Method for forming differential current based on current slow attenuation of differential current zero crossing identification CT (computed tomography)
A technology of differential current and current, which is applied in the field of identifying differential current formed by the slow decay of CT current in the power system, and can solve problems such as differential protection misoperation
Active Publication Date: 2013-04-10
STATE GRID ELECTRIC POWER RES INST
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[0006] The technical problem to be solved by the present invention is: to solve the problem that the slowly attenuating DC component on the secondary side of the CT when the primary current of the TPY level CT is cut off may cause the differential protection to malfunction
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[0017] Such as figure 1 As shown, when the primary current of the TPY-level CT is cut off, the slowly decaying DC component generated by the secondary side of the CT is also a slowly decaying DC component in the differential current, and its main feature is that the differential current is always biased to one side of the time axis , the differential current has no zero-crossing point, and the waveform of the differential current is discontinuous after being differentiated. When an internal fault occurs, no matter whether there is branch CT saturation, the differential current is continuous, there is a zero-crossing point, and the waveform after the difference is uninterrupted, that is, there is a differential current around the zero-crossing point, so the present invention is mainly based on the above-mentioned characteristics To identify the differential current formed by the slow attenuation generated when the primary current is cut off, so as to avoid the misoperation of t...
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The invention relates to a method for forming differential current based on the current slow attenuation of a differential current zero crossing identification CT (computed tomography), which comprises the following steps: 1) calculating the three-point integral value of the present point differential current according to the formula S(n)=x(n) + x(n-1) + x(n-2) and comparing the three-point integral value with the three-point integral value S(n-1) of the former point differential current, wherein the present point is judged to zero crossing point if S(n)*S(n-1) is less than 0; and 2) the internal faults are judged if more than two zero crossing points occur in the time T, otherwise, the differential protection is locked. The incorrect false operation of the differential protection in the course of the differential current resulted from the current flow attenuation of the CT is realized when the external faults are eliminated.
Description
technical field [0001] The invention relates to a method for identifying differential current formed by slow decay of CT current in a power system. Background technique [0002] At present, there are two main types of CTs used for relay protection in domestic power systems: ordinary P-level CTs and TP-level CTs considering transient characteristics. [0003] P-level CT adopts closed iron core without air gap, and the manufacturing material is usually cold-rolled silicon steel sheet, which has a small magnetic permeability. This type of CT can only guarantee the steady-state error, and its accurate characteristics are determined by the composite error when the primary current is the steady-state current. When a short-circuit fault occurs in the transient process of a system, non-periodic components will appear in the excitation current of CT, and the amplitude of this current can be up to dozens of times the amplitude of the steady-state excitation current. Under the action...
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IPC IPC(8): H02H7/26G01R19/10G01R19/175
Inventor 吴崇昊戴魏李志坚谭凌朱宁郑玉平
Owner STATE GRID ELECTRIC POWER RES INST