Short Circuit Fault Characteristic Analysis And Transient Overvoltage Suppression Technology Of Pmsg Connected To Weak Current Network

With the rapid development of power grid construction and the access of a large number of new energy units,the power grid strength in the new energy gathering area gradually weakens,and the transient characteristics of power system become more complex.The tradit ional power system transient characteristics analysis method based on thermal power units is no longer applicable to the high proportion of new energy power system.Due to its higher generation efficiency and reliable performance,direct drive wind turbine is more and more widely used in wind power collection areas.However,the transient characteristics of direct drive wind turbine during and after power grid short-circuit fault may have a certain impact on the safe operation of its grid connected system.In this paper,the single machine infinite bus system with direct driven wind turbine incorporated into weak current network is taken as the research object,the transient characteristics of the generator terminal at the time of occurrence and clearing of symmetrical short-circuit fault are analyzed,and the possible occurrence mechanism and influencing factors of transient overvoltage are studied.Firstly,the paper establishes the transient mathematical model of direct drive wind turbine,focuses on the fan input power,the relationship between AC and DC side electrical quantities and typical control logic,deduces the differential equations of fan fault transient current,analyzes the influence of PLL dynamic characteristics and fan control system PI para meters on fault current,and analyzes the coupling relationship between inner loop current command value and generator terminal voltage during low voltage ride through,The numerical solution of transient current in fault process is solved by Runge Kutta a lgorithm.The accuracy of the proposed calculation method is verified by comparing the theoretical value with the simulation value.At the clearing time after the short-circuit fault occurs in the weak current network,due to the untimely withdrawal of rea ctive current,the generator terminal will produce transient overvoltage,which may lead to the risk of fan disconnection.In this paper,based on the solution results of short-circuit fault transient current,the numerical solution of generator terminal t ransient voltage amplitude is calculated,and the effects of fan capacity,fault voltage drop degree,short-circuit impedance and PLL dynamic characteristics on transient overvoltage level are analyzed.A transient overvoltage suppression strategy based on PLL parameter slope rise and phase fixed value feedforward is proposed.Taking a direct drive wind farm in North China as an example,the MATLAB / Simulink model simulation verifies the effectiveness of the control strategy proposed in this paper.