Contents lists available at ScienceDirect Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes Review Ground/sea return with electrode systems for HVDC transmission M. Marzinotto a , G. Mazzanti b , M. Nervi c, ⁎ a TERNA S.p.A., Via della Marcigliana 911, 00138 Roma, Italy b Dept. Electrical, Electronic and Information Engineering, Univ. of Bologna, V.le Risorgimento 2, Bologna 40136, Italy c Dept. of Electrical, Electronic, Telecommunications Engineering and Naval Architecture – (DITEN), Via all'Opera Pia 11a, 16145 Genova, Italy ARTICLE INFO Keywords: DC power transmission Ground return Ground electrodes Sea electrodes Shore electrodes ABSTRACT HVDC transmission is booming worldwide. Despite electrode systems are one of the frontiers of DC technology, being a key component of HVDC links, in the experience of the Authors many aspects of their manufacturing, design and operations are less known among the HVDC community with respect to other components, like cables and converters. Aiming at filling this gap, this paper is a survey on ground/sea return with electrode systems for HVDC links, particularly focused on the preliminary design. The configurations of HVDC systems (monopolar, homopolar, bipolar) are recalled first, highlighting the advantages of ground/sea return. Then, ground elec- trodes are treated, illustrating the basic parameters and geometrical arrangements for the design of ground electrodes. Later, shore and sea electrodes are illustrated. Considerations relevant to ground current close the paper. 1. Introduction Although dating back to the 1930s [1], High Voltage Direct Current (HVDC) transmission systems are now becoming more and more com- petitive compared to High Voltage Alternate Current (HVAC) trans- mission systems, especially for bulk power transmission over long dis- tances [2]. Indeed, the improvements in reliability, performances, costs and losses of AC/DC converters have highlighted the many technical and commercial advantages of HVDC transmission. Thus, many HVDC systems have been put in service recently [3,4]. However, the design of electrodes for HVDC transmission is not trivial, since many requirements have to be taken into account, e.g. low electrical resistance to cut transmission losses, reduced and easy maintenance, high environmental compatibility for the safety of human beings and fauna [1,5,6]. Furthermore it is important to select the electrode site so as to not interfere with the grounding systems of the converter station and to prevent the risk of electrolytic corrosion of buried/immersed metallic structures. Several aspects of HVDC transmission are treated in many papers and books [1–4,7], with great emphasis on lines and converters. However, as for the Authors’ experience, many aspects of electrode systems manufacturing, design and operations are less known among the HVDC community. Nevertheless ground/sea return and electrode systems are fundamental in the performances of HVDC links, since they enable the operation of the system as a monopole with ground/sea return. This leads to two main advantages also for bipolar systems: (1) the bipolar dc systems can be realized in sequence, i.e. one pole first and the second later, so as to put the system in service as a monopole until the commissioning of the second pole; (2) in the final bipolar operation, in case of outage of one converter or one pole. In this con- dition, the DC system can transmit half its rated power; this greatly enhances the reliability and the flexibility of bipolar systems, and al- lows multiple configurations and operations of the dc link [2]. In ad- dition bipolar systems equipped with electrode for ground/sea or dedicated metallic return (unbalanced bipolar systems), can switch immediately from bipolar to monopolar operation (e.g. when a fault occurs in one converter or pole) without shutting the whole system down. This paper is a survey on ground/sea return with electrode systems for HVDC transmission, based on reference literature papers and books. The various HVDC schemes (monopolar, bipolar, homopolar) are re- called in Section 2 with peculiar reference to ground/sea return. Ana- lytical and numerical methods to study the behavior of HVDC ground electrodes are compared in Section 3. The basic parameters and the geometrical arrangements for the design of ground electrodes are dealt with in Section 4. The design of sea electrodes is illustrated in Section 5. Section 6 is devoted to particular issues related to ground current, i.e. electrolytic corrosion, chemical aspects, interactions with pipelines, electrical effects. Conclusions can be found in Section 7. https://doi.org/10.1016/j.ijepes.2018.02.011 Received 13 September 2017; Received in revised form 11 January 2018; Accepted 8 February 2018 ⁎ Corresponding author. E-mail address: mario.nervi@unige.it (M. Nervi). Electrical Power and Energy Systems 100 (2018) 222–230 0142-0615/ © 2018 Elsevier Ltd. All rights reserved. T