Guest Editorial

Abstract

Wind energy is a mature technology among renewables and currently plays a significant role in global electricity production. The installed capacity has increased exponentially from 6.1 GW (gigawatt) in 1996 to 486 GW by 2016 [1]. The majority of this capacity is located onshore, but offshore is also being promoted in Europe, and will approach 20% of the total wind energy installed capacity by the end of 2016. In certain countries like Denmark, wind power has become a significant part of the energy mix with 36.8% of current penetration (19 and 16% in Spain and Germany, respectively) [2]. In this paper of high diversity in the type of converters, generators and control strategies, electrical engineers are challenged to improve the performance of wind energy conversion systems (WECSs) in terms of robustness, efficiency, fault tolerance, and controllability. Different special sections have been devoted to bringing new solutions to wind power applications, either with a general scope [5, 6] or specifically focusing on the power electronics [7], electrical machines [8], and control and grid integration methods [9]. This Special Issue (SI) updates the knowledge in the field and provides some new solutions for multi-MW WECSs. The call for papers resulted in 27 submissions, of which 7 papers are included in this SI. The accepted papers cover various aspects, and a categorisation is provided in Table 1 in order to ease the identification of the content. The research topic of paper and corresponding reference number are shown in Table 1. Since each paper typically encompasses different topics, the topic related to the main contribution of the paper is shown in bold font. The research and development trends in the current wind energy industry are also reflected in Table 1, which include different types of converters (e.g. multi-level VSCs), electric machines (e.g. multi-phase modular PMSGs), control techniques (e.g. fault-tolerant predictive control), and topologies (e.g. series-connected modules). The SI commences with a review paper that provides an overview of PMSG-based WECSs [10]. Although there is no standard in the wind energy industry, WTs based on PMSGs are the most popular option among full-scale converter-based full-variable-speed ones, hence the choice of the scope in this initial survey paper. First the paper shows the most common types of power converters, including low/medium-voltage converters that are cost-effective below/above 3.0 MW. The different types of converters and their arrangement are reviewed, showing that diverse types of multi-level and multi-phase options are available in the wind energy industry. Grid integration of offshore WECSs is covered next, detailing different possibilities to connect the WTs and perform the transmission, either by high-voltage AC or high-voltage DC (HVAC or HVDC). Control issues in PMSG-based WECSs are also included in the survey, covering the two most popular methods: field-oriented control and direct-torque control for machine-side converter, and voltage-oriented control and direct-power control for grid-side converter. Fault ride through issues are addressed next, detailing the methods to allow the WTs to comply with grid code requirements. The section on future trends finally exposes the good prospect of direct-drive technology, PMSG-based systems and high-temperature superconducting synchronous generators, medium-voltage converters, gallium nitride and silicon carbide devices, and HVDC transmission in the next generation WTs rated for 10–15 MW range. Even though increasing attention is paid to PMSGs in wind applications, DFIGs with partial-scale converter topologies have been an industry standard for the last few decades and much research is still going on this type of WECS. The research papers of [11, 12] deal with DFIGs aiming to improve the WT performance by means of two new control approaches. While Gonçalves et al. [11] suggest the use of finite-control set model predictive control (MPC), Martinez et al. [12] tackle the control approach implementing a variant of the sliding mode control (SMC). In the former paper, the main objective is to make the WT fault tolerant against open-circuit and short-circuit faults, whereas in the latter paper the focus is placed on improving the performance of the WTs in the presence of unbalanced and distorted grid voltages. Fault tolerance is highly appreciated in WECS and Gonçalves et al. [11] provide both a fault detection technique for open-circuit faults and a fault-tolerant control proposal. Since the rotor side is fed by a three-level NPC converter, it is possible to continue operating in the case of a power switch fault. The detection is based on the voltage errors and the post-fault control simply avoids the switching states that are no longer available, fully exploiting the discrete nature of MPC. On the other hand, the robustness against grid disturbances is also a trending research topic and Martinez et al. [12] suggest the use of a second-order SMC for machine- and grid-side converters. The proposed control scheme is tested against unbalanced/distorted grid voltage conditions and parameter detuning confirming the good properties of second-order SMC. Aiming to further enhance the performance of DFIG-based WECSs, the brushless doubly-fed induction generator (BDFIG) appears as an alternative that aspires to maintain the DFIG features and improve the performance during unbalanced grid voltage conditions and LVRT. The application of BDFIGs for WTs is covered next in [13]. The content in [13] is that of a review paper, hence the paper is devoted to making a survey on the prospect of BDFIGs in WECSs. Apart from an historical perspective of BDFIGs, Strous et al. [13] provide a description of the operating principles, design considerations, modelling, and control issues. A performance comparison with other alternative generators (PMSGs, DFIGs etc.) is provided and the main research challenges are identified. The remaining papers [14-16] deal again with PMSG-based WECSs, which were identified in [10] as the main trends in wind energy applications. While the authors in [14, 15] focus on the power electronics and the topology for multi-MW systems, Li et al. [16] aim to improve the WT performance by design considerations. A full-scale power converter topology for direct-drive medium-voltage 10 MW WTs is suggested in [14] with a five-level Vienna rectifier on the machine side and a five-level neutral point clamped (NPC) converter on the grid side. The voltage balancing of the capacitors is achieved using a flying-capacitor auxiliary bridge leg. The proposed topology is compared to other alternatives in terms of the number of silicon devices, estimated costs and efficiency. A control method with special focus on the voltage balancing of the DC-link neutral points is also included and experimentally tested. Another full-scale transformerless converter topology is presented in [15], where half-bridge modules are cascaded in series to generate medium voltage with relatively low-voltage semiconductor switches. As in [14], a detailed comparison with other popular topologies can be found in [15]. The proposed topology is accompanied with a per-phase control scheme that accounts for the capacitors voltage balancing. Even though the proposal includes a six-phase PMSG, the series connection of the converter modules does not provide fault-tolerant capability in order to elevate the DC-link voltage. The topologies and control approaches in [14, 15] are different, but some common features include the transformerless operation at medium voltage using a full-scale arrangement with PMSGs. Multi-phase PMSGs can be found in [16], but in this case the focus of research is shifted to the design aspects. The paper provides guidelines for the design of fractional slot multi-phase modular PMSGs with single-layer concentrated windings, exploring different slot/pole number combinations in order to find an optimum value. The electromagnetic performance, such as winding factors, air-gap magneto-motive force, back electro-motive force, cogging torque, average torque, and torque ripple are investigated for different arrangements. The finite element and experimental analysis allow the establishment of general rules that can be extended to the design of multi-MW PMSGs for WECSs. The Guest Editors hope that this SI will provide a further insight into the challenges in multi-MW WECSs and will stimulate new research ideas in aspects such as generator design, novel topologies, use of modern control approaches, enhanced behaviour against grid disturbances, and the achievement of fault-tolerant WTs. The Guest Editors take this opportunity to thank all the authors who responded to the call for papers for this SI. The Guest Editors were deeply indebted to the reviewers, whose inputs were indispensable. They also thank the IET Editorial Office and Dr. Emil Levi for their help and support that made this SI possible. Mario J. Duran was born in Bilbao, Spain, in 1975. He received the M.Sc. and Ph.D. degrees in Electrical Engineering from the University of Málaga Spain, in 1999 and 2003, respectively. He is currently an Associate Professor with the Department of Electrical Engineering at the University of Málaga. His research interests include modelling and control of multiphase drives and renewable energies conversion systems. Samir Kouro received the M.Sc. and Ph.D. degrees in Electronics Engineering from the Universidad Tecnica Federico Santa Maria (UTFSM), Valparaiso, Chile, in 2004 and 2008, respectively. He is currently an Associate Professor at UTFSM. From 2009 to 2011, he was a Postdoctoral Fellow with the Department of Electrical and Computer Engineering, Ryerson University, Toronto, ON, Canada. He is the Principal Investigator of the Solar Energy Research Center (SERC Chile) and Titular Researcher of the Advanced Center of Electrical and Electronics Engineering (AC3E), both being Centers of Excellence in Chile. He has coauthored over 150 refereed journal and conference papers. Dr. Kouro received the IEEE Industrial Electronics Society Bimal Bose Award in 2016, the J. David Irwin Early Career Award in 2015, the IEEE Power Electronics Society Richard M. Bass Outstanding Young Power Electronics Engineer Award in 2012, the IEEE Industry Applications Magazine First Prize Paper Award in 2012, the IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS Best Paper Award in 2011, and the IEEE Industrial Electronics Magazine Best Paper Award in 2008. Venkata Yaramasu (S'08–M'14) received his Ph.D. degree in Electrical Engineering from the Ryerson University, Toronto, Canada, in 2014. From 2014 to 2015, he was a Postdoctoral Research Fellow at the Ryerson University. He joined Northern Arizona University, USA, in 2015, where he is currently an Assistant Professor of Electrical Engineering in the School of Informatics, Computing, and Cyber Systems. He has authored/coauthored more than 50 peer-reviewed papers, a Wiley – IEEE Press book on Model Predictive Control of Wind Energy Conversion Systems, 2 book chapters, and 10 technical reports. He is a recipient of over 15 teaching and research excellent awards, including Second Prize Paper Award from the IEEE JOURNAL OF EMERGING AND SELECTED TOPICS IN POWER ELECTRONICS in 2015. His research interests include renewable energy, high power converters, variable-speed drives, electric vehicles, smart grid, energy storage, and model predictive control.