Guest Editorial Converters and Semiconductor Circuit Breakers for HVDC and DC grids (2016)

We are very much thankful to the IET Power Electronics editorial board for instructing this Special Issue on “Converters and Semiconductor Circuit Breakers for HVDC and DC grids” and we are glad to be contributing as editors. We are grateful to editors Prof Volker Pickert and Dr Mohamed Dahidah for their support, to all the reviewers, and to the IET editorial office, who helped build this Special Issue into its final shape. In the final version for publishing, 16 papers have been selected for this journal, consisting of: six papers related to MMC converters, four papers dealing with DC Circuit breakers, four papers related to DC/DC converters and two papers on the topic of DC grid or multiterminal HVDC. It is good to see that industry participates in five of the selected papers while one paper has lead authors from industry. Two major manufactures (ALSTOM and SIEMENS) have contributed in the articles on the DC CB topic, and also two transmission system operators, RTE France and SSE Scotland are associated with papers dealing with DC CB and high power DC/DC converters. The MMC converter topology has transformed HVDC in the past 10 years and it has enhanced interest in HVDC applications. Because of the recent improvements in performance of HVDC MMC converters, there is now a real prospect of developing DC transmission grids. There is however significant scope for further technology development and this Special Issue makes good contribution with six articles. Two articles are focused on control and modulation developments with MMC converters. It is of interest that operation with low modulation ratio has been identified for its importance and the article “Bifurcate modular multilevel converter for low-modulation-ratio applications” proposes new MMC topology and control for such operating conditions. The article “Characteristics of nearest level modulation method with circulating current control for modular multilevel converter” identifies that adequate nearest level control with circulating current suppression can also achieve other performance improvements, such as THD reduction or fundamental AC voltage increase. The fault conditions are extremely important with high-power converters, and much ongoing world-wide research is centred on this topic. The article “Fast fault detection method for modular multilevel converter semiconductor power switches” demonstrates theoretically, and on a hardware prototype, that faster and more accurate fault detection is possible with alternative sensor positions and new control algorithms are adopted. Two articles research loss calculation in modular converters, but different topologies are employed. The article “Analytical determination of conduction power loss and investigation of switching power loss for modified flying capacitor multicell converters” provides an in-depth evaluation of loss in a Modified Flying Capacitor Multilevel converter. The second paper “Analysis and calculation on switching frequency and switching losses of modular multilevel converter with maximum sub-module capacitor voltage deviation” recognises the important impact of non-ideal submodule voltages, and develops a method for detailed loss calculation considering capacitor voltage ripple. The hardware demonstration confirms feasibility of the proposed method. There is significant potential benefit in developing a hybrid HVDC converter, with valves consisting of mix of thyristor switches and half/full bridge submodules, as demonstrated by increasing world-wide research on the topic. The article “Enhanced line commutated converter with embedded fully controlled sub-modules to mitigate commutation failures in high voltage direct current systems” further contributes on the topic by demonstrating that a hybrid converter can reduce probability of commutation failure. Semiconductors will play an important role in most high-power DC circuit breakers, but the performance requirements will be much different from traditional applications. Significant further research and development is required in this area and it is a very good to see that four papers in this issue contribute to modelling and technology development. The editors hope that semiconductor DC CB development will become an established topic in this journal. The article “Successful fault current interruption on DC circuit breaker” deals with new generation of mechanical DC CBs which utilise power electronics for timely initiating resonant circuits. Some very important results are obtained both theoretically and on a small-scale prototype, which increase understanding of current commutation and current interruption in DC CBs. A very important contribution towards understanding of application of DC CBs is made by a major industry player in the article: “System integration aspects of DC circuit breakers”. The maximal interrupting current in DC CB semiconductors, and the maximal VSC converter current below blocking level are the major constraints, which will require significant tradeoffs in other parameters like; component insulation level, inductance size or size of surge arrester. In the article “Hybrid HVDC circuit breaker with self-powered gate drives” the energy from the fault path is used to charge local capacitors which provide power for the driver of the main semiconductor switch. The design method improves reliability of the DC CB and the concept is proven on a 2.7kV, 700A prototype. An accurate system-level hybrid DC CB model is presented in the article “Modelling of high-power hybrid DC circuit breaker”. The model considers all internal subunits (main and auxiliary valve, and two mechanical switches) and provides detailed control/interlocking logic which enables accurate studies of DC faults in large DC grids and protection system development. The internal failure modes for protection reliability can also be studied. The DC/DC converters will take an important role in future DC grids though they require substantial new technology developments as it is demonstrated in the four papers on this topic. Three articles are focused on optimising DC/DC converter design towards reducing losses, which highlight the importance of component losses in high-power applications. The paper “High power modular multilevel converter optimal design for DC/DC converter applications” emphasises that MMC design for DC/DC applications will be substantially different from 50Hz MMC technologies, and it provides advice for reducing losses in DC/DC converters. This is achieved by trading power quality which may not be essential requirement in the inner circuit of DC/DC converters. The MMC bridge design and control optimisation inside DC/DC converters is further researched in the paper “Design and implementation of the low computational burden phase-shifted modulation for DC-DC modular multilevel converter”. A theory behind single switching of submodule per fundamental cycle is presented and benefits of such operation are demonstrated on a small-scale hardware. Some notable DC/DC converter loss reduction is achieved using zero voltage switching in the article “Voltage balancing control of isolated modular multilevel DC-DC converter for use in DC grids with zero voltage switching”. A comprehensive up-to-date review of high power DC/DC converters with 120 references is presented in the article “Review of DC-DC converters for multi-terminal HVDC transmission networks”. The development of high-power DC grids will place new demand on performance of AC/DC MMC converters and also require new power flow controlling components as is demonstrated in two papers on this topic. The question of communication requirements between protection units in a small 4-terminal DC grid consisting of full-bridge MMC is addressed in the article “Application of multilevel full bridge converters in HVDC multiterminal systems”. Full bridge MMC enables control of fault current magnitude, and it is demonstrated that this enables longer fault clearing times and minimal communication requirements across the DC grid without a compromise in protection selectivity. The article “Operation and control of an insulated gate bipolar transistor-based current controlling device for power flow applications in multi-terminal high-voltage direct current grids” illustrates that DC grids may require a range of new semiconductor-based units (apart from VSC AC/DC converters) on various locations inside the grid and in either series or shunt connections. A Current Flow Control Device is presented, along with full control system, and power balancing between three DC cables is demonstrated. A small scale prototype provides hardware demonstration. We are very pleased with the publication of our first Special Issue in IET Power Electronics. The Special Issue includes a variety of the latest research from both academia and industry on an interesting and timely topic. This will positively contribute to the quality and the development of the Journal in particular and the research community at large. Volker Pickert Editor-in-Chief Mohamed Dahidah Deputy Editor Professor Dragan Jovcic is a director of Aberdeen HVDC Research centre at University of Aberdeen, UK. His main research interests are HVDC power transmission, DC transmission grids and FACTS. He is author of the book “High Voltage Direct Current Transmission, Converters Systems and DC grids” published by Wiley 2015, and author of around 100 journal/conference articles. Professor Xu Cai, Department of Electrical Engineering, Shanghai Jiao Tong University (SJTU), China. Director, Wind Power Research Center of SJTU. Vice Dean, Energy Research Academy of SJTU. His main research interests are power conversion and grid integration of renewable energy, DC grid integration of large wind farm, and grid integration of energy storage system.