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مدیر وبلاگ : pc7a
نویسندگان

A high voltage transmission line in proximity to a pipeline will have an electrical influence on the pipeline during both steady state and fault conditions. This electrical

 

influence, commonly referred to as coupling, is important for the following reasons:

 

· individuals touching a pipeline subject to the influence of the transmission line may be at risk from electric shock,

 

· the pipeline may have voltages established between the pipeline and the coating outer surface; if this voltage becomes too high, coating damage may occur,

 

· a continuous flow of AC from the pipeline may also be established: this flow of current may accelerate pipeline corrosion,

 

· if the voltage of a pipeline rises with respect to the surrounding soil, equiptment (such as a transformer/rectifier in an impressed current cathodic protection system) connected to the pipeline may be darnaged

 

1

 

A high voltage transmission line influences a pipeline by three types of coupling:

 

· capacitive,

 

· inductive

 

· conductive.

 

All of these types of coupling arise from the fact that a transmission line carries power using high voltages and high currents

 

2

 

In general, capacitive coupling is:

independent of pipeline length,

proportional to the transmission line voltage,

inversely proportional to transmission line / pipeline separation,

lower, but not eliminated, when a three phase balanced system is used,

easily mitigated by earthing the pipeline, completely avoided by burial.

 

 

3

 

In brief, inductive coupling:

 

- affects both above and below ground pipelines,

is proportional to pipeline length and is Therefore more important for electrically continuous pipelines

 

· is proportional to transmission line current,

 

· is more significant during power system fault condition,

 

· can be mitigated to a degree by earthing the pipeline,

is more significant in higher resistivity soils

 

4

 

3.2 Resistive Coupling(Electrolytic)

 

3.2.1 Grounded structures of an AC power system

share an electrolytic environment with other

underground or submerged structures. Coupling

effects may transfer AC energy to a metallic structure

in the earth in the form of alternating current or

potential.   Whenever a power system with a

grounded neutral has unbalanced conditions, current

may flow in the earth. Substantial currents in the

earth may result from phase-to-phase or phase-to-ground faults. A metallic structure in the earth may carry part of this current. Also, a structure in the earth coated with an insulating material may develop a significant AC potential across the coating.

 

5

 

4.6 Distributed Anodes

 

4.6.1 Whenever distributed galvanic anodes are

used as part of the grounding system to reduce the

AC potential between a structure and earth, they

should be installed in close proximity to the protected

structure and away from power system grounds.

Connecting anodes directly to the affected structure,

without test connections, may he desirable Direct

connection reduces the number of points at which

people can come in contact with the structure, and

offers the shortest path to ground. Should it be

desirable, for measurement purposes, to open the

circuit between the distributed grounding system and

the structure, the test lead connection should be

made in an accessible, dead-front test box. When

galvanic anodes are used as part of a grounding

system, the useful life of the electrode material

should be considered. Dissipation of the anode

material increases the grounding system resistance.





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