Project description Title: Power Electronics for Reliable and Energy efficient Renewable Energy Systems OBJECTIVES Principal objective Provide competence and decision basis for enabling reliable and energy efficient power electronics components and systems for renewable power generation in harsh offshore environments. Sub-goals: 1. Contribute to advances in energy efficiency of power electronic converters and power systems dominated by converters. 2. Contribute to improved reliability of power electronic components and systems containing power electronics. 3. Provide decision basis for optimizing AC and DC power grid structures (hybrid grids) for interconnecting and transmission of offshore wind power. 4. Provide the required advanced control strategies for grid connected and islanding power electronics converters. 5. Verify attainment of project goals by accomplishing realistic scenario analysis supported by numerical simulations and energy laboratory experiments. 6. Contribute to education by engaging one PhD, one Post doc and supervising master students. 2 FRONTIERS OF KNOWLEDGE AND TECHNOLOGY The project applicants are aware of the huge amount of international ongoing R&D work on technologies for renewable energy systems generally, and wind power energy systems in particular. However, if remote high power renewable energy projects like offshore wind parks shall be realised, quite a few new challenges regarding power electronics components and systems will need to be met. These challenges are further argued in PART 2 of this application. The specified sub-goals are highlighting the most fundamental challenges. Converters for grid operation For several years now, power electronics converters with high control response has been available for various grid operations. Examples are front-end converters for e.g. motor drives and UPS, HVDC converters, Static VAr compensators, active filters and power line conditioners. Especially for the Voltage Source Converters (VSC), the unit power is continuously increasing as result of advances in high voltage power semiconductor technologies (especially IGBTs) and exploitation of new power circuit topologies (multi-level converters, series connection of IGBTs etc). Therefore there is a trend that thyristor based Line Commutated Converters (LCC) are replaced by VSCs in a continuously increasing power range. It is assumed that renewable energy system grids, e.g. offshore wind power systems, will benefit from the new converters. This however, calls for proper selection and combination of Research activities Power Electronics for Renewable Energy Systems 17/12/2009 page 1/5
technologies. SINTEF Energy Research has quite a lot of experience from analysis of complex power grids dominated by power electronics converters. This is mostly from projects for industry clients on analysis, verification and problem solving. SINTEF, together with NTNU, has also a good international network that will be exploited for the applied project. Energy efficient conversion systems Offshore wind energy is one of the most promising new renewable energy sources in Norway, both with respect to energy and industry potential as well as environmental effects. Energy from offshore wind parks located far from shore is, however, assumed to become expensive compared to present energy production systems. Therefore there will be need for developing energy production and transmission system components with higher efficiency than what is normal today. Today the efficiency of a high power converter (several MW) will be in the range 90 to 95 %. Especially in future grids with extensive usage of power electronics converters, the overall transmission losses may become significant, and there will be a push for optimizing the overall efficiency. Since increased component efficiency generally result in higher component costs, there will be need for technical support for making the required tradeoffs between individual component costs, cost for dissipated energy etc. The impression from available international publications on this area, is that SINTEF Energy Research is far ahead when it comes to expertise and methods for advanced dynamic analysis of thermal losses in switching converters (ref. section 3). Reliable power electronics Power systems containing a high percentage of power electronics loads are known to be complex systems with a lot of uncertainties concerning worst case electrical and thermal stress applied to the individual components, and the component ability to survive specific stress conditions. SINTEF Energy Research has quite a lot of experience from projects for industry clients on fault analysis of power electronics. The impression based on this experience is that fault causes often has a complex character, and are composed of several weak points as described in section 3. SINTEF Energy Research can contribute significantly to improved reliability for power electronic components and system containing power electronics, provided good cooperation with the vendor industry. The most significant shortage in our competence is within component fault mechanisms involving semiconductor physics. To build up this required competence SINTEF Energy Research and NTNU will utilize a cooperation with Josef Lutz at Chemnitz University of Technology. He is one of the leading experts on fault mechanisms of power semiconductors, and reliability of power electronic devices is the main field of his research. Control strategies for grid connected electronics converters Quite a lot of international work has been done or is in progress on control strategies for grid connected electronics converters, especially on control of active front end converters. In a Strategic Institute Project (SIP) at SINTEF Energy Research in the period 2000-2004 (see http://www.energy.sintef.no/prosjekt/ke/), a lot of competence was gained on control methods for grid connected converters. A quite new powerful universal digital hardware platform, based on the latest FPGA technology is about to be realised at SINTEF Energy Research as part of a self-financed project. This will be a powerful and flexible basis for implementing and testing complex converter control algorithms. The flexibility makes it suitable for nearly all applications for grid-connected converters, e.g. front-end converters for windmills, as well as HVDC-converters. The pre-existing knowledge will be available for contributing to significant progress on robust control strategies for grid connected converters. Research activities Power Electronics for Renewable Energy Systems 17/12/2009 page 2/5
Efficient and accurate software and hardware simulation tools Transient and harmonic analysis of complex systems containing power electronics is normally carried out using electromagnetic simulation tools such as PSCAD/EMTDC. Major difficulties include the wide-band representation of certain components and the combination of a fine time step and long simulation runs. Current research focuses on overcoming these limitations via black-box representation via rational fitting methods. SINTEF Energy Research has been a major contributor to these developments over the last decade. An IEEE PES Task Force of which SINTEF Energy Research is member investigates alternative ways of extracting and realizing low order network equivalents. As mentioned above SINTEF Energy Research has quite a lot of experience from fault analysis of power electronics for industry clients. Most of these analysis projects are accomplished by using advanced numerical simulations of detailed switching power electronics in complex power grids. These analysis also includes combined electrical, thermal and mechanical dynamics (rotating machineries). Base on this experience SINTEF Energy Research see a big potential in further extension of advanced component modelling. For such modelling good cooperation with component vendors, and laboratory experiments for verifying component modelling is needed. Due to physical dimensions and/or manufacturing time, some components cannot be implemented in a laboratory. Thus, required analysis and design of new systems in a laboratory becomes difficult to carry out. By combining accurate models with fast FPGA based real-time computer algorithms and amplifiers and other devices, a real-time terminal physical equivalent of a dynamic multi-port component can emulate and replace these components. Applications: Cables and overhead lines, transformers, feeding network. Energy laboratory The most important tools for achieving the goals for this project are advanced numerical simulations combined with laboratory experiments for examinations, verifications, education and demonstrations. NTNU and SINTEF Energy Research together is planning to establish a world class laboratory to serve research and development activities related to renewable energy generation components and their integration into the electric network. Status and plans for this laboratory are available at: www.sintef.no/energylab. This also includes special qualities compared to other similar facilities. This laboratory is aimed to service this and a wide range of other technology projects at NTNU and SINTEF Energy Research. 3 RESEARCH TASKS This section gives a brief presentation of the proposed research tasks in the project. Task 1 Energy efficient conversion and transmission There will be an initial subtask aiming to clarify state of the art and prospective regarding near future obtainable converter power and obtainable converter efficiency. A survey of what is obtainable from vendors today for high power, high voltage conversion will be made. Key words are high-voltage semiconductors, series connection of semiconductors and multi-level converters. The goal for the subtask is to examine how converter efficiency above 95 % can be achieved regarding topology related losses, switching frequency considerations, filter considerations, additional high frequency losses etc. Basis for calculating costs for achieving specific converter efficiencies will also be provided. Next, analysis will be carried out on different Research activities Power Electronics for Renewable Energy Systems 17/12/2009 page 3/5
options for optimizing plant efficiency and transmission system efficiency by utilizing advanced converter control options. This will mainly be done by numerical analysis of case scenarios. This task will be closely coordinated with Task 3. This task will utilize experience from a work in an ongoing Petromaks project (Subsea power systems, No: 176024/S30) on development and verification of numerical simulation modules for combined electrical and thermal dynamic simulation of power electronics converters. This project also includes switching losses and thermal conditions for the switching power semiconductors. The simulation studies will be supported by laboratory experiments, communication with component vendors and correspondence with international partners. Task 2 Reliable Power Electronics for Renewable Energy Systems This task will focus on reliability of power electronics and systems containing power electronics converters in a broad range of disciplines. As mentioned in section 2 the experiences from industry projects at SINTEF Energy Research is that that fault causes often has a complex character, and is composed of several weak points like: Insufficient specifications from appliance user Unknown response to failures, e.g. due to unintended interaction between grid components from different vendors Lack of knowledge regarding fault mechanisms for individual converter components, especially the switching devices Converter design weaknesses Degradation of switching devices due to stress or external influence/exposure Regarding complex power systems for renewable energy systems, it is assumed that the reliability problems need to be focused more than in other industry applications, due to low availability and high criticality, new component and system behaviour, new and especially harsh environments (e.g. sea water), and control difficulties due to long communication lines and poor media for electromagnetic waves. As mentioned in section 2, the project will cooperate with Chemnitz University of Technology (CUT) for competence building regarding Task 2. Especially the power cycling capability is of highest importance, since it limits the device operation time. Reliability data of the manufacturers are gathered due to applications in traction converters. CUT will build up a system for power cycling of high voltage high power modules with current load up to 1.6 ka. After run-in, the system will be transferred to SINTEF Energy Research. Further more, the obtained new competence will be combined with the other research tasks, e.g. numerical simulations of power cycling and temperature cycling stress, fault analysis (e.g. converter component stress following overloads and faults) and finding criteria for condition monitoring and protection. Task 3 Analysis of advanced hybrid AC-DC-grids A general experience from the analysis projects mentioned in section 2 is that that there is a risk that components from different vendors installed in the same grid may give unintended interaction problems, even though the individual components are working perfect as standalone units. Another experience is that close to all electrical and thermal problems related to complex power electronic networks are discoverable if the proper numerical simulation tool are available, and if sufficient data is available from component vendors. This task will focus on analysis of complete power systems for renewable energy, containing converters for power flow control, transient control and voltage control. E.g. for offshore power grids, all the way from wind generators to onshore tie-in point. The analysis will aim to optimize system control based on predefined criteria, such as optimized grid efficiency. It will also aim to unveil Research activities Power Electronics for Renewable Energy Systems 17/12/2009 page 4/5
possible problems during normal operation and in fault situations. The analysis will be accomplished as scenario analysis of grids, specified in cooperation with the project partners. This task will be closely linked to all the other tasks. Task 4 Advanced control strategies for grid connected converters In this task the project will do research work on suitable control strategies for grid connected converters. Special focus will be given to strategies for establishing robust reference frames for the converter control systems for applications where the grid impedance is strongly varying. Alternative presented approaches will be studied and the project will utilize existing experience, hardware FPGA-platform and ideas for advanced converter control as briefly explained in section 2 for realization, testing and demonstration of promising control strategies. In addition to the realization of control hardware and software, this task will involve numerical simulations of detailed control algorithms on switching converters, and laboratory experiments of control strategies implemented on converter prototypes installed in the Renewable Energy system Laboratory. The task will have a close cooperation with PhD student Jon Are Suul at NTNU. Task 5 New models for numerical simulation The project has high ambitions for utilizing numerical transient analysis for electrical and thermal phenomena in complex power grids, including detailed switching power electronics. Special models are about to be developed as part an ongoing Petromaks KMB project (Subsea power systems, No: 176024/S30). However, new possibilities and ideas has emerged on numerical modelling, that will benefit this new project. This includes robust algorithms for high frequency modelling of power components, based on input from measurements or computations. Candidate components: EMI filters, transformers, and feeders. The project will create a complete procedure for measurements, rational modelling, and generation of PSCADcompatible simulation models. In addition, the project will address the handling of components and systems with weakly observable modes. This task will also contribute in development and verification of numerical simulation modules for combined electrical, thermal and mechanical dynamic simulation of power electronics converters and drive trains. Experience from an ongoing Petromaks KMB project (Subsea power electronics, No: 176025/S30) will be utilised. This activity includes switching losses and thermal conditions for the switching power semiconductors. For offshore wind parks this is of special importance since if the wind is low and the rotor is moving with low speed, the rectifier modules will have a very low frequency (in the range of 1 Hz). This can create power cycles with significant temperature swing that will influence the power semiconductor life time. Task 6 Demonstrations by simulations and laboratory experiments Results from different tasks will be demonstrated by performing different realistic scenario analysis using developed simulation tools. These analysis will be supported by or verified in the laboratory, where developed emulator(s) and converter control platform will be important components. Task 7 PhD and Post doc. scholarships The project is planning two scholarships, one for a PhD and one for a Post doc. Topic for the PhD work will be Fault mechanisms and reliability of power semiconductors (within Task 2). The post doc work (two years) will be within medium voltage converters. Research activities Power Electronics for Renewable Energy Systems 17/12/2009 page 5/5