Study On Commutation Failure And Its Prevention Strategy In UHVDC

Commutation failure is one of the most common faults for HVDC transmission system,which would increase the DC current,decrease the DC voltage and power,even block the system.The voltage level and transmission capacity of HVDC projects were usually less than 500kV and 3000MW in its early application.As a new technology,the short-circuit ratios(SCR)of the AC system at sending and receiving are usually bigger than 3.In this condition,the impact of commutation failure on AC system is not significant due to the high strength of AC system.With the development and wide application of HVDC transmission technology,the DC voltage level and transmission capacity have been continuously improved.The UHVDC transmission project will be hierarchically accessed to different AC systems,to optimize the power flow distribution and provide more voltage support in the receiving end system.With a number of±800kV/10000MW UHVDC projects,and the ±1100kV/12,000MW project planned to,put into operation,the relative strength of AC grid will be greatly reduced,the coupling characteristic of AC system and DC system will be closer,and the interaction between AC and DC system has been aggravating.The effect of commutation failure on the AC/DC system has become increasingly prominent,and the commutation failure process has become more complicated.To some extent,the conventional measures for preventing commutation failure cannot meet the requirements of operation.The commutation failure caused by electromagnetic coupling between DC lines and transformer excitation inrush current has occurred many times during the UHVDC operation.With the increase of DC voltage and transmission capacity,the problems above will become more and more prominent.In additional,UHVDC hierarchical connection system is a new structure,which brings new challenges for avoiding commutation failure in another converter when commutation failure occurs in one converter.It is urgent to study the new issue of UHVDC commutation failure,and propose the effective method to prevent commutation failure.This paper integrated theory with practice,studied the mechanism,characteristics and predictive control of commutation failure caused by UHVDC hierarchical connection mode,electromagnetic coupling between DC lines and transformer inrush current,main contents are as follows.The mechanism,characteristics and fault process of commutation failure are studied,and the relationship between DC current,commutating voltage,commutating reactance and extinction angle are quantitatively analyzed,and the critical values that caused commutation failures are proposed.The sensitivity of various parameters to the extinction angle is analyzed.The total differential equation of the extinction angle is established by comprehensively considering DC current,commutating voltage and advance angle.The ability of HVDC transmission system to resist the commutation failure under different operating conditions can be clearly reflected according to its analytical equation.Based on full differential equation with extinction angle,a method for judging commutation failure considering more variables is presented.The mathematical model of hierarchical connecting UHVDC is established according to the characteristics of UHVDC and the practical project.Then,a ±1100kV UHVDC project hierarchically connected to the 500kV/1000kV AC system is built based on the electromagnetic transient program PSCAD/EMTDC.The simulation model is verified by the steady state and transient characteristic test.According to the node voltage equation of the hierarchical system and combining with the multi-infeed interaction factor(MIIF)proposed by CIGRE,the calculation method of voltage interaction between UHVDC hierarchical connection systems is derived and proposed.The influence of coupling between different accesses systems on commutation failure for hierarchical connecting UHVDC system is analyzed based on hierarchical connection voltage interaction factor(HVIF).And based on that,a coordination strategy between high voltage and low voltage converters for preventing commutation failures is proposed.The strategy comprehensively considers the voltage coupling between high voltage and low voltage converters on AC side and current coupling of series structure on DC side,the ability of UHVDC to prevent commutation failure has been improved by solving the inconsistent response of the traditional commutation failure prediction control in hierarchical system.The effectiveness and adaptability of the proposed strategy are verified by the electromagnetic transient model.The electromagnetic induction characteristics between bipolar transmission lines are analyzed,when one pole failed and lead to a commutation failure for the other pole,the measures of improved extinction angle controller combining with DC voltage variation are proposed.The strategy is based on the characteristics of bipolar line coupling,and the characteristics of inducting DC current and DC voltage of non-fault pole,the extinction angle controller is improved.At the same time,the index of DC voltage variation is used to discriminate the fault pole.By compensating the extinction angle,the commutation failure of non-fault pole can be avoided.The strategy can improve the response speed of fault pole identification;it has strong practicability for project.The effectiveness and adaptability of the proposed strategy are verified by the electromagnetic transient model.The mechanism and characteristics of excitation inrush current of large capacity UHV transformer when energized are analyzed.A measure to prevent the commutation failure based on a sliding-window harmonic voltage principle analyzed by iterative algorithm is proposed.The measures based on the harmonic voltage characteristics of the commutation bus when UHV large capacity transformer is being energized,and the characteristic harmonics are extracted by iterative algorithm of Discrete Fourier transformation through the sliding window.The extinction angle is compensated according to the proportion of the harmonic content.The limitation of conventional commutation failure control based on the zero sequence voltage detection is solved in inrush current attenuation process to avoid the continuous fluctuation of DC power and commutation failure.The effectiveness of the proposed measure is verified by electromagnetic transient simulation and digital-analog real-time simulation,which has been applied in the Tianshan-Zhongzhou UHVDC project.