Studies On A Series Of Improved Expressions Of Summation Impedance Criterion And Their In-depth Applications

High voltage transmission lines consist of the backbone of grid interconnection and the long-distance power transmission network,of which protective relay plays an important role in maintaining the secure and stable operation of power grids.With the continuous expansion of the scale of the power grid,such problems as remarkable capacitance current,current transformer saturation,power swing and high resistance grounding fault are becoming more and more serious,which will bring more serious challenges to existing line protections Therefore,it is urgent to study and develop new line protection.In recent years,a new relay concept-summation impedance relay,which combines the advantages of protections implemented with different principles,has been emerged.This type of relay is not only capable of removing the fault of lines rapidly,but needing less information of both line sides compared with other protections.However,the basic design of this criterion is still preliminary,and its performance improvement has great potential for mining.Accordingly,the design of different expressions of impedance relay,together with their applications under different operating conditions and data quality scenarios,are studied in this thesis.In order to reduce the fail-to-trip zone of existing summation impedance relay during single-phase ground faults near the receiving end under light load conditions,the negative influence of a fixed capacitance current threshold on the sensitivity of impedance and current based pick-up criterion is first analyzed in this thesis.On this basis,a new capacitor current compensation model is proposed,and a new adaptive floating threshold based on the real-time voltage phasor of opposite terminal and measuring impedance of this terminal is designed.By this means,Category I improved expression of summation impdance criterion is put forword.In order to eliminate the advese impact of asynchronism between two-side dada on Category I improved expression of summation impdance criterion,a new round deduction is carried out,and Category II improved expression of summation impdance criterion,which only depends on the modulus information of voltage,is put forward.For the purpose of overcoming the negative influence of TA saturation on the performance of summation impedance relay,the variation law of the operating value of the summation impedance protection during the process of TA saturation and desaturation is first deeply explored,and appropriate supplementary criteria against TA saturation is proposed.In order to completely eliminate the adverse impact of capacitance branching effect of long-distance transmission line on summation impedance criterion,based on the compensated line current,Category III improved expression of summation impdance criterion is put forward.The abilitis of this criterion against fault resistance and the sensitivity in response to internal faults during power swings are analyzed and discussed theoretically.Aiming at solving the problem of this crierion unable to reflect metallic faults,a measuring element capable of responding metallic faults and faults with low resistance sensitively is introduced.By virtue of the combination and coordination of above criteria,a pilot main protection of transmission line,which is highly secure,capable of responding various internal faults occurred on any position within this line,and immune to abnormal system operation conditions,is put forward.On the above basis,in order to effectively deal with the scenario that pilot main protection cannot work due to complete breakdown of data synchronization system,the risk of complete asynchronization among electrical quantities with the whole substation is analyzed.Then,Category IV improved expression of summation impdance criterion,which is an impedance modulus based criterion without depending upon phase information,is put forward.In view of the limitation of this type of criterion,a so-called line mid-part protection crierion is developed.By combing above criteria with instantaneous over-current protection,an emergency line main protection shceme against protection oriented data sychrounization system breakdown is proposed.Case studies and simulation test results show that the proposed a series of improved expressions of summation impedance criterion have high security and ability against fault resistance.The pilot line main protection consisting of these criteria are able to cover various complicated fault including single-phase-to-ground fault with very high resistance,which provides a good solution for improving the performance of the line main protection under complicated system operation conditions and bad data qualify.