EXPERT TIP #10: BASICS OF INDUCED AC ON PIPELINES
Induced AC (Alternating Current) is a common problem that occurs when buried pipelines are installed in a common corridor or near energized high-power transmission lines.
There are two main concerns with induced AC on metallic pipelines:
- Induced voltages can present a shock hazard to technicians who physically touch the pipeline or metallic devices connected to the pipeline.
- Induced AC is known to be the direct cause of soil side corrosion on buried pipelines.
AC power on a pipeline is a result of two methods:
- Inductive refers to an indirect electrical coupling from the electromagnetic field generated by high power transmission lines that result in voltages induced on the pipeline.
- Conductive refers to a direct resistive coupling between a power system and the pipeline. Typically, this would include a power line-to-ground, creating ground fault currents, or a lightning strike that travels to the pipeline through the earth.
Induced AC Voltage
The magnitude of the induced AC voltage or “power” depends on many variables:
- The relative positioning and alignment of the pipeline and power lines
- The length that the pipeline parallels the power lines
- The power demand on the power lines
- The balance between phases on the power system
- Soil resistivity as low soil resistivity provides a good path to earth
- The dielectric strength and quality of the pipeline coating (A well-coated pipeline is more susceptible to induced voltages than a bare pipeline.)
- Like DC corrosion, AC corrosion is normally concentrated at coating flaws (also named holidays).
- Abnormal Operating Conditions in the form of a local or direct lightning strike as well as a downed power line (These can result in a very high, i.e. > 1000 volts, induced or direct conductive coupling.)
Induced voltages are proportional to the magnitude of current on the power lines. Therefore, the AC pipeline-to-soil potentials will be notably higher during the hottest summer months because of higher power demand.
Induced AC on a pipeline can vary significantly in a 24-hour period. Typically, the maximum power demand is between 2pm to 7pm with the highest on Tuesday, Wednesday and Thursday. Therefore, it is reasonable to deduce that variable induced power events will occur at all the locations tested. To provide a better understanding of the range of induced voltages, a data logger can be used to capture AC voltage trends.
During routine pipeline surveys, AC pipe-to-soil potentials should be measured any time the pipeline is in or near an AC power corridor. The method for measuring an AC pipeline-to-soil potential is identical to measuring a DC pipeline-to-soil potential except that your digital multimeter is set to the AC voltage function.
How Much AC is Too Much?
NACE SPO177-2007 addresses many of the issues and effects of induced AC and lightning on metallic structures and corrosion control systems. Section 5: Personnel Protection, Subsection 5.2: “15 VAC (RMS) open circuit or a source current capacity of 8 mA (milliamps) or more are considered to constitute a possible shock hazard”.
Induced Voltage/Current Concerns
How dangerous the induced voltage/current is a function of how much power is available. There is a series of tests that must be performed to determine actual power and the possibility for personnel shock hazard. This topic will be addressed in a future tip.
In addition to the staff safety issues, there is also the concern of how much, if any, corrosion may be occurring on the pipeline. Corrosion caused by induced AC continues to be a controversial subject. In recent years, it has been determined that AC corrosion of cathodically protected pipelines can and does occur.
AC Current Densities
The corrosion that can occur from induced AC is not necessarily proportional to the induced voltage. The concern is the magnitude of AC current density that is induced on to the pipeline, which is expressed as follows:
- Less than 20 Am2 = No induced AC corrosion
- Greater than 20 Am2 but less than 100 Am2 = Corrosion is unpredictable and influenced by many environmental factors
- Greater than 100 Am2 = AC corrosion is likely to occur
AC corrosion is like DC corrosion in that the AC discharges at coating flaws or “holidays”. Better coatings, i.e., smaller and fewer coating holidays, yield smaller areas of bare metal in contact with the earth, resulting in higher current densities per unit area of steel. Other contributing factors to the AC corrosion rate are low soil resistivity soils, porosity and geometric factors in the interface between the soil and the coating holiday.
Monitoring the current density and available power, rather than just AC voltages, is key to assessing the AC current-related corrosion risk.
AC Mitigation
Providing worker safety is the number one priority. To do so, the pipeline operator must provide a dedicated, low resistance-to-earth ground, which is essential to ensure that the induced AC voltages are reduced to safe levels. Historically, galvanic anodes (magnesium or zinc) were connected directly to the pipe to provide an electrical path to ground. This allowed the AC to be reduced without imposing any load on the CP system. With the development of the solid-state decoupler that passes AC while providing DC isolation, the problem with depressed cathodic protection potentials can be eliminated. Therefore, any type of grounding electrode material can be used, including newer linear grounding electrode systems specifically designed for AC mitigation.
An AC mitigation system design may include any or all the following:
- Gradient control equipment to provide personnel safety against step and touch voltages
- An AC mitigation or grounding electrode system
- AC solid state decouplers
- AC current density monitoring equipment
In conclusion, it is important that pipeline operators monitor for the presence of induced AC and take necessary actions to provide the required mitigation to protect their expensive assets and more importantly, all personnel working on or near the pipeline.