Control of Grid-Side Power Converters
Prof. Dr. Slobodan N. Vukosavic
Department of Electrical Engineering
University of Belgrade
Slobodan N. Vukosavic, Ph.D. EE, was born on January 27, 1962. He obtained his B.Sc. and Ph.D. degrees at the School of Electrical Engineering, University of Belgrade in 1985 and 1989, respectively. He is elected an associate professor at the University Belgrade in 1998, and for full professor in 2003. He was the Head of The Power Engineering Department. He was visiting professor, lecturer at postgraduate courses, and gave seminars at technical institutes and universities in Boston (NEU), Novi Sad, in Italy (TO, GE), and Banja Luka. With Vickers Electric since ‘91, his R/D team developed motion control products for industrial robots in use at major EU car manufacturers. His interests include Electrical machines, motion control (MC) technologies applied to general automation, embedded DSP solutions in power electronics and electrical drives (PED), power conversion, clean and renewable energy technologies. In the field of electrical machines, design and control resulting in an increased efficiency and reliability, multiphase machines, SR machines, and the application of DSP technologies in monitoring and diagnostics of large machines. MC research is focused on transmission-less structures with linear motors, and performance improvement of conventional robots by anti-resonant controllers, suppressing the mechanical resonance in compliance-critical, flexible transmission. Proprietary control & tuning for MC systems extend the bandwidth, reduce stiffness and allow for shorter cycles. His R/D activity in PED include the motor-converter integration, efficiency optimized control, switching techniques reducing the insulation stress, state reconstruction for sensorless drives and parameter estimation focused on efficiency, robustness and diagnostics. Efforts in the field of energy conversion include novel topologies and embedded control providing reduced conversion losses, and concede savings on iron, copper and power semiconductors. His interest include electrostatic precipitation (ESP) applied to filtering pollution gasses released by power plants and industry. He has been elected corresponding member of Serbian academy of sciences and arts since 2015. His over 100 scientific papers are cited in leading international publications, including Wiley Encyclopaedia of E&E Engineering). Author of patented technical inventions. Member of the Serbian national academy of engineering, adjunct professor at the North Eastern University (Boston), IEC TC9 member, IEEE and IEE reviewer, member of the of the Belgrade University Council, Head of the Power engineering department. His students won the 1st prize at the IEEE “FEC” contest in 2005. He published several textbooks and monographs, the most recent one being Digital Control of Electrical Drives, published in 2007. by Springer. He founded The Laboratory for Digital Control of Electrical Drives and Power Converters at the School of Electrical Engineering, at The University of Belgrade (DDC Lab), with the goal of applying the DSP technology in power electronics and drives, with the motto to save on iron and copper by smart use of silicon and software. The Laboratory founded by Slobodan Vukosavić works in the field of DSP-controlled power conversion, general purpose drives, Automotive, Military, Appliances, Electrical drives, Motion control systems, UPS, electrical traction, fork lifts, golf carts, alternative energy. Technical expertise includes long, mature experience in designing servo drives and motion control systems. Consulting in power electronics and industrial drives. Analytical studies, experimental and prototyping capability in building low-cost appliance drives designed to the norms. Complete product development cycle, potentials and risks, rapid prototyping, Compliance to applicable standards. Providing design and manufacturing documentation tested and qualified samples. Development and design of converter topologies, Hardware, software & firmware, Analog & digital, Circuit & PCB designs. Reliability, safety and compliance. Fresh ideas are generated from a synergy of DDC-Lab mature experience of our engineers and the energy brought in by our new University postgraduates. Experience and flexibility we gained over the years turned us into a cost-effective, one-stop service provider for your product design, prototyping, qualification and production needs. Our outsource design capacity and flexibility ensure correct product integration, technical and post-production support. Our technical expertise and references are the unique benefits, completing our service provider offer. The DDC Lab offers quick and simple one-stop product design services, with both the cost and technical effectiveness. The Lab performed research for Sever-Subotica, Sprade-COM India, Serbain Railways, Nikola Tesla Power Plant, Zastava-automobili, Vickers-Electric, MOOG-Electric, International Rectifier, Lord-Baladyne Corp., Emerson Electric, Msemicon-IRL, GND-UPS Taiwan, Semicron, ELGE-Mi, Polimotor-GE, Iskra-avtoelektrika, Atech-SI, and other domestic and international companies and centers.
The ac grids include an ever increasing share of static power converters. These electronically controlled devices include the source-side converters, the bus converters and the load-side converters. With the advent of distributed and local accumulation and considering the regeneration needs of electrical drives, the load-side have to be bidirectional, capable of supplying electric energy into the grid during brief intervals of time. For this reason, the functionality of all the grid-side converters is reasonably similar. When interfacing the ac grids, most grid-side converters inject the ac currents in synchronism with the grid voltages. The properties of the synchronization device affect the response of the grid-side converter output power to the grid transients. Traditional sources in ac grids are the synchronous generators. Therefore, the grid controls and protections are designed so as to suite the static and dynamic properties of synchronous generators. An increased share of grid-side power converters with dynamic properties different than the ones of the synchronous machines could have an adverse effects on the grid operation and stability.
Along with the new level of flexibility and programmability, grid connected power converters also introduced several problems caused by imperfection of controls and components. Injection of parasitic dc currents and harmonics raises the power quality issues, increases the losses, jeopardizes distribution transformers and disturbs some sensitive ac loads. Volatile nature of wind-power and solar-power inputs calls for the means of energy accumulation, some of them also using the static power converters as the interface towards the ac grid.
An increased amount of electronically controlled power generation and consumption introduces a quite new stability problem in the grid. In electronically controlled grid-connected static power converters, the power is maintained unaffected by the ac voltage. As a consequence, their model includes negative resistance. Moreover, the most common ways of synchronizing the grid-connected converters introduces the frequency-power relation which is quite different than in traditional synchronous generators. Both issues have adverse effect on the stability, and the risks permanently increase with dissemination of electronically controlled sources and loads.
Contemporary power electronics and digital control technology provides the flexibility in conceiving and applying control, synchronization and protection mechanisms of grid-connected static power converters. Instead of being the source of parasitic dc voltage and harmonics, digitally controlled converters can be turned into absorbers of harmful voltage and current components. Moreover, the negative resistance characteristic can be removed by the control means. At the same time, the line-synchronization algorithms can be suited so as to aid the system stability. Provided with sufficient overload capability, the grid connected static power converters can be programmed to emulate the synchronous generators in supplying the fault spots with the fault currents sufficient to trigger the standard ac-grid devices for coordinated protection. Certain advanced features, such as the local voltage regulation by means of electronically controlled distributed power sources is already included into applicable standards (IEEE 1547). Grid-connected static power converters and other electronically controlled sources, loads and accumulators are the key element in modern electric power industry. Further development of their control and protection mechanisms will have a considerable impact on transmission and distribution systems.
Modular Multilevel Converter Based Conversion for MVDC Applications
Prof. Dr. Drazen Dujic
Power Electronics Laboratory - PEL
Swiss Federal Institute of Technology – EPFL
Drazen Dujic is an Assistant Professor and Head of the Power
Electronics Laboratory at EPFL. He received the Dipl.Ing. and MSc degrees from the University of Novi Sad, Novi Sad, Serbia in 2002
and 2005, respectively, and the PhD degree from Liverpool John Moores University, Liverpool, UK in 2008.
From 2003 to 2006, he was a Research Assistant with the Faculty of Technical Sciences at University of Novi Sad. From 2006 to 2009, he was a Research Associate with Liverpool John Moores University. After that he moved to industry and joined ABB Switzerland Ltd, where from 2009 to 2013, he was Scientist and then Principal Scientist with ABB Corporate Research Center in Baden-Dättwil, and from 2013 to 2014 he was R&D Platform Manager with ABB Medium Voltage Drives in Turgi. He is with EPFL since 2014. His research interests include the areas of design and control of advanced high power electronic systems and high-performance drives, predominantly for the medium voltage applications related to electrical energy generation, conversion and storage. He has authored or co-authored more than 90 scientific publications and has filed eleven patents.
In 2014, he received The Isao Takahashi Power Electronics Award for Outstanding Achievement in Power Electronics, presented at International Power Electronics Conference, IPEC-Hiroshima 2014, Japan. He is Senior Member of IEEE, EPE Member, and serves as Associate Editor for IEEE Transactions on Power Electronics, IEEE Transactions on Industrial Electronics and IET Electric Power Applications.
Medium Voltage Direct Current (MVDC) power distribution networks are currently being considered for various applications, from large wind and solar power collection and distribution grids to marine on-board electrical systems. Generally, one may consider DC voltage levels from 1.5kV to 50kV to be representative of MVDC voltage range, with power ranging from few MWs to several tens of MWs. Research activities in these areas reveal various technological gaps, predominantly the lack of suitable conversion and protection equipment. In contrast to Solid State Transformers (SSTs), characterized with highly modular converter structures comprising multiple galvanically isolated sub-converter stages, there are other possibilities to realize high-power medium voltage isolated converters employing a single transformer for isolation. The keynote talk will provide an overview of various topologies proposed in the literatures, as well as novel topologies proposed by author’s research laboratory, which will be compared regarding their performances and suitability for various MVDC applications. The Modular Multilevel Converter (MMC), with its excellent voltage scalability through series-connection of cells, offers interesting possibilities for realization of various conversion structures needed in MV applications. From basic characteristics of MMC topology, and its operating principles, various high power MMC-based DC-AC and DC-DC galvanically isolated converters for MVDC applications will be presented and elaborated.