CN106505523A - A method for identification of excitation inrush current suitable for transformers in traction network - Google Patents

Abstract

The invention discloses a kind of excitation flow recognition method suitable for Traction networks transformer, including：Step 1：Collection tractive transformer winding three-phase electric current, and difference processing is carried out to each phase current；Step 2：Calculate the three-phase current rate of change in power frequency period T；Step 3：T/2 cycle data windows are arbitrarily read, and pointwise sequentially elapses the difference and and value for calculating that three-phase current rate of change is adjacent the three-phase current rate of change absolute value corresponding to the sampling number of half period backward；Step 4：Using N/2 difference accumulated value Δ i '_{diff‑φ‑1}, N/2 and value accumulated value Δ i '_{diff‑φ‑2}, shove and internal fault according to the identification of further criterion.The method that a kind of utilization interval angle of the present invention and asymmetry compound characteristics recognize excitation surge current, reduces the impact of DC component, and the method principle is simple, and amount of calculation is little, and recognition accuracy is higher, is easily applied in engineering practice.

Description

A method for identification of excitation inrush current suitable for transformers in traction network

【Technical field】

The invention belongs to the technical field of relay protection for traction transformers in traction power supply systems, and in particular relates to an excitation inrush identification method suitable for traction network transformers.

【Background technique】

Traction transformer is the most important part of the traction power supply system, its reliable and safe operation is of great significance to the safe operation of the entire traction power supply system. The excitation inrush current is one of the main factors causing the maloperation of the differential protection of the transformer main protection. How to quickly and accurately identify transformer excitation inrush current and internal faults will help improve the reliability of transformer protection in traction network.

In recent years, scholars at home and abroad have proposed many methods for transformer identification inrush current, which can be divided into: 1) current characteristic method; 2) voltage characteristic method; 3) current-voltage compound characteristic method.

1. The excitation inrush identification method of current characteristics: mainly including the second harmonic method, discontinuous angle method, waveform symmetry method, etc. The second harmonic method has the characteristics of simple principle and easy implementation, but it is greatly affected by transient current characteristics and transformer magnetization characteristics, and it is difficult to set the second harmonic braking threshold. The discontinuity angle method is mainly based on the fact that the differential current has obvious discontinuity angle characteristics when the traction transformer generates excitation inrush current, but the differential current has no obvious discontinuity angle characteristics when a fault occurs. Waveform symmetry does not have a definite basis for the selection of the threshold, but it is selected based on engineering experience.

2. The excitation inrush current identification method of voltage characteristics: mainly including magnetic flux method, differential active work method, transformer equivalent circuit method, etc. The magnetic flux method can better reflect the transient process of the transformer, but it has a strong dependence on the characteristics and parameters of the transformer itself. The differential active power method makes full use of the voltage and current information of the transformer to identify the state of the transformer, but it is difficult to set the threshold. Although the transformer equivalent circuit does not require the magnetization curve of the transformer, it has higher requirements on the winding parameters of the transformer.

3. The excitation inrush current identification method based on the compound characteristics of voltage and current: mainly including magnetic flux characteristics, power differential principle, etc. Due to the difficulty in obtaining the residual magnetism of the transformer, the ψ- _id curve obtained in the case of inrush current will deviate from the magnetization curve, which will lead to misjudgment of the magnetic flux characteristic method. At the same time, there are difficulties in partitioning and complex setting of the braking coefficient threshold. The power differential method needs to avoid the charging process of the transformer in the first cycle of the inrush current, which leads to a delay in discrimination; it is difficult to accurately calculate the copper loss during the excitation inrush current, so it is difficult to use this principle to identify the threshold value setting of the inrush current.

In addition, some scholars have combined intelligent technologies such as fuzzy logic, wavelet transform, and neural network to carry out excitation inrush current identification methods. However, intelligent algorithms to identify inrush currents have large uncertainties, and the implementation of algorithms is complex and computationally intensive, so it is difficult to effectively apply them in practice in the short term. project.

【Content of invention】

Aiming at the deficiency of existing traction system transformer excitation inrush current identification, the purpose of the present invention is to provide an excitation inrush current identification method suitable for traction network transformers, which solves the problem of transformer differential protection misoperation existing in the prior art.

The object of the invention is achieved through the following technical solutions:

A method for identifying inrush current suitable for traction network transformers, comprising the following steps:

Step 1: Collect the three-phase current i _φ (k) of transformers A, B, and C, where φ=A, B, and C, and perform differential processing on each phase current;

Step 2: Calculate the three-phase current change rate i′ _diff-φ (k) within a power frequency cycle T;

Step 3: Read the T/2 period data window arbitrarily, and calculate the three-phase current change rate i′ _diff-φ (k) and the three-phase corresponding to the number of sampling points of adjacent integral multiples of half a period and move backward point by point. Phase current change rate Calculate the difference of the absolute value of the rate of change separately and the absolute value and value of the rate of change Among them, k is the number of sampling points, N is the number of sampling points of a power frequency calculation cycle, n=1,2,...;

Step 4: The cumulative difference value Δi′ _diff-φ-1 and the cumulative sum value Δi′ _diff-φ-2 of the three-phase current change rate calculated according to the three-phase current change rate and the sampling points of the adjacent half cycle , to identify inrush currents and internal faults according to the following inrush current identification criteria,

Among them, K _res is the braking threshold; n is an integer value; within a period, as long as one phase of the A, B, and C three-phase current meets the above inrush current identification criteria, it will be judged as inrush current and the transformer differential protection will be blocked; otherwise, it will be judged as for internal failure.

Step 1 is specifically: collect the three-phase currents i _A (k), i _B (k), and i _C (k) of the transformer, and calculate the differential current of each phase i _diff-A (k)=i _A (k)- i _A (k-1), i _diff-B (k)=i _B (k)-i _B (k-1), i _diff-C (k)=i _C (k)-i _C (k-1 ), where k=2,3,4....

Step 2 is specifically: Calculate the current change rate i′ _diff-φ of the differential currents i _diff-A (k), i _diff-B (k), and i _diff-C (k) in a power frequency calculation period T (k):

Current change rate:

Among them, k is the number of sampling points, and T _s is the sampling period.

Step 3 is specifically: the difference between the three-phase current change rate calculated at a certain sampling point in step 2 and the absolute value of the three-phase current change rate corresponding to the number of sampling points in the adjacent half cycle, and the N/2 calculated values Accumulated to get Δi′ _diff-φ-1 , namely:

Then the absolute value of the three-phase current change rate corresponding to this sampling point and the three-phase current change rate corresponding to the number of sampling points in the adjacent half cycle is summed, and the N/2 calculation values are accumulated to obtain Δi′ _{diff-φ- 2} , ie

The value of K _res is 0.2, and n is 1-3.

Compared with the prior art, the present invention has the following advantages:

The present invention is an excitation inrush identification method utilizing discontinuity angle and asymmetrical composite features, which makes full use of the waveform characteristics of traction transformer excitation inrush current, adopts differential calculation processing to amplify the discontinuity of excitation inrush current and internal fault current under weak current conditions The difference between corner and peak waveform characteristics is expected to improve the inrush current identification performance of differential protection of traction system transformers. The excitation inrush identification method of the present invention realizes accurate identification of inrush current and internal faults in view of the differences in discontinuity angles and peak wave characteristics of transformer excitation inrush current and inrush current during internal faults. In the event of an internal fault, the fault current waveform after differential processing shows a significant periodic sine wave feature. In any half of the power frequency calculation period, there is at least one current change extreme point and the current has a symmetrical feature; the transformer is switched on without load. When the excitation inrush current is generated, due to the discontinuous angle of the inrush current and the characteristics of the peak wave, the differentiated current only has the extreme point of the current change in the peak wave region and the current does not have the characteristic of symmetry. Therefore, using this current change feature can realize the reliable discrimination of inrush current and internal fault, and reduce the influence of DC component. Moreover, the method has simple principle, small calculation amount, high identification accuracy, and is easy to be applied in engineering practice. Compared with other excitation inrush current identification principles, the present invention has the following significant advantages:

(1) The applicability of using current characteristics to identify inrush current is improved by using the compound characteristics of the discontinuity angle and cusp of transformer excitation inrush current;

(2) By using whether the differentiated current still has a discontinuity angle and whether the extremum point of current change has symmetry, the exciting inrush current can be identified quickly and accurately.

【Description of drawings】

Fig. 1 is a flow chart of a method for identifying inrush current using discontinuity angle and asymmetrical compound features of the present invention.

【detailed description】

The specific embodiment of the present invention will be described in detail below in conjunction with the accompanying drawings, but the present invention is not limited to this embodiment. In order to provide the public with a thorough understanding of the present invention, specific details are specified in the following preferred embodiments of the present invention.

The principle of the invention is: the excitation circuit of the transformer is equivalent to the fault branch of the internal fault of the transformer. When the iron core of the transformer is saturated, a large excitation inrush current will be generated and flow into the differential relay, causing the differential protection to malfunction. Because the excitation current is very large, if the action current is used to avoid its influence, the sensitivity of the differential protection will be reduced when the transformer internal fault occurs. In view of this, the present invention uses the characteristic difference between the exciting inrush current and the internal fault current waveform to identify the exciting inrush current and prevent the differential protection from malfunctioning caused by the exciting inrush current. In the event of an internal fault, the fault current after differential processing has at least one current change extreme point within any half power frequency calculation cycle and the current has a symmetrical feature; when the transformer is closed with no load, the differential processed The current has the characteristics of discontinuous angle, and there are only extreme points of current change in the peak wave region, and the current does not have the characteristic of symmetry. Therefore, reliable discrimination between inrush current and internal fault can be realized by using this current change characteristic.

The present invention is a method for identifying the excitation inrush current by using discontinuity angle and asymmetric composite features. The specific process is shown in Fig. 1 and implemented according to the following steps:

Step 1: Collect the three-phase current i _φ (k) of the transformer, where φ=A, B, and C represent one phase of the three-phase circuit of the transformer, i.e. i _A (k), i _B (k), and i _C (k), calculate the differential current of each phase i _diff-A (k)=i _A (k)-i _A (k-1), i _diff-B (k)-i _B (k)-i _B (k- 1), i _diff-C (k)=i _C (k)-i _C (k-1), wherein, k=2,3,4...;

Since the transformer winding is inductive, both the excitation inrush current and the internal fault current contain some non-periodic components, and differential processing is used to reduce the influence of non-periodic components.

Step 2: Calculate the three-phase current change rate i′ _diff-φ (k) within the power frequency period T, specifically:

Calculate the current change rate i′ _diff-φ (k) of the differential currents i _diff-A (k), i _diff-B (k), and i _diff-C (k) in a power frequency calculation cycle T of each phase, namely

Current change rate:

Among them, k is the number of sampling points, and T _s is the sampling period;

Step 3: Identify inrush currents and internal faults using the criteria:

The difference between the three-phase current change rate calculated at a certain sampling point in step 2 and the absolute value of the three-phase current change rate corresponding to the number of sampling points in the adjacent half period is made, and the N/2 calculated values are accumulated to obtain Δi′ _{diff -φ-1} ie

Then the absolute value of the three-phase current change rate corresponding to this sampling point and the three-phase current change rate corresponding to the number of sampling points in the adjacent half cycle is summed, and the N/2 calculation values are accumulated to obtain Δi′ _{diff-φ- 2} namely

Step 4 is specifically:

According to the three-phase current change rate in step 3 and the sampling points of the adjacent half cycle, the difference accumulation value Δi′ _diff-φ-1 and the sum value accumulation value Δi′ _diff-φ-2 of the three-phase current change rate calculated , identify inrush currents and internal faults according to the following inrush current identification criteria:

Among them, K _res is the braking threshold. Considering the measurement error and calculation error of the current transformer, the value of K _res is 0.2;

In one cycle, since the differentially processed current of the excitation inrush only has current change extremum points in the peak wave region and the current does not have symmetry characteristics, while the internal fault current after differential processing has at least one current change pole value point and the extreme point has the characteristic of symmetry. Therefore, as long as one phase of the three-phase current meets the above inrush identification criteria, it is judged as inrush current, and the differential protection of the transformer is blocked; otherwise, it is judged as an internal fault.

The method of the invention utilizes the characteristic difference between the excitation inrush current and the internal fault current of the traction transformer, and proposes an identification method for the excitation inrush current and the internal fault by using discontinuity angle and asymmetry characteristics. In this method, when the traction transformer generates excitation inrush current, the differential current has obvious discontinuous angle characteristics and the extremum point of current change does not have the characteristics of symmetry, while there is no obvious discontinuous angle characteristic and at least one extremum point in the differential current when there is a fault. And the extremum point has the characteristics of symmetry to distinguish the angular characteristics of the excitation inrush current and the fault current. This method has a simple principle, a small amount of calculation, and has engineering application value.

The above are only preferred embodiments of the present invention, and are not limited to the implementation scope of the present invention. All equivalent changes and modifications made according to the content of the patent scope of the present invention shall fall within the technical scope of the present invention.

Claims (5)

1. A method for identifying inrush currents applicable to traction network transformers, characterized in that it may further comprise the steps: Step 1: Collect the three-phase current i _φ (k) of transformers A, B, and C, where φ=A, B, and C, and perform differential processing on each phase current; Step 2: Calculate the three-phase current change rate i′ _diff-φ (k) within a power frequency cycle T; Step 3: Read the T/2 period data window arbitrarily, and calculate the three-phase current change rate i′ _diff-φ (k) and the three-phase corresponding to the number of sampling points of adjacent integral multiples of half a period and move backward point by point. Phase current change rate Calculate the difference of the absolute value of the rate of change separately and the absolute value and value of the rate of change Among them, k is the number of sampling points, N is the number of sampling points of a power frequency calculation period, and n is a positive integer; Step 4: The cumulative difference value Δi′ _diff-φ-1 and the cumulative sum value Δi′ _diff-φ-2 of the three-phase current change rate calculated according to the three-phase current change rate and the sampling points of the adjacent half cycle , to identify inrush currents and internal faults according to the following inrush current identification criteria, $\frac{{\mathrm{<span\; class="notranslate">\; \&Delta;i</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&phi;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}{{\mathrm{<span\; class="notranslate">\; \&Delta;i</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&phi;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}}^{<span\; class="notranslate">\; \&prime;</span>}}<span\; class="notranslate">\; ></span>{\mathrm{<span\; class="notranslate">\; K</span>}}_{\mathrm{<span\; class="notranslate">\; r</span>}\mathrm{<span\; class="notranslate">\; e</span>}\mathrm{<span\; class="notranslate">\; the\; s</span>}}<span\; class="notranslate">\; ;</span>$ Among them, K _res is the braking threshold; n is an integer value; within a period, as long as one phase of the A, B, and C three-phase current meets the above inrush current identification criteria, it will be judged as inrush current and the transformer differential protection will be blocked; otherwise, it will be judged as for internal failure. 2. A kind of excitation inrush identification method suitable for traction network transformer according to claim 1, is characterized in that, step 1 is specifically: to the three-phase current i _A (k), i _B (k), i of transformer _C (k) collects and calculates the differential current of each phase i _diff-A (k)=i _A (k)-i _A (k-1), i _diff-B (k)=i _B (k)-i _B (k-1), i _diff-C (k)=i _C (k)-i _C (k-1), where k=2, 3, 4 . . . 3. A kind of excitation inrush identification method suitable for traction network transformer according to claim 1, is characterized in that, step 2 is specifically: calculate each phase differential current i _diff-A (k), i _diff-B (k ), i _diff-C (k) current change rate i′ _diff-φ (k) in a power frequency calculation cycle T: Current change rate: Among them, k is the number of sampling points, and T _s is the sampling period. 4. A method for identifying inrush current suitable for traction network transformers according to claim 1, wherein step 3 is specifically: the rate of change of the three-phase current calculated at a certain sampling point in step 2 is adjacent to it The absolute value of the three-phase current change rate corresponding to the number of sampling points in half a cycle is made a difference, and the N/2 calculated values are accumulated to obtain Δi′ _diff-φ-1 , namely: ${\mathrm{<span\; class="notranslate">\; \&Delta;i</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&phi;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\overset{{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; |</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&phi;</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; -</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&phi;</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; ;</span>$ Then the absolute value of the three-phase current change rate corresponding to this sampling point and the three-phase current change rate corresponding to the number of sampling points in the adjacent half cycle is summed, and the N/2 calculation values are accumulated to obtain Δi′ _{diff-φ- 2} , ie ${\mathrm{<span\; class="notranslate">\; \&Delta;i</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&phi;</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 2</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; =</span>\underset{\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}}{\overset{{\mathrm{<span\; class="notranslate">\; k</span>}}_{\mathrm{<span\; class="notranslate">\; 0</span>}}<span\; class="notranslate">\; +</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; 1</span>}}{\mathrm{<span\; class="notranslate">\; \&Sigma;</span>}}}<span\; class="notranslate">\; |</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&phi;</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; +</span><span\; class="notranslate">\; |</span>{\mathrm{<span\; class="notranslate">\; i</span>}}_{\mathrm{<span\; class="notranslate">\; d</span>}\mathrm{<span\; class="notranslate">\; i</span>}\mathrm{<span\; class="notranslate">\; f</span>}\mathrm{<span\; class="notranslate">\; f</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; \&phi;</span>}}^{<span\; class="notranslate">\; \&prime;</span>}<span\; class="notranslate">\; (</span>\mathrm{<span\; class="notranslate">\; k</span>}<span\; class="notranslate">\; -</span>\mathrm{<span\; class="notranslate">\; no</span>}<span\; class="notranslate">\; \&Center\; Dot;</span>\frac{\mathrm{<span\; class="notranslate">\; N</span>}}{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; )</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; |</span><span\; class="notranslate">\; .</span>$ 5 . A method for identifying inrush current suitable for traction network transformers according to claim 1 , wherein the value of K _res is 0.2, and n is 1-3. 6 .