DC Decoupling from Utility Grounding Systems

INTRODUCTION

Within facilities such as pipeline stations and tank farms, cathodic protection (CP) is most easily achieved when the protected structure is electrically isolated from other structures and grounding systems. This allows for lower current requirements from CP current sources, such as rectifiers or galvanic anodes, and generally allows the operator to achieve the required voltage criteria for protection against corrosion.

However, electrical equipment connected to the structure, such as motor operated valves and instrumentation, must be electrically grounded to provide protection against AC faults and lightning or to provide a suitable ground reference for instrumentation. If such equipment and the associated grounding systems are properly isolated from the cathodically protected structure through the use of isolation joints and/or dielectric fittings, there is no impact on the CP system.

However, it is not uncommon for many cathodically-protected structures to be inadvertently bonded to other structures and grounding systems and this results in the CP system having to protect not only the coating defects on the intended structure, but also the other structures and grounding systems to which it is bonded.

In addition to the local facility grounding system, the power utility grounding system to which the facility is bonded presents a large and unknown amount of buried metallic surface area in the form of copper ground rods and other grounding electrodes, as illustrated in Figure 1. Moreover, the facility is also bonded to all the other utility customers’ grounding systems. This vast bare surface area becomes part of the cathodically protected structure from a current-requirement perspective, overwhelming the CP design goals, and allowing unintended interaction to occur. High rectifier output to meet the current demand can also cause excessive voltage gradients in the soil that result in interference with other nearby structures, including both owned and foreign systems, affecting CP potentials, and possibly causing corrosion.

These negative effects can be prevented by judicious application of DC decouplers. Solid-state decouplers provide effective DC isolation while maintaining a safe grounding path for AC faults and lightning. Decouplers are bi-directional, two-terminal devices that feature power semiconductor-based isolation and switching. Fail-safe designs ensure that any exposure to conditions that exceed the device ratings result in an uneventful fail-shorted condition, hence the term “fail-safe”. Under normal operating conditions, the device blocks DC and conducts AC up to a predetermined voltage threshold of typically several volts. During an over-voltage event, upon reaching the voltage threshold, the decoupler instantly switches to the fully conducting mode, clamping voltages (AC and DC) to low and safe levels. After the event, the voltage drops below the threshold and the decoupler automatically switches back to the DC blocking/AC conducting mode.

Two accepted installation methods exist for applying decouplers to isolate CP protected facilities from grounding systems, each with its relative advantages: Installation in the grounding conductors of individual electrical devices and installation at the transformer feeding power to the facility.