Research On Novel Criteria And Schemes For Strengthening Weak Links Of Power Grid Protection Action Chain

After decades of development and engineering practice,China’s protective relaying technology is becoming more and more perfect.The excellent performance of traditional power grid protection has been fully verified in power grids of various voltage levels.However,with the increasing scale and complexity of power grid,higher requirements are put forward in terms of “the four characteristics” of protection.In the perspective of system theory,the overall performance of protection depends not only on the strict inherent logic,but also on its peripheral equipment.The whole chain of protection action can be roughly divided into data acquisition,data transmission,criterion calculation,control command issuing and condition monitoring.In the “acquisition and transmission” link,the upstream of the protection action chain,even if heavily fortified,some extreme situation,such as loss of data synchronism on both sides of the line,is difficult to be completely avoided,which will overturn the premise of various protections requiring phase comparison,such as current differential protection.Another extreme abnormal condition in the “acquisition and transmission” link is the loss of measurement information.In this case,the protection will partially or even completely lose the data basis required for decision-making.In order to avoid the protection mal-operation caused by the deterioration of synchronization quality or the lack of measurement information,the corresponding protection is usually blocked directly in the engineering practice,which will degrade the original dual configuration main protection into a single set of protection for high-voltage power grids,seriously reducing the reliability of protection system.As for the scenarios of single configuration of main protection,the pilot current differential protection with the best performance will be completely lost.With the rapid growth of system capacity,the short-circuit current level of power grids in China,no matter whether they are located in sending end or receiving end,is rising,approaching or even exceeding the maximum breaking capacity of circuit breakers,which brings major hidden dangers to the safe and stable operation of power grids.For the “control” link located at the downstream of the protection action chain,i.e.,the execution link,the traditional protection will directly trip the line once the fault is judged as an internal one.However,if the fault current exceeds the breaking capacity of the corresponding circuit breaker,the consequences of forced interruption are unimaginable.Facing the challenge of the increased risk of protection mal-operation caused by the abnormality of the upstream and downstream links of the above-mentioned protection action chain,based on the perspective of improving the overall reliability of protection,a series of new criteria and schemes are put forward in this thesis to improve the adaptability of protection in extreme scenarios such as deterioration of synchronization quality,lack of measurement information and excessive short-circuit current,which is of great significance to further strengthen the position and role of “the first line of defense”of protective relaying.Aiming at the problem that the reliability of current differential protection decreases or even protection fails to work completely after the synchronization quality of the“acquisition and transmission” link is degraded,a comprehensive summation impedance protection criterion without needing strict data synchronization is proposed.Firstly,the operation characteristics of summation impedance relays based on different input quantities and different wiring forms are analyzed,and the reasons why the existing summation impedance protection do not have the abilities of self-phase-selection and response towards metal fault are revealed;On this basis,by making rational use of various types of summation impedance relays,a comprehensive summation impedance protection criterion that can effectively overcome the above defects are constructed.The simulation results show that the proposed criterion can realize the sensitive response and phase-selection tripping from metallic fault to high-resistance fault.Based on the above abnormal working conditions,aiming at the problem that the performance of the pilot main protection is difficult to maintain after the voltage measurement of the “acquisition and transmission” link is lost,two kinds of improved current amplitude differential protection criteria are proposed.Firstly,the inadaptability of the existing current amplitude differential protection criteria when applied to transmission lines and distribution feeders respectively is analyzed;Furthermore,on the one hand,compensating completely the distributed capacitance current of transmission lines by using only one-sided voltage information is realized based on Bergeron model,so as to construct the type-I improved current amplitude differential protection criterion,which is completely immune to the distributed capacitance current of transmission lines;On the other hand,the decaying ratio of differential current is introduced to effectively extract the impulse current characteristics of induction motors,so as to construct the type-II improved current amplitude differential protection criterion,which is completely immune to the load branch current of distribution feeders.The two proposed criteria are well adapted to the loss of voltage measurement,and significantly improve the operation reliability of current amplitude differential protection.Aiming at the problem of protection issuing trip command but the circuit breaker failing to open due to the short-circuit current exceeding the rated breaking capacity of circuit breakers,a flexible tripping strategy is proposed,which can deal with the uncertain over-limit short-circuit current.Firstly,the characteristics and mechanism of full current exceeding the limit caused by high aperiodic component of short-circuit current are analyzed,and the fundamental idea of flexible tripping strategy is clarified;Secondly,the improved recursive least square algorithm is used to on-line estimate the parameters of short-circuit current with decaying aperiodic component,so as to identify the scene of excessive short-circuit current before the protection issues the trip command,and on-line search for the optimal opening time of the circuit breaker;Finally,the overall process of protection flexible tripping is designed.The simulation results show that the proposed strategy can deal with the potential risk of over-limit short-circuit current due to high residual aperiodic component.Aiming at the blind area of the protection flexible tripping strategy,a dynamic limiting strategy based on the on-line optimization of line breaking combination,together with the accelerating strategy of circuit breaker failure protection,is proposed.Combined with the flexible tripping strategy,a protection initiative response scheme with multi-strategy collaboration to deal with the scenarios from slight over-limit short-circuit current to serious over-limit short-circuit current is formed.Firstly,the disadvantages of directly triggering circuit breaker failure protection when the primary means and the secondary means such as protection flexible tripping strategy fail to work are analyzed,and the basic idea of protection active response scheme is clarified;Secondly,a dynamic limiting strategy based on on-line equivalence of back-side system and real-time evaluation of current limiting effect is proposed,which can realize the optimization of minimum line breaking combination;Finally,the whole process of protection initiative response scheme by combining the proposed three strategies is designed.The simulation results show that the proposed protection initiative response scheme can adaptively select the optimal tripping logic to realize the reliable breaking of the over-limit circuit breaker.