Research On The Power Control And Dynamic Load Reduction Techniques For Variable-speed Fixed-pitch WECS

Among various kinds of renewable energy generation techniques, wind energy is by far the fastestgrowing energy for its abundant resources and maturity of turbine techniques; and, it has become aresearch focus and priority all over the world. To meet the electricity needs of the people in remoteareas and to support the developing of distributed generation and microgrids, it is more important todevelop the small-to medium-scale wind energy conversion system (WECS) rather than thelarge-scale ones. The essential control goal for small-to medium-scale WECS is to minimize the costof per generated power. To achieve this, optimum power control should be applied to the WECS.However, for the turbine structure is usually chosen as fixed-pitch concept, it has problems onlimiting the turbine power at high wind velocities and sustaining high dynamic load acting on theturbine shaft, which will shorten the service life of the system. Thus, the main contribution of thispaper is to lead a deep analysis to the power control and dynamic load reduction techniques for small-to medium-scale wind turbines.The turbine structure is firstly chosen in this paper, and then it is modeled as a two-mass model.The characteristics of the turbine operating in the full wind velocity range is obtained, which doeshelp to design the regulator and damping controller of the turbine.Then, to increase the annual energy yield and to comfirm its safety, a new maximum power pointtrcking (MPPT) control strategy, which can achieve unified control performance, is proposed in thispaper. Then, to solve the problem that the tracking speed is low and varies with wind velocity existedin the conventional OP control method, method to make the tracking bandwidth constant is proposed.With the proposed method, it is possible to conduct a systematic design procedure on the MPPTcontrol strategy. To compensate the reliability of the system at high wind velocities, a newcompensation method is also proposed in this paper. With the proposed method, constant poweroperation is realized.Besides the power control strategy, the dynamic load reduction rechniques are also required toincrease the service life of the system, thus to reduce the system cost. In this paper, the dynamic loadsare categoried in four parts according to their souces. And the corresponding strategies to reduce theloads are also discussed in detail: To optimize the transient load produced during MPPT process, theMPPT bandwidth is optimally designed with the constant bandwidth MPPT method proposed. Thesystem control with damping injection method is then proposed to reduce the aerodynamic loadcaused by wind shear and tower shadow and to get rid of the vibration operation mode. In addition, to reduce the transit load under turbulence, soft-stall control method is proposed, with which the transitload can obtain a nearly80%reduction compared with the passive stall method. Moreover, to reducethe high frequency load induced by the current harmonics generated by the diode rectifier, a low costdamping method is also proposed. All the dynamic load reduction methods proposed are beingdiscussed in detail in this paper.Finally, a10kW stand-alone WECS is established in the laboratory. The power control and dynamicload reduction method as well as a new decoupled energy management mechanism are all employedin this system. The good performance of the proposed method is validated by the experimental results.In addition, to set an example on how to conduct a whole stand-alone WECS, the start/stop procedure,yaw and untwist system are all illustrated at the end of this paper.