CN100413169C - Phase-separated zero-sequence braking method to prevent misoperation of multi-branch transformer differential protection - Google Patents

Abstract

The present invention relates to a phase-splitting zero-sequence braking method for preventing the unwanted operation of differential protection for multiple-branch transformers; ratio braking elements are respectively composed of three-phase differential electric currents in the differential protection for transformers and a maximum value of a zero-sequence electric current at each neutral point grounding side (or each branch); when a ratio value is smaller than a definite value K, a corresponding differential electric current is judged to be an error current caused by an external fault; the differential protection for a corresponding phase ratio is stopped to prevent the malfunction of the longitudinal differential protection for transformers. The application of the method can thoroughly avoid the malfunction of differential protection for transformers caused by an external grounding fault at a neutral point grounding side when the errors of a three-phase electric current transformer are not consistent.

Description

Phase-separated zero-sequence braking method to prevent misoperation of multi-branch transformer differential protection

technical field

The invention relates to a transformer protection method, in particular to a phase-separated zero-sequence braking method for preventing maloperation of multi-branch transformer differential protection.

Background technique

The existing power transformer differential protection device uses a ratio brake element, that is, the ratio of the differential current to the through current, to identify the fault outside the zone and the fault in the zone; The waveform or harmonic content of the differential current is used to identify the excitation inrush current and over-excitation current to achieve the purpose of identifying internal faults. For power transformers with neutral points directly grounded on two or more sides, when an external ground fault occurs on one neutral point grounding side, a large ground fault zero-sequence current may occur on multiple grounding sides. In order to prevent the zero-sequence current from causing differential protection misoperation, the method of transforming the three-phase current on the grounding side of the neutral point is widely used at present to eliminate the influence of the zero-sequence current. However, when the errors of the three-phase current transformers on the grounding side are inconsistent, and the transformer only has power supply on the grounding side where an external fault occurs, the traditional method cannot eliminate the influence of the zero-sequence current caused by the external grounding fault on the grounding side, and there will be a large The differential unbalanced current of the differential current, the ratio braking element and the inrush braking element cannot perform reliable braking, especially when the transformer load current is very small or even zero, the ratio braking element and the inrush braking element cannot perform reliable braking. Braking cannot be performed, and as long as the error current exceeds the threshold, the transformer differential protection will malfunction.

Contents of the invention

The purpose of the present invention is to provide a phase-separated zero-sequence braking method to prevent misoperation of multi-branch transformer differential protection, through the three-phase differential current in the transformer differential protection and the zero-sequence current The maximum value of each constitutes the ratio brake criterion. When the ratio value is less than the set value, the corresponding phase differential current is judged to be the error current caused by an external fault, and the corresponding phase ratio differential protection is braked to prevent transformer differential protection from malfunctioning. move. The application of this method can completely avoid the misoperation of the transformer differential protection caused by the ground fault outside the neutral point grounding side when the errors of the three-phase current transformers are inconsistent.

A phase-separated zero-sequence braking method for preventing misoperation of multi-branch transformer differential protection, which is characterized in that the three-phase differential currents I _da , I _db , and I _dc in the differential protection of the transformer are used to communicate with each neutral point grounding side Or the maximum value of the zero-sequence current 3I ₁₀ , 3I ₂₀ , ... 3I _no of the branches respectively constitutes the ratio braking criterion. When the ratio of the differential current to the maximum value of the zero-sequence current is less than the setting value K, the corresponding phase difference The dynamic current is the error current caused by the fault outside the area, and the corresponding phase braking signal is output to brake the corresponding phase ratio differential protection to prevent the transformer differential protection from malfunctioning;

The basis for the phase-separation discrimination caused by the differential current of the multi-branch transformer caused by the ground fault outside the area is as follows:

I _da /(3I ₁₀ , 3I ₂₀ ,...3I _no )max＜K

I _db /(3I ₁₀ , 3I ₂₀ ,...3I _no )max＜K

I _dc /(3I ₁₀ , 3I ₂₀ ,...3I _no )max＜K

In the formula, I _da , I _db , and I _dc are three-phase differential currents respectively, 3I ₁₀ , 3I ₂₀ , ... 3I _no are zero-sequence current instantaneous values 3i ₁₀ , 3i ₂₀ , ... 3i _{no is} the zero-sequence current obtained after filtering, (3I ₁₀ , 3I ₂₀ , ... 3I _no )max is the maximum value of the zero-sequence current of each neutral point grounding side or branch, K is the setting value, when the zero-sequence The current and the three-phase differential current are attributed to the same side. In the case of the same transformation ratio, the setting value K is 0<K<1/3, and n is the number of neutral grounding sides or branches of the transformer.

Compared with the prior art, the present invention has the advantage of proposing for the first time the use of the three-phase differential current and the maximum value of the zero-sequence current on the grounding side of each neutral point to form a ratio brake criterion. When the ratio value is less than the set value, the corresponding phase difference The dynamic current is the error current caused by the fault outside the area, and the corresponding phase ratio differential protection is braked to prevent the transformer differential protection from malfunctioning; this method can completely avoid the neutral point grounding when the errors of the three-phase current transformers are inconsistent. The misoperation of the transformer differential protection caused by the ground fault outside the side area can also prevent the misoperation of the transformer differential protection caused by the differential unbalanced current caused by the zero-sequence current under other non-internal fault conditions, and does not affect the fault situation in the area The correct action of the lower differential protection.

Description of drawings

Fig. 1 is a circuit block diagram of a phase-separated zero-sequence braking device for preventing misoperation of multi-branch transformer differential protection according to the present invention.

Detailed ways

Example 1

A phase-separated zero-sequence braking method for preventing misoperation of multi-branch transformer differential protection, which can be realized by the circuit of a phase-separated zero-sequence braking device for preventing multi-branch transformer differential protection misoperation in Figure 1 . The device is composed of filters 01, 02, 03, filters 11-13, maximum value circuit 2, multiplier 3, and comparators 41-43. The input terminals of filters 01, 02, and 03 are respectively input into the Zero-sequence current instantaneous value signals 3i ₁₀ , 3i ₂₀ , 3i ₃₀ on the grounding side (or branch) of the neutral point, the output terminals of which are respectively connected to the input terminals of the maximum value circuit 2, and one input terminal of the multiplier 3 is connected to the maximum value The output terminal of the value circuit 2, the other input terminal inputs the setting value K, the negative input terminals of the comparators 41-43 are connected to the output terminal of the multiplier 3, and the input terminals of the filters 11-13 are respectively input from the transformer differential protection The output terminals of the obtained three-phase differential current instantaneous value signals i _da , i _db , and i _dc are respectively connected to the positive input terminals of comparators 41-43, and the comparators 41-43 respectively output phase A, phase B, and phase C brake signal. When the positive input signal in the comparators 41-43 is smaller than the negative input signal, the comparator outputs 0, judges that the corresponding phase differential current is an error current caused by an external fault, outputs the corresponding phase brake signal, and controls the corresponding phase differential current. The protection performs braking to prevent the transformer differential protection from malfunctioning; otherwise, the comparator outputs 1, allowing the corresponding phase differential protection to operate. When the differential current and zero-sequence current are attributed to the same side and the same transformation ratio, the setting value K is 0.1. The advantage of applying this method is that it can completely avoid the misoperation of the transformer differential protection caused by the ground fault outside the neutral point grounding side, and does not affect the correct operation of the differential protection under the fault condition in the zone.

Claims (1)

1. a split-phase zero-sequence braking method that prevents the malfunction of multiple-limb transformer differential protection is characterized in that: with the three-phase differential current I in the transformer differential protection _Da, I _Db, I _DcZero-sequence current 3I with each neutral ground side or branch ₁₀, 3I ₂₀... 3I _NoIn maximum component ratio braking criterion respectively, when differential current and the peaked rate value of zero-sequence current during less than setting value K, declaring corresponding phase differential current is the error current that external area error causes, export corresponding phase brake signal, corresponding phase ratio differential protection is braked, prevented the transformer differential protection malfunction;

Multiple-limb transformer difference stream by the phase-splitting distinguishing rule that external ground fault in district's causes is:

I _da/(3I ₁₀、3I ₂₀、……3I _no)max＜K

I _db/(3I ₁₀、3I ₂₀、……3I _no)max＜K

I _dc/(3I ₁₀、3I ₂₀、……3I _no)max＜K

I in the formula _Da, I _Db, I _DcBe respectively the three-phase differential current, 3I ₁₀, 3I ₂₀... 3I _NoZero-sequence current instantaneous value 3i for each neutral ground side or branch ₁₀, 3i ₂₀... 3i _NoThe zero-sequence current that after filtering, obtains, (3I ₁₀, 3I ₂₀... 3I _No) max is the maximum in the zero-sequence current of each neutral ground side or branch, K is a setting value, when zero-sequence current and the reduction of three-phase differential current to the same side, under the situation of same no-load voltage ratio, setting value K is 0＜K＜1/3, and n is transformer neutral point ground connection side number or branches.

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