Research On The Control Mismatch Mechanism And Improvement Strategy Of CFPREV In HVDC

The mutual influence between AC and DC systems has brought huge challenges to the safe and stable operation of the power grid,and commutation failure is the most important problem.In practical engineering,HVDC usually adopts commutation failure prevention control(CFPREV)to improve the ability to resist commutation failure.However,from the results of field recording and simulation,the existing CFPREV has the phenomenon of control mismatch.Control mismatch not only limits the ability of the HVDC to resist commutation failures,but also triggers new commutation failures,which is contrary to the original intention when designing CFPREV.Therefore,it is urgent to clarify the correlation mechanism between the effect of CFPREV and its influencing factors,and to dig deeper into the causes of the control mismatch of CFPREV.Taking effective measures to avoid the control mismatch of CFPREV and to further improve the performance of CFPREV is of great significance for ensuring the safe operation of the power grid.Based on this background,this paper conducts an in-depth study on the problem of the control mismatch of CFPREV,and carries out related work from three levels: mechanism analysis,case analysis,and improvement strategy.The main progresses are as follows:Aiming at the unclear problems of factors and mechanisms of the control mismatch of CFPREV.The influencing factors of the control effect are sorted out,and the influence of a single factor on the control effect and the relationship with the control mismatch are analyzed from the mechanism,the shortcomings in the design of the existing CFPREV are found,and the directions for the subsequent improvement of CFPREV are pointed out.Aiming at the problem that multi-factor coupling easily leads to the control mismatch of CFPREV.Based on a large number of simulation examples,this paper sorts out and classifies the fault conditions corresponding to the control mismatch of CFPREV under different fault types,analyzes the different coupling types that cause the control mismatch of CFPREV,summarizes the laws and characteristics,and proves the correctness of the theoretical analysis.Aiming at the phenomenon of the control mismatch of CFPREV that is prone to trigger angle overshoot near the critical commutation failure boundary.The influence of harmonics on the commutation voltage after an AC failure is analyzed,and the commutation voltage curve is fitted with the Levenberg-Marquardt algorithm which takes the low-order harmonic components into account.According to the results of fitting,a suppression method of the control mismatch of CFPREV based on output value limiting is proposed,and its effectiveness is verified by simulation.Aiming at the problem that unreasonable output response time of CFPREV will lead to the control mismatch of CFPREV.The correlation between system parameters,CFPREV control parameters and the control mismatch of CFPREV is studied separately,and the critical commutation voltage expression that takes initial fault angle into account is derived.Based on the calculated critical commutation voltage,a control parameter optimization strategy of CFPREV considering the influence of the control mismatch of CFPREV is proposed,and its effectiveness is verified by simulation.