electromagnetic Coupling - Inductive (Remediation)
Installation of a low resistance grounding systems to reduce current and voltage levels
Grounding mats for test stations (safety)
Zinc ribbon
Copper wire with the use of PCR or ISP
Achieve at least a 25 ohm impedance system
Ideally one ohm system
Normally deeper is better

52. AC Stray Current – Interference Methods
Electromagnetic Coupling –
Inductive
Electrostatic Coupling –
Capacitive
Conductive Coupling
Direct path

53. Electrostatic Coupling – Capacitive
Any two conductors separated by a dielectric material (insulator) is considered a capacitor
Electrical Field Gradient between the transmission line and conductor takes place, builds up a electrical charge on the structure
Such as a capacitor function
Primary characteristics include :
Very High Voltage peaks on power lines

54. Electrostatic Coupling – Capacitive
Conductors acceptable to capacitive coupling
Pipelines suspended above ground on skids
Any above ground equipment isolated such as vehicles or backhoes with rubber tires
Electrostatic Coupling does not penetrate the earth
Long parallel exposure of buried metallic structures to power lines

55. Electrostatic Coupling – Capacitive
Electromagnetic charge (
Voltage Gradient – electrom-agnetic field
VAC
VAC
The voltage builds up until it has path to ground to discharge
Ground

56. Electrostatic Coupling – Capacitive
Circuit is open, the voltage charge will build to high voltage static capacity, until a ground source is provided.
Electromagnetic charge (
Voltage Gradient – electrom-agnetic field
VAC
VAC
The voltage builds up until it has path to ground to discharge
Ground

57. Electrostatic Coupling – Capacitive
Direct Path to ground –
You……
By touching the structure and ground at the same time.
Electromagnetic charge
Voltage Gradient – electrom-agnetic field (
VAC
VAC
The voltage discharges
Ground

58. Pipe suspended
500 VAC

59. Electrostatic Coupling – Capacitive (Remediation)
Temporary repair –
Ground vehicles and equipment
Use temporary grounding rods (copper rods) normally in 3 meters in length
Use #2 Cable
Use ½ in diameter rods in normal soils
Refuel away of influence area to prevent accidental ignition, bond to refueling tanks
Due to high resistance soil, you need to place multiple rods, space about 6 feet apart
Metal chains dragging from the vehicle’s bumper in High AC voltage corridors are commonly used

60. Pipe is being grounded by making contact to the soil

61. Electrostatic Interference – Capacitive (Remediation)
Permanent repair –
Above ground pipelines or valves
Install Zinc ribbon
Install Zinc Grounding or voltage gradient Mats
Grounding Rods
3 Meters in length
Design cable size based on potential fault currents
Set depth until achieved a minimum of 25 ohms impedance
Lower the impedance as low as possible
One ohm is desirable

62. Electrostatic Interference – Capacitive (Remediation)
In most cases, the Electrostatic charge can not generate enough body current to create a shock hazard, more of a nuisance shock similar to static electricity.

63. AC Stray Current – Interference Methods
Electromagnetic Coupling –
Inductive
Electrostatic Coupling –
Capacitive
Conductive Coupling
Direct path

64. Conductive Coupling
Occurs when line to ground shorts or faults take place
On High Voltage power lines faults normally occur during lighting strikes
Fault currents can occur by accidental contact with other structures
Such as construction equipment or cranes
Fault currents is conducted to the pipeline through its coating
Higher the coating dielectric strength, the less amount of the transfer current on the pipeline

65. Conductive Coupling Occurs in milliseconds
Voltage and current is higher than steady state but happens very briefly
.1 or a tenth of a second is the normal time frame that voltage is present due to the fault protection system

66. Conductive Coupling Failure to the pipeline Coating damage
Cracking and melting of the pipe wall

67. Materials Used for AC Mitigation
Two reasons for mitigation of AC influence
Prevent corrosion on the pipeline
Prevent of hazardous shock from contacting the pipeline
Materials commonly used
Zinc grounding and/or voltage gradient mats
Zinc ribbon or heavy gauge copper wiring
Blind face test stations
Galvanic anodes
PRC or Inductive capacitive coupling

68. Materials Used for AC Mitigation
Zinc Ribbon – to mitigate the AC currents from the pipeline to the soil
Zinc is used in some low resistivity areas as a galvanic anode to protect structures
The AC currents will take the path of less resistance to the ground
Zinc provides this path
Depending of soil resistance, distance to the tower, the location of structure to the towers and the amount of magnitude influence of the towers must be calculated in the design of the amount of Zinc ribbon needed and the location

69. Materials Used for AC Mitigation
Programs available to profile the pipeline for AC mitigation
PRCI

70. Materials Used for AC Mitigation
Zinc Ribbon –
Installation –
Placed below the pipeline
Depending on soil resistance
Place in the lowest resistance area
Minimum Two feet away from the pipeline
Make connection to the pipeline in a junction box or test station
Commonly used, a minimum of a no. 4 gauge wire connected to the pipeline and zinc ribbon
May need to increase size of cable due to greater magnitude of fault currents

71. Materials Used for AC Mitigation
Zinc Ribbon –
Installation –
Placed between the pipeline and tower
To mitigate fault currents and prevent coating damage
Splice zinc ribbon by striping the zinc off the wire and crimp the connections together
Make crimp repair with epoxy resin kits, heat shrink sleeve, or electrical rubber tape

73. Chart for Zinc Ribbon

74. Standard – ½ inch comes in wooden spools
Ribbon is bonded to the main in the test station to be able to test AC mitigation such as AC current density & grounding system resistance

75. Zinc ribbon is placed below the pipeline and at least two feet away

76. Installed at test station facility –
Coupon for AC measurements
Grounding mat or voltage gradient mat for test point reader safety
Zinc ribbon connection
Structure connections

78. What's wrong with the next slide?

79. Zinc ribbon
Slide “B”
Pipe line
Tower
Zinc ribbon
Slide “A”

80. Zinc Ribbon is on the wrong side of the pipeline
Tower
Zinc ribbon
Zinc ribbon is above the pipeline

81. Polarization cells or Insulated surge protections are great for grounding the pipeline or structure with out shorting out the DC cathodic protection currents. It will block the DC and allow the AC currents flow to ground.

82. Dead Front Test Stations
To prevent electrical shock in making contact with wire connection to mainline

83. Any Questions???
Thank YOU!!!!