CN102005744A - Method for forming differential current based on current slow attenuation of differential current zero crossing identification CT (computed tomography) - Google Patents

Abstract

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

Flowing through zero identification CT electric current based on difference slowly decays and forms the method for difference stream

Technical field

The present invention relates to a kind of poor method that flows that electric power system CT electric current is slowly decayed and formed of discerning.

Background technology

Used for relay protection CT mainly contains two types in the present domestic electric power system: the TP level CT of common P level CT and considering transient characteristic.

P level CT adopts iron core-closed, is not with air gap, and manufactured materials is generally cold-reduced silicon sheet, and its magnetic capacity is little.Such CT can only guarantee steady-state error, and its accurate characteristic is that the composite error when being steady-state current by primary current is determined.In primary system is short-circuited the transient process of fault, aperiodic component will appear in the exciting current of CT, and the amplitude of this electric current reaches as high as tens times of stable state exciting current amplitude.Under this function of current, the iron core of CT enters saturation condition rapidly after after a while, CT this moment progress of disease primary current truly just, and its secondary current waveform will distort.For differential protection,, cause differential protection misoperation with producing bigger differential current.In addition, in order to reduce the core loss of CT, core material generally adopts the narrower silicon steel sheet of magnetic hysteresis loop, but the remanent magnetism of this class silicon steel sheet is still bigger usually.Behind primary system excision short circuit current, leave the remanent magnetism of people's amount in the iron core; Behind the pending fault branch road reclosing, if be short-circuited once more, and the residual flux direction is identical with the flow direction that aperiodic component produces, and iron core will reach capacity more rapidly, and serious distortion, the easier generation misoperation of differential protection will take place secondary current.

In high pressure and extra high voltage system, because single-machine capacity is big, the transmission line wire cross section is big, the primary system time constant is bigger, even cry and to reach 0.25--0.5 second, cause the DC component of short-circuit current decay slowly, and amplitude of short circuit is bigger usually, can reach more than 30 times of CT rated current, because there are more above-mentioned shortcomings in P level CT, therefore use TPY level CT in the above system of 500kV more.

But use TPY level CT to have following problem: TPY level CT has better transient characterisitics than P level CT, is to realize These parameters, and there is air gap in TPY level CT iron core, so its remanent magnetism is much smaller than P level CT, and general provision TPY level CT remanent magnetism is no more than 10% of saturation flux.When system's generation external area error; circuit breaker is generally in primary current zero crossing excision fault, and this moment, the CT magnetic flux was generally maximum, because TPY level CT remanent magnetism is less; according to the principle of continuity of magnetic flux; magnetic flux can not suddenly change, because damping time constant is bigger, magnetic flux is decay slowly; therefore also can produce the very slow DC component of a decay at secondary circuit; referring to Zhu Shengshi, " high-voltage fence Principles of Relay Protection and technology (third edition) ", 2005.The DC component of this decay promptly is a difference stream to differential protection, may cause differential protection malfunction.

Summary of the invention

The present invention wants the technical solution problem to be: solve TPY level CT CT secondary side when primary current excises and produce the problem that the DC component that slowly decays may cause differential protection malfunction.

The technical solution adopted for the present invention to solve the technical problems is as follows:

Flow through zero identification CT electric current based on difference and slowly decay and form the method for difference stream, may further comprise the steps:

1) AC sampling: each current sampling data that obtains differential protection;

2) each electric current that participates in differential protection calculating is carried out differential filtering, concrete computing formula is y (n)=x(n)-x(n-2), x(n) current some current sampling data of expression is current point 2 current sampling datas before x(n-2), and y(n) expression is through the output of differential filtering;

3) calculate three with each branch current and differ stream through difference;

4) calculate three dot product score value S (n)=x (n)+x (the n-1)+x (n-2) of current some difference stream, and compare,, judge that then current point is a zero crossing as S (n) * S (n-1)＜0 with three dot product score value S (n-1) of more preceding poor stream;

5) as 2 above zero crossings in time T, occurring, then be judged as troubles inside the sample space, otherwise Blocking Differential Protection.

The described time T of step 5) is 17ms.

Beneficial effect of the present invention is as follows: the invention provides based on difference and flow through zero point and be interrupted feature differentiation electric power system troubles inside the sample space and slowly the decay method of the difference stream that produces of CT electric current, not malfunction of differential protection when guaranteeing when external area error excise the CT electric current slowly decay causing difference to flow.

Description of drawings

The aperiodic component that the CT secondary side produced when Fig. 1 was the excision of TPY level CT primary current.

Embodiment

As shown in Figure 1, CT is when primary current excises the TPY level, it also is a slowly DC component of decay that the DC component of the slow decay that the CT secondary side produces is reflected in the difference stream, its principal character shows as difference stream and is partial to time shaft one side all the time, the no zero crossing of difference stream, and difference stream occurs being interrupted through waveform after the difference.And during the generating region internal fault; no matter whether there is branch road CT saturated; its spill current is continuous; there is zero crossing; and through differentiated waveform free of discontinuities; be that difference stream is all arranged about zero crossing, so the difference stream that forms of the slow decay that produces when mainly discerning the primary current excision according to above-mentioned these features of the present invention, differential protection malfunction avoided.

Adopt the DC component of differential filtering filtering spill current.

By three dot product score values of scan difference current sampling data, when three dot product score value contrary signs of three dot product score values of current some spill current sampled value and more preceding spill current sampled value, promptly judge to have a zero crossing.

Consider that the troubles inside the sample space damping time constant is 120ms, when spill current is full migration, can have 2 zero crossings among the longest continuous 17ms of spill current, therefore as continuously 2 above zero crossings occur among the 17ms, then be judged as troubles inside the sample space, otherwise Blocking Differential Protection.

Below in conjunction with example the present invention is described in further detail.But the invention is not restricted to given example.

1) AC sampling: each current sampling data that obtains differential protection.

2) each electric current that participates in differential protection calculating is carried out differential filtering; concrete computing formula is y (n)=x(n)-x(n-2); x(n) current some current sampling data of expression is current point 2 current sampling datas before x(n-2), and y(n) expression is through the output of differential filtering.

3) calculate three with each branch current and differ stream through difference.

4) calculate three dot product score value S (n)=x (n)+x (the n-1)+x (n-2) of current some difference stream, and compare,, judge that then current point is a zero crossing as S (n) * S (n-1)＜0 with three dot product score value S (n-1) of more preceding poor stream.

5) continuous sweep spill current as 2 above zero crossings occur in the data window of 17ms, then is judged as troubles inside the sample space, otherwise Blocking Differential Protection.

Claims (2)

1. flow through zero identification CT electric current based on difference and slowly decay and form the method for difference stream, it is characterized in that, may further comprise the steps:

1) AC sampling: each current sampling data that obtains differential protection;

2) each electric current that participates in differential protection calculating is carried out differential filtering, concrete computing formula is y (n)=x(n)-x(n-2), x(n) current some current sampling data of expression is current point 2 current sampling datas before x(n-2), and y(n) expression is through the output of differential filtering;

3) calculate three with each branch current and differ stream through difference;

4) calculate three dot product score value S (n)=x (n)+x (the n-1)+x (n-2) of current some difference stream, and compare,, judge that then current point is a zero crossing as S (n) * S (n-1)＜0 with three dot product score value S (n-1) of more preceding poor stream;

5) as 2 above zero crossings in time T, occurring, then be judged as troubles inside the sample space, otherwise Blocking Differential Protection.

2. according to claim 1ly flow through zero identification CT electric current based on difference and slowly decay and form the method for difference stream, it is characterized in that the described time T of step 5) is 17ms.

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