| Power grids are responsible for regional interconnection,power transmission and distribution,and other important functions.Their corresponding protection relay systems play an irreplaceable role in maintaining the safety and stability of lines and power systems.The outstanding performance of existing power grid protection has been verified thoroughly in the past practice.Nevertheless,with the continuous growth of electricity demand of the whole society,the scale and complexity of power grid in China are increasing,which raises higher requirement to the protection performance.The time synchronization devices,electrical quantity acquisition devices,electrical quantity transmission channel / network and secondary DC power are the important protection oriented peripheral devices(POPD),of which working conditions will directly affect the protection performance.The power grid protection,especially the line pilot main protection,is inevitable to encounter various abnormal conditions of POPD,including the asynchronous sampling between both sides due to the failure of clock crystal,secondary breakage,disturbance and saturation of current transformer(CT),the sampling data loss due to the abnormities of data transmission channel / network,the outage of protection devices due to the secondary DC power failure,etc.Generally,the related protections will be blocked or outage directly when the abnormal conditions of POPD discussed above are identified on site.Whereas,it will lead to the serious deterioration of the protection performance.In addition,with the development of information and communication technology and the construction of intelligent substations,the power grid is becoming more and more networked and smart.The application of a large number of network devices and potential cyber security problems will bring more serious abnormal conditions of POPD,and the problems caused by the POPD abnormities will be more prominent.Thus,it is urgent to upgrade or reinforce the relevant functions of the protection system to improve its ability to deal with the POPD abnormities.Considering that the POPD abnormities are events with relatively low probability,the burden of the protection system will be undoubtedly increased if the functions related to these events are always enabled during the normal operation of the POPD.And the protection reliability will be reduced to a certain extent if the protection system becomes more complex.Thus,it is necessary to design the protection structure reasonably and the relevant functions should be designed as an emergency one.They cannot be put into operation only if the emergency condition of POPD abnormities occurs.In this way,the original protection which can no longer function normally will be replaced by the emergency protection.Therefore,aiming at the problem of the deterioration of the protection system performance caused by POPD abnormities discussed above,a series of grid emergency protection criteria and schemes are studied in this thesis.Aiming at the problem of the deterioration of the protection performance due to the unavailability of the line pilot differential protections caused by the time synchronization device abnormities,the superimposed instantaneous power difference is selected as one-dimensional time series for reconstructing the phase space trajectory based on the idea of phase space trajectory recognition.By analyzing the variation characteristics of phase space trajectory under different system conditions,a novel supplemental line pilot protection criterion based on the recognition of phase space trajectory formed by fault component instantaneous power is proposed.The proposed criterion is immune to the asynchrony of data on both sides,line capacitance current and reactive power compensation apparatus,and has the advantages of free setting,high speed and high-fault-resistance tolerance capability.Aiming at to solve the problem that the reliability of dual-configured high-voltage transmission line protections is significantly reduced when one set of protection is unavailable due to CT secondary breakage,an emergency protection scheme with high reliability based on multi-criteria redundancy is proposed with the help of substation area information and direct communication channel between substations.Compared with the emergency condition where only one set of protection works due to CT secondary breakage and the "1 out of 2" trip scheme of dual-configured protections commonly used on site,the proposed protection scheme can significantly reduce the maloperation and fail-to-trip probability of the protection in the meantime,and has the ability to tolerate single and multiple CT abnormities;Aiming at the problem that distribution line pilot main protection degenerates to the three staged over-current protection due to the abnormities of dedicated communication channel between substations,which has low reliability and is also affected by the electrical quantity acquisition devices abnormities,by modifying above multi-criteria redundancy based protection scheme,a localremote emergency dual-protections scheme for distribution line is proposed.The proposed scheme can not only improve the operating speed of the distribution line protection under the conditions of communication channel abnormities,but improve the ability of tolerating CT abnormity and cyber-attack;Aiming at the extreme abnormal condition of the incorrect operation of multiple line protections and even the whole substation protections due to the loss of sampling information of the whole station which is caused by the abnormities of electric quantity transmission network,an emergency protection system with high reliability and sensitivity is proposed.The compensation voltage difference criterion and measurement reactance percentage comparison criterion are designed for multi-terminal and double-terminal systems,respectively.Then the accurate fault identification is realized by combining the proposed protection criterion with the multi-terminal current differential protection or direction protection and local distance protection.The proposed emergency protection system has high reliability and sensitivity,and high response ability to the faults with fault resistance,which can effectively guarantee the continuous operation of the substation and the safety and stability of the regional power grid when sampling information of the whole station is unavailable;Aiming at the extreme abnormal condition of the complete failure of the protection sampling,calculation and tripping functions within the substation due to the loss of secondary DC power supply,two kinds of performance improvement schemes with high cost-effectiveness for the substation secondary system are proposed,which lay the foundation for the implementation of the line emergency main protection based on the remote tripping.On this basis,a novel emergency protection criterion based on the comparison of compensation voltage moduli is proposed,which does not rely on the synchronization and integrity of multi-terminal data.The proposed criterion has high sensitivity and can effectively cover most of the faults occurred on the protected line in the case of DC power loss by cooperating with local zone I distance protection.Its selectivity and speed are much higher than that of zone II distance protection. |
PDF Full Text Request