0885-8993 (c) 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication. Citation information: DOI 10.1109/TPEL.2017.2709259, IEEE Transactions on Power Electronics TPEL-Reg-2016-06-1144.R2 1 Abstract—The race to increase the efficiency and reduce the power losses in transmission systems has resulted in the substantial growth of High Voltage Direct Current (HVDC) transmission systems. Moreover, the interconnection of these transmission systems significantly increases their reliability. However, the control of these meshed grids is a key problem that usually is managed through the control of the VSC’s in those grids. But, the control of the VSC can be complemented with a reconfiguration algorithm. This paper proposes the use of the Particle Swarm Optimization (PSO) algorithm, in order to reconfigure meshed High Voltage Direct Current (HVDC) transmission systems and reduce losses. The proposed algorithm has been tested in the CIGRE benchmark grid, which comprises of several offshore wind farms that generate energy sent to the grid through several HVDC transmission lines. The results show that as the energy generation changes due to wind changes, the grid topology must be reconfigured in order to achieve the maximum efficiency. Doing this reconfiguration, power savings around 18-19% could be achieved. Index Terms—HVDC, Off-shore, Particle Swarm Optimization, Reconfiguration, Wind Farm. I. INTRODUCTION NERGY consumption in the world is constantly growing [1]-[2]. Simultaneously, environmental crisis caused by climate change should be mitigated. One way to reconcile these two facts is by using sustainable and clean energy sources, with high growth potential [3]-[4]. Moreover, the electricity production cost from coal, gas, nuclear plants and petroleum keeps rising, while the cost of renewable energy is clearly decreasing. For example, grid parity has already been achieved in many places, such as Hawaii [5]. Consequently, in the recent years, most countries have invested to improve the use of renewable energy, basically, solar photovoltaic, hydroelectric power, and wind energy. The Manuscript received June 17, 2016; revised November 17, 2016 and April 7, 2017; accepted May 16, 2017. This work has been supported by research projects CONPOSITE (ENE2014-57760-C2-2-R Ministerio de Economía y Competitividad) and PRICAM (S2013/ICE-2933 Consejería de Educación, Juventud y Deporte de la Comunidad de Madrid). A previous version of this paper has been presented at the International Conference on Renewable Energy Research and Application (ICRERA), 19-22 Oct. 2014, Milwaukee, WI (USA). Authors are with Electronics Department, Universidad de Alcalá, Alcalá de Henares, 28801 Madrid, Spain (e-mail: ines.sanz@depeca.uah.es; miguel.moranchel@depeca.uah.es; javier.moriano@depeca.uah.es; fjrs@depeca.uah.es; susel.fernandez@depeca.uah.es). main disadvantage of photovoltaic energy is the need for large tracts of lands to cover the demands of little urban zones, due to solar panels’ low efficiency. Concentrating on hydroelectric power, this kind of energy is not available in all countries, like the Middle Eastern countries, and it is limited to very specific places. Moreover, this technology has several disadvantages that are related to environment, which diminishes people’s interest in it. In contrast, wind energy uses a less amount of land to produce the same amount of energy. Nevertheless, wind energy has a great environment impact [1]. The energy produced by wind turbines is highly unpredictable due to multiple reasons, like wind behavior. To attenuate this variability, wind farms are being moved to the sea, where the wind regime is more consistent, and it is possible to occupy larger areas. In Northern Europe there are many large offshore wind farms with power levels between 100 MW and up to gigawatts [7]. The transmission of all this energy from the sea to the land is a big challenge that has to be faced. High Voltage Direct Current (HVDC) grids are some of the best choices to connect this type of renewable source from the sea to the land [8]-[9]. With technological advancement seen in semiconductors, HVDC transmission acquired importance, because it was becoming easier rise the voltage to unsuspected limits. Nowadays there are HVDC lines up to 800kV, also called Ultra High Voltage Direct Current (UHVDC) [10]. These types of lines are used when the amount of energy to be transmitted is very high. In order to increase the reliability of the transmission and minimize the losses, multi-terminal links are increasingly used to transmit energy from the farms to the land [11]-[12]. Multi- terminal links connect several wind farms together with several grids in the land. This produces a meshed transmission grid that allows the distribution of energy between different nodes. The multi-terminal VSC (Voltage Source Converter)- HVDC system can connect different offshore substations with similar voltages. In the case of the stations with different voltages, it is possible the use of intermediate stations that implement a DC-AC-DC voltage transformation, in order to change the DC voltage as desired, although it is not essential [13]. The development of this type of connections requires proper high level control methods [14]. The most-used control technique is the DC voltage-current droop control, which Reconfiguration Algorithm to Reduce Power Losses in Offshore HVDC Transmission Lines Inés Sanz, Miguel Moranchel, Javier Moriano, Student Member, IEEE, F. Javier Rodríguez, Member, IEEE, and Susel Fernández E