Research On Frequency Control Of Isolated Grid Based On Permanent Magnet Wind Turbine Converters

Microgrid is a small power generation,distribution system that integrates distributed power supplies,loads,energy storage devices,converters,monitoring and protecting devices.Microgrid solved the problem of decentralized grid,and improved the utilization factor of wind power.The frequency fluctuation is serious in microgrid,especially in islanded mode.This thesis researches on the problems of frequency drop and small amplitude oscillation,caused by load increasing and fluctuation of wind speed.This thesis has solved the problem of low frequency oscillation by improving the control strategy of converters.First of all,the mathematical models of permanent magnet generator and converters were established,the characteristics of active power control of permanent magnet wind turbine were analyzed.The vector control strategy was used to decouple active and reactive power,which provides theoretical basis for further researching.Next,this thesis analyzed the characteristics of isolated power grid and proposed a load reduction control strategy according to the problem of frequency second drop with inertia control strategy.In low wind speed areas,wind turbine generator tracks suboptimal power curve to operate,which reserves the active power to participate in islanding frequency modulation.In high wind speed areas,wind turbine generator combined with pitch angle control strategy and load reduction control strategy to stabilize itself and reserve sufficient active power.These methods solve the problem of frequency second drop at high and low wind speeds.Then,this thesis proposed a combination control strategy because a single load control strategy cannot solve the problem of low frequency oscillation effectively.In the machine side converter,the characteristic curve of power and frequency was analyzed,the load factor was ascertained according to the different wind speed and the problem of active power shortage was solved.The machine side control strategies enhance the frequency stability of isolated grid.In the grid side converter,fuzzy controller and fuzzy rules were designed,the relationship between frequency deviation,frequency change rate and compensation power of wind turbine was established.The grid side converter control strategies optimized input and output active power,restrained frequency drop effectively and solved the problem of low frequency.Finally,the simulation models of the wind power generation units participating in the islanding frequency modulation were established with different control strategies,in the Matlab/Simulink simulation environment.The results show that under the combination control strategy the operation state of permanent magnet wind turbine is the most stable,the frequency fluctuation is lowest.The thesis verifies the effectiveness and feasibility of the combination control strategy based the island grid experimental platform.