PRESENTATION ON AC & DC INTERFERENCE By r choudhury Ns bhattacharyya 7th – 8th May’ 2015

INTERFERENCE CURRENT (STRAY CURRENT) STRAY CURRENT OR INTERFERENCE CURRENT CAN BE CLASSIFIED AS BEING STATIC OR DYNAMIC. STRAY CURRENT CAN BE EITHER AC CURRENT OR DC CURRENT DYNAMIC STRAY CURRENT: VARY WITH AMPLITUDE OR CHANGE WITH DIRECTION IN THE DIRECTION OF CURRENT FLOW -CAN BE MAN MADE OR NATURAL ORIGIN

DYNAMIC INTERFERENCE -- SOURCES OF DYNAMIC STRAY CURRENT: TRANSIT SYSTEMS MINING DC WELDING MACHINES ELECTRIC POWER TRANSMISSION INDUSTRIAL PLANTS (CHLORINE AND ALUMINIUM PRODUCTION FACILITIES) TELLURIC CURRENT DETECTING DYNAMIC STRAY CURRENT: BY STRUCTURE TO ELECTROLYTE POTENTIAL OR BY LINE CURRENT MEASUREMENT --

DC INTERFERENCE (DYNAMIC)

DC INTERFERENCE (DYNAMIC)

STATIC INTERFERENCE -- B) STATIC (STEADY STATE ) INTERFERENCE CURRENT: -MAINTAIN COSTANT MAGNITUDE AND CONSTANT GEOGRAPHICAL PATH -STATIC TYPE INTERFERENCE CAN BE TWO TYPES OR COMBINATION OF TWO TYPES -- CATHODIC INTERFERENCE :WHEN A VOLTAGE GRADIENT OVERLAPS FOREIGN STRUCTURE AND IS NEGATIVE WITH RESPECT TO REMOTE EARTH, IT PROMOTES CURRENT DISCHARGE FROM THE FOREIGN STRUCTURE IN THE AREA OF INFLUENCE -- ANODIC INTERFERENCE: IF A FOREIGN STRUCTURE CROSSES A VOLTAGE GRADIENT AND IS POSITIVE RESPECT TO REMOTE EARTH, IT PROMOTES CURRENT PICK UP ON THE FOREIGN STRUCTURE WITHIN THE AREA OF INFLUENCE. --

DC INTERFERENCE (STATIC)

DC INTERFERENCE (STATIC)

DC INTERFERENCE

DC INTERFERENCE

STATIC INTERFERENCE DETECTING STATIC STRAY CURRENT EFFECT ON PIPELINE: -POTENTIAL PROFILE SHOWS ABNORMAL VARIATION THAN PREVIOUS SURVEY -LARGE NEGATIVE VALUES ARE NOTED REMOTE FROM ANY CATHODIC PROTECTION SYSTEM ON THE PIPELINE OR ARE NOTED ON UNPROTECTED PIPELINE. -UNUSUAL CURRENT ARE MEASURED ALONG THE PIPELINE. -LOW NEGATIVE OR POSITIVE POTENTIAL ARE PRESENT -CHANGE IN CURRENT OUTPUT OF NEARBY CATHODIC PROTECTION SYSTEM MAY CAUSE CHANGE IN STRUCTURE TO ELECTROLYTE POTENTIAL OF THE PIPELINE. --

AC INTERFERENCE Pipelines may share a common right-of-way with parallel high voltage AC power lines resulting in mutual interference effects.

AC INTERFERENCE THERE ARE THREE BASIC METHOD BY WHICH AC VOLTAGE AND CURRENT APPEAR IN METALLIC STRUCTURE NEAR AC POWER LINE : -ELECTROSTATIC OR CAPACITIVE COUPLING: STRUCTURE ACTS AS ONE SIDE OF CAPACITOR WITH RESPECT TO GROUND.THIS IS ONLY OF CONCERN WHEN THE STRUTURE IS ABOVE GROUND.( i.e PIPELINE SUPPORTED ON SKIDS , DURING CONSTRUCTION OR MAINTENANCE OF PIPELINE CARE TO BE TAKEN) -

ELECTROMAGNETIC INDUCTION: AC INTERFERENCE ELECTROMAGNETIC INDUCTION: STRUCTURE ACTS AS THE SINGLE TURN SECONDARY OF AN AIR CORE TRANSFORMER IN WHICH OVER HEAD POWER LINE IS PRIMARY.THIS TYPE OF INDUCTION MAY OCCUR WHEN THE STRUCTURE IS EITHER ABOVE GROUND OR BELOW GROUND . THE RESULTING STRAY CURRENT MAGNITUDE IS DIRECTLY PROPORTIONAL TO PHASE CURRENT(I Φ) AND THEIR RELATIVE MAGNITUDE AND TO THE LENGTH(L) OF MUTUAL EXPOSURE .IT IS INVERSELY PROPORTIONAL TO THE RELATIVE DISTANCE (d) BETWEEN POWER LINE CONDUCTOR AND STRUCTURE. V (INDUCED)= f (I Φ, 1/d, L) ; V0 (VOLTAGE TO GROUND) =V(INDUCED)/2 THE INDUCED VOLTAGE DOES NOT DEPEND ON DIRECTLY PROPORTIONAL TO POWER LINE VOLTAGE .HENCE,RELATIVELY LOW VOLTAGE AC POWER LINE CAN PRODUCE ELECTROMAGNETICALLY INDUCED CURRENT. WHEN A PIPELINE CLOSELY PARALLEL TO POWER LINE FOR SOME DISTANCE , INDUCED VOLTAGE PEAK WOULD BE EXPECTED WHERE THE PIPELINE AND POWER LINE SEPARATE

AC INTERFERENCE RESISTIVE COUPLING AC POWER IS TRNAMITTED TO GROUND AND THEN FLOWS ON AND OFF THE UNDERGROUND STRUCTURE. DURING POWER LINE FAULTS TO GROUND ,A LARGE AC CURRENT CAN BE TRANSMITTED TO THE EARTH THROUGH RESISTANCE COUPLING AND SUBSQUENTLY NEARBY STRUCTURE. THESE CURRENT WHICH CAN BE SEVERAL THOUSAND AMPERES , CAN CAUSE SUBSTANTIAL PHYSICAL DAMAGE TO STRUCTURE COATINGS, IN EXTREME CASE WHERE AC DENSITY IS HIGH , STEEL PIPING HAS BEEN KNOWN TO MELT.NORMALLY THESE FAULT OCCUR INFREQUENTLY AND ARE OF SHORT DURATION.

AC INTERFERENCE The capacitive, ohmic or inductive AC coupling between power lines and pipelines may result in: 1. Danger to the safety of personnel under normal operation (15 Volt limit)      2. Danger to the safety of personnel under fault conditions      3. Danger to the pipeline integrity under fault conditions      4. Risk of AC-enhanced corrosion under normal operation      5. Risk of damage to the coating from electrical stress under fault conditions

AC INTERFERENCE AT NORMAL OPERATING LEVELS, CATHODIC PROTECTION WILL NOT MITIGATE AC CORROSION TO ABOVE A CERTAIN AC CURRENT THRESHOLD DENSITY. AS A RESULT, AC MITIGATION IS OFTEN REQUIRED TO PREVENT THIS TYPE OF CORROSION. USING AC MITIGATION TECHNIQUES, THE AC CURRENT DENSITY IS MAINTAINED BELOW A CRITICAL VALUE TO PREVENT AC CORROSION OF A STEEL PIPELINE.

AC INTERFERENCE i AC ˂ 20 A/m2 (NO CORROSION) AC CURRENT DENSITY THRESHOLD AT WHICH AC CORROSION OCCUR AS FOLLOWS: i AC ˂ 20 A/m2 (NO CORROSION) 20 A/m2 ˂ iAC ˂ 100 A/m2 (CORROSION UNPREDICTABLE) iAC ˃ 100 A/m2 (CORROSION EXPECTED) THE ABOVE THREHOLDS ARE WITH CATHODIC PROTECTION APPLIED.

INTERFERENCE MITIGATION With rapid development of infrastructure in the recent years, more and more buried pipelines are laid parallel with or adjacent to high-voltage transmission lines, and severe AC interference is caused. Therefore, an effective mitigation scheme is very important in reducing the AC interference risks.

AC INTERFERENCE

MITIGATION OF AC INTERFERENCE .AC MITIGATION METHODS INCLUDE: -SIGNIFICANT SEPARATION BETWEEN PIPE AND HVAC SYSTEM - GROUND PIPE WITH USING DISTRIBUTED GALVANIC ANODE -GROUND PIPE WITH USING A METAL SUCH AS ZN, MG, STEEL AND CU WITH DC DECOUPLER DEVICE (E.G CAPACITOR, POLARISATION CELL).A DECOUPLER ALLOW TO PASS AC CURRENT AND BLOCK DC CURRENT. -PROTECTING DEVICES FOR ELECTRICAL ISOLATING DEVICES SUCH AS FLANGES KIT OR JOINTS. -STEP AND TOUCH PROTECTION SYSTEMS (GRADIENT MATS OR GRID)

EARTH MAGNETIC INTERFERENCE

EARTH MAGNETIC INTERFERENCE

INTERFERENCE MITIGATION RESOLVING INTERFERENCE PROBLEM: Interference problems are individual in nature and resolution should be agreeable to all parties involved. One or more of the following method can be used to mitigate interference problem. -Removal of detrimental effects of interfering current by installing a metallic return path - Counteracting the effect of interference current by applying cathodic protection. -

INTERFERENCE MITIGATION RESOLVING INTERFERENCE PROBLEM: -Consulting with utility co ordinating committee. -Removing or relocating the interfering current source -Preventing the pick up or limitation of flow of interfering current. -

INTERFERENCE MITIGATION

INTERFERENCE MITIGATION Pipeline modification involve: Installation of insulating joint Installation of Magnesium anode on the pipeline at the location where pipeline is going into stray current discharge. If magnesium anode is installed where the pipeline both pick up and discharge stray current, installation of diode with magnesium anode is necessary to assure that anode discharge stray current but do not collect stray current. Installation of potentially control rectifier and impressed current gound bed where pipeline is going into stray current discharge.

INTERFERENCE MITIGATION

INTERFERENCE MITIGATION

INTERFERENCE MITIGATION

STATIC INTERFERENCE DETECTING STATIC STRAY CURRENT EFFECT ON PIPELINE: -POTENTIAL PROFILE SHOWS ABNORMAL VARIATION THAN PREVIOUS SURVEY -LARGE NEGATIVE VALUES ARE NOTED REMOTE FROM ANY CATHODIC PROTECTION SYSTEM ON THE PIPELINE OR ARE NOTED ON UNPROTECTED PIPELINE. -UNUSUAL CURRENT ARE MEASURED ALONG THE PIPELINE. -LOW NEGATIVE OR POSITIVE POTENTIAL ARE PRESENT -CHANGE IN CURRENT OUTPUT OF NEARBY CATHODIC PROTECTION SYSTEM MAY CAUSE CHANGE IN STRUCTURE TO ELECTROLYTE POTENTIAL OF THE PIPELINE. --

DC INTERFERENCE MITIGATION (DYNAMIC)

INTERFERENCE MITIGATION

INTERFERENCE MITIGATION

CORROSION CONTROL

CATHODIC PROTECTION Thank You