| It is required for grid-connected wind power generation systems(GWPGSs)that their output power frequency must be consistent with that of the grid.In most of the currently-operating variable speed constant frequency(VSCF)GWPGSs,double-fed induction generators(DFIGs)with partly-rated converters or direct drive synchronous generators(DDSGs)with fully-rated converters are usually employed to realize the decoupling between variable wind rotor speed and the fixed grid frequency.Such solutions can meet the needs of the current wind energy industry.However,with the expansion of the industrial scale and the increasing of the penetration rate of wind power generation in the distribution network,some significant technical problems for the existing GWPGSs are increasingly severe.The required converters and control system might produce voltage flicker and harmonic current to reduce output power quality.The operating stability and security of wind power generation system can also be affected due to the poor low voltage ride through(LVRT)capability.In addition,the high control complexity as well as the expensive manufacturing and maintenance costs of high-power converter equipment are also the critical issues to be settled.Above-mentioned problems have caused great concern,but owing to fixed principles and structures of the existing GWPGSs,it is still difficult to have substantive breakthrough for all these challenges.In fact,a significant technical bottleneck for the existing GWPGSs exists.Therefore,research on a kind of new-type GWPGSs to meet the sustainable development requirements of the future wind power industry has great theoretical and practical significance.Against this backdrop,based on the developed mechanical transmission and speed control technologies,some novel grid-friendly wind turbine(WT)concepts,using speed regulating devices at front-end of synchronous generator,were proposed by researchers around the world.And then the principle feasibilities of these schemes were verified.Taking the proposed grid-connected wind power generation system with the speed regulating differential mechanism(SRDM)as research object,its key issues including the configuration method and mechanical transmission performance of transmission system,the control strategy of speed regulating motor as well as the system grid-connected operating behaviors are all studied in depth.As a result,the critical scientific problems in real-time speed regulation transmission of VSCF GWPGSs can be solved,which provides strong theoretical basis and technical support for the development of large-scale GWPGSs.The main works and outcomes of this paper are as follows(1)Research on transmission performances of SRDM-based GWPGSThe specific connection configuration of transmission system in SRDM-based GWPGs was determined according to analyzing the relations between speeds and powers of each feasible transmission forms of SRDM.Based on the optimal transmission train,its transmission performances were studied.The calculated equations and analysis methods of key design parameters such as speed regulation region,power distribution ratio and transmission efficiency were proposed.Additionally,the simplified triaxial dynamic model of transmission system is presented via lumped-parameter method.The corresponding dynamic equations are also derived.The theoretical design basis for transmission system of VSCF GWPGS is preliminarily established.(2)Research on the speed control strategy for speed regulating motor of SRDM-based GWPGSIn order to make sure that generator output power frequency is consistent with the that of power grid regardless of the variable external working conditions,a discontinuous projection-based adaptive robust backstepping controller(ARBC)was proposed according to the control requirements for speed regulating motor in SRDM.Moreover,an extended state observer(ESO)was designed together with ARBC to guarantee the control system global robustness with a feedback robust law.The proposed controller is able to well-handle the negative effects caused by variable wind rotor input load,parametric uncertainties and external disturbances without using the large nonlinear feedback gain or violating any constraints during transient period.Therefore,the control performances of speed regulating motor can be guaranteed and the synchronous generator output power frequency can well-meet the application requirements for grid-connection.A kind of physical experimental platform for the SRDM-based GWPGS was established by which the speed tracking performance of the proposed ARBC-ESO were verified.(3)Research on grid-connected operating performances of SRDM-based GWPGSSimulation analysis software of GWPGS is an indispensable tool for the overall design of wind turbines.The existing simulation software tools such as Bladed and HAWC are mainly developed for mainstream DFIG-based WTs or DDSG-based WTs.These simulation tools can not be used directly to the variable speed constant frequency SRDM-based GWPGS because its transmission system possesses the special structure in terms of multi-degrees of freedom and variable gear ratio.As a result,taking advantages of the research outcomes in the fields of transmission performance,dynamic modeling and advanced speed control strategy of SRDM-based GWPGS,the dedicated numerical simulation model of SRDM-based GWPGS was developed via SIMULINK,in which the energy conversion and transmission mechanism,speed regulating capacity,power consumption and grid-connected operating characteristics can all be systematically studied.Afterwards,a series of experimental studies were carried out to verify the principle availability of the established triaxial dynamic model as well as the proposed simulation model.Experimental results show the small modeling errors and the satisfactory practicality of the designed simulation model.Considering the International Electrotechnical Commission(IEC)standards,four different typical wind conditions including normal turbulent model,extreme turbulent model,extreme operating gusts and extreme coherent gusts with change in direction were generated via Fatigue,Aerodynamics,Structures,and Turbulence(FAST).Then,the speed regulating ability and the energy consumption required for speed regulating motor were studied in detail with the proposed 1.5 MW simulation model of SRDM-based GWPGS under four wind speed inputs.Simulation results shown the speed regulating performance of the proposed SRDM-based GWPGS was able to well-meet the provisions of national standards.The energy efficiency of the proposed GWPGS can also be significantly improved because the power electronic equipment with high energy consumption were removed.In order to validate the grid-connecting operation performances of SRDM-based WPGS,the electrical harmonic pollutions and system low voltage ride through capability under different voltage dip faults were all analyzed and then compared with that of the existing direct-drive or double-fed wind power generation systems. |
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