Research On Grid-connected Characteristics Of Front-end Speed Controlled Wind Turbine

Grid-connected characteristics are the references to the gird-connection of wind turbines,which can reflect the operational characteristics and stability of wind turbine directly. It is ofparticularly urgent and important to investigate on grid-connected characteristics of windturbines with the improvement of wind generation technology and the ever-increasing on-gridwind capacity. The regular wind generators, for instance, permanent magnetic synchronouswind generator (PMSG) and doubly-fed induction wind generator (DFIG), become much lesslikely meet the ever-improving grid connection requirements for wind turbines as there existsa series of problems like poor low voltage ride through (LVRT) capability, inadequatereactive power output capacity and so on. In contrast, the front-end speed controlled windturbine has a better performance such as high LVRT capacity, reactive power output capacityand good electric energy quality, etc. So, it is of great importance to take pains to study thegrid-connected characteristics of front-end speed controlled wind turbine, which can bebeneficial to promote the development of wind generation technology in our country.The mathematical wind turbine model is established on the basis of comprehending theoperation principle of front-end speed controlled wind turbine and an analysis of turbinecontrol structure and efficiency. Then, the control strategy is determined based onsummarizing the grid-connection control requirements for wind turbines of the grid codes,that is quasi-synchronization unloaded grid-connection control, optimal active power controlbased on genetic algorithms, voltage and reactive power control via double loop H∞(DL-H∞)strategy and direct speed control regional division, and simulations are also conducted,accordingly. Simulation results show that the control strategies used can meet with differentcontrol requirements in this thesis, which verifies the correctness and validity of the controlmethods or strategies applied.According to the various kinds of abnormal operation conditions that the wind turbinemay confront in actual operation, the different types of wind speed model are used asturbine-side disturbances and both symmetric and asymmetric ground fault are used as thegrid-side disturbances, separately. Simulations are conducted considering both turbine-sideand grid-side disturbances in comparison with a DFIG turbine with equivalent capacity.Simulation results suggest that the front-end speed controlled wind turbine has a bettergrid-connected operation performance that DFIG under various operation conditions.Analysis and simulation of front-end speed controlled wind turbine are conducted ontransient load angle stability, voltage stability and frequency stability in considering thepower system classification standards for grid-connected stability and the impact ofconventional wind turbines once connected on grid. In the transient load angle stability analysis, the previously established simulation model is translated to an equivalent model andis applied to the IEEE5machine14bus system for time-domain simulation. Analysis on bothstatic and steady-state stability are conducted with an equivalent calculations in the voltagestability aspect and corresponding simulations are also carried out. In the frequency stabilitystage, the main factors affecting the stability of the system are summarized with themechanism analysis, based on which, the simulation analysis is carried out. The simulationresults of front-end speed controlled wind turbine in transient load angle stability, voltagestability and frequency stability all indicate that the front-end speed controlled wind turbinehas a higher grid-connection stability and a better grid adaptability in comparison with thatDFIG wind turbines.