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Featured Application: The present paper evaluates 10 alternative shoreline pond electrode station designs using frames in a linear arrangement of electrodes for HVDC interconnections, against the typical design, according to CIGRE B4.61/2017 (parallel arrangement of electrode frames, with respect to the longitudinal axis of the breakwater), studying it from the point of view of electric field analysis (with emphasis on the near-field). During the design of a shoreline electrode station for High-Voltage Direct Current (HVDC) interconnections, the location of the electrodes plays a critical part, especially in the development of the near-electric field. The basic structure is their linear placement, in the form of successive frames, parallel to the longitudinal axis of the breakwater, as proposed by CIGRE and implemented in existing projects. However, this arrangement requires a considerable breakwater length, which may not be permissible, as in the case of Stachtoroi, one of the two electrode stations being built for the 1 GW, ±500 kV HVDC interconnection between Crete and mainland Greece. This troubled the preliminary study team of the electrode stations, which investigated other possible configurations. In this paper, configurations of linear placements of electrode frames are studied and compared at the preliminary study level in terms of electric field effects (especially the near-field), using an analytical simplified model and the superposition method, to determine the most appropriate arrangement of electrodes that will cover the respective requirements of CIGRE directives B4.61/2017. These arrangements are practically evaluated for two different electrode station locations at Korakia in Crete and at Stachtoroi in Aegina for the Crete–mainland-Greece interconnection, resulting in interesting alternative solutions. [ABSTRACT FROM AUTHOR]

In Greece, a new bi-polar high voltage direct current (HVDC) transmission system with a ground return was designed with nominal characteristics of ±500 kV, 1 GW, between Attica in the continental country and the island of Crete, which is an autonomous power system based on thermal diesel units. The interconnection line has a total length of about 380 km. The undersea section is 330 km long. In this paper, the use of the Aegean Sea as an active part of the ground return, based on shoreline pond electrodes, was proposed to avoid EUR 200 Μ of expenses. According to the general guidelines for HVDC electrode design by the International Council on Large Electric Systems (CIGRE) working group B4.61/2017, the electric field and ground potential rise of shoreline electrodes should be studied to analyze safety, electrical interference and corrosion impacts related to the operation of the electrodes. Two kinds of studies are available; one is a simplified approach based on a spherical/pointed electrode centered at the edge of the seashore and seabed, assuming it to be sloping to the horizontal, and the other is a detailed simulated model using a suitable electric field software package. The first approach usually gives more unfavorable results than the second one, especially in the near electric field, while it can not take into account obstacles, i.e., dams, near to electrode position. The second approach demands a detailed description of the wider installation area, which cannot be available during the preliminary study, significant computational time and considerable financial resources for the purchase of a reliable specialized software package. In this research, a two-step modification of the CIGRE simplified model was proposed. The first modification deals with the obstacles in the near electric field, and the second modification deals with the use of a linear current source (instead of a point one), which can give more accurate results. Additionally, the electric field for complex electrode formation is calculated by applying the superposition method, which can be easily achieved using a common software package, i.e., MATLAB. The proposed simplified approaches were applied on shoreline pond electrode locations for the Attica–Crete HVDC interconnection line (between Stachtoroi island in Attica and Korakia beach in Crete), allowing the preliminary study to be conducted swiftly, giving satisfactory results about electric field gradient, ground potential rise and resistance to remote earth of electrodes stations for the near and far electric field. [ABSTRACT FROM AUTHOR]