Royal Navy Dockyard Project
About the Royal Navy
A new temporary 11kV/LV substation is being installed by Eaton Electric Ltd at a Royal Navy dockyard. The new substation will be installed on the dock itself. The substation is required to operate for a five-year period to facilitate the construction process of upgrades to the dock.
PSE2 Consulting have been requested to carry out an earthing study and design to ensure High Voltage (HV) earth faults are safely dissipated and circuit protection operates correctly.
Customer Challenge
The usual practice for the design and installation of an HV earth electrode system is to install grading electrode in the vicinity of the new installation to meet touch voltage requirements. However, as the dock is concrete, it would not be practical to install a typical/formal electrode system in the ground.
The likelihood is that the earthing systems, structural steelwork, and utilities across the site will be interconnected via HV cable sheaths and LV network. This type of system would typically be defined as a global earthing system (GES) however, there is no record of connections between the substations, structures, etc, and the client did not want to rely on the GES for the new substation electrode system. Additionally, the historical measurements of the existing site electrodes were unreliable.
The new substation will be mounted above ground, on metallic supports. The ground itself is not natural soil and the dockyard is comprised of concrete, slate, and made-up ground. It would therefore be impractical and ineffective to install a typical earth grid in the vicinity of the new substation.
Solution
The proposed earthing system was to utilise two new earth plates submerged in the basin as the electrode system for the new substation. Research was carried out into the electrical properties of sea/estuary water to determine the composition of the plates. The result was to use at least two earth plates composed of non-ferrous stainless steel to give greater conductivity. Grade 316 was recommended to provide better corrosion resistance to salt. As it was a marine application, duplex was recommended (2507 stainless steel). The plates will need to be submerged below tidal level and may need to be anchored or connected to the structure of the dock to prevent movement.
