CN106129966B - The method and device for preventing transformer differential protection malfunction based on coefficient of kurtosis - Google Patents

Abstract

The invention discloses a method and device for preventing maloperation of transformer differential protection based on kurtosis coefficient. The method includes: obtaining the instantaneous sampling value of the secondary current; judging whether a fault occurs; obtaining the waveform of the differential current of each phase; Combined waveform of current; calculate the kurtosis coefficient of the combined waveform of the differential current of each phase of the transformer; judge whether there is saturation of the current transformer during the transformer fault process; block the differential protection of the transformer; open the differential protection of the transformer. The present invention properly transforms part of the waveform of the differential current, and uses the kurtosis coefficient to extract the differential current waveform characteristics of faults inside and outside the zone, and compares the kurtosis coefficient with the fixed value of the kurtosis coefficient to accurately identify the zone Internal faults and external faults under severe saturation, general saturation, and light saturation of current transformers can effectively prevent differential protection from malfunctioning when current transformers are saturated due to external faults of transformers.

Description

Method and device for preventing maloperation of transformer differential protection based on kurtosis coefficient

technical field

The invention belongs to the technical field of power systems, and in particular relates to a method and a device for preventing maloperation of transformer differential protection based on kurtosis coefficient.

Background technique

The P-class current transformer (CT) for protection is used as the measurement equipment of the primary current in the abnormal operation or fault state of the power system, and is widely used in the 220kV and below systems in our country. The P-class current transformer can ensure the accuracy of the transmission change during the steady-state operation of the system during the selection of the type, and make the transmission error not exceed the specified value, but it cannot guarantee the accuracy of the transient process. When the system fails, the primary current may be dozens to hundreds of times the normal operating current, and contains a DC component according to the law of exponential decay. Therefore, the operating point of the current transformer core will have a large range between the linear region and the nonlinear region. changes, the current transformer is saturated, the primary current of the transmission system cannot be accurately measured, and the actual current of the primary system cannot be truly reflected. Refusal to move or delayed action will seriously endanger the safe and stable operation of the power grid.

The transformer is the core equipment of the AC transmission system, and the differential protection, as the main protection for faults in the transformer area, should ensure the reliability of its operation. However, in the field operation, differential protection misoperation events occur from time to time, and the saturation of the current transformers on each side of the transformer is one of the important reasons for the significant increase of the differential current during external faults and thus the misoperation of the differential protection. Certain measures can identify the current transformer saturation and implement protection blocking to ensure the reliability of differential protection.

At present, a variety of technologies have been proposed to prevent the misoperation of transformer differential protection caused by current transformer saturation, such as Pu Nanzhen, etc. (Pu Nanzhen, Zhai Xuefeng, Yuan Yubo, etc. P-level TA saturation versus digital ratio The influence of differential protection on braking characteristics [J]. Electric Power Automation Equipment, 2003, 23 (04): 76-80.) It is proposed to start with the action characteristics of differential protection, and to carry out the ratio braking characteristics which have a certain anti-saturation ability. Correction, raise braking coefficient. However, the improvement of the braking coefficient will reduce the sensitivity of faults in the transformer area at the same time. Therefore, many scholars at home and abroad have proposed a variety of current Transformer saturation detection algorithm. For example, Shen Quanrong et al. (Shen Quanrong, Yan Wei, Liang Ganbing, etc. New principle and application of asynchronous current transformer saturation discrimination [J]. Electric Power System Automation, 2005, 29(16): 84-86.) through accurate positioning of faults Time and the moment when the differential current increases significantly, and judge whether the two are synchronized, and detect whether the current transformer is saturated, but when the current transformer is seriously saturated, the saturation time of the current transformer will be less than 1/4 cycle, The difference between the moment of fault occurrence and the differential current is very small. On the other hand, the detection result of the current transformer may be wrong due to inaccurate time positioning, which may lead to misoperation of the differential protection. Li Guicun et al. (Li Guicun, Liu Wanshun, Jia Qingquan, et al. A new method for detecting current transformer saturation using wavelet principle[J]. Electric Power System Automation, 2001,26(05):36-39+44.) Saturation detection by wavelet transform According to the difference of the maximum value of the wavelet modulus of the secondary current of the current transformer to detect whether the current transformer is saturated, but at the current zero-crossing point and the window critical point, the wavelet detection method may have a large deviation, which will affect the final saturation detection result . Wang Zhihong et al. (Wang Zhihong, Zheng Yuping, He Jiali. Determining the saturation of current transformers in busbar protection by calculating the harmonic ratio [J]. Acta of Electric Power Systems and Automation, 2000,12(5):19-24.) through calculation The harmonic ratio of the secondary current of the current transformer detects whether the current transformer is saturated, but it increases the amount of calculation to varying degrees and reduces the reliability of the saturation detection. E.M.dos Santos et al. (E.M.dos Santos,G.Cardoso Jr.,P.E.Farias and et al.CT saturation detection based on the distance between consecutive points in the plans formed by the secondary current samples and their difference-functions[J].IEEE Transactions on Power Delivery,2013,28(1):29-37.) By calculating the multi-order differential value of the secondary current sampling value of the current transformer to detect the saturation point and desaturation point of the current transformer, but the anti-interference ability is poor, reducing the The reliability of saturation detection and differential protection action.

Therefore, it is necessary to find a faster, more accurate, and less parameter-based method to detect CT saturation in order to prevent the differential protection from malfunctioning when the transformer is faulted outside the zone. The research results can be used as a useful supplement to the existing current transformer saturation detection scheme. It helps to improve the speed and reliability of differential protection.

Contents of the invention

In view of the above problems, the present invention proposes a method and device for preventing maloperation of transformer differential protection based on kurtosis coefficient, which solves the problem of low sensitivity, large amount of calculation and low reliability of existing methods for preventing maloperation of transformer differential protection Defects.

In order to achieve the above object, the technical scheme of the present invention is as follows:

A method for preventing maloperation of transformer differential protection based on kurtosis coefficient, comprising:

Step S1: Obtain instantaneous sampling values of the secondary currents of the current transformers of each phase in the star-side area and the corner-side area of the transformer.

Step S2: According to the instantaneous sampling value of the secondary current, it is judged whether the transformer is faulty; if yes, execute step S3; if no, execute step S7.

Step S3: Calculate the instantaneous value of the differential current of each phase of the transformer according to the instantaneous sampling value of the secondary current, and obtain the waveform of the differential current of each phase according to the instantaneous value of the differential current of each phase.

Step S4: forming a combined waveform of the differential current of each phase of the transformer.

Step S5: Calculating the kurtosis coefficient of the combined waveform of the differential current of each phase of the transformer.

Step S6: Judging whether there is a current transformer saturated during the transformer fault process: judging whether the kurtosis coefficient of any phase of the transformer is greater than the kurtosis coefficient fixed value; if yes, then judging that there is a current transformer during the transformer fault process Saturation occurs, and step S7 is executed; if no, it is determined that no current transformer is saturated during the transformer fault process, and step S8 is executed.

Step S7: blocking the differential protection of the transformer.

Step S8: Open the differential protection of the transformer.

Further, while executing step S3, the time when the fault occurs is recorded.

Further, step S4 includes:

Step S401: Record the moment corresponding to the first extreme point after the differential current of each phase increases significantly.

Step S402: Intercepting the waveform of the differential current of each phase from the time when the fault occurs to the time corresponding to the first extreme point.

Step S403: Taking the time when the fault occurs as the origin, the differential current waveforms intercepted by each phase are symmetrical about the origin to obtain a differential current waveform after rotation transformation.

Step S404: Combining the rotationally transformed differential current waveform of each phase with the original intercepted differential current waveform.

Step S405: normalize the combined differential current waveforms of each phase according to their amplitudes to form the combined waveform of each phase centered at the time when the fault occurs.

Further, in step S2, the judging whether the transformer fails includes: judging whether the real-time variation of the secondary current of any one phase is greater than the preset start-up current; if yes, judging that the transformer is faulty; If not, it is determined that the transformer is not faulty.

Further, the kurtosis coefficient is the ratio of the fourth-order center distance of the combined waveform to the fourth power of the standard deviation.

Further, the starting current is equal to 0.2 times of the rated current of the transformer.

Further, step S402: using a data window to intercept the waveform of the differential current of each phase from the time when the fault occurs to the time corresponding to the first extreme point.

A device for preventing maloperation of transformer differential protection based on kurtosis coefficient, including a secondary current acquisition module, a transformer fault detection module, a differential current generation module, a differential current combination module, a kurtosis coefficient calculation module, and a transformer saturation Detection module, differential protection drive module.

The secondary current acquisition module is used to obtain the instantaneous sampling value of the secondary current of the current transformers of each phase in the star side area and corner side area of the transformer. The transformer fault detection module is used to judge whether the transformer is faulty; if yes, control the differential current generating module; if not, control the differential protection drive module to send a differential protection blocking signal. The differential current generation module is used to calculate the instantaneous value of the differential current of each phase of the transformer according to the instantaneous sampling value of the secondary current, and obtain the waveform of the differential current of each phase according to the instantaneous value of the differential current of each phase. The differential current combination module is used to form the combined waveform of the differential current of each phase. The kurtosis coefficient calculation module is used to calculate the kurtosis coefficient of the combined waveform of the differential current of each phase of the transformer. The transformer saturation detection module is used to judge whether there is saturation in the current transformer during the transformer fault process: to judge whether the kurtosis coefficient of any phase of the transformer is greater than the kurtosis coefficient fixed value; A current transformer is saturated, and the differential protection drive module is controlled to send a differential protection blocking signal; if not, it is determined that no current transformer is saturated during the transformer fault process, and the differential protection drive module is controlled to send a differential protection open signal Signal. The differential protection drive module is used to control the opening or closing of the differential protection circuit of the transformer.

Further, the differential current combination module includes an extremum point recording unit, an interception unit, a rotation transformation unit, a combination unit, and a normalization unit connected in sequence.

The extreme point recording unit is used to record the moment corresponding to the first extreme point after the differential current of each phase increases significantly. The intercepting unit is used to intercept the waveform of the differential current of each phase from the time when the fault occurs to the time corresponding to the first extreme point. The rotation transformation unit is used to take the time when the fault occurs as the origin, and make the differential current waveform intercepted by each phase symmetrical about the origin to obtain the differential current waveform after rotation transformation. The combination unit is used to combine the differential current waveform after the rotational transformation of each phase with the original intercepted differential current waveform. The normalization unit is used to normalize the combined differential current waveforms of each phase according to their amplitudes, so as to form the combined waveforms of each phase centered at the time when the fault occurs.

Further, the kurtosis coefficient is the ratio of the fourth-order center distance of the combined waveform to the fourth power of the standard deviation.

The beneficial effects of the present invention are:

The invention properly transforms the partial waveform of the differential current, and uses the kurtosis coefficient to extract the differential current waveform characteristics of the faults inside and outside the zone, and compares the kurtosis coefficient with the fixed value of the kurtosis coefficient to accurately identify the zone Internal faults and external faults under severe saturation, general saturation, and light saturation of current transformers, effectively prevent differential protection misoperations caused by current transformer saturation in external faults of transformers; and the algorithm is simple, fast, and reliable High, can guarantee the reliability and rapidity of the relay protection device, can be used as a beneficial supplement to the existing saturation detection algorithm, and has certain engineering practical significance.

Description of drawings

Fig. 1 is a schematic flowchart of a method for preventing maloperation of transformer differential protection based on kurtosis coefficient.

Fig. 2 is a combined waveform diagram of differential current after rotation transformation and combination.

Fig. 3 is a simulation diagram of the power system in the embodiment.

Fig. 4 shows the current waveforms of A and B phases on the star side of the transformer in the embodiment.

Fig. 5 is the A-phase current waveform at the corner side of the transformer in the embodiment.

Fig. 6 is a phase difference current waveform of transformer A in the embodiment.

Fig. 7 is a structural block diagram of a device for preventing maloperation of transformer differential protection based on kurtosis coefficient.

Detailed ways

The embodiments will be described in detail below in conjunction with the accompanying drawings.

When there is a fault in the transformer area, the time when the differential current occurs is synchronized with the time when the fault occurs; when there is a fault outside the transformer area, if the current transformers on both sides are saturated to different degrees, the differential current will increase significantly, and the differential current will easily However, in this case, the time when the differential current increases significantly is not synchronized with the time when the fault occurs, that is, there is an obvious discontinuity angle in the differential current.

The present invention proposes a method and device for identifying saturation of a current transformer based on the kurtosis coefficient in statistics according to the waveform characteristics of the differential current when faults occur inside and outside the transformer zone.

As shown in Figure 1, it is a schematic flow sheet of the method of the present invention, the method includes:

Step S1: Obtain instantaneous sampling values of the secondary currents of the current transformers of each phase in the star-side area and the corner-side area of the transformer. Step S2: Judging whether the transformer is faulty: judging whether the real-time variation ΔI _φ of the secondary current of any one phase is greater than the preset start-up current I _QD ; And execute step S3; if not, it is determined that the transformer is not faulty, and execute step S7. Step S3: Calculate the instantaneous value of the differential current of each phase of the transformer according to the instantaneous sampling value of the secondary current, and obtain the waveform of the differential current of each phase according to the instantaneous value of the differential current of each phase. Step S4: forming a combined waveform of the differential current of each phase of the transformer. Step S5: Calculating the kurtosis coefficient of the combined waveform of the differential current of each phase of the transformer. Step S6: Judging whether there is a current transformer saturated during the transformer fault process: judging whether the kurtosis coefficient of any phase of the transformer is greater than the kurtosis coefficient fixed value; if yes, then judging that there is a current transformer during the transformer fault process Saturation occurs, and step S7 is executed; if no, it is determined that no current transformer is saturated during the transformer fault process, and step S8 is executed. Step S7: blocking the differential protection of the transformer. Step S8: Open the differential protection of the transformer.

The present invention will be described in detail below by taking the connection mode of the transformer Y0d11 as an example.

Specifically, in step S1, the secondary current of the three-phase current transformers in the star-side area and the corner-side area of the transformer is sampled, and a cycle of 80 sampling points is set, that is, the sampling frequency f _s =4kHz.

In step S2, it is judged whether the transformer is faulty or not according to the startup criterion of sudden change in phase current. The so-called starting criterion of phase current sudden change refers to whether the change of any phase current of the transformer is greater than the set starting current I _QD , and if it is greater, the differential protection is started.

Specifically, calculate the real-time phase current variation ΔI _φ (k) of any phase in the star-side region and the corner-side region _of the transformer. If the start-up current I _QD is fixed, it is judged that there is a fault, and the time when the fault occurs is recorded as t _f =k/f _s , the differential protection is started, and the step (3) is started; otherwise, return to step S1 to continue sampling.

Set the sudden start criterion of the phase current variation ΔI _φ :

ΔI _φ >I _QD (1)

The calculation formulas of phase current variation ΔI _φ and starting current I _QD are:

Where φ refers to the three phases A, B, and C, i _φ (k) is the instantaneous value of the phase current of φ phase at the kth sampling point, Ie is the rated current converted to the star side of the transformer, i _φ (k), Ie mean is the current value of the secondary side of the current transformer.

In step S3, according to the current instantaneous value of the secondary side of the three-phase current transformer in the star side area and corner side area of the transformer collected in step (1), the instantaneous value of the transformer three-phase differential current i _dA , i _dB , i _dC is calculated:

i _dA ＝(i _YA -i _YB )+i _dA

i _dB = (i _YB -i _YC )+i _dB

i _dC ＝(i _YC -i _YA )+i _dC (3)

Among them, i _YA , i _YB , i _YC are the instantaneous sampling values of the secondary side current of the three-phase current transformer on the star side of the transformer; i _dA , i _dB , and i _dC are the instantaneous sampling values of the secondary side current of the three-phase current transformer on the corner side of the transformer value.

In step S4, the partial waveforms of the differential currents of each phase of the transformer are intercepted, the partial waveforms of the intercepted differential currents of each phase are rotated and transformed, and the transformed differential current waveforms of each phase are combined with the original intercepted differential current waveforms, And normalize according to the amplitude to form the combined waveform.

Specifically, step S4 includes the following steps: Step S401: Record the moment corresponding to the first extreme point after the differential current of each phase increases significantly; Step S402: Intercept the differential current of each phase from the occurrence of the fault The waveform between time and the time corresponding to the first extremum point; Step S403: Taking the time when the fault occurs as the origin, the differential current waveforms intercepted by each phase are symmetrical about the origin, and the differential current waveform after rotation transformation is obtained. Current waveform; Step S404: Combine the differential current waveform after the rotational transformation of each phase with the original intercepted differential current waveform; Step S405: Normalize the combined differential current waveform according to the amplitude respectively to form The combined waveform of each phase centered at the time when the fault occurs.

In step S402, before the waveform is clipped, a data window for clipping data is set first, and the desired waveform is clipped using the data window. The normalization by amplitude described in step S405 is to divide the combined data by the maximum amplitude in the data window. The data window belongs to the prior art and will not be repeated here. In step S401, regarding the acquisition of the first extreme point after the differential current increases significantly, a threshold value of the differential current can be set in advance, and when the differential current exceeds the threshold value, record the extreme point reached by the current, That is, the first extreme point after the differential current increases. Fig. 2 is a combined waveform diagram of differential current after rotation transformation and combination.

In step S5, the kurtosis coefficient is the ratio of the fourth-order center distance of the combined waveform diagram to the fourth power of the standard deviation, expressed in Kur, and the calculation formula is as follows:

In the formula: n represents the total number of sampling points contained in the differential current combined waveform, x _i represents the differential current of each sampling point, Indicates the average value of the combined differential current, and i is the ith sampling point.

Preset the kurtosis coefficient to be 1.8; after calculating the kurtosis coefficient of the differential current combination waveform of each phase of the transformer, in step S6, compare the kurtosis coefficient of each phase with the kurtosis coefficient fixed value; if the transformer is any If the kurtosis coefficient of one phase is greater than the fixed value of the kurtosis coefficient, it is judged that the current transformer is saturated during the fault process; otherwise, it is judged that no current transformer is saturated during the fault process.

Current transformer saturation means that when the current of the current transformer exceeds a certain range, the iron core will be saturated. When the last coil of the iron core passes current, the excitation current generates magnetic flux in the iron core, and the change of the magnetic flux generates a transformed current on the secondary coil. Excessive excitation current, exceeding a certain value, the reluctance in the iron core will decrease, the iron core will be saturated, the increase of the current will not increase the magnetic flux obviously, and the current of the primary and secondary coils cannot be changed proportionally. Flux saturation occurs, i.e. current transformer saturation.

When it is determined that a current transformer is saturated during the transformer failure process, proceed to step S7 to block the differential protection of the transformer; otherwise, proceed to step S8 to enable the differential protection of the transformer.

The technical solution of the present invention is further described below through a specific embodiment.

This case considers the most serious situation, that is, in the case, the current transformer on the high-voltage side of the transformer is not saturated, and the B and C phase current transformers on the low-voltage side are not saturated. Only the saturation of the A-phase current transformer and the differential current of the A-phase are analyzed in detail. , but this simplified analysis does not affect the reliability and correctness of the application of the present invention.

(1) Establish a 220kV double-terminal power transmission system. The simulation diagram is shown in Figure 3. The transformer is connected with Y0d-11, the capacity is 240MVA, and the transformation ratio is 220/38.5kV. The current transformers on both sides of the transformer are simulated based on J-A theory Model, sampling 80 points per week.

(2) A three-phase short-circuit fault occurred outside the corner side of the transformer at 0.5685s. The current waveforms of phase A and B on the high-voltage side are shown in Figure 4, and the current waveform of Phase A on the low-voltage side is shown in Figure 5. The corresponding differential current of phase A is as follows Figure 6 shows.

(3) Part of the waveform of the intercepted differential current is rotated and transformed, and the transformed differential current waveform is combined with the original intercepted differential current waveform, and normalized according to the amplitude to form a fault-occurring time-centered Combined waveform, as shown in Figure 2.

(4) Calculating the kurtosis coefficient of the combined waveform in step (3) to obtain Kur=6.7534, and the kurtosis coefficient is fixed at 1.8.

(5) Since Kur is greater than the fixed value of the kurtosis coefficient, it can be judged that the current transformers on both sides of the transformer are saturated, and the differential protection is blocked.

Fig. 7 is the structural block diagram of described device of the present invention, and this device comprises: secondary current acquisition module 1, transformer fault detection module 2, differential current generation module 3, differential current combination module 4, kurtosis coefficient calculation module 5 , a transformer saturation detection module 6 , and a differential protection drive module 7 .

Secondary current acquisition module 1, transformer fault detection module 2, differential current generation module 3, differential current combination module 4, kurtosis coefficient calculation module 5, transformer saturation detection module 6, and differential protection drive module 7 are connected in sequence; And the transformer fault detection module 2 is connected with the differential protection drive module 7 .

The secondary current acquisition module 1 is used to acquire instantaneous sampling values of the secondary currents of the current transformers of each phase in the star-side area and corner-side area of the transformer.

The transformer fault detection module 2 is used to judge whether the transformer breaks down: judge whether the real-time variation ΔI _φ of the secondary current of any one phase is greater than the preset starting current I _QD ; if yes, then judge that the transformer breaks down, record When the fault occurs, and control the differential current generating module 3; if not, it is determined that the transformer is not faulty, and the differential protection drive module 7 is controlled to send a differential protection blocking signal.

The differential current generation module 3 is used to calculate the instantaneous value of the differential current of each phase of the transformer according to the instantaneous sampling value of the secondary current, and obtain the waveform of the differential current of each phase according to the instantaneous value of the differential current of each phase;

The differential current combining module 4 is used to form a combined waveform of the differential current of each phase.

The kurtosis coefficient calculation module 5 is used to calculate the kurtosis coefficient of the combined waveform of the differential current of each phase of the transformer.

The transformer saturation detection module 6 is used to judge whether the current transformer is saturated during the transformer fault process: whether the kurtosis coefficient of any phase of the transformer is greater than the fixed value of the kurtosis coefficient; if yes, then determine the transformer fault process Saturation occurs in any current transformer in the transformer, and the differential protection driving module 7 is controlled to send a differential protection blocking signal; Automatic protection open signal.

The differential protection driving module 7 is used to control the opening or closing of the differential protection circuit of the transformer. When the differential protection drive module 7 sends a differential protection blocking signal, the transformer differential protection circuit does not work; When the differential protection drive module 7 sends a differential protection open signal, the transformer differential protection circuit starts to work.

Wherein, the differential current combination module 4 includes an extremum point recording unit, an interception unit, a rotation transformation unit, a combination unit, and a normalization unit connected in sequence.

The extreme point recording unit is used to record the moment corresponding to the first extreme point after the differential current of each phase increases significantly. The intercepting unit is used to intercept the waveform of the differential current of each phase from the time when the fault occurs to the time corresponding to the first extreme point. The rotation transformation unit is used to take the time when the fault occurs as the origin, and make the differential current waveform intercepted by each phase symmetrical about the origin to obtain the differential current waveform after rotation transformation. The combination unit is used to combine the differential current waveform after the rotational transformation of each phase with the original intercepted differential current waveform. The normalization unit is used to normalize the combined differential current waveforms of each phase according to their amplitudes, so as to form the combined waveforms of each phase centered at the time when the fault occurs.

This embodiment is only a preferred specific implementation of the present invention, but the scope of protection of the present invention is not limited thereto, any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention , should be covered within the protection scope of the present invention. Therefore, the protection scope of the present invention should be determined by the protection scope of the claims.

Claims (8)

1. a kind of method for preventing transformer differential protection malfunction based on coefficient of kurtosis, which is characterized in that including：

Step S1：Obtain transformer star lateral areas, angle lateral areas each phase current mutual inductor secondary current instantaneous sampling value；

Step S2：Judge whether transformer breaks down according to the instantaneous sampling value of the secondary current；If it has, then executing step Rapid S3；If it has not, thening follow the steps S7；

Step S3：According to each phase difference current instantaneous value of the instantaneous sampling value calculating transformer of the secondary current, and according to Difference current described in each phase is instantaneously worth to the waveform of each phase difference current；

Step S4：Form the combined waveform of each phase difference current of transformer；

Step S5：The coefficient of kurtosis of combined waveform described in each phase difference current of calculating transformer；

Step S6：Judge that whether there is or not current transformers to be saturated during transformer fault：Judge the institute of any phase of transformer State whether coefficient of kurtosis is more than coefficient of kurtosis definite value；If it has, then having current transformer appearance during judgement transformer fault Saturation, and execute step S7；If it has not, there is no current transformer to be saturated during then judging transformer fault, and execute Step S8；

Step S7：It is latched transformer differential protection；

Step S8：Open transformer differential protection；

The coefficient of kurtosis is the fourth central of the combined waveform away from the ratio between the bipyramid with standard deviation.

2. the method for preventing transformer differential protection malfunction based on coefficient of kurtosis according to claim 1, it is characterised in that：

While executing step S3, the moment occurs for record failure.

3. the method for preventing transformer differential protection malfunction based on coefficient of kurtosis according to claim 2, it is characterised in that：

The step S4 includes：

Step S401：First extreme point is recorded after difference current described in each phase significantly increases at the time of correspond to；

Step S402：It intercepts difference current described in each phase and corresponds to the moment to first extreme point from the failure generation moment Between waveform；

Step S403：The moment occurs as origin using the failure, the difference current waveform that each phase is intercepted is obtained about origin symmetry Difference current waveform after to rotation transformation；

Step S404：By the difference current waveform combination of difference current waveform and former interception after each phase rotation transformation；

Step S405：By it is each it is combined after difference current waveform be normalized respectively by amplitude, form each phase and sent out with failure The combined waveform centered on the raw moment.

4. the method for preventing transformer differential protection malfunction based on coefficient of kurtosis according to claim 1, it is characterised in that：

It is described to judge whether transformer breaks down in the step S2, including：

Judge the real-time change amount Δ I of secondary current described in any one phase_φWhether preset starting current I is more than_QD；

If it has, then judgement transformer breaks down；If it has not, then judging that transformer does not break down.

5. the method for preventing transformer differential protection malfunction based on coefficient of kurtosis according to claim 4, it is characterised in that：

The starting current I_QDEqual to 0.2 times of transformer rated current Ie.

6. the method for preventing transformer differential protection malfunction based on coefficient of kurtosis according to claim 3, it is characterised in that：

The step S402：Difference current described in each phase is intercepted using data window, the moment occurs to described first from the failure Extreme point corresponds to the waveform between the moment.

7. a kind of device for preventing transformer differential protection malfunction based on coefficient of kurtosis, which is characterized in that including：

Secondary current acquisition module (1), transformer fault detection module (2), difference current generation module (3), difference current group Mold block (4), coefficient of kurtosis computing module (5), mutual inductor saturation detection module (6), differential protection drive module (7)；

The secondary current of each phase current mutual inductor of the secondary current acquisition module (1) for obtaining transformer star lateral areas, angle lateral areas Instantaneous sampling value；

Transformer fault detection module (2) is for judging whether transformer breaks down；If it has, then control difference current generates Module (3)；If it has not, then controlling differential protection drive module (7) sends out differential protection block signal；

Difference current generation module (3) is used for each mutually differential electricity of the instantaneous sampling value calculating transformer according to the secondary current The instantaneous value of stream, and according to the waveform for being instantaneously worth to each phase difference current of each phase difference current；

Difference current composite module (4) is used to form the combined waveform of each phase difference current；

The coefficient of kurtosis of combined waveform described in each phase difference current of the coefficient of kurtosis computing module (5) for calculating transformer；

Mutual inductor saturation detection module (6) is for judging that whether there is or not current transformers to be saturated during transformer fault：Sentence Whether the coefficient of kurtosis of any phase of disconnected transformer is more than coefficient of kurtosis definite value；If it has, then judgement transformer fault process In there is current transformer to be saturated, and control differential protection drive module (7) and send out differential protection block signal；If it has not, Then judge there is no current transformer to be saturated during transformer fault, and controls differential protection drive module (7) and send out difference Dynamic protection clearing signal；

Differential protection drive module (7) is for controlling being turned on and off for transformer differential protection circuit；

The coefficient of kurtosis is the fourth central of the combined waveform away from the ratio between the bipyramid with standard deviation.

8. the device for preventing transformer differential protection malfunction based on coefficient of kurtosis according to claim 7, it is characterised in that：

Difference current composite module (4) includes sequentially connected extreme point recording unit, interception unit, rotation transformation unit, group Close unit, normalization unit；

The extreme point recording unit for record difference current described in each phase significantly increase after first extreme point it is corresponding when It carves；

For intercepting difference current described in each phase from the failure occurs for the interception unit to first extreme point the moment Waveform between the corresponding moment；

The rotation transformation unit is used to the moment occur as origin using the failure, difference current waveform that each phase is intercepted about Origin symmetry obtains the difference current waveform after rotation transformation；

The assembled unit is used for the difference current waveform combination of difference current waveform and former interception after each phase rotation transformation；

The normalization unit be used for by it is each it is combined after difference current waveform be normalized respectively by amplitude, form each phase The combined waveform centered on the failure generation moment.