AC Power Grid electrical grounding is purposely joined to/gets electrically coupled with below-grade metal assets, and sometimes with cathodic protection (CP) systems. The typical North American power grid is supplied by 60-cycle-per-second (or Hertz, Hz) alternating current (AC) electricity. Larger facilities are typically receiving three-phase AC power, with three separate power-delivering legs, each of a different power phase, and one common neutral wire. In 2018-2019, Chapman Engineering studied an 8.5-mile-long, 12-inch-diameter crude oil pipeline suffering from significant AC power interference. Field measurements led to AC mitigation design and installation work, and the mitigation systems reduced AC interference magnitude by more than 85 percent. But the field information and modeling work could not explain how the pipeline was receiving such significant AC power build, with only medium-voltage AC (MVAC) distribution power lines as the source. A Chapman Engineering rep worked with an energy company lineman who helped identify various components in the electrical distribution system near the major AC interference area. This gentleman identified a bank of power conditioning capacitors, one of which was out of service due to damage. Cycling the capacitors through “on” and “off” patterns, the AC neutral butt ground wiring showed a change from 18 VAC to 0.9 VAC, respectively. With capacitors energized on two of the three primary power legs, an 18-volt “imbalance voltage” was present on the AC neutral system wiring in that area. This AC neutral system imbalance was the primary cause of elevated AC voltages on the pipeline, with the related risks for safety and AC-induced corrosion. This was not induction-driven power on the pipeline, but rather a conductively touching AC power load, almost certainly delivered through the rectifier’s AC power circuit neutral wire. The operator now holding this asset has yet to agree to new testing to verify this conductive tie-in. There were two major effects on the pipeline due to large AC neutral voltage. The first was elevated AC voltage on pipeline metal, likely caused at the rectifier by an accidental combining of the pipeline as structure, local grounding for the rectifier, and the AC neutral wire. The second issue, more insidious, was greatly increased shifts in DC structure-to-soil voltages on pipeline metal, due to neutral voltage back-feeding rectifier. This increased driving voltage in "on" cycle, but also created a non-zero voltage driving output during "off" cycle of current interruption. True "off" voltages could not be obtained on pipeline metal.
