This article, courtesy of Dairyland Electrical Industries, analyses the role of DC decouplers in corrosion prevention and safety.
Cathodic protection (CP), when properly applied, is an effective technique to minimise the natural corrosion process that occurs on pipelines, tanks, and other buried metallic structures. To maintain effective CP coverage with minimal current demand, the structure must be well-isolated from earth for DC current flow. However, these structures require electrical earthing for both personnel safety and protection of the structure from damage due to over-voltage conditions. These earthing bonds require the CP system to protect significantly more material surface area for which it was likely not designed. As a result, it is often difficult to maintain adequate CP potentials on the structure that is to be protected.
A practical and widely accepted solution is to install DC decoupling devices in series with the bonding connections between the cathodically-protected structure and the earthing system. Decouplers are designed to block CP current while allowing steady state AC, AC faults, and lightning to pass freely. This prevents CP current from passing to the earthing system and so minimises the amount of CP current required to protect the pipeline.
This article will examine the basic function of DC decouplers and over-voltage protection devices and explain how they should be used in conjunction with cathodic protection systems to obtain optimal corrosion and safety protection.
The challenges with cathodic protection and earthing systems
CP systems, and external corrosion protection in general, require that the protected structure be electrically well-isolated from earth. Since CP systems are designed to protect only relatively small defects in the coating of the protected asset, the degree of isolation determines the efficiency and effectiveness of the CP system in protecting the structure. Certainly, the less surface area of the structure that is directly in contact with earth, the less CP current is required for protection. This is the basic function of high resistance pipeline coatings. Additionally, every surface on the structure that interfaces other earthed structures must be insulated to minimise CP current flow through the other structures. This is accomplished using flange isolation kits and monolithic joints at piping connections, dielectric fittings for smaller piping connections, conduit and sensor connections and isolation pads for above-ground pipe supports.
In addition to these mechanical interfaces, there are electrical connections between the structure and earth for which continuity must be maintained to protect the structure and personnel from potential over-voltage conditions. These connections include earthing bonds for AC interference mitigation, AC faults and lightning protection and earthing bonds for electrical equipment that is electrically bonded to the structure.
AC mitigation earthing paths
The main intent of AC mitigation systems is to dissipate unwanted voltage along the pipeline resulting from induced AC from nearby power transmission lines and AC faults and lightning. The general technique for mitigating induced AC pipeline voltage is to connect the pipeline at appropriate locations to a suitably low impedance earthing system in order to collapse the voltage to a safe value. The earthing system is commonly bare zinc ribbon or copper wire run in parallel with the pipeline.
The design process typically begins with software modelling by specialised consultants, inputting various factors such as soil resistivity, lateral separation distance and power system characteristics (voltage and amperage) to arrive at an induced AC voltage map at all points along the pipeline. Then, by applying low impedance earthing points at various locations along the affected area, the AC effects under steady-state and fault conditions can be modelled, and the earthing system design can be optimised to address worker touch and step voltage safety, coating stress voltage, and AC current density issues. Depending on many variables – such as the separation distance and geometry between the pipeline and power lines, power levels, soil resistivity, pipeline coating, etc. – spacing of earthing connections may vary between a few hundred metres to several kilometres.
These mitigation bonds between the pipeline and the earthing system provide a low impedance path for AC interference to dissipate, but they also introduce additional material surface area for the CP system to protect. As a result, the rectifier often cannot support the increased current load and CP potentials can become compromised, leaving the structure inadequately protected.
Electrical equipment earthing
It is common for electrical equipment, such as motor operated valves, to be electrically bonded to pipelines through the power supply system and therefore the earthing system of the power supply. Consequently, the equipment, its earthing system, and everything to which it is bonded, including the utility earthing system, is bonded to the pipeline. These earthing systems then become additional paths that pickup CP current and so present additional material to be protected by the CP system. This can have a dramatic negative impact on CP system performance.
Electrical earthing conductors are needed to conduct AC fault current and prevent over-voltage conditions and must be left in place. National electrical codes require equipment earthing conductors to be 1) permanent and continuous, 2) rated to handle the anticipated AC fault current from the source, and 3) of low impedance to allow AC fault current to flow, permitting a clearing device (circuit breaker or fuse) to clear the fault. These define what is an “effective ground-fault current path” for safety, per the US National Electrical Code. To remove the offending conductor for convenience on the CP system is to allow an unacceptable shock hazard at the site during any over-voltage condition and would violate electrical codes.
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