Direct Stability Analysis Of Power System With Direct-drive Wind Turbine Generation

As the installed capacity of wind power rises year by year,while enjoying the concept of sustainable development brought about by clean energy,the problems it brings to the transient stability of the power system are becoming more and more complicated.At this time,directdrive wind turbines have been more and more widely used in wind power grid connection due to its technical advantages and lower cost.And then,an effective method is needed to analyze its stability.Currently,the transient stability analysis of the direct-drive wind turbine grid-connected system is mostly based on the time domain simulation method.However,its disadvantage of time-consuming makes the time domain method often only used for off-line analysis.At the same time,the direct method can be used as a complement to the time-domain method due to the fast simulation speed,and its ability to provide stability margins.This paper firstly established a power system model,including the direct-drive wind turbine model,where the wind turbine,the permanent magnet synchronous generator and the averaged converter model are used.The paper also used some control strategies to achieve the maximum power transmission from the wind turbine to the grid and the effectiveness of the control strategy under wind speed disturbances was verified on the 3-machine and 10-machine systems.Secondly,the energy function of the power system with direct-drive wind turbines was given by combining the definition of port energy.The wind turbine is equivalent to a dynamic load and the energy function of the direct-drive wind turbine as well as the whole system was calculated based on the voltage and power of the grid connection point.It is verified by simulations that,the obtained energy function decreased monotonically.Finally,the potential energy boundary surface method(PEBS),iterative potential energy boundary surface method(IPEBS)and the controlling unstable equilibrium point method(CUEP)are used to calculate the obtained energy functions respectively,and the detailed calculation steps are given.This paper analyzed the influence of generator permeability on the transient stability of the system where a fault occurred on the grid side.Also,the applicability of these methods were compared.The results show that the potential energy boundary surface method is effective for grid faults compared to the results of the time-domain method,but errors may occur in some cases.The iterations can improve the aggressive errors effectively,but cannot give optimization for errors that are too conservative.Although the results given by the controlling unstable equilibrium point method may be aggressive,the results are within an acceptable range.Compared with the PEBS method,the error range obtained is relatively narrow.When the wind power permeability gradually increases,the controlling unstable equilibrium point method shows more aggressive results and its effectiveness is reduced.