Alternating Current Interference Assessment and Mitigation

Contribute by Chris Isaacson

Alternating current (AC) interference is becoming an increasing concern as pipelines and high voltage transmission lines are required to share common easements and right-of-way.  This collocation leads to integrity concerns related to capacitive (pipeline construction), inductive and conductive interference.  

Mears has recently completed a turnkey AC interference assessment on an Operator’s pipeline collocated with a 138 kV HVAC transmission line, Figure 1. This project included the field testing, engineering assessment, mitigation design, installation and commissioning of an AC mitigation system. The assessment was based on assessing the threat of AC interference on over seven miles of an 8-inch diameter natural gas pipeline.  

 

Figure 1. 138kV HVAC Transmission Line

The AC interference assessment was triggered by historical AC voltages ranging from 5 VAC to more than 12 VAC. A targeted field test plan was developed as part of the assessment and included a close spaced AC voltage survey coupled with soil resistivity measurements. This approach allowed for a detailed analysis over the length of pipeline collocated with HVAC transmission lines. A detailed survey allows for a focused assessment in order to provide the most efficient design based on actual pipeline conditions.

 

Following the targeted field testing plan, Mears successfully assessed and designed a mitigation system to target the elevated AC voltages. This assessment included the review and evaluation of the surveyed voltages and included a historical data review to allow for an adequate understanding of the magnitude of voltages experienced on the pipeline. In addition, data loggers were used to evaluate the impact of interference on the pipeline as a result of various load demands on the overhead electric transmission lines.  The daily changes measured during this project are shown in Figure 2.

Figure 2. AC Voltage Data Log

Figure 3. AC Mitigation DC Decoupler

Mears’ construction team worked directly with the client to effectively install the Mears designed AC mitigation system while coordinating with Mears engineering to ensure and maintain efficiency and quality. This open communication between the client and our engineering design group provides our construction team a unique advantage to deliver a safe, timely cost-effective and high-quality mitigation system.

Following the installation, Mears’ engineers completed the testing and commissioning of the mitigation system.  This ensures proper component installation and to verify the effectiveness of the system. The results of the commissioning survey indicated that the designed AC mitigation system for this project was successful in reducing both the induced AC voltages and AC current densities on the pipeline.  As shown in Figures 4 and 5, the AC voltages were reduced to less than 4 VAC, and the calculated current densities to less than 14 A/m2 at the time of testing.

Figure 4. AC Mitigation System Commissioning Results – AC Voltages
Figure 5. AC Mitigation System Commissioning Results – AC Current Densities