Current control of a modular multilevel converter for HVDC applications Fernando Martinez-Rodrigo a, * , Santiago de Pablo b , L. Carlos Herrero-de Lucas a a Electronic Technology Department, University of Valladolid, Industrial Engineering School, Calle Francisco Mendizabal 1, 47014 Valladolid, Spain b Electronic Technology Department, University of Valladolid, Industrial Engineering School, Paseo del Cauce 59, 47011 Valladolid, Spain article info Article history: Received 23 March 2014 Accepted 14 April 2015 Available online 14 May 2015 Keywords: Current controlled converter HVDC transmission Modular multilevel converter (MMC) Multilevel converters Offshore transmission abstract Multi-modular converters (MMC) are an emerging and promising option for high voltage direct current (HVDC) transmission, connection of offshore wind farms and FACTS. For such converters, two new strategies for current control are proposed, in which a band is defined around the reference current of the three phases, and modules to be turned ON are chosen to keep the three phase currents within the bands. In the first strategy, only the voltage levels adjacent to the grid voltage level are chosen; this is called “constant excitation” and it is the most appropriate when the number of modules per arm is small. The second strategy uses an excitation proportional to the current error, and it is the most appropriate when the number of modules per arm is great. The theoretical foundation of the strategies and the simulation results within an external active and reactive power control loop are presented. Finally, the current control strategies were applied to HVDC transmission from offshore wind farm to the onshore grid. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The cost of an offshore wind farm (OWF) is mainly composed of wind turbines (30e50%), substation and electricity infrastructure (15e30%), foundations (15e25%) and installation (0e30%) [1]. As a part of the electricity infrastructure, the interest in high voltage direct current (HVDC) transmission has grown lately due to the increase in the construction of OWF. In general, the OWF can be connected to the mains via DC or AC transmission. The presence of power electronic converters makes DC transmission more expen- sive when the wind farm is near the coast, while the higher number of cables makes the transmission more expensive in AC when it is far from the coast. The boundary between the two situations is about 90 km [2]. Also, the length of AC shielded transmission cables is limited due to their capacitance, responsible for the high reactive power consumption; conversely, DC cables do not need reactive power [3]. Offshore wind farms can have a very large number of high- power turbines (up to 5 MW and even higher). When trans- mission is done by HVDC, the topology of transmission lines can be: one cable and the return by the earth, two active cables and return by the earth, one active cable for going and one for the return, and two active cables for going and one for the back. An offshore wind park with HVDC transmission is composed by a low or medium AC generator in the turbine, step-up transformer(s) and electronic converter(s) from AC to DC high voltage. All of them can be built according to several topologies: all the turbines parallelized in low AC voltage þ one transformer þ one electronic converter, groups of turbines parallelized in low AC voltage þ one transformer per group (groups are connected in the high voltage AC) þ one con- verter, groups of turbines parallelized in low AC voltage þ one transformer per group þ one converter per group (groups are connected in the high voltage DC); other configurations may be found in publications [4,5]. The converters used in HVDC transmission are of two types, while a third type is under investigation and being tested [6]. The two types of converters commonly used are line commutated converter (LCC) [7] and voltage source converter (VSC). The VSC converters can be two-level (2L) or multilevel. Under investigation and experimentation is the modular multilevel converter (MMC) (Fig. 1), first introduced for HVDC applications by Marquardt [8]. The VSC-MMC converters have the following advantages over other topologies used in HVDC transmission [9]: 1) The capacitive energy storage is distributed. 2) It is a modular topology, so it is easily scalable. * Corresponding author. Tel.: þ34 983423921; fax: þ34 983423490. E-mail address: fer_mart@tele.uva.es (F. Martinez-Rodrigo). Contents lists available at ScienceDirect Renewable Energy journal homepage: www.elsevier.com/locate/renene http://dx.doi.org/10.1016/j.renene.2015.04.037 0960-1481/© 2015 Elsevier Ltd. All rights reserved. Renewable Energy 83 (2015) 318e331