New Protection Scheme Of Transmission Line For MMC-HVDC Grid

With the shortage of fossil energy and the increasingly serious environmental pollution problems,more and more countries are working on the vigorous development of renewable energy and the adjustment and optimization of energy structure.China has a vast territory,and the renewable energy base and load demand are distributed inversely.The modular multilevel converter based high voltage direct current(MMC-HVDC)grid with overhead lines is an effective solution to realize the long-distance transmission of large-scale renewable energy.The application of overhead lines will greatly increase the probability of DC line failures.A complete and reliable DC line protection scheme is essential for the safe and stable operation of the MMC-HVDC grid.In this paper,funded by the Smart Grid Joint Fund and the National Natural Science Foundation of China,the requirements of the line protection for MMC-HVDC grid are analyzed,and the problems existing in the DC line protection in actual operation and existing research are summarized.The fault characteristics of the MMC-HVDC transmission system are revealed,based on which the distance protection for DC lines based on the online estimation of the criterion extreme value,the distance protection for DC lines in response to the outlet fault,and the current differential protection for DC lines based on the segment compensation of distributed capacitive current are proposed,forming a new protection scheme of MMC-HVDC grid.The main research of this paper includes:The study on the fault characteristics of the MMC-HVDC system is the basis for proposing new DC line protections.The temporal and spatial distribution characteristics of the DC line voltage are analyzed,the linear fitting method of the line distribution voltage is proposed,and the correlation between the linear fitting error and the cut-off frequency of the low-pass filter and the length of the fitted line is discussed.The analytical expression for the transient voltage of the equivalent capacitance of the converter after the DC line fault in the two-terminal MMC-HVDC system is derived,and it is proved that the transient voltage will keep unchanged during the short period after the fault.A numerical analysis model is established in the MMC-HVDC grid,with which the transient voltage of the equivalent capacitance of the converter can be calculated.The temporal and spatial distribution characteristics of the DC line voltage and the transient voltage fault characteristics of the equivalent capacitance of the converter are analyzed and verified.The conclusions obtained provide important theoretical support for proposing new DC line protections for MMC-HVDC grid.DC line protection in operation has a single principle and poor reliability.The fault characteristics that the main protection relies on only last for extremely short time,the setting relies on simulation,and there is no protection principle to respond to faults during the time interval between the main and backup protections.The distance protection does not require communication channels,can be used for a long time,and has a clear setting principle.Aiming at this problem,the polarity relationship between the compensating voltage and the measure voltage under the internal fault and external fault of the DC line is analyzed based on the temporal and spatial distribution characteristics of the voltage,and the method which uses the voltage ratio,the ratio of the compensating voltage to the measure voltage,to distinguish the fault is proposed.Aiming at the slow operation of the distance protection for DC lines,the protection is divided into several sections to insure the quick action under the near fault and the selectivity under the remote fault.In order to prevent the maloperation of the protection caused by the line parameter error and frequency change characteristics,the maximum voltage ratio is estimated online within the range of the line parameter.The simulation results show that the proposed distance protection can respond to non-outlet faults of the DC line quickly.The distance protections in current research and the distance protection based on the online estimation of the criterion extreme value are difficult to distinguish the outlet faults of DC lines.Aiming at this problem,the polarity relationship between the compensating voltage and the transient voltage of the equivalent capacitance of the converter under the fault at the outlet of the DC line is analyzed.Considering the fault characteristics of the transient voltage,and the influence of the line parameter error and frequency variation characteristics,a distance protection is proposed to distinguish the fault by using the polarity of the extreme value of the compensating voltage during the short period after the fault.Combining this protection with the distance protection based on the online estimation of the criterion extreme value,the overall distance protection scheme of the DC line is formed.The non-outlet faults of DC lines are identified by the distance protection based on the online estimation of the criterion extreme value,and the outlet faults are identified by the distance protection in response to the outlet fault.Simulation results show that the proposed overall DC line distance protection scheme can respond to outlet faults and non-outlet faults of DC lines quickly.The current differential protection for DC lines can be put into operation for a long time,respond to the faults continuously and has strong transition resistance withstand ability.The action speed of the current differential protection in actual operation is slow and the methods in research for dealing with distributed capacitive current still need to be improved.Aiming at these problems,the segment compensation method of distributed capacitive current is proposed based on the temporal and spatial distribution characteristics of the DC line voltage,which can insure the protection action speed as well as the compensation accuracy of the distributed capacitive current.To prevent the differential protection from maloperation due to synchronization errors,a synchronization error identification method based on waveform matching is proposed.By matching the waveforms of the currents at the compensation point,the synchronization error is calculated,which can reduce the dependence of protection on clock synchronization.Simulation results prove that the proposed current differential protection can identify internal and external faults quickly and reliably and will not be influenced by the synchronization error.Aiming at the problems in the existing DC line protection scheme,a new protection scheme of DC lines for MMC-HVDC grid is proposed.On the basis of retaining the existing traveling wave protection and differential undervoltage protection,the distance protection based on the online estimation of the criterion extreme value and the distance protection in response to the outlet fault are included in the new protection scheme,which can enrich the configuration of the single-end protection in the scheme and make it have the ability to respond to the faults for a long time based on single-end information.The current differential protection for DC lines based on the segment compensation of distributed capacitive current is also included in the scheme,which is based on two-end information,can respond to the faults continuously and has strong transition resistance withstand ability.The results of digital simulation show that the new protection scheme of DC lines can clear the fault before the converter is locked,without considering the present limit of the breaking capacity of the DC circuit breaker.The new protection scheme of DC lines for MMC-HVDC grid is also verified by the dynamic model test.