Research On Transient Voltage Control Strategy Of Wind Farms At The Sending End Under Commutation Failure Of HVDC Transmission System

In recent years,with the increasing demand for electricity in China and the largescale access of renewable energy,new energy power generation and high voltage direct current(HVDC)transmission technologies represented by wind power have been rapidly developed.When large-scale wind power is sent out via HVDC,if the AC system on the inverter side fails to cause DC commutation failure,it will cause a transient overvoltage in the DC transmission end system,which will cause the wind turbine to disconnect from the grid and cause a chain reaction.In order to suppress the transient overvoltage of the DC transmission end system,this thesis chooses the wind farm with doubly-fed wind turbines as the object to study the transient voltage control strategy of the transmission end wind farm under the commutation failure of the HVDC transmission system.Firstly,the mechanism of transient overvoltage generation after the DC commutation failure of the sending end system is deduced.The influence of DC control parameters and wind power control parameters on the transient overvoltage mechanism is analyzed.At the same time,the influence of the power grid of the sending end AC system on the transient is studied.The influence mechanism of overvoltage,on this basis,further extracts the main influence factors of the DC system and the sending end AC system on the overvoltage during the fault,and establishes the theoretical model between the degree of voltage change during the failure of DC commutation and its influence factors.Secondly,in order to suppress the transient overvoltage caused by DC commutation failure,a transient voltage control method of the synchronous condenser based on voltage prediction is proposed.Based on the establishment of a theoretical model between the change degree of the commutation failure voltage and the influencing factors,the Spearman correlation coefficient analysis method is used to analyze the correlation degree between the key power grid parameter characteristic values and the transient overvoltage.Aiming at the problem that the reactive power response is not timely due to the time delay of the excitation control of the synchronous condenser,the OS-ELM voltage joint prediction model is established,which combines the off-line prediction results of the electrical eigenvalue training with the online prediction results of the actual voltage measurement training.The change trend of the transient voltage is obtained,which provides a reference for the advanced excitation control of the synchronous condenser and effectively suppresses the transient voltage overvoltage can reduce the risk of wind turbine off grid.Finally,a coordinated control strategy of the synchronous condenser and SVC is proposed to suppress the transient overvoltage caused by commutation failure.Aiming at the shortcomings of reactive power compensation when the synchronous condenser is used as reactive power source during DC commutation failure,the reactive power response characteristics of the synchronous condenser and SVC during fault are analyzed.The advantages of the two reactive power sources are brought into full play,and the coordinated control of the two sources is established in the fault ride through and voltage recovery stages,so as to effectively suppress the transient overvoltage degree during fault and reduce the transient overvoltage during fault recovery period.In the simplified model of the actual power grid,the effectiveness and feasibility of the proposed control strategy are verified.