A method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics

Abstract

A method for suppressing fault removal differential protection malfunction by relying on hysteresis characteristics, characterized in that the method comprises: step 1, collecting the analog quantity of the circuit breaker in the differential protection circuit, and calculating the differential current of the differential protection circuit based on the collected analog quantity; step 2, sampling the differential current according to a preset sampling frequency, and obtaining the single-point sampling value difference at the current sampling point, and the integral sampling value difference at the current sampling point; step 3, summing and calculating the single-point sampling value difference or the integral sampling value difference within a half-cycle, and judging whether the differential current is a tail current based on the tail current identification criterion; step 4, when the tail current is identified, locking the differential protection within a preset time period. The method algorithm of the present invention is simple, the response is fast, the identification is accurate, the adaptability is strong, and it has good practicality.

Description

Technical Field

[0001] The invention relates to the field of relay protection of power systems, and more specifically to a method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics. Background Art

[0002] In the prior art, current transformer equipment (CT) is widely used in power grids as an important device for realizing current collection in the process of relay protection. However, when a system fault occurs, the protection system will send a trip signal. At the moment when the circuit breaker executes the trip signal and trips, the secondary current of the current transformer will not suddenly change to zero immediately, but will generate an exponentially decaying non-periodic DC component current in the secondary circuit, namely, the tail current. Due to the existence of the tail current, the differential protection of the line, the differential protection of the transformer, and the differential protection of the busbar are prone to malfunction when the fault is removed outside the execution area. Therefore, accurate identification of the tail current can overcome the malfunction of the differential protection.

[0003] In the prior art, there are several methods for identifying tail current, such as zero-crossing detection method, improved grid fractal identification method, Fourier filtering method, frequency domain identification method, etc. First, the zero-crossing detection method refers to the detection of whether the current crosses the zero point every other cycle after the differential protection is started. If not, it is determined that tail current has occurred, and the protection is locked at this time. However, since line faults are often accompanied by zero drift problems, even if the current is still mainly composed of periodic components, there is still a phenomenon that the zero point cannot be detected, so this detection method is not accurate enough.

[0004] Secondly, the improved grid fractal recognition method refers to obtaining current data within a preset time period and recording the non-sinusoidal degree of the tail current corresponding to the first sampling point after the switch action, thereby predicting the non-sinusoidal degree of the tail current at the nth sampling point. If the value of the non-sinusoidal degree gradually increases with the increase of the sampling point number, the current is determined to be a tail current. However, when the current transformer is saturated, in the nonlinear transmission area of ​​the current transformer, its fault current waveform may show the same waveform characteristics as the tail current of the current transformer, so this method cannot accurately distinguish the tail current.

[0005] Third, the Fourier filter method refers to filtering the current signal through the Fourier filter method, so as to accurately calculate the fundamental wave and harmonics in the input current signal. However, when the initial value of the tail current is large or the decay time constant is low, the false operation of the differential protection by the tail current cannot be eliminated within a short time when the circuit breaker is disconnected.

[0006] Finally, the frequency domain identification method refers to a method of effectively identifying the tail current by performing wavelet transformation on the tail current or fault current based on the significant difference in the wavelet energy ratio between the frequency band where the DC component is located and the frequency band where the power frequency periodic component is located in the frequency domain. Although the frequency domain identification method will not be affected by the saturation of the current transformer and the DC component of the fault, the calculation speed cannot be guaranteed due to the transformation of the current signal from the time domain to the frequency domain, and the identification of the fault current and the tail current requires a lot of time. This judgment method has a large amount of calculation and slow response speed.

[0007] Therefore, there is an urgent need for a method to suppress false operation of fault removal differential protection with high algorithm reliability, fast recognition speed and strong adaptability. Summary of the invention

[0008] In order to solve the deficiencies in the prior art, the purpose of the present invention is to provide a method for suppressing the false operation of fault-cutting differential protection by relying on hysteresis characteristics, by sampling the differential current, and calculating the single-point sampling value difference and the integral sampling value difference based on the sampled data, and accurately identifying the tail current based on simple and reliable criteria.

[0009] The present invention adopts the following technical solution.

[0010] A method for suppressing fault removal differential protection malfunction by relying on hysteresis characteristics, wherein the method comprises: step 1, collecting the circuit breaker analog quantity of the differential protection circuit according to a preset sampling frequency, and calculating the differential current of the differential protection circuit based on the collected analog quantity; step 2, sampling the differential current according to the preset sampling frequency, and obtaining the single-point sampling value difference at the current sampling point, and the integral sampling value difference at the current sampling point; step 3, summing and calculating the single-point sampling value difference or the integral sampling value difference within a half cycle, and judging whether the differential current is a tail current based on a tail current identification criterion; step 4, when a tail current is identified, locking the differential protection within a preset time period.

[0011] Preferably, the preset sampling frequency is an integer multiple m of the normal AC current frequency of the differential protection circuit.

[0012] Preferably, the calculation formula for the single-point sampling value difference is:

[0013] I C (N)=|I d (N-1)|-|I d (N)| (1)

[0014] Among them, I C (N) is the single-point sampling value difference of the current sampling point N,

[0015] I d(N) is the differential current sampling value of the current sampling point N,

[0016] I d (N-1) is the differential current sampling value of the previous sampling point N-1.

[0017] Preferably, the calculation formula of the integrated sampling value difference is:

[0018] I 3C (N)=|I 3d (N-1)|-|I 3d (N)| (2)

[0019] Among them, I 3C (N) The integrated sampling value difference of the current sampling point N,

[0020] I 3d (N) is the integral of the three-point differential current sampling value related to the current sampling point N, and I 3d (N)=I d (N)+I d (N-1)+I d (N-2),

[0021] I 3d (N-1) is the integral of the three-point differential current sampling value related to the previous point N-1 of the current sampling point N.

[0022] Preferably, the sum of the single-point sampling value differences within a half cycle is:

[0023]

[0024] Among them, I sC (N) is the sum of the sampling value differences of single point N,

[0025] N to N are the sampling numbers within the half cycle of the normal AC current of the differential protection circuit.

[0026] Preferably, the sum of the integrated sampling value differences within a half cycle is:

[0027]

[0028] Among them, I 3sC (N) is the sum of the differences of the three-point integrated sampling values ​​corresponding to the single point N,

[0029] N to N are the sampling numbers within the half cycle of the normal AC current of the differential protection circuit.

[0030] Preferably, the tail current identification criterion is:

[0031]

[0032] Among them, k l is the attenuation coefficient of the tail current.

[0033] Preferably, the tail current identification criterion is:

[0034]

[0035] Among them, k l It is the threshold coefficient of the tail current identification criterion.

[0036] Preferably, the threshold coefficient k of the tail current identification criterion is l =0.8.

[0037] Preferably, formula (5) and formula (6) are used to distinguish the periodic component and the exponential decay component in the differential current.

[0038] Preferably, the sampling frequency multiple m≥24.

[0039] Preferably, the preset time period is greater than or equal to two cycles.

[0040] The beneficial effect of the present invention is that, compared with the prior art, the method of the present invention, which relies on hysteresis characteristics to suppress the malfunction of fault removal differential protection, can sample the differential current, calculate the single-point sampling value difference and the integral sampling value difference based on the sampling data, and accurately identify the tail current based on the preset criterion. The method algorithm of the present invention is simple, the response is fast, the identification is accurate, the adaptability is strong, and it has good practicality.

[0041] The beneficial effects of the present invention also include:

[0042] 1. The present invention overcomes the problems existing in the prior art in the process of identifying tail current using the zero-crossing detection method, improved grid fractal identification method, Fourier filtering method, and frequency domain identification method. The provided tail current identification criterion has a very simple judgment method, a very small amount of calculation, and can be easily adapted to different current transformers according to the adjustment of the tail current attenuation coefficient, thereby accurately distinguishing between fault current and tail current.

[0043] 2. The present invention can pre-set the sampling frequency multiples. Slow protection can further reduce the complexity of the algorithm by reducing the sampling frequency, and fast protection can achieve more accurate discrimination of fault current and tail current by increasing the sampling frequency, thereby improving the adaptability of the algorithm.

[0044] 3. The present invention combines the three-point integration method of the sampling points to avoid the inaccurate calculation results caused by the bit errors in the sampling process, thereby improving the accuracy of the algorithm.

[0045] 4. The principle of the method of the present invention is reliable and can be widely applied to various types of protection such as busbars, lines and transformers, as well as failure protection of equipment such as circuit breakers, and meets the requirements of various types of ratio-restrained current differential protection. The protection criterion can realize the judgment logic in the whole process of protection action, with short locking time and fast action speed, and can prevent the protection malfunction caused by the removal of out-of-zone faults, thereby ensuring the safe and stable operation of the power system. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 A flow chart of the steps of a method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics of the present invention;

[0047] FIG2 is a schematic diagram of normal operating current and fault current in a method of suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics according to the present invention;

[0048] Figure 3 A schematic diagram of the change of fault current and differential protection action over time in a method of suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics of the present invention;

[0049] Figure 4 A schematic diagram of comparison of fault current and tail current in a method of suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics of the present invention;

[0050] Figure 5 A schematic diagram of a single-point sampling value difference and a summation calculation result thereof in accordance with a method for suppressing malfunction of a fault removal differential protection by relying on hysteresis characteristics according to an embodiment of the present invention;

[0051] Figure 6 A schematic diagram of the integrated sampling value difference and the summation calculation result thereof as the differential current changes in another embodiment of a method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics of the present invention; Figure 7 The present invention is a schematic diagram of differential protection interlocking and operation logic in a method for suppressing fault removal differential protection malfunction by relying on hysteresis characteristics. DETAILED DESCRIPTION

[0052] The present application is further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present application.

[0053] In the prior art, when a system fault occurs, the protection trips and the circuit breaker cuts off the primary current loop when the current passes through zero and the arc is extinguished. At this time, since the current timing values ​​of the excitation branch and the secondary winding loop are not necessarily zero when the primary device is disconnected, the energy stored in the inductor will be gradually released through the loop composed of the resistor and the inductor, that is, the CT secondary current tailing phenomenon occurs. This discharge loop is a typical first-order RL loop, and the attenuation trend of the secondary current is recorded in the following formula:

[0054] I=-I e(0) e -t / τ

[0055] Where, I is the secondary current of CT, I e(0) is the initial value of the excitation current after the primary current is cut off, and τ is the time constant of the CT secondary circuit. Generally speaking, the time constant τ is usually determined based on the total inductance and resistance in the circuit. Since the excitation reactance of the CT equipment is very large, the decay time constant of the CT secondary circuit is also large, generally between several hundred ms and several seconds.

[0056] It is understandable that the CT tail current is mainly composed of the decaying non-periodic component. Since there is usually residual energy in the CT secondary winding when the circuit breaker is disconnected, the CT secondary current tail phenomenon is inevitable, but the current size and decay speed are different. The initial size of the CT tail current is related to the initial current when the circuit breaker is tripped, and the decay time constant is related to the CT flux saturation, CT structure and core structure.

[0057] On the other hand, the fault current contains not only non-periodic components, but also periodic components during the fault. Specifically, the fault current is composed of a composite current of power frequency current, harmonic current and decaying transient components, which has obvious characteristic differences from the hysteresis decay current after the fault is removed, that is, the tail current.

[0058] According to the above characteristics of tail current and fault current, the present invention provides a new method for suppressing malfunction of fault removal differential protection.

[0059] Figure 1 FIG. 1 is a flow chart of the steps of a method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics of the present invention. Figure 1 As shown, a method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics includes steps 1 to 4.

[0060] Step 1: collect analog quantities of the circuit breaker in the differential protection circuit according to a preset sampling frequency, and calculate the differential current of the differential protection circuit based on the collected analog quantities.

[0061] Specifically, the differential protection circuit includes a circuit breaker for performing differential protection, and the circuit breaker performs opening and closing operations based on a differential protection signal generated in the circuit. Generally speaking, the differential signal is determined based on the differential current and the braking current in the differential protection circuit.

[0062] In the present invention, in order to obtain the differential current, the three-phase current of the circuit breaker related to the differential protection can be collected, and the differential current or the braking current can be calculated according to the protection principle. In other words, the analog data at both ends of the circuit breaker can be collected in the present invention, and the analog data is the analog data used to realize the differential current calculation.

[0063] In simple terms, when the analog data collected at both ends of the circuit breaker are consistent, such as the three-phase current, the differential protection is activated. At this time, the collected analog data, that is, the sum of the phase currents, can be used as the differential current to implement the judgment of the subsequent steps. When the analog data collected at both ends of the circuit breaker are inconsistent, it can be determined that the normal operation state is in place. At this time, either no fault has occurred or the fault is outside the scope of differential protection. At this time, there is no need to perform differential current judgment.

[0064] In the prior art, the differential protection principles for busbars, transformers and lines are different, but generally speaking, the protection principle of differential protection circuits is usually a ratio braking principle. In this principle, differential current and braking current are usually included at the same time. When the differential current is greater than a certain ratio of the braking current, the differential protection is activated.

[0065] For example, in a bus differential protection circuit, the logic of the differential protection action is shown in the following formula:

[0066]

[0067] As shown in the above formula, when the differential current is greater than or equal to K times the braking current i f When the differential current is equal to or greater than the differential current threshold value i0, the differential protection action can be determined. Where K is the braking coefficient.

[0068] In different ratio braking algorithms, there may be different braking coefficients K, but generally speaking, they will follow the above formula. That is, when the above formula is satisfied, it is judged that the current differential current meets the conditions for executing differential protection. When the above formula is not satisfied, the differential protection should be locked, that is, the corresponding bus, transformer or line equipment does not need differential protection.

[0069] Step 2: Sample the differential current according to a preset sampling frequency, and obtain the single-point sampling value difference at the current sampling point and the integral sampling value difference at the current sampling point.

[0070] After the differential current is calculated, it can be sampled at a preset frequency. Preferably, the preset sampling frequency is an integer multiple m of the normal AC current frequency of the differential protection circuit. Generally speaking, the multiple of the sampling frequency relative to the normal AC current frequency can be determined according to the actual calculation needs or the specific characteristics of the current transformer.

[0071] In one embodiment of the present invention, in order to achieve high accuracy of the algorithm, a sampling frequency multiple m ≥ 24 is selected. That is, when the sampling frequency multiple is 24, the number of sampling points for the differential signal in each cycle time is m or m+1, that is, 25. That is, when the first sampling point happens to fall at the moment when the current cycle of the signal is turned on, the number of sampling points is m+1=25, otherwise the number of sampling points in each cycle is m=24.

[0072] After sampling is implemented, the sampling value difference at the sampling point of the differential current can be calculated.

[0073] Preferably, the calculation formula for the single-point sampling value difference is:

[0074] I C (N)=|I d (N-1)|-|I d (N)| (1)

[0075] Among them, I C (N) is the single-point sampling value difference of the current sampling point N,

[0076] I d (N) is the differential current sampling value of the current sampling point N,

[0077] I d (N-1) is the differential current sampling value of the previous sampling point N-1.

[0078] Preferably, the calculation formula of the integrated sampling value difference is:

[0079] I 3C (N)=|I 3d (N-1)|-|I 3d (N)| (2)

[0080] Among them, I 3C (N) The integrated sampling value difference of the current sampling point N,

[0081] I 3d (N) is the integral of the three-point differential current sampling value related to the current sampling point N, and I 3d (N)=I d (N)+I d (N-1)+I d (N-2),

[0082] I 3d (N-1) is the integral of the three-point differential current sampling value related to the previous point N-1 of the current sampling point N.

[0083] In the present invention, two different methods can be used to calculate the sampling value difference. The first method is relatively simple, and the absolute value of the sampling value of the current sampling point and the previous sampling point can be calculated. Through this method, the difference between the values ​​of two consecutive sampling points can be obtained. In addition, this method flips all the signals falling on the negative half axis of the y-axis in the differential signal to the positive half axis. The second method is more complicated, and is obtained by integrating the sampling value data of three consecutive points, that is, summing them.

[0084] Since the integration process of discrete point signals is also a summation process, it can be seen that through such a method, the sampling signal can be obtained more accurately, preventing the problem of inaccurate sampling values ​​of one or more sampling points, and allowing a certain degree of error in the sampling process.

[0085] In addition, in order to eliminate the interference of individual point data disturbance on the overall algorithm, other multi-point integration methods can be similarly adopted, such as two-point integration, five-point integration, etc. Different multi-point integration methods can be changed according to factors such as the degree of support for algorithm speed of the device and the accuracy requirements of the judgment process.

[0086] Step 3, summing up the single-point sampling value difference or the integrated sampling value difference within the half cycle, and judging whether the differential current is a tail current based on the tail current identification criterion.

[0087] After calculating the sampling value differences of each sampling point in the present invention, a sum calculation can be performed on the differences to provide a basis for determining the tail current.

[0088] Preferably, the sum of the single-point sampling value differences within a half cycle is:

[0089]

[0090] Among them, I sC (N) is the sum of the sampling value differences of single point N,

[0091] N to N are the sampling numbers within the half cycle of the normal AC current of the differential protection circuit.

[0092] Preferably, the sum of the integrated sampling value differences within a half cycle is:

[0093]

[0094] Among them, I 3sC (N) is the sum of the differences of the three-point integrated sampling values ​​corresponding to the single point N,

[0095] N to N are the sampling numbers within the half cycle of the normal AC current of the differential protection circuit.

[0096] Since the single-point sampling value difference and the integral sampling value difference are different, in step 3, the summation operation of the single-point sampling value difference and the integral sampling value difference can be performed on the above two points of data respectively. Specifically, in the present invention, the sampling value difference of multiple points over a half-cycle time length can be selected as the basis for the summation operation. This is because, for a fault signal containing a periodic component, its current includes a periodic component that conforms to the sinusoidal wave trend. In the sinusoidal wave trend, no matter where the current moment is located in the signal cycle, after every half cycle, there will definitely be a trend reversal from gradually increasing to gradually decreasing, or from gradually decreasing to gradually increasing. In other words, after every half cycle, there will definitely be a highest or lowest point of the sinusoidal wave signal between each sampling point, or one of the sampling points will definitely fall on the highest or lowest point of the sinusoidal wave signal.

[0097] Therefore, in order to fully implement the subsequent judgment logic, the sampling data within half a cycle is selected as the judgment criterion.

[0098] Step 4: When the tail current is identified, the differential protection is locked within a preset time period.

[0099] Preferably, the tail current identification criterion is:

[0100]

[0101] Among them, k l is the attenuation coefficient of the tail current.

[0102] Preferably, the tail current identification criterion is:

[0103]

[0104] Among them, k l is the attenuation coefficient of the tail current.

[0105] FIG2 is a schematic diagram of normal operating current and fault current in a method of the present invention that relies on hysteresis characteristics to suppress misoperation of fault removal differential protection. FIG2(a) is a schematic diagram of normal operating current, and FIG2(b) is a schematic diagram of fault current. As shown in FIG2, in the normal operating current, the three-phase currents all follow the characteristics of a periodic sine wave, alternating between the maximum and minimum values ​​of the current. The amplitudes and periods of the three-phase currents are the same, and the phases differ by 120 degrees. As for the fault current, depending on the fault, the amplitude of one phase current in the three-phase current becomes larger, which makes the amplitudes of the other two phase currents smaller. The currents no longer have a fixed phase relationship.

[0106] When the differential protection circuit identifies a fault current, it usually removes the fault point, such as shielding the secondary circuit containing the fault current. When the removal is completed, the differential current will no longer have the characteristics of a fault current.

[0107] Figure 3 The figure is a schematic diagram of the change of fault current and differential protection action over time in a method of suppressing fault removal differential protection malfunction by relying on hysteresis characteristics of the present invention. Figure 3 As shown in the figure, Ic1, Ic2 and Ic3 are three branch currents respectively, and Icdd is the differential current. Before the differential protection is activated, the differential current is the fault current. When the differential protection is activated and the fault is removed, the differential current immediately becomes the tail current.

[0108] In order to prevent misjudgment of differential protection action caused by the tail current, the present invention adopts a comparison and judgment method of the fault current and the tail current.

[0109] Figure 4 FIG. 1 is a schematic diagram showing a comparison between differential current and tail current in a method of suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics according to the present invention. Figure 4 As shown in the figure, the left figure is the fault current. Since it has a periodic component, it shows an oscillating and attenuated change. Its envelope may gradually decay or be in an unstable state, but the signal itself is still in the process of periodic increase or decrease. In comparison, the right figure is the tail current after the fault is removed. This current shows a standard exponential characteristic, and the signal strength gradually decays.

[0110] Preferably, formula (5) and formula (6) are used to distinguish the periodic component and the exponential decay component in the differential current.

[0111] According to the difference between the single-point sampling value difference and the integrated sampling value difference, the criterion in the present invention is also different. However, the criterion includes two aspects. The first criterion, I 3c (K)>0, where The formula N can be used to determine the data of all sampling value differences within a half cycle to see whether all sampling value differences are positive numbers.

[0112] Generally speaking, for a periodic signal, it is difficult for all sampling value differences within a half cycle starting from any time to be positive numbers, which distinguishes the tail signal that does not contain a periodic component from the fault signal that contains a periodic component.

[0113] However, there is a special case, namely Each sampling point falls into a decreasing interval or an increasing interval in the periodic signal. For this situation, we know that Since the Nm / 2 sampling points and the N sampling points are located on the horizontal axis, that is, on both sides of the sampling value 0, there is also I 3sc (N)=|I 3d (N)+I 3d (Nm / 2)|. That is, I 3sc The actual value of (N) is very small, even close to 0.

[0114] However, at this time The value of is very large due to the decreasing or increasing periodic signal, and can usually be approximately twice the amplitude value of the periodic signal. At this time, for the periodic signal, it is impossible to satisfy the first criterion and the second criterion at the same time.

[0115] On the other hand, for the tail current, not only can the first criterion be met. e(0) e -t / τ The algorithm has a gradually decaying exponential characteristic and can simultaneously meet the second criterion. According to the above first and second criteria, the difference between the tail current and the fault current can be accurately distinguished. The algorithm is not only simple and fast, but also has a high accuracy rate.

[0116] Preferably, the threshold coefficient k of the tail current identification criterion is l =0.8. Specifically, in order to ensure that the criterion is not too sensitive, for example, when a certain degree of error occurs in the sampling of the sampling point, an erroneous judgment may be made, a threshold coefficient may be provided. The threshold coefficient can ensure the reliability of the criterion when it is in action.

[0117] Figure 5 The figure is a schematic diagram of the single-point sampling value difference and the summation calculation result thereof as the differential current changes in one embodiment of a method for suppressing the malfunction of the fault removal differential protection by relying on the hysteresis characteristic of the present invention. Figure 5 As shown, in one embodiment of the present invention, the absolute value of the single-point sampling value of the differential current, the single-point sampling value difference and the summed result thereof are respectively shown in the figure. Before the 40th sampling point, since the current still has a certain periodicity, the single-point sampling value difference and the summed result thereof cannot simultaneously meet the requirements of the criterion in the present invention. After the 40th sampling point, the current is in a process of gradual decay, at which time the criterion can be met, thereby achieving differential protection locking after the fastest half cycle.

[0118] Figure 6 The schematic diagram of the integrated sampling value difference and the summation calculation result thereof in another embodiment of the method of suppressing the malfunction of the fault removal differential protection by relying on the hysteresis characteristic of the present invention as the differential current changes. Figure 6As shown, similarly, similar results can be obtained by using the three-point integration method to calculate the sampling value difference and its sum. After the 43rd sampling point, the differential current can meet the criterion requirements, thereby achieving differential protection locking after the fastest half cycle.

[0119] Figure 7 The figure is a schematic diagram of differential protection locking and operation logic in a method of suppressing fault removal differential protection malfunction by relying on hysteresis characteristics of the present invention. Figure 7 As shown, the discrimination logic in the present invention can be used for discrimination during the operation of differential protection, and can be calculated and recursively discriminated point by point according to the sampling points. In addition to differential current, branch current can also be discriminated by this method. This criterion can be widely used in the judgment of various types of differential protection or circuit breaker failure protection.

[0120] In order to ensure the simplification of the calculation process, while using three-point integration to determine whether to lock the differential protection, a single-point sampling method can also be used to filter certain invalid judgment processes. For example, when it is judged that the sum of the single-point sampling value differences is not greater than the threshold value in the second criterion of the present invention, the count of the tail current can be directly cleared and the differential protection action can continue to be executed. Only when the sum of the three-point integration meets the second criterion of the present invention, the differential protection lock will be executed.

[0121] Preferably, the preset time period is greater than or equal to two cycles. Specifically, in the present invention, the preset time period can be at least two cycles, that is, when the tail current is detected, the differential protection lockout will be executed for at least two cycles. During this period of time, the differential protection will not be executed due to the differential protection circuit misdetecting the tail current.

[0122] The beneficial effect of the present invention is that, compared with the prior art, the method of the present invention, which relies on hysteresis characteristics to suppress the malfunction of fault removal differential protection, can sample the differential current, calculate the single-point sampling value difference and the integral sampling value difference based on the sampling data, and accurately identify the tail current based on the preset criterion. The method algorithm of the present invention is simple, the response is fast, the identification is accurate, the adaptability is strong, and it has good practicality.

[0123] The applicant of the present invention has made a detailed explanation and description of the implementation examples of the present invention in conjunction with the drawings in the specification. However, those skilled in the art should understand that the above implementation examples are only preferred implementation schemes of the present invention, and the detailed description is only to help readers better understand the spirit of the present invention, but not to limit the scope of protection of the present invention. On the contrary, any improvements or modifications based on the inventive spirit of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics, characterized in that: The method comprises: Step 1, collecting the circuit breaker analog quantity of the differential protection circuit according to a preset sampling frequency, and calculating the differential current of the differential protection circuit based on the collected analog quantity; Step 2, sampling the differential current according to a preset sampling frequency, and obtaining a single-point sampling value difference at a current sampling point, and an integral sampling value difference at the current sampling point; Step 3, summing up the single-point sampling value difference or the integrated sampling value difference within a half cycle, and judging whether the differential current is a tail current based on a tail current identification criterion; The tail current identification criterion is: Alternatively, the tail current identification criterion is: Among them, k l is the attenuation coefficient of the tail current; m is the sampling frequency multiple; N is the current sampling point; I c (·) is the single point sampling value difference of the sampling point, I 3c (·) is the integrated sampling value difference of the sampling point, I sc (·) is the sum of the single-point sampling value differences within half a cycle, I 3sc (·) is the sum of the integrated sampling value differences within half a cycle; Step 4: When a tail current is identified, the differential protection is locked within a preset time period, and the preset time period is greater than or equal to two cycles.

2. A method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics according to claim 1, characterized in that: The preset sampling frequency is an integer multiple m of the normal AC current frequency of the differential protection circuit.

3. A method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics according to claim 2, characterized in that: The calculation formula of the single point sampling value difference is: I C (N)=|I d (N-1)|-|I d (N)|(1) Among them, I C (N) is the single-point sampling value difference of the current sampling point N, I d (N) is the differential current sampling value of the current sampling point N, I d (N-1) is the differential current sampling value of the previous sampling point N-1.

4. A method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics according to claim 2, characterized in that: The calculation formula of the integral sampling value difference is: I 3C (N)=|I 3d (N-1)|-|I 3d (N)|(2) Among them, I 3C (N) The integrated sampling value difference of the current sampling point N, I 3d (N) is the integral of the three-point differential current sampling value related to the current sampling point N, and I 3d (N)=I d (N)+I d (N-1)+I d (N-2), I 3d (N-1) is the integral of the three-point differential current sampling value related to the previous point N-1 of the current sampling point N.

5. A method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics according to claim 3, characterized in that: The sum of the single-point sampling value differences within the half cycle is: Among them, the I sC (N) is the sum of the sampling value differences of the current sampling point N, N to N are sampling numbers within a half cycle of the normal alternating current of the differential protection circuit.

6. A method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics according to claim 4, characterized in that: The sum of the integrated sampling value differences within the half cycle is: Among them, the I 3sC (N) is the sum of the three-point integrated sampling value differences corresponding to the current sampling point N, N to N are sampling numbers within a half cycle of the normal alternating current of the differential protection circuit.

7. A method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics according to claim 1, characterized in that: The attenuation coefficient k of the tail current is l =0.

8.

8. A method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics according to claim 7, characterized in that: The formula (5) and the formula (6) are used to distinguish the periodic component and the exponential decay component in the differential current.

9. A method for suppressing malfunction of fault removal differential protection by relying on hysteresis characteristics according to claim 2, characterized in that: The sampling frequency multiple m ≥ 24.

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