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AC Interference

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

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AC Interference

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  1. 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 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 0 0 0 Half Cycle Single Phase Peak of negative side of cycle

  4. Basic Electric - AC 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 0 0 0 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

  7. 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

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

  9. 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

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

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

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

  13. 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

  14. 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

  15. 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)

  16. Safety I (Fault Current) f 10 kV Ouch!!! Potential Touch voltage = 2kV 9 kV 8 kV 7 kV

  17. Safety I (Fault Current) f Ouch!!! 10 kV Potential Step voltage = 1kV 9 kV 8 kV 7 kV

  18. 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)

  19. Safety – Step and Touch Voltage Calculation • Maximum voltage that human body can tolerate by touch or step – • Step formula - • VStep = (1000 + 6) 0.157/  ts( for a 70 kg body) • VStep = (1000 + 6) 0.116/ ts ( for a 50 kg body) • Touch formula - • VTouch = (1000 + 1.5) 0.157/ ts( for a 70 kg body) • VTouch = (1000 + 1.5) 0.116/ ts( for a 50 kg body)

  20. 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?

  21. Safety – Step and Touch Voltage Calculation • VTouch = (1000 + 1.5) 0.116/ ts - ( for a 50 kg body) • VTouch = (1000 + 1.5 • 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

  22. Safety I (Fault Current) f Cool !!! 10 kV Potential Step voltage = 0 kV Voltage gradient Mat = 10 kV 10 kV 10 kV 7 kV

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

  24. Zinc Grounding Mat 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

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

  26. 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

  27. 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

  28. 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).

  29. Safety -(Calculation for Arcing) If •  ( = < 100-m) • R(m) = 0.08 • Use this formula with lower resistivity • R(m) = 0.047 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

  30. 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)

  31. Lightening Pipeline 90 Meters Fault Currents

  32. Lightening 90 Meters Pipeline Fault Currents Zinc Ribbon

  33. 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

  34. Lightening Fault Currents Pipeline

  35. Lightening Fault Currents Zinc Pipeline Zinc Ribbon

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

  37. 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 - 100 A/m²) = Corrosion is unpredictable • (< 20 A/m²) = Corrosion will not result

  38. AC Corrosion on Pipelines • There has been documented cases of pipe to soil potentials being above -1.170VCSE 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..

  39. 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

  40. 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 5Vac in a soil resistivity of 10 Ohms m (1000 ohms CM) • = 127 amp/m² ((( Corrosive))) • But below the 15 Vac

  41. Documented cases of AC Corrosion Found - Pipe to soil potential readings were above –1.0v CSE DC Pipe met DOT criteria for CP – above .850- V CSE

  42. Documented cases of AC Corrosion Found -

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

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

  45. 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

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

  47. 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

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