Research On Modeling And Application Of Converter Operation State Based On Temporal Features Of Valve-side Current

The line commuted converter is the core equipment in high-voltage direct current(HVDC)transmission system,and it achieves the AC/DC conversion.However,with the operation of actual project,the events of thyristor valves damage and converter block occur frequently,then the accurate and reliable extraction of the converter fault characteristics is of great significance to improve the reliability of converter protection.Therefore,this paper focuses on significant demands of the converter safety and the system stability.Firstly,as for the defect that the characteristics of terminal differential current is insufficient to detect and locate fault,considering the converter valve itself,the converter operating state model is constructed by comprehensively utilizing the internal relationship between the amplitude features and temporal features of the converter terminal currents and each valve state.Furthermore,the characteristic quantity can change from the electrical quantities of the converter terminal to the operating state of each valve,and the model of the converter operating state is constructed.On this basis,the detection of commutation failure,fault location of valves level and virtual measurement of valve current based on temporal features of the converter operation state are studied,which provides a set of fundamental methods to detect valve states and extract fault characteristics for control and protection system.The main contents are as follows:(1)As for the problem that the differential current amplitude characteristics of the converter terminal cannot match the converter operating state,the operating state model of converter is constructed based on temporal features of converter terminal currents.Taking the current intersection of two commutating valves currents as the demarcation point,the characteristics of AC maximum current is constructed by using the relative relation between the unilaterally polarity current of AC current on the converter terminal and the half of the maximum value of the three-phase AC current.The characteristic quantity of the AC maximum possesses the non-zero characteristic of conducting state and the constant-zero characteristic of blocking state,and the normal operation possesses the constant temporal features.However,the constant temporal features are broken after the fault occurs.The characteristic quantity of AC maximum current can be used to track the conducting process or blocking process of each valve,which can clearly characterize the characteristics of the operating state of each valve before and after the fault occurs,and realize the detection of the converter operating state.The constructed operating state model of the converter provides an underlying method for commutation failure detection and fault location.(2)As for the problem that the differential current of the converter terminal currents is easily affected by fault factors as operating conditions of AC-DC systems and fault conditions,which renders the insufficient sensitivity of the detection method.A novel method for commutation failure detection is proposed based on the interlocking relationship of two commutating valves states.The virtual conducting state of the valve is constructed by using half of the amplitude of the characteristic quantity of AC maximum current.Then the accumulated width is calculated by integrating it in a period to determine whether the virtual conducting state of the valve is abnormal.The interlocking relationship between the lengthened and shortened virtual conduction width is used to determine the commutation failure during the commutation process.Through the analysis of disturbance and fault transiency,the threshold for commutation failure is determined.Based on the practical project and wave recorded data,the results show that the proposed detection method is reliable to detect commutation failure.Compared with the detection method based on differential current,the proposed method for detecting commutation failure is less affected by factors such as operating conditions of AC and DC system and fault moment under different fault conditions.This method provides more margin for the adjustment of the control system,and helps to optimize the delay accumulation strategy for commutation failure protection.(3)As for the problem of the insufficiency for the converter protection to locate the fault and of the strategy of random bypass pair in the output of the converter protection,based on research(1),a method for fault location of converter valve level based on the differences of valves state in the horizontal and vertical is proposed.Taking the inverter as the research object,the state characteristics of the circuit at different fault locations inside the converter are analyzed.In the structure of the converter,horizontally,the conduction width of AC current is constructed according to the location of the commutation groups where the multi-valve is located,and in the longitudinal direction;vertically,the temporal features are constructed according to the states difference of AC current between the single bridge or the two bridges.Firstly,the fault location is identified by the amplitude characteristics of the converter terminal AC and DC currents and the abnormal characteristics of valve states.Then,the fault of the converter internal is located by using the horizontal difference of conduction width difference and the vertical temporal features of the two bridges.Thirdly,the faulty phase or faulty valve is located by the valve that has the greatest virtual conduction width within the time window interval.Based on the practical project and wave recorded data,the results show that the method has strong adaptability and will not be affected by the fault moment.The proposed method for location valve level fault provides a reference for the optimization research on the strategy of random bypass pair in the output of the converter protection.(4)As for the problem of the failure of the converter operating state under bypass pairs and the inadaptability to determine the valve state under converter complicated operating,a virtual measurement method of the valve current based on the temporal features of the converter the terminal AC and DC current is proposed.The actual conduction state or blocking state of each valve is estimated by using temporal features of the terminal AC and DC current.Then,the number of bypass pairs of the converter is calculated according to the estimated valve state,and the bypass loop equation is constructed based on the parallel relationship between the multi-phase bypass.Thirdly,the conducting state and valve current is corrected using the valve’s unidirectional conductivity to correct for valve blocking state.The results based on the practical project and wave recorded data show that,under different valve states and bypass groups,the proposed method for virtually measuring valve current can accurately calculate valve current in the transient process of commutation failure and its fault recovery,which verifies the effectiveness and reliability of the virtual measurement method of the valve current.The method improves the capability of detecting the converter operating state,and provides fundamental results for the optimization of the control system and the calculation of the heat accumulation of the valve.