| High voltage line protection will start the failure protection at the same time.The protection will return immediately to realize the backup protection function when the line fault is isolated and the failure protection current element detects no line current.However,there are inductive elements in the secondary current circuit of current transformer(CT).The energy stored in the inductance is released gradually at the moment of tripping,and the decaying aperiodic DC component(i.e.CT tailing current)is generated in the secondary circuit of CT when the circuit breaker jumps off.The failure protection may lead to maloperation due to the continuous satisfaction of current criterion when CT tailing current is serious.If the tailing current can not be restrained or identified in time,the fault range may be further expanded due to the maloperation of failure protection,which will bring serious security risks to the high-voltage power grid.Therefore,it is necessary to find a more rapid and reliable method of tailing current identification in case of failure protection maloperation caused by tailing current.The improved scheme of failure protection is obtained by integrating the locking function with CT tailing current flag in the traditional failure protection device,which is based on the analysis of the mechanism of current transformer tailing current.Two new algorithms for tailing current recognition are proposed based on the comprehensive characteristics of current waveform.Once the algorithm identifies the tailing current,the tailing current is marked at position 1 to block the failure protection and prevent the failure protection from maloperation.Two new algorithms for CT tailing current recognition are as follows:An algorithm of tailing current identification based on multi-point slope characteristic ratio is proposed to solve the problem that tailing current may lead to the maloperation of failure protection.The slope sequence of sampling current in 1/4 cycle data window is selected.Based on the difference of the sampling value and the symbol of slope value between the fault current and the tailing current,a fault current identification criterion is constructed to eliminate fault current quickly.Then the equilibrium slope sequence is obtained by discarding the partial extremum of the slope sequence.Counting the number of elements when the absolute value of the tailing current slope is less than the threshold value of the tailing current slope.After that the slope characteristic ratio is obtained,then the tailing current identification criterion is constructed.By using this criterion,the tailing current can be identified quickly and the position 1 of the tailing current can be marked.Then,the tailing current flag bit is connected to the failure protection to quickly block the failure protection.In order to solve the problem of lacking anti-interference ability of the tailing current recognition method based on the characteristic ratio of multi-point slope,a tailing current recognition algorithm based on fréchet distance algorithm is proposed.The algorithm is constructed according to the waveform difference between the tailing current and the fault current.The start point and the end point in the short data window sampling current sequence are selected to form the template straight line,and then the time difference t between the zero crossing time and the start point of the template straight line is obtained.Then the fréchet distance between the normalized current sequence and the template line sequence is calculated.The tailing current recognition criterion is composed of t and distance value's complement set.Based on PSCAD/MATLAB simulation software,the current transformer tailing current model in various scenarios is established.The simulation results of the two algorithms are verified.The results show that the new algorithm is simple and reliable.Besides,it is not affected by fault DC component and CT saturation.Furthermore,the new algorithm has strong anti-interference ability. |
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