An electrical substation copper grounding system has suffered from a severe corrosion due to the presence of direct stray current, originated from an aluminum plant. A study was done to demonstrate the presence of dynamic direct stray current as the cause of the corrosion of the copper grounding system.
To mitigate the influence of the stray direct current over the copper grounding system and to facilitate the application of cathodic protection, the electrical substation grounding mat was first disconnected from the aluminum plant grounding mat and then reconnected through polarization cell replacement (PCR) devices. Since the stray direct current could not be interrupted to measure structure to soil true potentials, four different types of IR drop free test stations were designed and installed.
To protect the substation grounding mat from corrosion, three impressed current cathodic protection (ICCP) systems with distributed anodes were installed and adjusted. In order to ensure the copper grounding system was protected, a cathodic polarization potential criterion of at least -300 mV with respect to a Cu/CuSO4 (CSE) was used.
Aluminum plants may create large amounts of stray direct current (DC) interference during the production of this valuable metal. The grounding electrical substation's mat that was built adjacent to and north of the aluminum plant has suffered from severe corrosion within a 13-years period of time since its operation due to the close proximity to this source of stray direct current interference, see Figure 1 below. It was determined that a new copper grounding system would have to be installed in conjunction with a cathodic protection system to protect the copper against stray current corrosion and increase the longevity of the copper grounding mat.
The electrical substation is divided into three sections: east section, middle section and west section. The three sections are electrically continuous. The scope of the work performed for the client included design of an ICCP system for each of the three sections of the electrical substation grounding.
The focus of this paper includes the scope of work performed for the client, including: CP current requirement tests to assess current distribution with anode distribution, the design of the ICCP system for each section, and the performance testing of the CP systems to ensure that the grounding system is being properly protected against stray current corrosion.
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Based on the NACE SP0169-2007 standard in order for a copper structure to be cathodically protected there must be a minimum of -100 mV of cathodic polarization between the structure surface and a stable reference electrode contacting the electrolyte.
Since the copper structure in consideration is constantly affected by dynamic stray DC interference, the native copper grounding-to-soil potential could not be measured (i.e., with no DC interference). Given the fact that the native copper-to-soil potential with no interference or cathodic protection applied is approximately -200 mV, with reference to Cu/CuSO4 (CSE) electrode.