2012 FEPA Presentation: Jeff Didas


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2012 FEPA Presentation: Jeff Didas

  1. 1. AC Interference and MitigationFlorida Energy Pipeline Association
  2. 2. Pipelines & HVAC Lines Collocated• Collocated Utilities  Pipelines and HVAC Power Lines often share the same right of way  There are problems that must be addressed when HVAC and buried pipelines share the same right of way
  3. 3. Enhanced Regulatory Scrutiny• Regulators are focusing more on this issue given recent risk findings by operators and enhanced pipeline safety regulations• NACE Standard RP0177 (Latest Revision) - Recommended Practice on Mitigation of Alternating Current & Lightning Effects on Metallic Structure and Corrosion Control Systems. Also, ANSI/IEEE Standard 80 specifies safety design criteria for determining maximum acceptable touch and step voltages during fault conditions.
  4. 4. Three Distinct Issues• Health and Safety of Personnel/Public/Livestock  Well known and easily fixed• AC Fault Currents  Short duration occurring at a particular tower location• AC Induced Corrosion  Not well understood but affects well coated pipe  Steady state condition  Can be quite damaging and intense
  5. 5. How do Pipelines and AC Interact• Electrostatic Coupling  Capacitive nuisance effect• Conductive Coupling  Fault Currents at tower footings• Inductive Coupling  Steady State Induced AC voltage buildup
  6. 6. Electrostatic Coupling Pipe and Power line create a circuit of two capacitors in series. A capacitor is a passive electronic component consisting of a pair of conductors separated by an insulator (air)
  7. 7. Electrostatic Coupling Can generate very high AC voltage levels – but there is not enough power to do much more than create a minor electrostatic shock. Generally a nuisance, however can be an issue and grounding may be required.
  8. 8. How do Pipelines and AC Interact• Electrostatic Coupling  Capacitive nuisance effect• Conductive Coupling  Fault Currents at tower footings• Inductive Coupling  Steady State Induced AC voltage buildup
  9. 9. Conductive Coupling AC Fault Conditions •Relatively rare •Short duration •Generally due to weather (lightning and high winds) •Can be structural failure Causes intense stressing of pipeline coating and possibly the pipeline wall
  10. 10. Conductive Couplings• Rare occurrences that can result in significant current discharging through the ground• Separation distance of the pipeline from the fault is critical• Soil resistivity is critical – note soil layering can affect current path• Requires arcing through the soil for a current path – not easy
  11. 11. How do Pipelines and AC Interact• Electrostatic Coupling  Capacitive nuisance effect• Conductive Coupling  Fault Currents at tower footings• Inductive Coupling  Steady State Induced AC voltage buildup
  12. 12. Electromagnetic Induction •A function of Line Current not Voltage •Power transferred is I1 •Proportional to line current •Proportional to parallelism φ •Inversely proportional to separation distance •Can result in high voltages on I2 long sections of pipeline even if the pipeline is grounded
  13. 13. Electromagnetic Induction • Current through the HVAC lines generate a Longitudinal Electric Field (LEF)A B C • The separation between the phase conductors has a significant effect on the LEF and increases with separation • Bundled buried conductors have no separation and provide only a minimal effect on pipelines
  14. 14. Electromagnetic Induction• The arrangement of A B A B phases on multiple C C circuit HVAC lines can have a large impact on Center Line Symmetric the LEF A B C B C A Center Point Symmetric
  15. 15. Electromagnetic Induction• If all characteristics are perfectly uniform along the pipeline/HVAC then there will be a zero voltage at the mid point 0 L and peaks where the L/2 HVAC and pipeline separate if the pipeline is electrically “short”
  16. 16. Electromagnetic Induction• If the length is electrically “long” it would look more like this… 0 L L/2
  17. 17. What are the effects of Coupling• Electrostatic Coupling  Capacitive nuisance effect• Conductive Coupling  Fault Currents at tower footings  Safety Concern  Stress Voltage• Inductive Coupling  Steady State Induced AC voltage buildup  Safety Concern  AC Induced Corrosion
  18. 18. Stress Voltage AC Fault Conditions •Relatively rare •Short duration •Rapid localized increase in voltage •Can cause significant coating damage •Could result in a direct arc from tower footing to pipeline heating the metal quickly (burning a hole)
  19. 19. Steady State AC Corrosion• Until recently not a concern for pipeline operators• Published studies pre-1990s discounted AC corrosion as a possibility• Regulators not focused on this as a risk until quite recently
  20. 20. German Experiments• Pipeline failures in Germany in the early 90s on well protected new pipelines puzzled investigators• Previously, AC corrosion rates were not considered a threat• Testing on coupons with 1 cm2 holidays in low resistivity soils found corrosion rates of 210 mpy on steel polarized to 1800-2000 mV cse
  21. 21. Morphology of AC CorrosionRound crater like corrosion withdeep pits typical of very activecorrosionMay have some false indications ofMicrobiologically Induced CorrosionOccurs in the presence of ACTransmission and in some casesDistribution linesLikely in lower soil resistivities
  22. 22. Optimum Coating Holiday• Testing has found that the optimum coating holiday size for high AC Corrosion rates is between 1-3 cm 2 coating holiday.• AC Current density is the key consideration  0-20 A/m2 no corrosion  20-100 A/m2 corrosion risk unpredictable  Above 100 A/m2 corrosion can be expected
  23. 23. Sample Calculation Sample calculation for a 1 cm2 holiday in 10 ohm- m soil Even at very low AC voltage levels this could yield corrosion rates in excess of 20 mpy even with good CP applied
  24. 24. AC Voltage vs. Soil ResistivityThis graph shows theholiday size and ACVoltage required toexceed the 100 A/m2“corrosion can beexpected” threshold atvarying soil resistivities.
  25. 25. AC Induced Corrosion• The higher the quality of the coating the greater the risk of AC induced corrosion• AC Induced corrosion with well coated pipelines can create significant and rapid corrosion even at low levels of induced AC Voltage even with good CP levels on pipeline• Corrosion mechanism still being researched but evidence is clear that it occurs
  26. 26. Safety Concerns• High voltage levels either from induction (steady state) or from fault conditions (rare and short duration) present a danger to personnel  15 VAC threshold is well established by NACE  Based on release threshold calculations  Gradient Mats are well established for a long time in the industry
  27. 27. Step and Touch PotentialDuring a fault condition or evensteady state AC Voltage presence on 10 kVthe pipeline can create a safetycondition at above ground structures(test stations, valves, etc…)The person touching the structure isexposed to 2 kV touch potential while 9 kV 8 kVthe man standing is exposed to 1 kV in 7 kVthis diagram
  28. 28. Gradient MatsCreatesequipotentialenvironment forpersonnel
  29. 29. AC Mitigation AC Mitigation typically involves installation of one or more grounding devices to allow AC current to readily discharge off of the pipeline thus minimizing coating stress during fault conditions and reducing the inductive voltage levels to well below any threshold for personnel safety or AC induced corrosion.
  30. 30. Key Issues 1. Step and Touch potentials at above ground appurtances (15 VAC NACE criteria) 2. Conductive coupling dumping excessive Fault Current onto pipeline causing damage 3. Induced Voltage discharging through smaller holidays on well coated pipelines causing AC induced corrosion
  31. 31. AC Modeling• Very complex mathematically to model  Numerous variables  Some very difficult to quantify  Requires input from utility on the operating conditions  Requires field data gathering  Different modeling software  PRCI, SES/CDEGS, ARC Engineering, Dabkowski, others…
  32. 32. Goals for AC Modeling• Calculating Fault Condition Stress Values• Calculating Induced Voltage at various points along the model• Evaluating Impact of Mitigation Measures  Where  How much  How effective
  33. 33. Shortcomings of Modeling• Modeling is only as good as the data being used• Modeling is only as good as the assumptions being made• Modeling has to focus on worst case conditions
  34. 34. What is being modeled• The power line  Peak loads, winter and summer  Max fault current (line to ground)  Shield wire data – type and geometry (mostly for fault analysis – only minor impact on steady state)  Phase wire data  Phase imbalance data  Tower data
  35. 35. What is being modeled• The pipeline  Pipe diameter  Wall thickness  Depth of cover  Coating resistance and thickness (generally a guess since it is not practical to measure this)  Centerline distance from towers
  36. 36. What is being modeled• The environment  Soil resistivity along the colocation  Soil resistivity at various depths (used in some of the more sophisticated modeling)  Foreign structures of note (multiple pipelines and multiple HVAC lines)
  37. 37. Typical Modeling
  38. 38. Stress Currents• The concern is elevated short duration coating stress. Different coatings have different coating stress limits• Computer modeling is very complex and requires numerous assumptions  Geometry  Soil Resistivity and layering  Transmission Fault data
  39. 39. Modeling of Mitigation
  40. 40. Modeling Results
  41. 41. Conductive Coupling Modeling• Dabkowski – Corrosion 2003 presented the following:
  42. 42. Risk Assessment w/o Modeling• Look for changes that will cause voltage spikes  Changes in the pipeline to HVAC distance from each other  Changes in the HVAC line (phase transpositions)  Changes in soil resistivity• Identify what your concerns might be  Stress voltages during fault conditions  Steady state AC induced corrosion
  43. 43. Risk Assessment w/o Modeling• Corrosion risk - Zero in on areas where voltage spikes can be anticipated and there is low soil resistivity.• Fault current risk – Zero in on areas with the least separation between tower footings and pipeline• Modeling may not be required
  44. 44. Field testing of LEF• It is possible to take AC voltage readings and to measure the induced longitudinal electrical field (LEF) by placing a calibrated insulated cable on the ground parallel to the pipeline, grounding it, and using an high impedance voltmeter to measure the open circuit potential• The value received reflects the operating conditions at the time
  45. 45. AC Mitigation Project
  46. 46. AC Test Coupons• Designed to replicate a 1 cm2 holiday• Can be used to determine the actual current density being picked up at the pipeline before applying mitigation and after installing mitigation• Uses the same coating and geometry as the pipe
  47. 47. PCR Installation
  48. 48. Copper Ground Wire Detail
  49. 49. Optimum AC Mitigation• Modeling is only as good as the model, the assumptions and the data being input• Gradient control line(s) parallel to the pipeline for new well coated pipelines are recommended to minimize AC Corrosion risk• Short lines at the tower footings are best for fault condition mitigation and can be used in conjunction with long gradient control lines
  50. 50. MATCOR’s MITIGATOR™What is the MITIGATOR™?Looks like the SPL™ LinearAnode.Not an anode but a coppergrounding cableSpecial backfill
  51. 51. InstallationInstallation of the MITIGATOR™ along aWilliams (Transco) Gas pipeline in NorthernNew Jersey.The pipeline is actually to the left of theMITIGATOR™ trench.The MITGATOR™ provides for easy installation,a much larger surface area for dischargingcopper, and the copper conductor is housed ina special backfill with corrosion inhibitors.
  52. 52. From the Plattline™ Website Life expectancy of Plattline in this application would be quite long and would generally be determined by Plattline as a projected cathodic protection system. The most common sizes of Plattline for AC mitigation are plus and standard.
  53. 53. Zinc Ribbon SIZE SURFACE AREA COST Standard 54.0 mm2 $2.50/ft. Plus 76.2 mm2 $5.00/ft. Super 114.3 mm2 $9.50/ft. MITIGATOR™ 119.6 mm2 $5.25/ft.
  54. 54. Concerns with Zinc• Zinc can passivate and should have a special backfill when used for AC Mitigation• Zinc is much more difficult to handle and install relative to the Mitigator™• Must use a torch to make connections• Requires more frequent use of decouplers• Will consume over time – not as long a life as copper
  55. 55. Areas for more investigation• Sophisticated modeling of Mitigator™ vs. Zinc• Investigation of “propagation constant” and the spacing of decouplers for zinc vs. Mitigator™
  56. 56. Summary of AC Interference• There are three key threats  Safety (15 V AC Threshold)  Fault Conditions (rare but potentially damaging)  AC Corrosion – for new well coated pipelines this can easily be the most challenging and difficult threat to control and can cause damage even at lower levels of AC
  57. 57. Summary of AC Interference• Modeling may not be fully effective – especially for AC Corrosion• AC Coupons give information based on current operating conditions – changes in electrical flow affect the AC Induced Voltage• Risk Assessment can often be performed without expensive modeling
  58. 58. Questions Questions?