www.sait-group.com
BSS Technologies
BSS Technologies
P.O. Box : 33464, Abu Dhabi, U.A.E.
Tel : 00971 2 446 0700
bsstech@bsstechnologies.com
Global Location
AC Mitigation Modelling

Computer Modelling:


The computer modelling of the AC interference between overhead powerlines and the buried pipelines has been carried out using the Safe Engineering Services software “Right of Way - Induced AC Prediction Program”.


Power line Data Required for Computer Modelling:


General:

Power line cross section with conductor spacing details.

Datasheet/Details of conductors and ground wires. 

Substation and Power plant locations with GPS coordinate.

Substation and power plant grounding system.

Structure, substation details and grounding details & ground resistance value, earth impedance and Power Distribution.

PhaseTranspositions in the power line.

Single line diagram of associated networks.

Tower earthing expected/designed resistance.

Tower earthing design details.

Tower Design with Phase wires, earth wires design impedances.

Phase normal load, emergency load and fault load at towers.

Pipeline diameters, wall thickness, coating type, and thickness, burial depths.


Normal Load Condition:

Phase to Phase energizing voltage

Magnitude and angle of present and future peak load currents

Maximum load unbalance for each circuit

Maximum harmonic currents


Faulted Condition:

Current contribution – Fault Current Phase to Ground

Future Expansion

Fault duration & fault type

X/R ratio



Site Based data collections:


Soil Resistivity:

At  depth of 1m, 5m, 10m, 20m,30m,40m

At each 20km intervals

At GB locations (Existing or proposed)

At all exposed structure locations such as

valve sites

Pig launchers

Pig receivers

metering stations

regulating stations

Compression stations etc

Locations where one more powerlines deviating significantly from pipeline & vice versa

At phase transposition locations

At powerline crossings

At power plant locations

At substation location

At location where the pipeline is so close to the power line structures


Power line Parallelism:

Power line span & rating.

Distance of own pipeline from the center of power structure (Centre of tower or pole).

Distance of own pipeline from the edge of power structure (from outside of the nearest

tower leg).


Power line Crossing:

Power line location 

Power line rating.


Pipeline Parallelism:

Pipeline dia & span.

Distance from New pipeline


Pipeline Crossing:

Pipeline crossing location

Pipeline dia


Pipelines running into close proximity with electric power transmission and distribution systems will occur electromagnetic field created by the alternating current (AC) (expands and collapses and changes direction 120 times per second). An alternating electromagnetic field will exhibit an induced voltage on the pipeline.

Possible electrical effects on Pipeline:

Electrical Energy from an over head power line can be transferred to a pipeline by three possible electrical effects,

   •Capacitive Coupling (Or) Electrostatic Coupling
   •Inductive Coupling (Or) Electromagnetic Coupling
   •Resistive Coupling (Or) Conductive Coupling

Capacitive Coupling:

Any two materials separated by a dielectric material can be considered as a capacitor. Capacitance is the ability to store electrical charge between to conductors relative to the voltage between the conductors. In this case air is the dielectric and power line is one conductor while the pipeline is the other conductor.

This capacitive coupling is of most concern on above ground pipeline or such as during construction of pipeline.

 

                AC mitigation

Inductive Coupling:


Inductive (or) electromagnetic coupling where the pipeline is a conductor in an electrical field and thus induced voltage may build up beyond acceptable threshold limits.

In this case pipeline act as a secondary and overhead power lines are secondary. The induced voltage appears on the pipeline

 

                  AC mitigation

Resistive Coupling:


During power line faults to ground, large AC currents can be transmitted to the earth through resistive coupling and subsequently into nearby underground structures. These AC currents which can be several thousand amperes can cause physical damage to the structure coatings. These faults are occur infrequently and   are of short duration. 

                 AC mitigation

Need of Mitigation:


AC interference on the pipeline due to inductive, capacitive and resistive coupling between the power line and the pipeline produces the following risks:

    •Shock to personnel under normal (steady state) operation

    •Shock to personnel under fault conditions

    •Electrical arcing under fault conditions causing puncture or damage to pipeline

    •AC-enhanced corrosion under steady state operation

    •Damage to the coating due to electrical stress under fault condition



 

Need of AC Mitigation Measures (For Parallelism):

As per the NACE 0177-2007 the steady state induced AC voltage not more than 15V.   

Based on the AC Mitigation Computer Modelling results, if the steady state induced voltage more than 15V there will be mitigation measures are required.
AC Mitigation Grounding Materials:
  •  Polarization Cell Replacement (PCR)
  •  Zinc Grounding Cell
  •  Zinc Ribbon Anode


Polarization Cell Replacement (PCR)



Polarization cell Replacement widely used as a DC decoupler and AC mitigation device that allows AC current to pass to a ground while blocking the DC current intended for the cathodic protection of the structure. The cell consists of a container filled with a Potassium hydroxide solution (the electrolyte) into which stainless steel plates are immersed and alternately connected to the cell terminals. One of the cell terminals is connected to the structure and the other to ground (or if used to protect an insulating joint from AC, the terminals are connected across the insulating joint).

 

 

 


Zinc Grounding Cell



Zinc Grounding Cells are used for AC Mitigation. This is used to bleed the induced AC from structures. Zinc Grounding cell can be installed at High Voltage O/H line parallelism or crossing.Zinc Grounding Cell consisting of one, two or four standard zinc Anodes separated by 1” insulating spacers. Nominal resistance of a two Anode Grounding Cell in a wet area is 0.2 to 0.6Ω. Nominal resistance of a four Anode cell is one fourth to one half that of an Anode cell. The Grounding Cells are packed in Standard Zinc backfill (75% Gypsum, 20% Bentonite and 5% Sodium Sulphate) either in a unit of one, two or four.

 



Zinc Ribbon Anode



Zinc Ribbon Anode is used in a variety of applications; for cathodic protection especially in marine applications, AC Mitigation on structures. To overcome the small impurities which are always present in the base material, conventional zinc alloys contain Aluminium.Zinc Ribbon Anode is a very simple, cost effective maintenance free method to Corrosion control for buried or immersed metals useful for unattended applications; where an external power source is not applicable.

 

 

 

     
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