CGL ENGINEERING CO., LTD.
Mitigation of AC Induced Voltage
On Buried Metallic Pipelines
Mr. Dusit Thongjun
Managing Director
CGL Engineering Co., Ltd.
1
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
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
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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
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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
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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)
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Touch and Step Potentials
Touch Potential
Step Potential
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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
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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
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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
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Example of AC Induced Corrosion
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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
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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
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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
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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
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
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
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Equipotential Grounding Mat to
Prevent Shock Hazards
Test Station
Water Pipeline
Ground Mat
Connected to Pipe
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Zinc Grounding Mats
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Distributed Galvanic Anodes
(Point Grounding)
Overhead AC Transmission Line
Underground Pipeline
Distributed Sacrificial Anodes
Induced Voltage
Without Anodes
With Anodes
Distance
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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
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Double Zinc Ribbon Installation
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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
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
CGL ENGINEERING CO., LTD.
RIGHT OF WAY SCHEMATIC
5
Crossings not shown for clarity
4.5
4
AC Circuit 3 - 115KV
AC Circuit 2 - 230KV
AC Circuit 1 - 500KV
Cypress Pipeline
AC Circuit 7 - 500KV
AC Circuit 6 - 500KV
AC Circuit 5 - 115KV
AC Circuit 4 - 230KV
3.5
3
2.5
2
1.5
1
0.5
0
0
20
40
60
80
100
120
140
160
140
160
STEADY STATE PIPE TO EARTH VOLTAGE
120
No Mitigation
Single Ribbon Mitigation
Double Ribbon Mitigation
15-Volt Maximum
VOLTS
90
60
30
15-Volt Maximum
0
0
20
40
60
80
100
CONSTRUCTION MILEPOST
120
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Touch Potentials Predictions at Valve
Station Under Fault Conditions
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Thank You
Can I answer any questions?