| Offshore wind power has the advantages of high power generation,stable op-eration,no occupation of land resources,and low transmission cost.Thus,it has huge development potential.Increasing the power level of the wind turbine is one of the effective means to reduce the levelized cost of energy.Currently,10MW systems are available in the market,and numerous research activities aim at 10-15MW level for offshore applications.Direct-drive permanent magnet syn?chronous wind power generation system has simple structure,high power density,and wide speed operation range,which is an attractive configuration for future high-power offshore wind turbine systems.Power converter control is offshore wind power’s key part,which will dom-inantly impact system power quality,efficiency,and safety operation.Conven-tional control techniques suffers from their "post-error-correction" and "limited-objective-optimization" principles,illustrating inevitable bottlenecks to further explore.Model Predictive Control(MPC)has the advantages of flexible design,good dynamic performance and strong expandability,which overcomes these bot-tlenecks of narrow control bandwidth and difficult global optimization posed on conventional control techniques.In this thesis,the principles of vector control and model predictive control are illustrated.The two methods are tested in the 10MW offshore wind power plant at wide operating points with the help of hard-ware in the loop.The comparative results proves that model predictive control is a one of the ideal control method for high power,low switching frequency offshore wind power system.However,there are still two challenges in the application of MPC to offshore wind power system.Firstly,the switch frequency of model predictive control is unfixed.The predictive control combines the control objective optimization and switching state decision processes into one step,which reduces the complexity of the controller design.However,the modulator-less mode of switching vector selection leads to unfixed switching frequency.The spectrum distribution is wide,hindering the filter design.In addition,spread harmonics may lead to unexpected resonance,noise pollution,and even reduce the stability of the system.Secondly,as a model-based solution,predictive control is sensitive to parameter variations and has poor robustness.When the system parameters are not accurate,the prediction of the system future behavior will be inaccurate.Hence,the predictive controller may select the incorrect switching vector,resulting in the deteriorated control performance.In this thesis,the predictive control theory for direct-drive permanent magnet synchronous wind power converters is investigated from two key aspects:fixed switching frequency and improved robustness.For the first challenge,predictive period control solution is proposed by including the switching period term into the cost function.The switching frequency of the proposed solution is fixed,while retaining the advantages of high dynamic performance of predictive control.For the second challenge,this paper proposed a parameter-free model predictive control method.The proposed method uses state variables measurements at k-1 and k-2 instants and the wind power system’s small signal model,to reconstruct the dynamic model of the wind power system.As a result,the proposed solution does not need any system parameters to predict the system’s future behavior,showing high robustness to the parameter variations.To sum up,this thesis designs a robust predictive control solution for high-power offshore wind power system with fixed switching frequency,which provides technical support for high-performance,efficient operation of large-scale wind power systems. |
