Study On The Influence Of Firing Control On Subsequent Commutation Failure Of HVDC System

Commutation failure(CF)is one of the most common faults of LCC-HVDC systems,A large number of studies and operational experience show that most CFs occur as a consequence of AC faults on the inverter side.It takes only a few milliseconds from an AC fault to a CF,so the first CF is difficult to be avoided.The subsequent CFs may occur when the control system is not correctly adjusted after the first CF,which seriously threatens the safety of the power grid.As the basis of the HVDC control system,firing control system is used to regulate the system's operation state by changing the trigger phase of the converter valve.Therefore,in-depth analysis of HVDC firing control is of great significance to reduce the probability of subsequent CF of HVDC and realize the fast and reliable recovery after the first CF of HVDC.Firstly,this paper introduces the hierarchical control strategy of HVDC control system.Based on CIGRE HVDC benchmark model,the basic principle of HVDC pole control system is introduced.Then,the three common HVDC firing control systems of CIGRE,ABB,and SIEMENS are analyzed in detail from two aspects: phase-locked loop and firing pulse generation.Then,the influence of the conventional PLL on the subsequent CF of LCC-HVDC systems is analyzed and it's found that the conventional PLL has some problems such as slow dynamic response,significant frequency fluctuation,and coupling of phase detection and frequency under fault conditions.A novel PLL is proposed to address the aforementioned problems.Cascade delayed signal cancellation operators are combined with a mathematical filter,forming a hybrid pre-filter to extract fundamental positive sequence voltage for the PLL.To reduce the fluctuation of PLL frequency during faults,the estimated frequency is fixed when the fault is detected by a fault detector.As a consequence,the coupling between phase detection and frequency detection is realized,and the proposed PLL's dynamic response performance is enhanced.Finally,the proposed PLL is applied to the CIGRE HVDC Benchmark model and Three Gorges–Shanghai HVDC project model and Guiguang II HVDC project model.Simulation results show that the proposed PLL can provide accurate phase reference for the DC control system and decrease the probability of subsequent CF of LCC-HVDC systems.In addition,the Small-signal model with discrete form of the ABB firing control is established.The influence of PI parameter and commutation voltage synchronization correction factor k on firing control are analyzed.An improved firing control is proposed to deal with the problems of the existing firing control,such as slow dynamic performance of PLL,and the firing control cannot trigger according to the output phase of the PLL quickly and accurately.The improved firing control adopts CDSC-PLL to synchronize the commutation voltage phase quickly,and the firing method is improved to makes the firing control can quickly trigger accurately according to the output phase of the PLL,which is beneficial to the precise regulation the HVDC control system.Finally,the improved firing control is implemented in CIGRE HVDC benchmark model,and the simulation results show that the improved firing control can reduces the probability of subsequent CF and improve the recovery performance of the HVDC system.Finally,the above researches are summarized,and further research works about firing control is prospected.