## 1 Introduction  
Depending on the design of the HVDC interconnection configuration, the question of the presence (or not) of a ground return current arises. Non-use of a metallic return conductor leads to a considerable reduction in construction costs for great interconnection lengths [1]. After all, this was the reason why the Hellenic Independent Power Transmission Operator (IPTO), in 2018-2019 during its study of the bi-directional interconnection, between the island of Crete and mainland Greece in the region of Attica (with nominal power characteristics of 1 GW, voltage \(\pm\)500 kV DC using voltage source converters), opted for a bipolar heteropolar configuration, with a ground return (especially through the sea), since from a total length of approximately 380 km (the largest part of which (330 km) being underwater), the ground return conductor can be limited by, at least, 310 km [2,3]. Thus, around 200 million euros is saved, as the construction of two electrode stations in the sea is more economic. The latter is also confirmed by the study [4], where the limit of using a metal conductor, over using the ground, in HVDC interconnections is in the order of 20 km. The locations of the stations (the island of Stachtoroi in Attica and Korakia beach in Crete) meet a number of criteria, such as close proximity to the conversion stations, and sufficient distance from a residential area or an area with man-made facilities, to avoid unwanted repercussions resulting from their operation such as electrochemical corroisons, or regarding the possibility of issuing building/environmental permits [1]. In [2], the classic configuration of the shoreline electrode station was first proposed, with the electrodes placed in frames in a straight arrangement and the frames in turn placed parallel to the longitudinal axis of the breakwater. This is the basic configuration that has been implemented in the Muskrat Falls Project [5, 6, 7] and is recommended in the guidelines of CIGRE B4.61 675: 2017 [1] and in the literature [8, 9, 10]. However, this is not necessarily the case, as other underwater devices made of fibre-reinforced polymer concrete have been proposed, whose electrodes are not directly accessible [11]. In the present case [2], however, the following problems were identified:  
* _Marginal positioning of the electrode stations_: In both areas, but especially in the area of Stachtoroi, the necessary lengths of the electrode stations can only be ensured marginally by reaching the shores, requiring dredging of the pond under construction, etc. * _Exchange of water within the pond_: The construction of a rubble-mound dam, permeable to water through a suitable layering of rubble, does not ensure the complete exchange of water within the pond, due to the small waves in the Argosaronic gulf and the small tidal effect, in contrast to the Muskrat Falls Project [5,6]. * _Dam height_: The initial height of the dam was significantly shorter than the expected maximum wave height, especially in the area of Korakia, so this naturally leads to an increase in the height of the dam and, by extension, to an increase in its other parameters. Therefore, in this paper, various ways of positioning the frames, with a linear arrangement of electrodes in a shoreline station with emphasis on the near-field, are explored. Effects concerning the far-field, such as the effect of corrosion on the railway network and gas pipelines, due to the presence of a ground direct-current return from HVDC systems [12, 13, 14], are essentially independent of the positioning of the electrodes within the station; hence, they are not considered here.