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 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 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 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) CGL ENGINEERING CO., LTD. Touch and Step Potentials Touch Potential Step Potential 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 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 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 CGL ENGINEERING CO., LTD. Example of AC Induced Corrosion 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 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 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 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 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 CGL ENGINEERING CO., LTD. Equipotential Grounding Mat to Prevent Shock Hazards Test Station Water Pipeline Ground Mat Connected to Pipe CGL ENGINEERING CO., LTD. Zinc Grounding Mats 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 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 CGL ENGINEERING CO., LTD. Double Zinc Ribbon Installation 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 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 CGL ENGINEERING CO., LTD. Touch Potentials Predictions at Valve Station Under Fault Conditions CGL ENGINEERING CO., LTD. Thank You Can I answer any questions?