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CGL ENGINEERING CO., LTD. Mitigation of AC Induced Voltage On Buried Metallic Pipelines. Mr. Dusit Thongjun Managing Director CGL Engineering Co., Ltd. CGL ENGINEERING CO., LTD. Outline of Presentation. Causes of AC voltages on pipelines Problems associated AC interference

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slide1

CGL ENGINEERING CO., LTD.

Mitigation of AC Induced Voltage

On Buried Metallic Pipelines

Mr. DusitThongjun

Managing Director

CGL Engineering Co., Ltd.

outline of presentation

CGL ENGINEERING CO., LTD.

Outline of Presentation
  • Causes of AC voltages on pipelines
  • Problems associated AC interference
  • Mechanisms of AC induced voltages
  • Mitigation options
  • AC mitigation modeling
what is ac interference

CGL ENGINEERING CO., LTD.

What is AC Interference ?
  • Electrical currents and electromagnetic fields from the powerline in close proximity of a pipeline can produce AC voltages on pipelines
  • The magnitude and location of induced AC on a pipeline is a function of numerous conditions and is difficult to predict
  • The induced voltages can exist during normal or abnormal operation of the powerline
shared right of ways row

CGL ENGINEERING CO., LTD.

Shared Right of Ways (ROW)
  • Clear / unoccupied Right-of-Way's are difficult to obtain for new pipelines
  • An attractive option is for new pipelines to a share an existing ROW with an overhead electric power transmission systems
issues with induced ac on pipelines in shared row s

CGL ENGINEERING CO., LTD.

Issues with Induced AC on Pipelines in Shared ROW’s
  • Safety
    • Induced voltage can provide shock hazards to personnel safety
  • Pipeline Integrity
    • Coating damage
    • AC induced corrosion
personnel safety

CGL ENGINEERING CO., LTD.

Personnel Safety
  • Safety standards for personnel are based on Touch and Step potentials which can result in shock hazards.
    • Step potential is the voltage difference measured between two points on the earth separated by a distance of 1 pace (1 meter)
    • Touch potential is the voltage difference between a metallic structure and a point on the earth separated by a distance of the normal reach of human (1 m)
north american standards

CGL ENGINEERING CO., LTD.

North American Standards
  • NACE RP0177
    • Considers AC step potentials >15 volts to be a potential shock hazard. Voltages above that level require mitigation or evidence that a potential shock hazard does not exist.
  • ANSI / IEEE 80
    • Provides safety criteria related to heart fibrillation
    • Safety limits are inversely proportional to fault duration and directly proportional to surface resistivity
coating damage

CGL ENGINEERING CO., LTD.

Coating Damage
  • Fault currents from powerlines can be collected and discharged from pipelines at coating holidays
  • Coating damage can occur when voltage exceed the dielectric strength of the coating
  • Arching stresses may damage the pipeline
ac induced corrosion

CGL ENGINEERING CO., LTD.

AC Induced Corrosion
  • Recent finding indicate that AC current can cause 2% of the equivalent DC electrolysis problems on steel pipelines
  • AC induced corrosion is primarily a function of current density on steel structures. The general likelihood of AC induced corrosion is:
    • Current density < than 30A/m² : no or low
    • Current > than 30A/m² < 100 A/m² : medium
    • Current density > than 100A/m² : very high
ac interference mechanism

CGL ENGINEERING CO., LTD.

AC Interference Mechanism
  • Generation of AC induced voltages on a pipeline typically occur by one of the following mechanisms:
    • Capacitive coupling
    • Resistive coupling (electrolytic)
    • Inductive coupling
capacitive coupling

CGL ENGINEERING CO., LTD.

Capacitive Coupling
  • Caused by accumulation of electrostatic charge resulting in capacitive coupling between the powerline and a coated pipeline
    • Typically occurs during construction when coated and ungrounded sections of pipe are near a HV powerline
    • Unlikely on buried pipeline because the of the low pipe-to-earth capacitance
resistive coupling electrolytic

CGL ENGINEERING CO., LTD.

Resistive Coupling (Electrolytic)
  • Unbalance phase to phase or phase to ground faults of a powerline may cause current flow through the earth
  • Underground metallic structures in the vicinity may conduct some of the current
  • Typical occurs during abnormal operating conditions of the powerline
inductive coupling

CGL ENGINEERING CO., LTD.

Inductive Coupling
  • Current flow in the powerline creates an electromagnetic field surrounding the conductors
  • An AC voltage can be induced on a metallic structure positioned in the magnetic field
  • Occurs during normal operating conditions of the powerline
  • The induced potential on the affected pipeline can reach 100’s of volts and present shock hazards
  • Pipelines within 350m to a HV powerline should be investigated
factors contributing to ac interference

CGL ENGINEERING CO., LTD.

Factors Contributing to AC Interference
  • Soil resistivity values
  • Magnitude of steady state current in powerline
  • Separation distance and orientation
  • Powerline operating characteristics
  • Magnitude and duration of fault currents
  • Grounding characteristic
  • Pipeline coating type
ac interference mitigation

CGL ENGINEERING CO., LTD.

AC Interference Mitigation
  • Equipotential gradient control mats
    • Test stations
    • Main line valves
    • Meter stations
    • Casing vents
  • Parallel zinc ribbon grounding electrodes
  • Point grounding with sacrificial anodes
  • DC de-couplers devices
  • Dead front construction of test stations
  • Non-metallic casing vents
slide18

CGL ENGINEERING CO., LTD.

Equipotential Grounding Mat to Prevent Shock Hazards

Test Station

Ground Mat

Connected to Pipe

Water Pipeline

slide20

CGL ENGINEERING CO., LTD.

Distributed Galvanic Anodes (Point Grounding)

Overhead AC Transmission Line

Underground Pipeline

Distributed Sacrificial Anodes

Induced Voltage

Without Anodes

With Anodes

Distance

dc de coupling device

CGL ENGINEERING CO., LTD.

DC De-coupling Device
  • Conducts AC current & blocks DC current
  • AC current is dissipated to earth
  • Electronic Polarization Cell Replacements (PCR)

Power Cable Sheath

Insulating Flange

slide22

CGL ENGINEERING CO., LTD.

DoubleZincRibbon Installation

ac interference modeling

CGL ENGINEERING CO., LTD.

AC Interference Modeling
  • CDEGS Interference Analysis Software
    • Estimates interference due to inductive and conductive coupling
    • Calculates steady state voltage conditions
    • Define phase to ground fault condition
    • Predicts step and touch potentials
    • Determines high risk areas e.g. phase transpositions, powerline crossings of ROW
    • Recommends AC mitigation options
input for ac interference modeling

CGL ENGINEERING CO., LTD.

Input for AC Interference Modeling
  • Soil conditions
  • Pipeline characteristics
  • Pipeline and power system alignments
  • Power system characteristics
    • Operating voltage
    • Fault currents
    • Phase transpositions
    • Tower configurations
    • Static wire
    • Grounding design
    • Substation locations