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 Fault current/voltage
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 ohms 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. Local Mitigation
High
Low
Voltage on Pipeline

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: 1V
AC fault: 2 to 10kA
Lightning: kA

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 Road Metering Station Casing I J Power line Substation
Pipeline
I J

40. Example site Road Metering Station 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 – 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: