Research On Low Voltage Ride Through Control Of VSCF Wind Turbine Based On A Leading-edge Modular Drive Train Concept

Although there is much research on low voltage ride through (LVRT) for doubly-fedinduction generator and permanent magnetic synchronous generator at home and abroad, thesame research for the front-end speed controlled wind turbine isn’t much yet. The front-endspeed controlled wind turbine has the substantial development potential and will be the newdirection for the future development of wind turbine technology on account of its number ofadvantages. Therefore, the analysis and research on LVRT and the dynamic reactive supportcapability for the front-end speed controlled wind turbine are the urgent hot issues needed tobe addressed.The dissertation takes the front-end speed controlled wind turbine as an object to study.The dissertation does research on the LVRT control strategies as well as the characteristics inthe condition of different fault and different voltage dips. Also the effectiveness of controlstrategies is verified by simulation.Firstly, the dissertation analyzes operating theory of the front-end speed controlled windturbine, which is formulated mainly in four aspects. The basic principle, the mathematicalmodel, the efficiency and the wind farm model of the front-end speed controlled wind turbinewere studied. The unit structure was formulated and the regulating principle of the guide vanewas analyzed. The dissertation focused on the operating theory of the front-end speed contr-olled wind turbine. On this basis, the mathematical model of main components was establish-ed, the whole efficiency of the unit was deduced and the modeling method of the wind farmwas discussed.Secondly, the fast and accurate detection methods are utilized for various types ofvoltage sags. Detection method of voltage sags duration based on information-entropy andself-adaptive threshold is proposed. Detection method of voltage sags duration based onfractal number measurement and self-adaptive threshold is presented. Simulations arerespectively done for these types detection method.Moreover, the control structure of wind turbine is determined according to the front-endspeed controlled wind turbine’s characteristics. On this basis, the LVRT control of the front-end speed controlled wind turbine is mainly studied. How to effectively achieve dynamicreactive support capability of wind turbine is discussed during fault. Three subsystems areconsidered to achieve LVRT, which are electric pitch control of the pitch system, vane controlof the hydrodynamic torque converter, excitation control of the brushless electric excitationsynchronous generator, respectively. And these are three aspects for addressing LVRT of thefront-end speed controlled wind turbine. These above three aspects have something incommon, which can reduce power evenness. Pitch control and vane control both can decrease input mechanical power of prime mover, but brushless excitation control can increaseelectromagnetic power of synchronous generator. According to these different aspects, greypredictive model based pitch control with LVRT function and variable universe fuzzy controlof hydrodynamic speed control system based on multiple population genetic algorithmtogether with predictive fuzzy PID control of brushless excitation system based on greymodel are proposed respectively. Variable universe control is used for pitch control and vanecontrol, multiple population genetic algorithm (MPGA) is utilized to optimize contraction-expansion factor’s parameters, the intelligent optimization of contraction-expansion factor’sparameter is achieved. Predictive fuzzy PID control is exploited in brushless excitationsystem; imperialist competitive algorithm (ICA) is used to optimize quantity proportionfactors. Specific control strategies and the design of controllers are elucidated thoroughly. Thecontrol systems of the front-end speed controlled wind turbines switch from normal operatingstate control model to fault operating state, that is, reactive current priority mode during gridfault. Corresponding operations are carried out by main controller of wind power system,which takes the voltage of point of coupling common (PCC) as a trigger condition. On thisbasis, a coordinated LVRT control strategy for the front-end speed controlled wind turbinegenerating unit is proposed during grid fault. And system level simulation model isestablished. The simulation analysis of the LVRT operating characteristics for front-end speedcontrolled wind turbine is done. Simulation results show that the control strategies proposedcan fulfill control objects of each studied system better. The front-end speed controlled windturbine has better LVRT characteristics, which adapts with different faults and differentvoltage dip and meets the stringent grid code-the technology regulation of wind farmsintegrating in power system. Simulation results verify the effectiveness of the applied controlstrategies.Finally, grid support capability of the front-end speed controlled wind turbine basedwind farm is discussed. This dissertation deals with the effectiveness of fault induced currentinjection into the system by wind turbines. Firstly, a brief review of the grid code requirementon wind turbines to support voltage profile during fault is reviewed. Secondly, from the viewof grid integration model, based on first principles, analytical expressions quantifying theeffectiveness and the limits of the voltage support capability are formulated. This has thenbeen extended to include the limits imposed on output current by the transient stabilityrequirements of the wind park, in which stability constrained current limits through the linkwind park-the point of the interconnection and the boundary conditions for stable operationhave been derived. Using sample computations, the effect of the violation of this stabilitylimit during fault on the wind park has been analyzed. Moreover, a control scheme has beenproposed, which on the basis of the voltage dip experienced by the network, reduces the real part of the wind park output current with the objective of enhancing the transient stabilitymargin. Again, the control scheme for the system is proposed to follow the requirementsdefined by the grid codes. The dissertation presents a novel output feedback based method fornonlinear excitation controller and a structure based on voltage dependent active powerreduction to enhance the LVRT capability of the variable speed constant frequency (VSCF)wind turbines with directly coupled synchronous generator and hydrodynamic torqueconverter. The transient stability of the front-end speed controlled wind turbine based windpark is simulated using CA-2.0MW-WD based dynamic model. Simulation results show thatthe front-end speed controlled wind turbine has better LVRT characteristics, which adaptswith the symmetrical and asymmetrical grid fault situation with different voltage dips andmeets the stringent grid code-the technology regulation of wind farms integrating in powersystem. Simulation results demonstrate the effectiveness of the applied control strategies.