Control method and device for differential protection of power system and transformer and medium

Abstract

The invention discloses a control method and device for differential protection of a power system and a transformer and a medium, and the control method comprises the steps: extracting positive sequence current and negative sequence current of three-phase current of the transformer when the amplitude of differential current is greater than a preset value; constructing a positive sequence current-negative sequence current two-dimensional curve; and according to the positive sequence current-negative sequence current two-dimensional curve, differential protection locking or action of the transformer is controlled. According to the technical scheme, the excitation inrush current and the fault current can be accurately identified under the working condition of air-drop in a slight internal fault and a slight turn-to-turn fault, and the reliability of differential protection can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of power distribution networks, and in particular to a control method, device and medium for differential protection of a power system and a transformer. Background Art

[0002] Transformers are indispensable electrical equipment in power systems. Transformer failures will have a serious impact on power supply reliability and safe operation of the system. At present, differential protection is widely used as the main protection for transformers in engineering. When the transformer is closed under no-load or the voltage is restored after the external fault is removed, the transformer core may be severely saturated, resulting in magnetizing inrush current, which will cause the differential protection to malfunction. Therefore, how to accurately distinguish between transformer magnetizing inrush current and internal fault current has become the focus of transformer differential protection research.

[0003] With the increase of transformer capacity, the decrease of magnetic saturation point of core material and the access of reactive power compensation device in power system, the content of second harmonic in magnetizing inrush current decreases, the discontinuity angle decreases and the waveform distortion becomes serious. As a result, it is difficult for traditional differential protection criteria such as second harmonic braking principle, discontinuity angle braking principle and waveform symmetry principle to ensure that differential relay does not malfunction.

[0004] At present, in order to solve this problem, experts and scholars at home and abroad have proposed many methods, which are mainly divided into the following categories: 1. Identification methods based on current characteristics or voltage characteristics; 2. Identification methods based on artificial intelligence. For the first type of methods, there are mainly differential power method, equivalent loop equation method, equivalent parameter identification method and flux characteristic identification method. However, this type of method usually requires adding a voltage transformer to the circuit to measure the transformer parameters, which brings the problem that the voltage transformer may be disconnected, and the requirements for the measuring device are relatively high. The second type of method currently has limited actual case records of transformer excitation inrush current and internal faults in real power systems, and it is often difficult to show ideal performance, which limits the application of artificial intelligence technology in the field of system protection in the store. Summary of the invention

[0005] The present invention provides a control method, device and medium for a power system and transformer differential protection, so as to solve the problems existing in the prior art, realize accurate identification of excitation surge current and fault current, and improve the reliability of transformer differential protection.

[0006] In a first aspect, the present invention provides a control method for transformer differential protection, comprising:

[0007] When the amplitude of the differential current is greater than a preset value, extracting the positive sequence current and the negative sequence current of the three-phase current of the transformer;

[0008] Construct a two-dimensional curve of positive sequence current and negative sequence current;

[0009] According to the positive sequence current-negative sequence current two-dimensional curve, the differential protection lockout or action of the transformer is controlled.

[0010] Optionally, extracting the positive sequence current and the negative sequence current of the three-phase current of the transformer includes:

[0011] A signal delay method is adopted to extract the positive sequence current and the negative sequence current of the three-phase current.

[0012] Optionally, controlling the differential protection lockout or action of the transformer according to the positive-sequence current-negative-sequence current two-dimensional curve includes:

[0013] Based on the positive-sequence current-negative-sequence current two-dimensional curve, constructing a rate change curve Speed ​​and a rate threshold curve Thres of the two-dimensional curve;

[0014] According to the rate change curve Speed ​​and the rate threshold curve Thres, the differential protection lockout or action of the transformer is controlled.

[0015] Optionally, based on the positive-sequence current-negative-sequence current two-dimensional curve, constructing a rate change curve Speed ​​and a rate threshold curve Thres of the two-dimensional curve, including:

[0016] Calculate the first rate, second rate, mean and standard deviation of each data point in the positive-sequence current-negative-sequence current two-dimensional curve; wherein the first rate is the rate component of the data point along the horizontal axis, and the second rate is the rate component of the data point along the vertical axis;

[0017] Performing vector synthesis of the first rate and the second rate of each data point to obtain a true rate of each data point, and constructing a rate change curve Speed ​​of the two-dimensional curve;

[0018] The rate threshold curve Thres is constructed according to the mean and the standard deviation.

[0019] Optionally, controlling the differential protection lockout or action of the transformer according to the rate change curve Speed ​​and the rate threshold curve Thres includes:

[0020] Calculate the area Sy enclosed by the speed change curve Speed ​​and the time axis and the area Sb enclosed by the speed threshold curve Thres and the time axis within a preset time period;

[0021] Determine whether Sy>Sb holds;

[0022] If so, it is determined that the differential current is an excitation inrush current, and the differential protection element of the transformer is controlled to be locked.

[0023] Optionally, the transformer differential protection control method further includes:

[0024] If Sy≤Sb, the differential current is determined to be a fault current, and the differential protection action of the transformer is controlled.

[0025] Optionally, when the amplitude of the differential current is greater than a preset value, before extracting the positive sequence current and the negative sequence current of the three-phase current of the transformer, the method further includes:

[0026] Collecting currents on both sides of the transformer and calculating the differential current;

[0027] Determining whether the differential current is greater than the preset value;

[0028] If yes, the step of obtaining the three-phase currents on both sides of the transformer is performed;

[0029] If not, the process returns to the step of collecting the currents at both sides of the transformer and calculating the differential current.

[0030] In a second aspect, the present invention provides a control device for transformer differential protection, comprising:

[0031] A three-phase current acquisition module, used for acquiring the three-phase currents on both sides of the transformer when the amplitude of the differential current is greater than a preset value;

[0032] An extraction and construction module, used for extracting the positive sequence current and negative sequence current of the three-phase current, and constructing a positive sequence current-negative sequence current two-dimensional curve;

[0033] The differential protection element control module is used to control the differential protection lockout or action of the transformer according to the positive sequence current-negative sequence current two-dimensional curve.

[0034] In a third aspect, the present invention provides a power system, comprising: at least one transformer and a controller;

[0035] The controller is used to execute any of the above-mentioned control methods for transformer differential protection.

[0036] In a fourth aspect, the present invention provides a computer-readable storage medium, wherein the computer-readable storage medium stores computer instructions, and the computer instructions are used to implement the control method of transformer differential protection described in any one of the above when executed by a processor.

[0037] The technical solution of the present invention extracts the positive-sequence current and negative-sequence current of the three-phase current of the transformer when the amplitude of the differential current of the transformer is greater than a preset value, and constructs a positive-sequence current-negative-sequence current two-dimensional curve. Since there are differences in the positive-sequence current-negative-sequence current two-dimensional curve under different working conditions, the working condition of the transformer can be determined according to the positive-sequence current-negative-sequence current two-dimensional curve, thereby controlling the differential protection lockout or action of the transformer. In this way, the excitation inrush current and fault current of the transformer can be accurately identified, the differential protection of the transformer can be prevented from malfunctioning, and the reliability of the differential protection of the transformer can be improved.

[0038] It should be understood that the contents described in this section are not intended to identify the key or important features of the embodiments of the present invention, nor are they intended to limit the scope of the present invention. Other features of the present invention will become easily understood through the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0040] Figure 1 A single-phase transformer no-load closing model provided by an embodiment of the present invention;

[0041] Figure 2 A schematic diagram of differential current when the air-drop is subjected to a 3% turn-to-turn fault provided by an embodiment of the present invention;

[0042] Figure 3 A schematic diagram of the proportion of the second harmonic in the differential current when the air is dropped to a 3% turn-to-turn fault according to an embodiment of the present invention;

[0043] Figure 4 A flowchart of a control method for transformer differential protection provided by an embodiment of the present invention;

[0044] Figure 5 A two-dimensional curve of positive-sequence current and negative-sequence current when a transformer with magnetizing inrush current occurs provided by an embodiment of the present invention;

[0045] Figure 6 Provide a two-dimensional curve of positive-sequence current and negative-sequence current when the transformer is switched on without load due to an internal fault for an embodiment of the present invention;

[0046] Figure 7 A two-dimensional curve of positive-sequence current and negative-sequence current when a 20% inter-turn fault occurs in phase A of the transformer provided by an embodiment of the present invention;

[0047] Figure 8A two-dimensional curve of positive-sequence current and negative-sequence current when a phase A ground fault occurs in a transformer protection zone provided by an embodiment of the present invention;

[0048] Fig. 9 A flow chart of another transformer differential protection control method provided by an embodiment of the present invention;

[0049] Fig.10 A schematic diagram of a simulation model of magnetizing inrush current and internal faults provided by an embodiment of the present invention;

[0050] Fig.11 It is the differential current on both sides of the transformer when the transformer is air-dropped;

[0051] Fig.12 This is the positive and negative sequence current curve corresponding to the no-drop transformer;

[0052] Fig.13 The waveform diagram of Thres and Speed ​​changing with time when the transformer is unplugged;

[0053] Fig.14 It is the differential current on both sides of the transformer when the transformer is dropped due to a minor internal fault;

[0054] Fig.15 This is the positive and negative sequence current curve corresponding to the transformer being air-dropped due to a slight internal fault;

[0055] Fig.16 The waveform diagram of Thres and Speed ​​changing with time when the transformer is air-dropped due to a minor internal fault;

[0056] Fig.17 It is the differential current on both sides of the transformer when the transformer is dropped to a turn-to-turn short circuit fault;

[0057] Fig.18 It is the positive and negative sequence current curve corresponding to the transformer being dropped to a turn-to-turn short circuit fault;

[0058] Fig.19 The waveform diagram of Thres and Speed ​​changing with time when the transformer is dropped to a turn-to-turn short circuit fault;

[0059] Fig. 20 It is the differential current on both sides of the transformer when a single-phase ground short circuit fault occurs in the transformer;

[0060] Fig.21 It is the positive and negative sequence current curve corresponding to the transformer when a single-phase ground short circuit fault occurs;

[0061] Fig. 22 The waveform diagram of Thres and Speed ​​changing with time when a single-phase ground short circuit fault occurs in the transformer;

[0062] Fig.23 A schematic diagram of the structure of a control device for negative-sequence directional protection of a power grid provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0063] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0064] It should be noted that the terms "first", "second", etc. in the specification and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein.

[0065] Figure 1 A single-phase transformer no-load closing model is provided in the embodiment of the present invention. Under the combined effect of residual magnetism in the transformer core and different closing angles, when the transformer is no-load closed, the core may be saturated, thereby generating a large excitation current. Figure 1 As shown, u s is the power supply voltage, R s is the internal resistance of the power supply, L s is the internal reactance of the power supply, R 1σ is the leakage resistance of the primary side of the transformer, L 1σ is the leakage reactance of the primary side of the transformer, R m is the transformer excitation branch resistance, L m is the transformer excitation branch reactance. Assume the power supply voltage u s =U m sin(wt+α), according to the loop voltage equation and magnetic flux formula, we get formula (1):

[0066]

[0067] Among them, α is the closing angle of the power supply, i is the excitation branch current, R = R s +R 1σ +R m , L=L S +L 1σ +L m .

[0068] By solving the differential equation, the flux expression of the transformer core can be obtained:

[0069] ψ=-ψ m cos(ωt+α) +ψ m +ψ r (2).

[0070] in, ψ r It is the residual magnetism of the iron core.

[0071] Since the reactance of the transformer is much larger than the resistance, ignoring the resistance, we can get ψ m =U m / w.

[0072] When the magnetic flux in the transformer core exceeds the saturation magnetic flux, an excitation inrush current will be generated. The expression of the excitation inrush current is:

[0073]

[0074] Among them, ψ s It is the core saturation flux. It can be seen that the amplitude of the excitation surge current can reach 6-8 times the rated current, thus posing a serious threat to the differential protection of the transformer.

[0075] At present, since the excitation inrush current contains a large amount of second harmonic components, the excitation inrush current is usually identified based on the second harmonic braking principle in engineering to prevent the transformer differential protection from malfunctioning due to the excitation inrush current.

[0076] The second harmonic braking principle criterion is I2 / I1>K, where I1 and I2 are the magnitudes of the fundamental and second harmonic amplitudes in the differential current, respectively, and K is the braking coefficient, which is usually 0.15-0.20. When the criterion is established, that is, when the second harmonic content in the differential current exceeds the braking coefficient, the transformer differential protection will be locked. The second harmonic braking principle criterion can usually identify the excitation inrush current and conventional fault current within tens of milliseconds, but when the transformer is dropped to a slight turn-to-turn fault, the second harmonic braking principle criterion will fail. The specific failure analysis is as follows.

[0077] Figure 2 A schematic diagram of differential current when the air-drop is subjected to a 3% turn-to-turn fault provided by an embodiment of the present invention, Figure 3 A schematic diagram of the proportion of second harmonics in the differential current when the air is dropped to a 3% turn-to-turn fault according to an embodiment of the present invention. Figure 2 As shown in the figure, when the transformer is dropped into a slight inter-turn fault with an inter-turn short-circuit ratio of 3% at 0.2s, the differential current is composed of the superposition of the excitation surge current and the fault current. Since the fault is slight and the fault current is small, the excitation surge current component in the differential current dominates and the differential current waveform presents a peak state. Figure 3As shown, the second harmonic content of the fault current is always higher than the braking coefficient, so the second harmonic criterion fails and the protection cannot operate correctly.

[0078] In order to solve the above problems, this embodiment provides a control method for transformer differential protection. Figure 4 A flowchart of a control method for transformer differential protection provided by an embodiment of the present invention, referring to Figure 4 As shown, the control method includes:

[0079] S110. When the amplitude of the differential current is greater than a preset value, extract the positive sequence current and the negative sequence current of the three-phase current of the transformer.

[0080] The differential current can be understood as the current difference measured between the transformer windings, which is used to monitor whether the transformer has a fault or short circuit.

[0081] The preset value can be understood as the minimum current amplitude for the differential protection element to operate. In different power systems, the preset value may be the same or different. The size of the preset value may be set according to the needs of the power system.

[0082] The positive sequence current is a current in a three-phase symmetrical power system in which the three-phase currents have the same phase angle, are equal in magnitude, and are arranged in the order of abc. The negative sequence current is a current in a three-phase symmetrical power system in which the three-phase currents have the same phase angle, are equal in magnitude, and are arranged in the reverse order of cba. The method for extracting the positive sequence current and negative sequence current of the three-phase current of the transformer may include, but is not limited to, extracting the positive sequence current and negative sequence current of the three-phase current of each sampling point by using a symmetrical component method or a transient analysis method, as long as the positive sequence current and negative sequence current of the three-phase current of the transformer can be extracted, and this embodiment does not make specific limitations on this.

[0083] S120. Construct a positive-sequence current-negative-sequence current two-dimensional curve.

[0084] The positive-sequence current-negative-sequence current two-dimensional curve is a curve formed by the positive-sequence current and the negative-sequence current at each sampling point in a coordinate system with the positive-sequence current as the horizontal axis and the negative-sequence current as the vertical axis.

[0085] Figure 5 A two-dimensional curve of positive-sequence current and negative-sequence current when a transformer with magnetizing inrush current occurs provided by an embodiment of the present invention. Figure 6 The embodiment of the present invention provides a two-dimensional curve of positive-sequence current and negative-sequence current when the transformer is closed under no-load conditions due to an internal fault. Figure 7 The positive-sequence current-negative-sequence current two-dimensional curve when a 20% inter-turn fault occurs in phase A of the transformer provided by the embodiment of the present invention is Figure 8 A two-dimensional curve of positive-sequence current and negative-sequence current when a phase A ground fault occurs in the transformer protection zone provided by the embodiment of the present invention. Figure 5-Figure 8 As shown, it can be seen that when the transformer has an excitation inrush current, the sampling points on the positive-sequence current-negative-sequence current two-dimensional curve are distributed irregularly and the distance between two adjacent sampling points changes greatly within the same sampling interval, and the rate change at the sampling points is also relatively large; when the transformer is closed at no-load due to a fault or an internal fault occurs in the transformer, the sampling points on the positive-sequence current-negative-sequence current two-dimensional curve are distributed more regularly and present an elliptical shape, and the distance between two adjacent sampling points changes less, and the rate change at the sampling points is also smaller within the same sampling interval.

[0086] S130. Control the transformer differential protection to lock or operate according to the positive sequence current-negative sequence current two-dimensional curve.

[0087] Among them, when the differential protection is locked, the differential protection action element will not be activated when the differential current reaches the set action threshold, thereby avoiding the differential protection from malfunctioning. Differential protection action means that when the differential current reaches the set action threshold, the differential protection action element is activated, thereby disconnecting the power supply of the faulty equipment to protect the power system.

[0088] Since there is a large difference between the positive-sequence current-negative-sequence current two-dimensional curve when the transformer has an excitation inrush current and the positive-sequence current-negative-sequence current two-dimensional curve when the transformer has a fault, the state of the transformer can be determined based on the positive-sequence current-negative-sequence current two-dimensional curve, and then the differential protection lockout or action of the transformer can be controlled.

[0089] In an exemplary embodiment, when the sampling points in the positive-sequence current-negative-sequence current two-dimensional curve are irregularly distributed and the rate change at the sampling points is large, it is determined that an excitation inrush current occurs in the transformer. At this time, the differential protection of the transformer is controlled to be locked, thereby preventing the differential protection from malfunctioning. On the contrary, when the sampling points in the positive-sequence current-negative-sequence current two-dimensional curve are relatively regularly distributed in an elliptical shape and the rate change at each sampling point is small, it is determined that the transformer is closed at no-load due to a fault or an internal fault occurs in the transformer. At this time, the differential protection of the transformer is controlled to operate so that the faulty equipment is disconnected from the power supply to protect the power system.

[0090] Specifically, when the amplitude of the differential current is greater than the preset value, it indicates that the transformer may have a fault. In order to further determine whether the transformer has an excitation inrush current or a fault, the positive-sequence current and negative-sequence current of the three-phase current of the transformer are extracted, and a positive-sequence current-negative-sequence current two-dimensional curve is constructed. Since there is a large difference between the positive-sequence current-negative-sequence current two-dimensional curve when the transformer has an excitation inrush current and the positive-sequence current-negative-sequence current two-dimensional curve when the transformer has a fault, the transformer differential protection lockout or action can be controlled according to the positive-sequence current-negative-sequence current two-dimensional curve, so that the transformer can be excitation inrush current to prevent the differential protection from malfunctioning, and the transformer can be closed at no-load when the transformer is faulty or the transformer has an internal fault, so that the faulty equipment is disconnected from the power supply to protect the power system.

[0091] In this embodiment, when the amplitude of the differential current of the transformer is greater than a preset value, the positive-sequence current and negative-sequence current of the three-phase current of the transformer are extracted, and a positive-sequence current-negative-sequence current two-dimensional curve is constructed, wherein the positive-sequence current-negative-sequence current two-dimensional curve is different under different working conditions, so that the working condition of the transformer can be determined according to the positive-sequence current-negative-sequence current two-dimensional curve, thereby controlling the differential protection locking or action of the transformer, so that the excitation inrush current and fault current of the transformer can be accurately identified, the differential protection of the transformer can be prevented from malfunctioning, and the reliability of the differential protection of the transformer can be improved.

[0092] Fig. 9 A flowchart of another control method for transformer differential protection provided by an embodiment of the present invention. Based on the above embodiment, this embodiment further adds steps of how to extract the positive sequence current and negative sequence current of the three-phase current of the transformer, and how to control the transformer differential protection lockout or action according to the positive sequence current-negative sequence current two-dimensional curve. Fig. 9 As shown, the control method includes:

[0093] S210, collecting currents on both sides of the transformer and calculating differential current.

[0094] The current on both sides of the transformer can be collected by, but is not limited to, a current transformer. The differential current refers to the current difference measured between the transformer windings. When there is a fault or short circuit inside the transformer, the current will be unbalanced between the transformer windings, thereby generating a differential current.

[0095] S220, determine whether the differential current is greater than a preset value; if so, execute S230; if not, return to execute S210.

[0096] The preset value can be understood as the minimum current for differential protection to operate. When the differential current is greater than the preset value, it indicates that there may be a fault in the power grid and differential protection needs to operate. When the differential current is less than or equal to the preset value, it indicates that the power grid is operating normally and differential protection will not operate. It can be understood that in different power systems, the preset value can be set to the same value or different values. The specific value of the preset value can be determined according to the characteristics of the power system, and this embodiment does not specifically limit this.

[0097] S230. Using a signal delay method, extract the positive sequence current and negative sequence current of the three-phase current of the transformer.

[0098] Among them, the signal delay method refers to delaying the voltage of the power system by 1 / 4 cycle in a two-phase stationary coordinate system, thereby achieving the purpose of separating the positive sequence component and the negative sequence component of the power system. The signal delay method for separating positive sequence current and negative sequence current has the advantages of simple implementation and small calculation amount.

[0099] In an optional embodiment, using a signal delay method, extracting the positive sequence current and the negative sequence current of the three-phase current of the transformer includes:

[0100] In the abc coordinate system, the instantaneous value expressions of the positive and negative sequence components can be obtained according to the symmetrical component method:

[0101]

[0102] In the formula and are the three-phase positive sequence component and negative sequence component in the abc coordinate system, T + and T - is the transformation matrix, I abc is the three-phase instantaneous value in the abc coordinate system,

[0103] The abc coordinate system can be transformed to the αβ0 coordinate system using Clark transformation:

[0104]

[0105] Among them, I αβ0 represents the instantaneous value of the three-phase current in the αβ0 coordinate system, I abc It represents the instantaneous value of three-phase current in the abc coordinate system.

[0106] The instantaneous values ​​of the positive and negative sequence components in the αβ0 coordinate system can be expressed as:

[0107]

[0108] Substituting equations (4)-(6) into equation (7), we can obtain the relationship between the three-phase positive sequence, negative sequence components and their instantaneous values ​​in the αβ0 coordinate system:

[0109]

[0110]

[0111] in, and They represent the positive and negative sequence currents in the αβ0 coordinate system, is the rotation factor.

[0112] According to the positive and negative sequence components in the αβ0 coordinate system, the Clark inverse transformation can be performed to obtain the positive and negative sequence components in the abc coordinate system:

[0113]

[0114] in, and They represent the positive and negative sequence components of the current in the abc coordinate system, respectively. and are the positive sequence α and β components of the current respectively, and They are the negative sequence α and β components of the current respectively.

[0115] Combining the above formulas, the instantaneous value expression of positive and negative sequence current calculated based on phase a can be obtained as follows:

[0116]

[0117] S240, construct a positive sequence current-negative sequence current two-dimensional curve.

[0118] S250 . Based on the positive-sequence current-negative-sequence current two-dimensional curve, construct a rate change curve Speed ​​and a rate threshold curve Thres of the two-dimensional curve.

[0119] The rate change curve Speed ​​of the two-dimensional curve is a curve formed by each sampling point and its corresponding rate. In an optional embodiment, the value of the rate of each sampling point in the positive sequence current-negative sequence current two-dimensional curve is first extracted, and then the value of the rate of each sampling point is connected into a curve to form the rate change curve Speed.

[0120] Since the sampling points on the positive-sequence current-negative-sequence current two-dimensional curve are irregularly distributed when the transformer has an excitation inrush current, and the distance between two adjacent sampling points changes greatly within the same sampling interval, the rate change at the sampling point is also relatively large. When the transformer is closed at no-load or an internal fault occurs in the transformer, the sampling points on the positive-sequence current-negative-sequence current two-dimensional curve are more regularly distributed in an elliptical shape, and the distance between two adjacent sampling points changes less. Within the same sampling interval, the rate change at the sampling point is also small. Therefore, a rate threshold curve can be set for the positive-sequence current-negative-sequence current two-dimensional curve. The rate threshold curve Thres is a curve formed by the threshold change of each sampling point and its corresponding rate.

[0121] It should be noted that the construction of the speed change curve Speed ​​and the speed threshold curve Thres of the two-dimensional curve may be performed simultaneously or in different time periods, which is not specifically limited in this embodiment.

[0122] It should be noted that the rate of a sampling point on the positive-sequence current-negative-sequence current two-dimensional curve can be calculated using vector synthesis considerations. The rate threshold of a sampling point on the positive-sequence current-negative-sequence current two-dimensional curve can be determined using statistical principles.

[0123] In an optional embodiment, based on the positive-sequence current-negative-sequence current two-dimensional curve, constructing a rate change curve Speed ​​and a rate threshold curve Thres of the two-dimensional curve includes: calculating the first rate, second rate, mean and standard deviation of each data point in the positive-sequence current-negative-sequence current two-dimensional curve; vector synthesizing the first rate and the second rate of each data point to obtain the true rate of each data point, and constructing the rate change curve Speed ​​of the two-dimensional curve; constructing the rate threshold curve Thres according to the mean and standard deviation.

[0124] The first rate is the rate component of the data point along the horizontal axis, and the second rate is the rate component of the data point along the vertical axis. Standard Deviation

[0125] The curve speed Speed ​​can be defined as follows:

[0126] Among them, i p and i n represents the positive sequence current and the negative sequence current respectively, k represents the kth sampling point of the current sequence, and T is the time interval between two adjacent sampling points. In an optional embodiment, the sampling frequency is set to 4kHz (80 points in one cycle), that is, the sampling time interval is 2.5×10 -4 s, thus making the differential protection action faster and more accurate.

[0127] In statistics, the mean can reflect the central tendency of data, the standard deviation can reflect the degree of dispersion of data, and the mean plus standard deviation can represent the upper limit of the overall degree of data distribution. For a set of data with smaller changes, the mean plus standard deviation curve is closer to the upper limit of this set of data, that is, the upper limit of the confidence interval in mathematical statistics. Therefore, based on the mean and standard deviation, constructing the rate threshold curve Thres can specifically include: rate threshold curve Thres = μ + σ.

[0128] S260 . Control the transformer differential protection to lock or actuate according to the rate change curve Speed ​​and the rate threshold curve Thres.

[0129] Since the rate of each sampling point in the positive-sequence current-negative-sequence current two-dimensional curve changes differently when the transformer has an excitation inrush current or a fault, the differential protection lockout or action of the transformer can be controlled according to the rate change curve Speed ​​and the rate threshold curve Thres. Specifically, when the rate of a sampling point on the rate change curve Speed ​​exceeds the rate threshold, it means that the rate change of the sampling point is fast, so that it can be determined that the transformer has an excitation inrush current, and the differential protection of the transformer is controlled to be locked at this time; on the contrary, when the rate of a sampling point on the rate change curve Speed ​​is lower than the rate threshold, it means that the rate change of the sampling point is slow, so that it can be determined that the transformer is closed at no-load or an internal fault occurs in the transformer, and the differential protection of the transformer is controlled to act at this time.

[0130] In an optional embodiment, according to the rate change curve Speed ​​and the rate threshold curve Thres, controlling the transformer's differential protection lockout or action includes: calculating the area Sy enclosed by the rate change curve Speed ​​and the time axis and the area Sb enclosed by the rate threshold curve Thres and the time axis within a preset time period; judging whether Sy>Sb holds; if Sy>Sb holds, determining that the differential current is an excitation surge current, and controlling the transformer's differential protection element to lockout; if Sy>Sb does not hold, determining that the differential current is a fault current, and controlling the transformer's differential protection action.

[0131] The preset time period can be determined according to the requirements for the sensitivity and accuracy of the differential protection. When the preset time period is set to a smaller value, the sensitivity of the differential protection is higher, but the accuracy is lower, and when the preset time period is set to a larger value, the sensitivity of the differential protection is reduced, but the accuracy is high. In an exemplary embodiment, the preset time period is 15s.

[0132] When Sy>Sb is established, it means that the rate change of each sampling point on the positive-sequence current-negative-sequence current two-dimensional curve is large, so the differential current is determined to be an excitation surge current. At this time, the differential protection element of the transformer is controlled to be locked to prevent the differential protection from malfunctioning. When Sy>Sb is not established, that is, Sy≤Sb, it means that the rate change of each sampling point on the positive-sequence current-negative-sequence current two-dimensional curve is small, so the differential current is determined to be a fault current. At this time, the differential protection element of the transformer is controlled to operate to disconnect the power supply of the faulty equipment and achieve the purpose of protecting the power system.

[0133] In this embodiment, the positive-sequence current and negative-sequence current of the three-phase current of the transformer are extracted by adopting a signal delay method, which is simple to implement and has a small amount of calculation. Based on the positive-sequence current-negative-sequence current two-dimensional curve, a rate change curve Speed ​​and a rate threshold curve Thres of the two-dimensional curve are constructed, and according to the rate change curve Speed ​​and the rate threshold curve Thres, the differential protection locking or action of the transformer is controlled, thereby further improving the accuracy of identifying the excitation surge current and fault current of the transformer, and further improving the reliability of the differential protection of the transformer.

[0134] The effectiveness of the control method for transformer differential protection provided in this embodiment is verified by square analysis below.

[0135] This embodiment builds a transformer excitation inrush current and internal fault simulation model based on the PSCAD / EMTDC platform. Fig.10 Schematic diagram of the excitation inrush current and internal fault simulation model provided by the embodiment of the present invention. Fig.10 As shown in the figure, QF1 and QF2 are circuit breakers, CT1 and CT2 are current transformers, and LOAD is the load side. The turn-to-turn fault simulation is built by three single-phase three-winding transformers, and the primary winding is composed of winding #2 and winding #3 in series. By changing the voltage of these two windings, different degrees of turn-to-turn faults can be achieved. The specific simulation parameters are shown in Table 1.

[0136] Table 1. Simulation parameters

[0137] name Parameter Value Transformer wiring method Dyn-11 Transformer rated capacity / MVA 100 Transformer ratio 35kV / 230kV Frequency / Hz 50 Leakage reactance / pu 0.1 Iron loss / pu 0.00823 Copper loss / pu 0.002 Power supply voltage / kV 35 Power supply internal impedance / Ω 0 Power frequency / Hz 50 Load / MW 60 Sampling frequency / kHz 4

[0138] 1. When the transformer is air-dropped

[0139] The transformer is closed at no-load at t=0.2017s, that is, the closing angle is 30°. There is no residual magnetism in each phase before closing. At this time, the differential current of phase A is a unidirectional excitation surge current, which is biased to one side of the time axis.

[0140] Fig.11 is the differential current on both sides of the transformer when the transformer is air-dropped, Fig.12 This is the positive and negative sequence current curve corresponding to the no-drop transformer. Fig.13Waveform diagrams of Thres and Speed over time when the transformer is energized with no load. The simulation results are as Figure 11-13 shown. It can be seen that at this time, the differential current is greater than the preset value, the criterion is activated, and after 15 ms, it is calculated that Sy = 16.77 and Sb = 16.53, which satisfy the operating criterion Sy > Sb, and it is determined to be inrush current, and the differential protection is reliably blocked.

[0141] 2. When the transformer is energized with no load into a slight internal fault

[0142] The transformer is switched on with no load at t = 0.2 s, that is, the closing angle is 0°, and there is a 3% turn-to-turn fault on the angle side of phase A of the transformer.

[0143] Fig.14 is the differential current on both sides of the transformer when the transformer is energized with no load into a slight internal fault, Fig.15 is the positive and negative sequence current curve diagram corresponding to when the transformer is energized with no load into a slight internal fault, Fig.16 is the waveform diagram of Thres and Speed over time when the transformer is energized with no load into a slight internal fault. The simulation results are as Figure 14-16 shown. At this time, the differential current is the superposition of the fault current and the inrush current, and the positive and negative sequence current curve is an irregular ellipse. According to the operating criterion calculation, Sy = 48.63 and Sb = 58.95, which satisfy Sy < Sb, and it is determined to be the fault current, and the protection is opened and operates quickly.

[0144] 3. When energized with no load into an internal fault

[0145] At t = 0.1 s, a 1% turn-to-turn short circuit fault occurs on the angle side of phase A of the transformer.

[0146] Fig.17 is the differential current on both sides of the transformer when the transformer is energized with no load into a turn-to-turn short circuit fault, Fig.18 is the positive and negative sequence current curve diagram corresponding to when the transformer is energized with no load into a turn-to-turn short circuit fault, Fig.19 is the waveform diagram of Thres and Speed over time when the transformer is energized with no load into a turn-to-turn short circuit fault. The simulation results are as Figure 17-19 shown. According to the operating criterion calculation, at this time Sy = 7.19 and Sb = 14.41, which satisfy Sy < Sb, and it is determined to be the fault current, and the protection is opened and operates quickly.

[0147] 4. When a fault occurs within the protected area

[0148] At t = 0.1 s, a single-phase ground short circuit fault occurs on the star side of phase A of the transformer.

[0149] Fig. 20 is the differential current on both sides of the transformer when a single-phase ground short circuit fault occurs in the transformer, Fig.21It is the positive and negative sequence current curve diagram corresponding to the single-phase grounding short-circuit fault of the transformer. Fig. 22 It is the waveform diagram of Thres and Speed changing with time when the single-phase grounding short-circuit fault occurs in the transformer. The simulation results are as Figure 20-22 shown. After the amplitude of the differential current is greater than the preset value, the protection criterion starts. It is calculated that at this time, Sy = 54.23 and Sb = 60.83. Since Sy < Sb, it is determined as the fault current, and the protection acts quickly.

[0150] Based on the same inventive concept, this embodiment also provides a control device for negative-sequence directional protection of a power grid. This determination device can be implemented in the form of hardware and / or software and can be integrated into a controller. Fig.23 It is the structural schematic diagram of the control device for negative-sequence directional protection of the power grid provided by the embodiment of the present invention, as Fig.23 shown. This control device includes:

[0151] A three-phase current acquisition module 310, configured to acquire the three-phase currents on both sides of the transformer when the amplitude of the differential current is greater than the preset value.

[0152] An extraction and construction module 320, configured to extract the positive-sequence current and negative-sequence current of the three-phase current and construct a two-dimensional curve of positive-sequence current - negative-sequence current.

[0153] A differential protection element control module 330, configured to control the locking or action of the differential protection of the transformer according to the two-dimensional curve of positive-sequence current - negative-sequence current.

[0154] The control device for transformer differential protection provided by the embodiment of the present invention can execute the control method for transformer differential protection provided by any embodiment of the present invention, and has the corresponding functional modules and beneficial effects of the execution method. The same parts can be referred to the above description.

[0155] The embodiment of the present invention also provides a power system, which includes at least one transformer and a controller; the controller can be integrated with the control device for transformer differential protection provided by any embodiment of the present invention and can execute the control method for transformer differential protection provided by any embodiment of the present invention.

[0156] Since the power system provided by the embodiment of the present invention includes at least one transformer and a controller, and the controller can be integrated with the control device for transformer differential protection provided by the embodiment of the present invention and can execute the control method for transformer differential protection provided by the embodiment of the present invention, it can have the corresponding structure and features of the control method for transformer differential protection provided by the embodiment of the present invention and can achieve the beneficial effects of the control method for transformer differential protection provided by the embodiment of the present invention. The same parts can be referred to the above description.

[0157] Based on the same concept, an embodiment of the present invention further provides a computer-readable storage medium, which stores computer instructions, and the computer instructions are used to enable a processor to implement the control method provided by any of the above embodiments when executed.

[0158] In the context of the present invention, a computer-readable storage medium may be a tangible medium that may contain or store a computer program for use by or in combination with an instruction execution system, device or equipment. A computer-readable storage medium may include, but is not limited to, electronic, magnetic, optical, electromagnetic, infrared, or semiconductor systems, devices or equipment, or any suitable combination of the foregoing. Alternatively, a computer-readable storage medium may be a machine-readable signal medium. A more specific example of a machine-readable storage medium may include an electrical connection based on one or more lines, a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber, a portable compact disk read-only memory (CD-ROM), an optical storage device, a magnetic storage device, or any suitable combination of the foregoing.

[0159] The above specific implementations do not constitute a limitation on the protection scope of the present invention. It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and substitutions can be made according to design requirements and other factors. Any modification, equivalent substitution and improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A control method for transformer differential protection, characterized in that: include: When the amplitude of the differential current is greater than a preset value, extracting the positive sequence current and the negative sequence current of the three-phase current of the transformer; Construct a two-dimensional curve of positive sequence current and negative sequence current; According to the positive sequence current-negative sequence current two-dimensional curve, the differential protection lockout or action of the transformer is controlled.

2. The control method for transformer differential protection according to claim 1, characterized in that: Extracting the positive sequence current and the negative sequence current of the three-phase current of the transformer, including: A signal delay method is adopted to extract the positive sequence current and the negative sequence current of the three-phase current.

3. The control method for transformer differential protection according to claim 1, characterized in that: According to the positive sequence current-negative sequence current two-dimensional curve, controlling the differential protection lockout or action of the transformer includes: Based on the positive-sequence current-negative-sequence current two-dimensional curve, constructing a rate change curve Speed ​​and a rate threshold curve Thres of the two-dimensional curve; According to the rate change curve Speed ​​and the rate threshold curve Thres, the differential protection lockout or action of the transformer is controlled.

4. The control method for transformer differential protection according to claim 3, characterized in that: Based on the positive-sequence current-negative-sequence current two-dimensional curve, a rate change curve Speed ​​and a rate threshold curve Thres of the two-dimensional curve are constructed, including: Calculate the first rate, second rate, mean and standard deviation of each data point in the positive-sequence current-negative-sequence current two-dimensional curve; wherein the first rate is the rate component of the data point along the horizontal axis, and the second rate is the rate component of the data point along the vertical axis; Performing vector synthesis of the first rate and the second rate of each data point to obtain a true rate of each data point, and constructing a rate change curve Speed ​​of the two-dimensional curve; The rate threshold curve Thres is constructed according to the mean and the standard deviation.

5. The control method for transformer differential protection according to claim 3, characterized in that: Controlling the differential protection lockout or action of the transformer according to the rate change curve Speed ​​and the rate threshold curve Thres, including: Calculate the area Sy enclosed by the speed change curve Speed ​​and the time axis and the area Sb enclosed by the speed threshold curve Thres and the time axis within a preset time period; Determine whether Sy>Sb holds; If so, it is determined that the differential current is an excitation inrush current, and the differential protection element of the transformer is controlled to be locked.

6. The control method for transformer differential protection according to claim 5, characterized in that: Also includes: If Sy≤Sb, the differential current is determined to be a fault current, and the differential protection action of the transformer is controlled.

7. The control method for transformer differential protection according to claim 1, characterized in that: When the amplitude of the differential current is greater than a preset value, before extracting the positive sequence current and the negative sequence current of the three-phase current of the transformer, the method further includes: Collecting currents on both sides of the transformer and calculating the differential current; Determining whether the differential current is greater than the preset value; If yes, the step of obtaining the three-phase currents on both sides of the transformer is performed; If not, the process returns to the step of collecting the currents at both sides of the transformer and calculating the differential current.

8. A control device for transformer differential protection, characterized in that: include: A three-phase current acquisition module, used for acquiring the three-phase currents on both sides of the transformer when the amplitude of the differential current is greater than a preset value; An extraction and construction module, used for extracting the positive sequence current and negative sequence current of the three-phase current, and constructing a positive sequence current-negative sequence current two-dimensional curve; The differential protection element control module is used to control the differential protection lockout or action of the transformer according to the positive sequence current-negative sequence current two-dimensional curve.

9. A power system, characterized in that: include: at least one transformer and a controller; The controller is used to execute the control method for transformer differential protection according to any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores computer instructions, and the computer instructions are used to enable a processor to implement the control method for transformer differential protection according to any one of claims 1 to 7 when executed.

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