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High Voltage Riddle No.1 - HV power line and metallic structure interference
Are there any hazardous interference (induced voltages) between power line and vicinity metallic structures or any recommendation about it?
Author : Shahroz - From: Iran
Mon, March 8th, 2010 - 18:56
Overhead AC power line field may induce voltages on the metallic pipelines running in close vicinity leading to serious adverse effects due to sharing of the right-of-way. This electromagnetic interference is present both during normal operating conditions as well as during faults. The coupling of the field with the pipeline takes place either through the capacitive path or through the inductive or conductive paths.
In the present work, the induced voltages due to capacitive and inductive coupling on metallic pipelines running in close vicinity of high voltage power transmission lines have been computed. The conductor surface field gradients calculated for the various phase configurations have been presented in the thesis. Also the electric fields under transmission lines, for both single circuit and double circuit (various phase arrangements) have been analyzed. Based on the above results, an optimum configuration giving the lowest field under the power line as well as the lowest conductor surface gradient has been arrived at and for this configuration induced voltage on the pipeline has been computed using the Charge Simulation Method (CSM). For calculating the induced voltage due to the inductive coupling, electromotive force (EMF), induced along the pipeline due to the magnetic field created by the transmission line has been calculated. The potential difference between the pipeline and the earth, due to the above induced EMFs, is then calculated. As the zones of influence are generally formed by parallelism, approaches, crossings as well as removals, the computation involves subdividing the zone into several sections corresponding to these zones. The calculation of voltages is carried out at both the ends of the sections. Each section is represented by an equivalent п electrical network, which is influenced by the induced EMF. The induced EMF is calculated during faulted conditions as well as during steady state conditions. Inductive coupling calculations have been carried out for the following cases:

- Perfect parallelism between power line and pipeline.
-Zone of influences formed by parallelism, approaches, crossings and removals.

It has been observed that when the pipeline is approaching the HV transmission line at an angle, then running parallel for certain distance and finally deviating away, the induced voltage is maximum at the point of approach or removal of the pipeline from the transmission line corridor.
The induced voltage is almost negligible near to the midpoint of the zone of influence. The profile of the induced voltage also depend on whether the pipeline is grounded or left open circuited at the extremities of the zone of influence.

About other metallic structures, the electric field associated with potentials on power conductors can develop a potential on an inadequately grounded structure in the vicinity of the power system. The potential that the structure attains because of capacitive coupling varies with the power conductor potential and depends on many factors, including the geometric configurations of the structures involved. During construction, when the structure is aboveground or in an open trench, it may reach a dangerously high potential. When the structure is buried or submerged, the capacitive coupling effect usually is not significant unless:
1- The soil resistivity is high
2- The structure is electrically isolated
3- The structure is very long
AC current flow in power conductors producers an alternating magnetic field around these conductors. Thus, an AC potential can be induced in an adjacent structure within this magnetic field, and current may flow in that structure. The magnitude of the induced potential depends on many factors including the overall geometric configuration of the structures involved, the magnitude of the current in the power circuit, and any current imbalance. If the currents in a three-phase power system are equal (balanced) and the affected structure is equidistant from each of the conductors, the total induced voltage is zero. This, however, is seldom the case, and induced AC voltage is usually present on the affected structure. Greater electromagnetically induced potentials may occur during a phase-to-ground or phase-to-phase fault in multiphase circuits because of the higher magnitude of fault current in these systems. The leakage conductance to ground, caused by the resistive coupling of the affected structure, allows AC current to flow between that structure and earth. This phenomenon, combined with other factors, results in different values of AC structure-to-electrolyte potential along the affected structure. The higher the dielectric strength and resistance of the coating and the higher the soil resistivity, the greater the induced AC potential.
According NACE standard RP0177-2000, during the construction of metallic structures in areas of AC influence, the following minimum protective requirements are prescribed:
1-On long, metallic structures paralleling AC power systems, temporary electrical grounds shall be used at intervals not greater than 300 m, with the first ground installed at the beginning of the section. Under certain conditions, a ground may be required on individual structure joints or sections before handling.
2-All temporary grounding connections shall be left in place until immediately prior to backfilling. Sufficient temporary grounds shall be maintained on each portion of the structure until adequate permanent grounding connections have been made.
Author : Hamid - From: Iran- Firouzabad in Fars
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