Alternative Simplified Analytical Models for the Electric Field, in Shoreline Pond Electrode Preliminary Design, in the Case of HVDC Transmission Systems

Abstract

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.

Comparison of Different Configurations for Shoreline Pond Electrode Station for HVDC Transmission Systems—Part I: Electric Field Study for Frames of Linear Electrode Arrangement Based on a Simplified Analytical Model

“…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:…”

“…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 ±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.…”

“…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].…”

“…The second case is based on the Rusk methodology [8,[28][29][30][31], taking into account the electrode resistance and the electric current intensity in water and soil, while the Uhlmann method disregards the effect of the soil and seabed [8,32]. In [2], the respective analytical methods of point current sources are unified. Furthermore, the addition of the breakwater to the analytical methods [1,27], as well as the use of a linear current source (instead of a point source), with a cylindrical water zone of specific height, were also proposed to the study of the near-electric field.…”

“…In this paper, the preliminary study of shoreline pond electrode stations with electrode arrangements other than the typical one (where the vertical or inclined electrodes linearly placed form frames, and then the frames are placed linearly, parallel to the longitudinal axis of the breakwater) is carried out in order to address the space limitations of the specific locations (Stachtoroi and Korakia for the HVDC interconnection of Crete, Attica), covering the limits of the potential gradient for the protection of marine life and divers in marine electrodes in steady and transient states and of the absolute potential, in relation to remote earth, for the steady state. To calculate the electric field gradient, ground potential rise, and resistance to remote earth of electrode stations, the method proposed in [2] is used, which means that for the near-field, a linear current source is used for each electrode, which extends cylindrically in a sea/soil or dam zone of constant circumference. The respective mathematical background of a point current source, in the form of an appropriate wedge of sea on the ground, is used for the far-field (extension of [1,27]).…”