AC Mitigation Overview

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