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1
Mr. Dusit ThongjunManaging Director
CGL Engineering Co., Ltd.
CGL ENGINEERING CO., LTD.
Mitigation of AC Induced VoltageMitigation of AC Induced VoltageOn Buried Metallic PipelinesOn Buried Metallic Pipelines
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Outline of Presentation
Causes of AC voltages on pipelines
Problems associated AC interference
Mechanisms of AC induced voltages
Mitigation options
AC mitigation modeling
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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 damageAC 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 StandardsNACE 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 80Provides safety criteria related to heart
fibrillationSafety limits are inversely proportional to
fault duration and directly proportional to surface resistivity
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Coating DamageFault 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 CorrosionRecent 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 CGL ENGINEERING CO., LTD.
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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 CouplingCurrent 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
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Factors Contributing to AC Interference
Soil resistivity valuesMagnitude of steady state current in powerline Separation distance and orientation Powerline operating characteristicsMagnitude and duration of fault currents Grounding characteristic Pipeline coating type
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AC Interference MitigationEquipotential gradient control mats
Test stationsMain line valvesMeter stationsCasing vents
Parallel zinc ribbon grounding electrodes Point grounding with sacrificial anodesDC de-couplers devicesDead front construction of test stationsNon-metallic casing vents
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Ground Mat Connected to Pipe
Test Station
Water Pipeline
Equipotential Grounding Mat to Prevent Shock Hazards
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Zinc Grounding MatsCGL ENGINEERING CO., LTD.
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Distributed Galvanic Anodes (Point Grounding)
Overhead AC Transmission Line
Underground Pipeline
Distributed Sacrificial Anodes
Induced VoltageWithout Anodes
With Anodes
Distance
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DC De-coupling DeviceConducts AC current & blocks
DC currentAC current is dissipated to
earth Electronic Polarization Cell
Replacements (PCR)
Power Cable Sheath
Insulating Flange
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Double Zinc Ribbon Installation CGL ENGINEERING CO., LTD.
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AC Interference Modeling
CDEGS Interference Analysis Software
Estimates interference due to inductive and conductive coupling
Calculates steady state voltage conditionsDefine phase to ground fault conditionPredicts step and touch potentials Determines high risk areas e.g. phase
transpositions, powerline crossings of ROWRecommends AC mitigation options
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Input for AC Interference ModelingSoil conditionsPipeline characteristicsPipeline and power system
alignmentsPower system characteristics
Operating voltageFault currentsPhase transpositionsTower configurationsStatic wireGrounding designSubstation locations
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STEADY STATE PIPE TO EARTH VOLTAGE
15-Volt Maximum
0
30
60
90
120
0 20 40 60 80 100 120 140 160
CONSTRUCTION MILEPOST
VO
LT
S
No MitigationSingle Ribbon MitigationDouble Ribbon Mitigation15-Volt Maximum
RIGHT OF WAY SCHEMATIC
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
0 20 40 60 80 100 120 140 160
AC Circuit 3 - 115KV AC Circuit 7 - 500KV
AC Circuit 2 - 230KV AC Circuit 6 - 500KVAC Circuit 1 - 500KV AC Circuit 5 - 115KV
Cypress Pipeline AC Circuit 4 - 230KVCrossings not shown for clarity
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Touch Potentials Predictions at Valve Station Under Fault Conditions
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Thank You
Can I answer any questions?
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