This project is to examine the subject of A C Mitigation


Induced AC voltage is thought to cause corrosion.

In recent years the use of high performance, fusion bonded, coatings has become widespread in the pipeline industry. These coatings provide pipelines with excellent isolative characteristics, but when pipelines are co-located along high voltage AC transmission corridors, the high dielectric effect of these coatings can result in AC voltages induced onto the pipeline. This induced voltage might create a safety hazard to operating personnel and the general public.

Cathodic protection connects each section of every pipeline to earth through the transformer rectifier or sacrificial anode system.

To address these effects, AC mitigation grounding systems are typically installed to lower induced AC voltage to safe levels, and to ensure that AC current densities do not exceed the levels at which corrosion can occur.

The pipeline is already grounded and each pipeline section must be regarded as a very low resistance conductor in each path of each circuit.

---

Alternating current = AC

Current is the passage of energy measured as electrical charges passing through a conductive path.

Electrical energy is generated or caused by mechanically passing magnetic fields to induce the energy in other metal to move. This metal is conductive and allows the energy to pass into a circuit where it is made to work.

The mechanism to move the magnetic field is circular and divided so that it pumps energy first in one direction and then in the opposite direction around the circuit in which the work is done. The current alternates in direction but it is simply energy. This is called AC.

Direct current = DC

Corrosion is an electrochemical source of energy that causes electrical charges to pass from a high potential to a low potential. The chemical reaction releases charges that can only be measured by electrical measuring instruments. These charges are sometimes described as electromotive force or EMF and can be measured by comparing the total in one item to the total in another item and the difference is measured in 'voltage' which is the potential difference between the two items.

We must break down what is written about AC mitigation to be understood within the concept that there is only one sort of electricity and that it is governed by the codified laws that have made modern electronics and all the benefits of the use of electricity possible.

Having read in detail the information on the linked websites, there seems to be many issues that demand explanation.

It is first necessary to define the problem in fact by gathering and sharing information relating to the manifestation of Alternatin Current on pipelines and cathodic protection circuitry. This should be divided into anecdotal information from all sources such as word of mouth and news media reports, and factual reports that should include photos and recorded data and studies that must included repeatedly observable data in recorded format.

We must break down all of the claims made in all publications and put them into scientific logic.

This question is always avoided. How is current density measured? We cannot know the surface area that is exposed to the electrolyte.

---

The following has been written by a company that is selling, installing, commissioning and training with respect to 'AC Mitigation'.

The following statements all need examining to seek answers that satisfy the laws of science that used in the electrical and electronics industries.

Over time numerous conditions may change that could negatively impact the performance of the mitigation system including significant fluctuations in AC voltage levels due to daily and seasonal changes in electrical load, cable disconnection, soil conditions, ground faults, and failure of the systems.

A major pipeline company has been continously verifying the operation of AC mitigation systems on pipelines throughout the Chicago, IL region for several years.

While troubleshooting erratic readings at some of the test stations, it was discovered that the AC mitigation on the pipeline had failed and that excessive AC voltage was affecting the measurements at the stations.

To resolve the issue, the pipeline company requested that a company add the capability to measure induced AC voltage to the remote monitoring equipment in the field.

Test sites were set up in locations known to have excessive induced AC voltage levels.

Mitigation systems were already in place at the sites selected, and induced voltage levels were reduced to below 15 volts AC. The monitoring/data-logger systems were installed along with dual coupon test stations to measure induced AC voltage potential and AC current density.
Typical test point measurements, including DC coupon to soil poten-tial, instant off potential, and native coupon potential were also measured.

The monitoring systems were configured to log all test site values at one minute intervals.

Data was stored on 2GB cards inside the devices.

Every three hours readings were transmitted via GSM cellular telemetry to the secure web site.

Additionally, any out-of-range alarm readings were transmitted to the website for immediate notification to the pipeline technician.

The pipeline company requires mitigation to be below the NACE (National Asso-ciation of Corrosion Engineers)15 volts safe touch standard, and AC current density below 20A/mē.

Using the graphing tool on the website, the technician was able to verify over time whether the mitigation was consistently reducing the AC voltage and current to acceptable levels.

To test a mitigation failure, the system was disconnected at one site location for a two day period.

The graphs in Figures 1 and 2 are screen shots from the one-click graphing tool.

These graphs clearly show the changes in induced voltage and current P2S-AC

When underground metallic pipelines are in close proximity to high voltage power transmission lines, an electromagnetic field is created by the alternating current (AC).

The potentially devastating effects of power line operations can include

compromising personnel safety,

equipment malfunction and negatively impacting pipeline reliability.

AC voltages greater than 15 volts are a shock hazard, raising safety and liability concerns.

While rare, pipe failures caused by excessive voltages under power line fault conditions have been reported.

AC interference can result in severe corrosion of the pipeline.

Depending on soil characteristics and other factors, the corrosion can occur at voltages less than those considered tolerable relative to electrical safety.

Extensive AC-caused corrosion is becoming more common, particularly for pipelines that have operated in collocated rights-of-way for many years without appropriate mitigation.

In certain instances, AC-caused corrosion was detected in as little as four years after the pipeline was put into operation.

This includes multi-channel AC/DC close interval potential survey capabilities that allow for simultaneously determining AC interference and cathodic protection levels at 3-foot intervals along a given pipeline. Anomalous conditions are flagged with sub-meter GPS for further investigation.

Intermediate Training: Safety Considerations for AC Mitigation Designs

This three part training session focuses on key mitigation concepts including touch/step potentials, lightning & AC fault considerations, ground mat design, and the use of decouplers in mitigation design.

Hazardous (Classified) Locations are defined in the US National Electric Code (NEC), Articles 500-505, and in other international standards such as the IEC, and describe the requirements of electrical products used in these locations. Decisions on the classification of a particular site depend on several factors,

Measuring Induced AC

Any site with induced AC voltage to levels that require mitigation needs a product that provides AC grounding, without compromising the cathodic protection voltage. The device must be suitable for conducting steady-state AC current while blocking DC current, when connected between pipeline and ground.

Determining AC Fault Current

AC fault current available from an electrical source can follow unintended paths back to the source, affecting cathodically protected structures. A solid, metallic connection between the faulting source and the cathodically protected structure can result in heavy current flow, but even an isolated pipeline or structure can pick up moderate currents flowing in the soil. This latter case is an example of a pipeline in a common corridor with a power line.

Many user sites are formally classified as hazardous locations, according to various standards. Other sites may commonly have explosive vapors or gases present, but haven't been identified as a "hazardous location" due to unfamiliarity with the classification requirements.