Research On Prediction And Suppression Method For Commutation Failure Of High Voltage DC Transmission System

Considering the reverse distribution of energy resources and load centers in China,DC transmission technology has been widely used in large capacity,long-distance transmission and regional power grid interconnection.Line commutated converter based high voltage direct current（LCC-HVDC）is the main form of HVDC transmission.Due to the LCC-HVDC converter station adopts thyristor without self-turning ability as converter element,the receiving end AC grid fault is easy to cause commutation failure of inverter station.If the commutation failure is not effectively suppressed in time,it is easy to lead to subsequent commutation failure,which will cause repeated power impact on the AC grid,resulting in converter station locking.Meanwhile,considering that the DC system inverter stations are densely located,the interaction between different DC systems and the interaction between AC and DC systems is becoming more and more complicated.AC fault can easily lead to simultaneous or secondary commutation failure of multi circuit DC,and then lead to DC blocking.When the AC system bearing capacity is weak,converter blocking may lead to the transfer of active power flow,making the normal operation of AC transmission line protection malfunction,resulting in cascading failure.At the same time,the converter station blocking causes the power imbalance between the sending and receiving end of the power grid,which may force the system to cut off the generator and reduce the load or even automatically split,which brings severe challenges to the safe operation of the power system.The controller equipped by LCC HVDC converter station can restrain the occurrence of commutation failure to some extent.However,because the controllers all adopt fixed parameters,they can not adjust according to the severity of failure,the dynamic change of electrical quantity during the commutation and the interaction between multiple controllers,and the suppression effect is seriously limited.Therefore,this paper analyzes the development process of LCC HVDC commutation failure,studies the mechanism and influencing factors of commutation failure,puts forward the idea of predicting the whole process of commutation failure,and puts forward a control method based on the prediction results to dynamically adjust the controller parameters and implement the whole process suppression of commutation failure.The main contents are as follows:（1）In view of the lack of theoretical basis for the selection of the existing preventive control parameters for the first commutation failure and the imperfect consideration of the influencing factors,the influence of the fault occurrence time,the dynamic change of DC current and the response of the extinction angle controller on the commutation process is deeply analyzed;Starting from the calculation of the critical commutation voltage value that caused the first commutation failure and the selection of the firing angle adjustment considering the severity of the fault,an improved strategy for the prevention control of the commutation failure of the DC system is proposed,which can enhance the immunity ability of DC system to the greatest extent.（2）Aiming at the problem that the subsequent commutation failure suppression measures cannot take into account the severity of the fault and the DC self-regulating effect and the effect is limited,a subsequent commutation failure prediction method is proposed,and the DC control parameters are adaptively adjusted based on the prediction results:1)The paper presents a prediction method for subsequent commutation failure of DC system under AC grid fault.Firstly,the predictability of subsequent commutation failure of DC system is demonstrated,and the idea of predicting subsequent commutation failure through commutation voltage comparison is proposed.Secondly,the calculation formula of subsequent commutation failure threshold voltage is derived through the analysis of commutation recovery process.Finally,the prediction method of subsequent commutation failure of DC system based on threshold commutation voltage is proposed.2)In order to fully improve the suppression effect of subsequent commutation failures,a new idea of adaptively adjusting the slope of the U_d-I_d-ord curve based on the prediction results to avoid subsequent commutation failures is proposed.This paper analyzes the causes and influencing factors of subsequent commutation failures,and the U_d-I_d-ord critical slope calculation formula is derived by considering the initial state of the failure and the influence of the inverter station controllers.Further considering the impact of DC system current control on the voltage and power angle stability of the receiving end,a subsequent commutation failure current limiting control method based on adaptive starting voltage is proposed.（3）As the coupling between multiple LCC-HVDC inverter stations fed into the same area strengthened,secondary commutation failures occurred in LCC-HVDC.Aiming at the problem of lack of mechanism research and quantitative calculation and analysis of secondary commutation failures,methods for predicting and suppressing successive commutation failures are respectively proposed:1)This paper finds and demonstrates the new phenomenon of successive commutation failure caused by commutation failure and DC return under the fault of receiving end weak current network;analyzes the dynamic reactive power characteristics of inverter station under the action of DC control system,and analyzes the generation mechanism of successive commutation failure;establishes the dynamic reactive power model of DC system during commutation failure,and derives the commutation failure of adjacent DC system Furthermore,a fast prediction method for successive commutation failure of multi infeed DC system is proposed.2)The consistency between LCC-HVDC subsequent commutation failure and adjacent LCC-HVDC successive commutation failure is analyzed.Based on the prediction results of successive commutation failure,the idea of coordinated suppression of successive commutation failure considering the influence of subsequent commutation failure is proposed.By deducing the maximum reactive power exchange capacity of adjacent LCC-HVDC under fault,the maximum reactive power consumption of its own DC inverter station is described;Combined with the upper and lower limit constraints of the DC current command value and the leading firing angle command value of the inverter station control system,an adjustable security region of the inverter station control output considering the influence of subsequent commutation failure is proposed,and based on this security region,a coordinated control method of successive commutation failure is proposed.The proposed method not only suppresses the subsequent commutation failure of lcc-hvdc,but also avoids the successive commutation failure of adjacent LCC-HVDC,and comprehensively improves the safety and stability of multi infeed LCC-HVDC.