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AC Mitigation Overview. Mike Tachick Dairyland Electrical Industries, Inc. Mitigation requirements. Connection to low impedance grounding DC isolation of CP system Addressing lightning, AC faults Complying with codes. AC Power Effects. Steady-state induction Can be measured

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AC Mitigation Overview


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    1. AC Mitigation Overview Mike Tachick Dairyland Electrical Industries, Inc.

    2. Mitigation requirements • Connection to low impedance grounding • DC isolation of CP system • Addressing lightning, AC faults • Complying with codes

    3. AC Power Effects • Steady-state induction • Can be measured • Fault current/voltage • Usually estimated/modeled • Conductors, grounds sized for estimated fault current

    4. Background: AC and Lightning Compared Amplitude Time (milliseconds) Time (microseconds) Alternating Current Lightning

    5. AC Voltage Mitigation • Create a low impedance AC path to ground • Have no detrimental effect on the CP system • Provide safety during abnormal conditions (AC fault, lightning)

    6. Factors Affecting Mitigation Key Factors… • Soil resistivities, layers • Proximity to power lines • Power line loading • Quality of pipeline coating …and many others

    7. AC Mitigation Design • Best performed by consulting engineers using specialized software • Evaluates coating stress, step and touch potentials, fault current, steady-state current, grounding arrangements, etc. • Typical for larger projects, new construction

    8. AC Mitigation Design • CP personnel can roughly estimate or experiment • Practical for small projects, single locations • Trade-off: optimizing design vs. efficiently installing estimated requirements

    9. AC Mitigation Design • Induced voltage is ongoing nuisance • AC faults are significant risk • Design should consider lightning hazards • Control step and touch potentials

    10. Step Potential

    11. Touch Potential

    12. Typical Field Values • Open circuit voltage: up to 50V • Short circuit current: up to 30A Can exceed these values

    13. Field Measurements • Open circuit voltage: voltmeter from pipe to grounding system • Short circuit current: AC ammeter – reads current in a bond between pipe and ground

    14. Mitigation Example Problem: • Open-circuit induced AC on a pipeline = 40 V • Short-circuit current = 15 A • Then, source impedance:R(source) = 40/15 = 2.7 ohms Solution: • Connect pipeline to ground through decoupler

    15. Mitigation Example • Typical decoupler impedance:X = 0.01 ohms0.01 ohms << 2.7 ohm source 15A shorted = 15A with decoupler • V(pipeline-to-ground) = I . X = 0.15 volts • Result: Induced AC on pipeline reduced from 40 V to 0.15 V

    16. Local Mitigation • Reduces pipeline potentials at a specific point (typ. accessible locations) • Commonly uses existing grounding systems • Needs decoupling

    17. High Voltage on Pipeline Low Local Mitigation

    18. Continuous Mitigation • Reduces pipeline potentials at all locations • Provides fairly uniform over-voltage protection • Typically requires design by specialists

    19. Continuous Mitigation Gradient control wire choices: • Zinc ribbon • Copper wire • Not tower foundations!

    20. Mitigation Wire Installation

    21. Reasons to DC Decouple From Grounding Systems If not decoupled, then: • CP system attempts to protect grounding system • CP coverage area reduced • CP current requirements increased • CP voltage may not be adequate But simultaneous AC grounding is also needed…

    22. Decoupler Characteristics • High impedance to DC current • Low impedance to AC current • Passes induced AC current • Rated for lightning and AC fault current • Fail-safe construction • Third-party listed to meet electrical codes

    23. Typical Decoupler Ratings • Threshold: 2-3V peak • AC impedance: 0.01Ω • DC leakage: <<1mA @ 1V • AC fault: 2 to 10kA • Lightning: 75-100kA

    24. Typical Decouplers

    25. Typical Decouplers

    26. Zinc Ribbon for Mitigation • Provides CP if not isolated • Affects CP if impressed current system is used • Doesn’t allow instant-off readings when not isolated • Grounding effectiveness changes with zinc consumption • Can pick up stray currents

    27. Copper for Mitigation • Common, low cost material • Must be decoupled with suitable device

    28. Grounding Materials If decoupled… • CP system not affected • Allows instant-off readings • Lengthens grounding system life • Avoids stray current pick up

    29. Typical Mitigation Site

    30. Typical Mitigation Site

    31. Typical Mitigation Site

    32. Typical Mitigation Site

    33. Typical Mitigation Site

    34. Other Grounds • Casings at road crossings • Station grounding system • Existing metallic vault • Abandoned pipeline • Culvert • Other metallic structure with low resistance to earth

    35. Mitigation - Other Issues • Risks at insulated joints • Other affected facilities • Hazardous locations

    36. Mitigating at an Insulated Joint

    37. Mitigating at an Insulated Joint • Provides AC mitigation for pipeline • Provides over-voltage protection for insulated joint • Easy location to test, install

    38. Other Affected Facilities • Transfer of AC fault conditions to other structures • Goal is to keep voltage from rising, control current flow • Doesn’t mean that current will not get on your pipeline or others • Accomplished by bonding, grounding

    39. Example site Metering Station Road Casing I J Power line Substation Pipeline I J

    40. Example site Metering Station Road Casing I J Power line Substation Pipeline I J

    41. Hazardous Locations • Accomplish mitigation while complying with codes • Determine your site classification • Use certified (listed) products and methods • Keep conductors short to limit over-voltage, possible arcing, due to lightning

    42. Hazardous Locations • CFR 192.467 – combustible atmospheres and insulated joints • CFR 192 incorporates the National Electrical Code “by reference” • NEC defines hazardous locations and product requirements

    43. Questions? • For follow up questions: mike@dairyland.com