Ac mitigation overview l.jpg
This presentation is the property of its rightful owner.
Sponsored Links
1 / 43

AC Mitigation Overview PowerPoint PPT Presentation


  • 634 Views
  • Updated On :
  • Presentation posted in: General

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

Download Presentation

AC Mitigation Overview

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


Ac mitigation overview l.jpg

AC Mitigation Overview

Mike Tachick

Dairyland Electrical Industries, Inc.


Mitigation requirements l.jpg

Mitigation requirements

  • Connection to low impedance grounding

  • DC isolation of CP system

  • Addressing lightning, AC faults

  • Complying with codes


Ac power effects l.jpg

AC Power Effects

  • Steady-state induction

    • Can be measured

  • Fault current/voltage

    • Usually estimated/modeled

  • Conductors, grounds sized for estimated fault current


Background ac and lightning compared l.jpg

Background: AC and Lightning Compared

Amplitude

Time (milliseconds)

Time (microseconds)

Alternating Current

Lightning


Ac voltage mitigation l.jpg

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)


Factors affecting mitigation l.jpg

Factors Affecting Mitigation

Key Factors…

  • Soil resistivities, layers

  • Proximity to power lines

  • Power line loading

  • Quality of pipeline coating

    …and many others


Ac mitigation design l.jpg

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


Ac mitigation design8 l.jpg

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


Ac mitigation design9 l.jpg

AC Mitigation Design

  • Induced voltage is ongoing nuisance

  • AC faults are significant risk

  • Design should consider lightning hazards

  • Control step and touch potentials


Step potential l.jpg

Step Potential


Touch potential l.jpg

Touch Potential


Typical field values l.jpg

Typical Field Values

  • Open circuit voltage: up to 50V

  • Short circuit current: up to 30A

    Can exceed these values


Field measurements l.jpg

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


Mitigation example l.jpg

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


Mitigation example15 l.jpg

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


Local mitigation l.jpg

Local Mitigation

  • Reduces pipeline potentials at a specific point (typ. accessible locations)

  • Commonly uses existing grounding systems

  • Needs decoupling


Local mitigation17 l.jpg

High

Voltage on Pipeline

Low

Local Mitigation


Continuous mitigation l.jpg

Continuous Mitigation

  • Reduces pipeline potentials at all locations

  • Provides fairly uniform over-voltage protection

  • Typically requires design by specialists


Continuous mitigation19 l.jpg

Continuous Mitigation

Gradient control wire choices:

  • Zinc ribbon

  • Copper wire

  • Not tower foundations!


Mitigation wire installation l.jpg

Mitigation Wire Installation


Reasons to dc decouple from grounding systems l.jpg

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…


Decoupler characteristics l.jpg

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


Typical decoupler ratings l.jpg

Typical Decoupler Ratings

  • Threshold: 2-3V peak

  • AC impedance: 0.01Ω

  • DC leakage: <<1mA @ 1V

  • AC fault: 2 to 10kA

  • Lightning: 75-100kA


Typical decouplers l.jpg

Typical Decouplers


Typical decouplers25 l.jpg

Typical Decouplers


Zinc ribbon for mitigation l.jpg

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


Copper for mitigation l.jpg

Copper for Mitigation

  • Common, low cost material

  • Must be decoupled with suitable device


Grounding materials l.jpg

Grounding Materials

If decoupled…

  • CP system not affected

  • Allows instant-off readings

  • Lengthens grounding system life

  • Avoids stray current pick up


Typical mitigation site l.jpg

Typical Mitigation Site


Typical mitigation site30 l.jpg

Typical Mitigation Site


Typical mitigation site31 l.jpg

Typical Mitigation Site


Typical mitigation site32 l.jpg

Typical Mitigation Site


Typical mitigation site33 l.jpg

Typical Mitigation Site


Other grounds l.jpg

Other Grounds

  • Casings at road crossings

  • Station grounding system

  • Existing metallic vault

  • Abandoned pipeline

  • Culvert

  • Other metallic structure with low resistance to earth


Mitigation other issues l.jpg

Mitigation - Other Issues

  • Risks at insulated joints

  • Other affected facilities

  • Hazardous locations


Mitigating at an insulated joint l.jpg

Mitigating at an Insulated Joint


Mitigating at an insulated joint37 l.jpg

Mitigating at an Insulated Joint

  • Provides AC mitigation for pipeline

  • Provides over-voltage protection for insulated joint

  • Easy location to test, install


Other affected facilities l.jpg

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


Example site l.jpg

Example site

Metering Station

Road

Casing

I J

Power line

Substation

Pipeline

I J


Example site40 l.jpg

Example site

Metering Station

Road

Casing

I J

Power line

Substation

Pipeline

I J


Hazardous locations l.jpg

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


Hazardous locations42 l.jpg

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


Questions l.jpg

Questions?

  • For follow up questions:

    [email protected]


  • Login