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AC Mitigation Overview

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Presentation on theme: "AC Mitigation Overview"— Presentation transcript:

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:


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