AC power is not typically found on pipelines. If an AC voltage is present on a pipeline, it has come from one of three effects: inductive, capacitive or conductive coupling. For any of these three couplings to occur, the pipeline must be relatively close to a power line or to other metallic structure carrying AC current or it is bonded to another structure that is being affected by AC. This is different from the 120 Hz ripple left on pipes from single-phase cathodic protection rectifiers or the 360 Hz ripple left from three-phase rectifiers, which may present itself as an RMS AC voltage when referenced to the earth. A check of the voltage frequency usually can be used to rule out cathodic protection system ripple. If odd multiples of 60 Hz are present on the pipeline, this has likely come from coupling to a power line that has a component of harmonic distortion.
Except within the first few metres from the power line, it is always the case that the further a pipeline is from a power line, the less of an influence the power line will have on the pipeline. This is true for all three modes of coupling.
The electric fields (due to the power line operating voltage) around a power line lead to charges on nearby objects such as wires, fences, swing sets and above ground pipelines. Large AC voltages can arise on the objects due to these charges, but unless the objects have sufficient capacitance, or the electric field strength at the objects is sufficiently high, these voltages are typically not immediately dangerous.
AC currents flowing through power line conductors cause alternating magnetic fields to be set up around the conductors. These magnetic fields in turn induce opposing AC voltages in any metallic object that is within the influence of the power line. The effect is known as mutual inductance and gives rise to voltagesof opposite phase to the power linecurrent. The amount of inductionthat occurs is dependent on many factors with the length of parallelism, the distance from power line to objectand the soil resistivity being some of the most important factors. AC voltages along the object that arise due to induction tend to peak around changes in the mutual system impedance. Some examples of changes in system impedance would be at locations where the power line or the pipeline turns away from one or the other, the pipeline ends or contains an electrical insulating coupling or at termination points of the power line. Many other system impedance changes are possible.For a simple system containing a single isolated pipe section parallel to a power line, the pipe ends will generally have the highest voltages.
Conductive coupling is a form of direct connection between a pipeline, a person or a piece of equipment and the power line conductors either through direct physical contact, air or soil arcing or by being on or in the soil near a point of current transfer between the earth and the power line or power system equipment. In these cases, there is a direct path of power line current through a person or an object. This current is in phase with the power line as opposed to induction.
Power lines are designed to operate in a certain voltage range. The current they carry depends on the load to which they are connected, the power system impedance and the voltage setting. This is called steady state operation and for the most part is steady over short time periods. Load demand changes will change the line current levels over time.
In steady state operation usually only inductive and capacitive coupling takes place. Conductive coupling only occurs under steady state if the phase currents are not balanced or if there is a form of direct contact with a phase conductor (physical or air arcing) and equipment or between the conductor and nearby earth.
Occasionally, power lines suffer an upset such as a fault or a switching event. In these situations, large unbalanced currents exist momentarily in the power line conductors. These currents have to return to their sources by either entering the earth at grounding locations or by direct cable connections. While flowing in the phase conductors, the currents can induce very large voltages on parallel metallic objects. Additionally, as the current returns to earth at pole grounds, large soil potentials (voltages) will occur. This ground potential rise (GPR) is somewhat additive to any induced voltages that may occur on nearby objects leading to a more dangerous situation.
The effects of AC on human beings are defined in terms of current, as this is the quality of AC that directly affects our nervous system and skin. Nerves communicate by sending electrical signals along their cell bodies. AC current travelling in our tissues causes pain receptors to fire and disrupts the flow of electrical signals. At a high enough amount of steady current, 15 to 20 milliamperes, the muscles become rigid. This is considered the “limit of letting go”.
As the amount of steadily applied current increases, deeper tissues are affected resulting in laboured breathing at between 20 and 100 milliamperes and possibly heart fibrillation, at 50 to 100 milliamperes. Once in fibrillation, the heart generally does not start beating normally again without intervention. This will result in death.
Above about 100 milliamperes our breathing stops and our hearts can no longer beat. If the current is removed soon enough, our hearts may begin to beat again and there may not be any permanent damage.Our skin begins to burn above 300 milliamperes. As the current increases above an ampere, skin, muscle and internal organ damage will occur.
Pipe walls can be affected by AC interference through AC corrosion, external coating damage and pipe wall damage. Various papers have been published regarding AC corrosion. They all agree that AC corrosion is a real and measurable phenomenon and they all agree that lowering the induced AC voltage on pipes will reduce the amount of corrosion occurring. Other than those two points, most papers disagree on why corrosion occurs and on the safe operating limits for pipelines. A good summary paper on the topic has been published by NACE International and is titled “AC Corrosion State-of-Art: Corrosion Rate, Mechanism, and Mitigation Requirements”, Jan. 2010.
Pipe coating damage from AC voltage and arcs is better understood than AC corrosion, but again there are various opinions as to what are safe limits to be applied to a pipe coating. NACE International has published AC voltage limits in the Standard Practice SP0177-2007 and coating test voltage limits in SP0490-2007 and SP0274-2011. ASTM coating test voltage limits appear in their standard G62. Other associations around the world have also published voltage limits on pipeline coatings that generally relate to Paschen’s Law for electrical breakdown voltage of air.
Pipe wall damage due to AC interference is rare and usually requires significant fault current availability or very close proximity of pipeline and power line poles. Guidelines for minimum offsets can be recommended by Northern Grounding on a case-by-case basis.