New DC decoupling technology provides operators with a solution to capacitance-related data accuracy challenges seen with traditional devices when recording structure to electrolyte potentials. When CP current sources are interrupted for systems that utilize DC decouplers, the capacitors discharge. If structure to electrolyte potentials are taken prior to full discharge, data inaccuracies can result. Data accuracy challenges have historically been difficult to predict definitively, which can present challenges in selecting the appropriate device prior to mitigation installation. Computer modelling in the mitigation design phase can proactively predict the magnitude of capacitance and rate of capacitive discharge. Further, different interruption cycles and their effect on capacitive discharge can be analyzed to determine the magnitude of the issue, so the optimal DC decoupler can be chosen, minimizing project inefficiencies and costs.
Background – CP and Grounding Systems
Extensive and increased collocation of high voltage AC (HVAC) electrical transmission lines, coupled with advances in coating technology, has resulted in the emergence of the possibility of transfer of electrical energy from the HVAC line to paralleling utilities through electrical induction. That transfer of energy can result in safety risks for personnel, as well as corrosion risks for below grade assets.1,2 In order to mitigate those risks, operators ground the induced AC using grounding electrodes, typically consisting of bare copper cabling or zinc ribbon.
Cathodic Protection is commonly used in the utility industry to prevent external corrosion of below grade metallic assets as a result of interaction with the environment. Cathodic protection works by using a DC current to polarize a structure's cathodic sites to the open circuit potential of its most anodic site.3 The current required to achieve cathodic protection is dependent on multiple factors, including the surface area of bare metallic structure in contact with the electrolyte.
When operators use grounding electrodes to reduce induced AC voltages, those grounding electrodes function as bare metallic structure in contact with the earth, which increases cathodic protection current requirements. In order to prevent that increase in cathodic protection current requirement, the DC decoupler was invented. The DC decoupler is AC-continuous but DC blocking, so installation of the device between the asset and the grounding electrode allows for grounding of AC voltages without resultant DC CP current requirement increases. The downside to DC decouplers is their effect on monitoring CP efficacy during interrupted surveys involving rapid changes in DC potentials. DC decouplers utilize a capacitor in their design, and when CP systems are ON, that capacitor accumulates charge. When CP systems are interrupted to test functionality, that capacitor discharges, affecting CP potential reads until current discharge from the capacitor has ended. If not accounted for, that capacitance can affect the accuracy of cathodic protection surveys, as this appears as an additional current source that has not been interrupted.
