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    A Controller for Optimum Electrical Power Extraction from a Small Grid-Interconnected Wind Turbine

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    [EN] Currently, wind power is the fastest-growing means of electricity generation in the world. To obtain the maximum efficiency from the wind energy conversion system, it is important that the control strategy design is carried out in the best possible way. In fact, besides regulating the frequency and output voltage of the electrical signal, these strategies should also extract energy from wind power at the maximum level of efficiency. With advances in micro-controllers and electronic components, the design and implementation of efficient controllers are steadily improving. This paper presents a maximum power point tracking controller scheme for a small wind energy conversion system with a variable speed permanent magnet synchronous generator. With the controller, the system extracts optimum possible power from the wind speed reaching the wind turbine and feeds it to the grid at constant voltage and frequency based on the AC-DC-AC conversion system. A MATLAB/SimPowerSystems environment was used to carry out the simulations of the system. Simulation results were analyzed under variable wind speed and load conditions, exhibiting the performance of the proposed controller. It was observed that the controllers can extract maximum power and regulate the voltage and frequency under such variable conditions. Extensive results are included in the paper.This work was partially supported by the Spanish Ministry of Education, Culture and Sports-reference FPU16/04282.García-Sánchez, TM.; Mishra, AK.; Hurtado-Perez, E.; Puche-Panadero, R.; Fernández-Guillamón, A. (2020). A Controller for Optimum Electrical Power Extraction from a Small Grid-Interconnected Wind Turbine. Energies. 13(21):1-16. https://doi.org/10.3390/en13215809S1161321Fernández-Guillamón, A., Villena-Lapaz, J., Vigueras-Rodríguez, A., García-Sánchez, T., & Molina-García, Á. (2018). An Adaptive Frequency Strategy for Variable Speed Wind Turbines: Application to High Wind Integration Into Power Systems. Energies, 11(6), 1436. doi:10.3390/en11061436Fernández-Guillamón, A., Sarasúa, J. I., Chazarra, M., Vigueras-Rodríguez, A., Fernández-Muñoz, D., & Molina-García, Á. (2020). Frequency control analysis based on unit commitment schemes with high wind power integration: A Spanish isolated power system case study. International Journal of Electrical Power & Energy Systems, 121, 106044. doi:10.1016/j.ijepes.2020.106044Huber, M., Dimkova, D., & Hamacher, T. (2014). Integration of wind and solar power in Europe: Assessment of flexibility requirements. Energy, 69, 236-246. doi:10.1016/j.energy.2014.02.109Fernández-Guillamón, A., Martínez-Lucas, G., Molina-García, Á., & Sarasua, J.-I. (2020). Hybrid Wind–PV Frequency Control Strategy under Variable Weather Conditions in Isolated Power Systems. Sustainability, 12(18), 7750. doi:10.3390/su12187750Fernández‐Guillamón, A., Vigueras‐Rodríguez, A., & Molina‐García, Á. (2019). Analysis of power system inertia estimation in high wind power plant integration scenarios. IET Renewable Power Generation, 13(15), 2807-2816. doi:10.1049/iet-rpg.2019.0220Fernández-Guillamón, A., Das, K., Cutululis, N. A., & Molina-García, Á. (2019). Offshore Wind Power Integration into Future Power Systems: Overview and Trends. Journal of Marine Science and Engineering, 7(11), 399. doi:10.3390/jmse7110399Muñoz-Benavente, I., Hansen, A. D., Gómez-Lázaro, E., García-Sánchez, T., Fernández-Guillamón, A., & Molina-García, Á. (2019). Impact of Combined Demand-Response and Wind Power Plant Participation in Frequency Control for Multi-Area Power Systems. Energies, 12(9), 1687. doi:10.3390/en12091687Gil-García, I. C., García-Cascales, M. S., Fernández-Guillamón, A., & Molina-García, A. (2019). Categorization and Analysis of Relevant Factors for Optimal Locations in Onshore and Offshore Wind Power Plants: A Taxonomic Review. Journal of Marine Science and Engineering, 7(11), 391. doi:10.3390/jmse7110391Molina-Garcia, A., Fernandez-Guillamon, A., Gomez-Lazaro, E., Honrubia-Escribano, A., & Bueso, M. C. (2019). Vertical Wind Profile Characterization and Identification of Patterns Based on a Shape Clustering Algorithm. IEEE Access, 7, 30890-30904. doi:10.1109/access.2019.2902242Global Wind Report 2019https://gwec.net/global-wind-report-2019/Chagas, C. C. M., Pereira, M. G., Rosa, L. P., da Silva, N. F., Freitas, M. A. V., & Hunt, J. D. (2020). From Megawatts to Kilowatts: A Review of Small Wind Turbine Applications, Lessons From The US to Brazil. Sustainability, 12(7), 2760. doi:10.3390/su12072760Culotta, S., Franzitta, V., Milone, D., & Moncada Lo Giudice, G. (2015). Small Wind Technology Diffusion in Suburban Areas of Sicily. Sustainability, 7(9), 12693-12708. doi:10.3390/su70912693Nazir, M. S., Wang, Y., Bilal, M., Sohail, H. M., Kadhem, A. A., Nazir, H. M. R., … Ma, Y. (2020). Comparison of Small-Scale Wind Energy Conversion Systems: Economic Indexes. Clean Technologies, 2(2), 144-155. doi:10.3390/cleantechnol2020010García-Sánchez, T., Muñoz-Benavente, I., Gómez-Lázaro, E., & Fernández-Guillamón, A. (2020). Modelling Types 1 and 2 Wind Turbines Based on IEC 61400-27-1: Transient Response under Voltage Dips. Energies, 13(16), 4078. doi:10.3390/en13164078Fernández-Guillamón, A., Martínez-Lucas, G., Molina-García, Á., & Sarasua, J. I. (2020). An Adaptive Control Scheme for Variable Speed Wind Turbines Providing Frequency Regulation in Isolated Power Systems with Thermal Generation. Energies, 13(13), 3369. doi:10.3390/en13133369Tiwari, R., Padmanaban, S., & Neelakandan, R. (2017). Coordinated Control Strategies for a Permanent Magnet Synchronous Generator Based Wind Energy Conversion System. Energies, 10(10), 1493. doi:10.3390/en10101493Sajadi, M., De Kooning, J. D. M., Vandevelde, L., & Crevecoeur, G. (2019). Harvesting wind gust energy with small and medium wind turbines using a bidirectional control strategy. The Journal of Engineering, 2019(17), 4261-4266. doi:10.1049/joe.2018.8182Chavero-Navarrete, E., Trejo-Perea, M., Jáuregui-Correa, J. C., Carrillo-Serrano, R. V., & Ríos-Moreno, J. G. (2019). Expert Control Systems for Maximum Power Point Tracking in a Wind Turbine with PMSG: State of the Art. Applied Sciences, 9(12), 2469. doi:10.3390/app9122469Orlando, N. A., Liserre, M., Mastromauro, R. A., & Dell’Aquila, A. (2013). A Survey of Control Issues in PMSG-Based Small Wind-Turbine Systems. IEEE Transactions on Industrial Informatics, 9(3), 1211-1221. doi:10.1109/tii.2013.2272888Daili, Y., Gaubert, J.-P., Rahmani, L., & Harrag, A. (2019). Quantitative Feedback Theory design of robust MPPT controller for Small Wind Energy Conversion Systems: Design, analysis and experimental study. Sustainable Energy Technologies and Assessments, 35, 308-320. doi:10.1016/j.seta.2019.08.002Zhang, X., Huang, C., Hao, S., Chen, F., & Zhai, J. (2016). An Improved Adaptive-Torque-Gain MPPT Control for Direct-Driven PMSG Wind Turbines Considering Wind Farm Turbulences. Energies, 9(11), 977. doi:10.3390/en9110977Shafiei, A., Dehkordi, B. M., Kiyoumarsi, A., & Farhangi, S. (2017). A Control Approach for a Small-Scale PMSG-Based WECS in the Whole Wind Speed Range. IEEE Transactions on Power Electronics, 32(12), 9117-9130. doi:10.1109/tpel.2017.2655940Oliveira, T. D., Tofaneli, L. A., & Santos, A. Á. B. (2020). Combined effects of pitch angle, rotational speed and site wind distribution in small HAWT performance. Journal of the Brazilian Society of Mechanical Sciences and Engineering, 42(8). doi:10.1007/s40430-020-02501-4Battisti, L., Benini, E., Brighenti, A., Dell’Anna, S., & Raciti Castelli, M. (2018). Small wind turbine effectiveness in the urban environment. Renewable Energy, 129, 102-113. doi:10.1016/j.renene.2018.05.062Jeong, H. G., Seung, R. H., & Lee, K. B. (2012). An Improved Maximum Power Point Tracking Method for Wind Power Systems. Energies, 5(5), 1339-1354. doi:10.3390/en5051339Zhu, Y., Cheng, M., Hua, W., & Wang, W. (2012). A Novel Maximum Power Point Tracking Control for Permanent Magnet Direct Drive Wind Energy Conversion Systems. Energies, 5(5), 1398-1412. doi:10.3390/en5051398Chen, J.-H., & Hung, W. (2015). Blade Fault Diagnosis in Small Wind Power Systems Using MPPT with Optimized Control Parameters. Energies, 8(9), 9191-9210. doi:10.3390/en8099191Syahputra, R., & Soesanti, I. (2019). Performance Improvement for Small-Scale Wind Turbine System Based on Maximum Power Point Tracking Control. Energies, 12(20), 3938. doi:10.3390/en12203938Aubrée, R., Auger, F., Macé, M., & Loron, L. (2016). Design of an efficient small wind-energy conversion system with an adaptive sensorless MPPT strategy. Renewable Energy, 86, 280-291. doi:10.1016/j.renene.2015.07.091Lopez-Flores, D. R., Duran-Gomez, J. L., & Chacon-Murguia, M. I. (2020). A Mechanical Sensorless MPPT Algorithm for a Wind Energy Conversion System based on a Modular Multilayer Perceptron and a Processor-in-the-Loop Approach. Electric Power Systems Research, 186, 106409. doi:10.1016/j.epsr.2020.106409Urtasun, A., Sanchis, P., San Martín, I., López, J., & Marroyo, L. (2013). Modeling of small wind turbines based on PMSG with diode bridge for sensorless maximum power tracking. Renewable Energy, 55, 138-149. doi:10.1016/j.renene.2012.12.035Kot, R., Rolak, M., & Malinowski, M. (2013). Comparison of maximum peak power tracking algorithms for a small wind turbine. Mathematics and Computers in Simulation, 91, 29-40. doi:10.1016/j.matcom.2013.03.010Muhsen, H., Al-Kouz, W., & Khan, W. (2019). Small Wind Turbine Blade Design and Optimization. Symmetry, 12(1), 18. doi:10.3390/sym12010018Qi, Z., & Lin, E. (2012). Integrated power control for small wind power system. Journal of Power Sources, 217, 322-328. doi:10.1016/j.jpowsour.2012.06.039Doll, C. N. H., & Pachauri, S. (2010). Estimating rural populations without access to electricity in developing countries through night-time light satellite imagery. Energy Policy, 38(10), 5661-5670. doi:10.1016/j.enpol.2010.05.014Zhang, S., & Qi, J. (2011). Small wind power in China: Current status and future potentials. Renewable and Sustainable Energy Reviews, 15(5), 2457-2460. doi:10.1016/j.rser.2011.02.009Rehman, S., & Sahin, A. Z. (2012). Wind power utilization for water pumping using small wind turbines in Saudi Arabia: A techno-economical review. Renewable and Sustainable Energy Reviews, 16(7), 4470-4478. doi:10.1016/j.rser.2012.04.036Park, J. H., Chung, M. H., & Park, J. C. (2016). Development of a small wind power system with an integrated exhaust air duct in high-rise residential buildings. Energy and Buildings, 122, 202-210. doi:10.1016/j.enbuild.2016.04.037Simic, Z., Havelka, J. G., & Bozicevic Vrhovcak, M. (2013). Small wind turbines – A unique segment of the wind power market. Renewable Energy, 50, 1027-1036. doi:10.1016/j.renene.2012.08.038Parag, Y., & Sovacool, B. K. (2016). Electricity market design for the prosumer era. Nature Energy, 1(4). doi:10.1038/nenergy.2016.32Kortabarria, I., Andreu, J., Martínez de Alegría, I., Jiménez, J., Gárate, J. I., & Robles, E. (2014). A novel adaptative maximum power point tracking algorithm for small wind turbines. Renewable Energy, 63, 785-796. doi:10.1016/j.renene.2013.10.036Emejeamara, F. C., Tomlin, A. S., & Millward-Hopkins, J. T. (2015). Urban wind: Characterisation of useful gust and energy capture. Renewable Energy, 81, 162-172. doi:10.1016/j.renene.2015.03.028Britter, R. E., & Hanna, S. R. (2003). FLOW AND DISPERSION IN URBAN AREAS. Annual Review of Fluid Mechanics, 35(1), 469-496. doi:10.1146/annurev.fluid.35.101101.161147Askarov, A., Andreev, M., & Ruban, N. (2020). Impact assessment of full-converter wind turbine generators integration on transients in power systems. THERMOPHYSICAL BASIS OF ENERGY TECHNOLOGIES (TBET 2019). doi:10.1063/5.0000832Pillay, P., & Krishnan, R. (1988). Modeling of permanent magnet motor drives. IEEE Transactions on Industrial Electronics, 35(4), 537-541. doi:10.1109/41.9176Shariatpanah, H., Fadaeinedjad, R., & Rashidinejad, M. (2013). A New Model for PMSG-Based Wind Turbine With Yaw Control. IEEE Transactions on Energy Conversion, 28(4), 929-937. doi:10.1109/tec.2013.2281814Ata, R., & Kocyigit, Y. (2010). An adaptive neuro-fuzzy inference system approach for prediction of tip speed ratio in wind turbines. Expert Systems with Applications, 37(7), 5454-5460. doi:10.1016/j.eswa.2010.02.068Anelion SW 3.5 GThttps://www.wind-turbine-models.com/turbines/950-anelion-sw-3.5-gtSalles, M. B. C., Hameyer, K., Cardoso, J. R., Grilo, A. P., & Rahmann, C. (2010). Crowbar System in Doubly Fed Induction Wind Generators. Energies, 3(4), 738-753. doi:10.3390/en3040738Kim, Y.-S., Chung, I.-Y., & Moon, S.-I. (2015). Tuning of the PI Controller Parameters of a PMSG Wind Turbine to Improve Control Performance under Various Wind Speeds. Energies, 8(2), 1406-1425. doi:10.3390/en8021406Widanagama Arachchige, L., Rajapakse, A., & Muthumuni, D. (2017). Implementation, Comparison and Application of an Average Simulation Model of a Wind Turbine Driven Doubly Fed Induction Generator. Energies, 10(11), 1726. doi:10.3390/en10111726Kim, C., Gui, Y., Zhao, H., & Kim, W. (2020). Coordinated LVRT Control for a Permanent Magnet Synchronous Generator Wind Turbine with Energy Storage System. Applied Sciences, 10(9), 3085. doi:10.3390/app10093085Das, K., Hansen, A. D., & Sørensen, P. E. (2016). Understanding IEC standard wind turbine models using SimPowerSystems. Wind Engineering, 40(3), 212-227. doi:10.1177/0309524x1664205

    A Maximum Power Point Tracking Control Algorithms for a PMSG‐based WECS for Isolated Applications: Critical Review

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    This chapter deals with a comprehensive overview study of the direct‐driven (DD) permanent magnet synchronous generator (PMSG) for wind‐energy generation system for stand‐alone applications. The dynamic model of PMSG is presented, and different maximum power point tracking (MPPT) algorithms have been realized in the aim to compare their performance. A comparison of performances of the conventional P&O MPPT and the fuzzy logic P&O (FLC P&O) MPPT is presented. Control technique for the presented system is presented and analyzed for the generator side converter. The simulation results carried out using Matlab/Simulink software show the effectiveness of the wind turbine control system

    A novel scheme for control by active and reactive power utilized in gearless variable speed wind turbine system with PMSG connected to the grid

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    Introduction. As a result of increasing fossil fuel price and state-of-the-art technology, more and more residential and commercial consumers of electricity have been installing wind turbines. The motivation being to cut energy bills and carbon dioxide emissions. Purpose. The main goal of this work is developing a control scheme for a variable speed wind turbine generator in order to produce utmost power from varying wind types, and variable wind speed. Novelty. This research paper presents an IGBT power converter control scheme for active power in relation to wind speed and reactive power by adjusting Q-reference (Qref) value in a gearless variable speed wind turbine with permanent magnet synchronous generator. Methods. An effective modelling and control of the wind turbine with the suggested power converter is executed by utilizing MATLAB/Simulink software. The control scheme consists of both the wind turbine control and the power converter control. Simulation results are utilized in the analysis and deliberation of the ability of the control scheme, which reveals that the wind turbine generator has the capability to actively sustain an electric power grid network, owing to its ability to independently control active and reactive power according to applied reference values at variable wind speed. Practical value. This research can be utilized for assessing the control methodology, the dynamic capabilities and influence of a gearless variable-speed wind energy conversion system on electric power grids. A case study has been presented with a (3×10 MW = 30 MW) wind farm scheme.Вступ. Внаслідок зростання цін на викопне паливо та використання найсучасніших технологій, дедалі більше побутових та комерційних споживачів електроенергії встановлюють вітряні турбіни. Мотивація полягає в тому, щоб скоротити рахунки за електроенергію та викиди вуглекислого газу. Мета. Основною метою цієї роботи є розробка схеми управління вітряним генератором зі змінною швидкістю для отримання максимальної потужності від різних типів вітру та змінної швидкості вітру. Новизна. У даній дослідницькій роботі представлена схема управління силовим IGBT перетворювачем для активної потужності в залежності від швидкості вітру та реактивної потужності шляхом регулювання значенняQ-еталона (Qref) у безредукторній вітровій турбіні з регульованою швидкістю та синхронним генератором із постійними магнітами. Методи. Ефективне моделювання та керування вітровою турбіною з запропонованим перетворювачем потужності здійснюється з використанням програмного забезпечення MATLAB/Simulink. Схема управління складається з управління вітряною турбіною і з управління силовим перетворювачем. Результати моделювання використовуються для аналізу та обговорення можливостей схеми управління, що показує, що генератор вітрової турбіни здатний активно підтримувати електроенергетичну мережу завдяки своїй здатності незалежно контролювати активну та реактивну потужність відповідно до застосовуваних еталонних значень при змінній швидкості вітру. Практична цінність. Це дослідження може бути використане для оцінки методології управління, динамічних можливостей та впливу безредукторної системи перетворення енергії вітру зі змінною швидкістю на електричні мережі. Наведено тематичне дослідження зі схемою вітряної електростанції (3×10 МВт = 30 МВт)

    Control structure for single-phase stand-alone wind-based energy sources

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    This paper is analyzing the operation of a standalone wind turbine system with variable speed Permanent Magnet Synchronous Generator (PMSG) and a system for storing energy during wind speed and load variations. Energy storage devices are required for power balance and power quality in stand alone wind energy systems. Initially, the holistic model of the entire system is achieved, including the PMSG, the boost converter and the storage system. The power absorbed by the connected loads can be effectively delivered and supplied by the proposed wind turbine and energy storage systems, subject to an appropriate control method. The main purpose is to supply 230 V/50 Hz domestic appliances through a single-phase inverter. The simulation results, validated by experimental testing, show a good prediction of the electrical parameter waveforms. The control system is implemented on a dSPACE DS1103 real-time board. Furthermore, the results confirm the stability of the supply

    Dynamic Modeling and Performance Analysis of PMSG- based Variable Speed WTG: Case Study of Adama Wind Farm I, Ethiopia

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    In this paper, the performance of Permanent Magnet Synchronous Generator (PMSG) -based Variable Speed Wind Turbine Generator (WTG) at Adama Wind Farm I (WTG), connected to a grid is studied. To study the performance of the WTG, both machine and grid side converters are modeled and analyzed very well. On the machine side, maximum power point tracking (MPPT) for maximum energy extraction is done using the direct speed control (DSC) technique, which is linked with the optimal tip speed ratio for each wind speed value considered. On the grid side, dc-link voltage and reactive power flow to the grid are controlled. For this purpose, first, the simulation model of the system is prepared in MATLAB Simulink considering the dynamic mathematical model of the PMSG, and Wind Turbine Aerodynamic model using the user-defined function blocks. Then, the PI regulators designed for direct speed, torque (current) control, and dc-link voltage are employed in the model. Moreover, to study and analyze the behavior of the system in a variable speed operation, a wind speed starting from cut-in wind speed (3m/s) to the rated wind speed (11m/s) is applied in 4s. The simulation result of the existing system model shows that the actual values of performance variables correspond well with the analytical values of the system. In addition, the chosen control algorithms applied in the control system of the generator-side converter are hence verified

    Offshore Wind Farm-Grid Integration: A Review on Infrastructure, Challenges, and Grid Solutions

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    Recently, the penetration of renewable energy sources (RESs) into electrical power systems is witnessing a large attention due to their inexhaustibility, environmental benefits, storage capabilities, lower maintenance and stronger economy, etc. Among these RESs, offshore wind power plants (OWPP) are ones of the most widespread power plants that have emerged with regard to being competitive with other energy technologies. However, the application of power electronic converters (PECs), offshore transmission lines and large substation transformers result in considerable power quality (PQ) issues in grid connected OWPP. Moreover, due to the installation of filters for each OWPP, some other challenges such as voltage and frequency stability arise. In this regard, various customs power devices along with integration control methodologies have been implemented to deal with stated issues. Furthermore, for a smooth and reliable operation of the system, each country established various grid codes. Although various mitigation schemes and related standards for OWPP are documented separately, a comprehensive review covering these aspects has not yet addressed in the literature. The objective of this study is to compare and relate prior as well as latest developments on PQ and stability challenges and their solutions. Low voltage ride through (LVRT) schemes and associated grid codes prevalent for the interconnection of OWPP based power grid have been deliberated. In addition, various PQ issues and mitigation options such as FACTS based filters, DFIG based adaptive and conventional control algorithms, ESS based methods and LVRT requirements have been summarized and compared. Finally, recommendations and future trends for PQ improvement are highlighted at the end

    Fault analysis and protection for wind power generation systems

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    Wind power is growing rapidly around the world as a means of dealing with the world energy shortage and associated environmental problems. Ambitious plans concerning renewable energy applications around European countries require a reliable yet economic system to generate, collect and transmit electrical power from renewable resources. In populous Europe, collective offshore large-scale wind farms are efficient and have the potential to reach this sustainable goal. This means that an even more reliable collection and transmission system is sought. However, this relatively new area of offshore wind power generation lacks systematic fault transient analysis and operational experience to enhance further development. At the same time, appropriate fault protection schemes are required. This thesis focuses on the analysis of fault conditions and investigates effective fault ride-through and protection schemes in the electrical systems of wind farms, for both small-scale land and large-scale offshore systems. Two variable-speed generation systems are considered: doubly-fed induction generators (DFIGs) and permanent magnet synchronous generators (PMSGs) because of their popularity nowadays for wind turbines scaling to several-MW systems. The main content of the thesis is as follows. The protection issues of DFIGs are discussed, with a novel protection scheme proposed. Then the analysis of protection scheme options for the fully rated converter, direct-driven PMSGs are examined and performed with simulation comparisons. Further, the protection schemes for wind farm collection and transmission systems are studied in terms of voltage level, collection level wind farm collection grids and high-voltage transmission systems for multi-terminal DC connected transmission systems, the so-called “Supergrid”. Throughout the thesis, theoretical analyses of fault transient performances are detailed with PSCAD/EMTDC simulation results for verification. Finally, the economic aspect for possible redundant design of wind farm electrical systems is investigated based on operational and economic statistics from an example wind farm project

    Comparison between unipolar and bipolar single phase grid-connected inverters for PV applications

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    An inverter is essential for the interfacing of photovoltaic panels with the AC network. There are many possible inverter topologies and inverter switching schemes and each one will have its own relative advantages and disadvantages. Efficiency and output current distortion are two important factors governing the choice of inverter system. In this paper, it is argued that current controlled inverters offer significant advantages from the point of view of minimisation of current distortion. Two inverter switching strategies are explored in detail. These are the unipolar current controlled inverter and the bipolar current controlled inverter. With respect to low frequency distortion, previously published works provide theoretical arguments in favour of bipolar switching. On the other hand it has also been argued that the unipolar switched inverter offers reduced switching losses and generates less EMI. On efficiency grounds, it appears that the unipolar switched inverter has an advantage. However, experimental results presented in this paper show that the level of low frequency current distortion in the unipolar switched inverter is such that it can only comply with Australian Standard 4777.2 above a minimum output current. On the other hand it is shown that at the same current levels bipolar switching results in reduced low frequency harmonics
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