Pilot reactance protection method based on time domain model identification

Abstract

The invention belongs to the field of new energy grid-connected relay protection, and particularly relates to a pilot reactance protection method based on time domain model recognition. Presetting a pilot reactance protection action area according to the inherent impedance and capacitive reactance parameters of the power transmission line; acquiring three-phase current and voltage sampling values at two sides of the power transmission line, synthesizing differential voltage and differential current data, and calculating the variable quantity of two adjacent sampling points of the differential current; whether longitudinal reactance protection is started or not is determined according to the differential current variable quantity, and waveform similarity detection is conducted on currents on the two sides after protection is started; when a waveform similarity detection result is greater than a threshold value, an external fault is judged, and pilot reactance protection is locked; otherwise, establishing an R-L equivalent circuit model equation, performing parameter identification on the short-circuit impedance, and when the vector positions of the pilot resistor and the pilot reactance in the complex plane fall into a preset pilot reactance protection action area, performing pilot reactance protection section entering, and performing pilot reactance protection action after confirmation.

Description

Technical Field

[0001] The present invention belongs to the technical field of new energy grid-connected relay protection, and in particular relates to a longitudinal reactance protection method based on time domain model identification. Background Art

[0002] With the construction of new power systems, renewable energy generation, represented by wind power, photovoltaics, and energy storage, has grown rapidly, with grid connection scale penetrating from low voltage levels to 220kV and above. The power supply structure, control, and operation of renewable energy sources differ significantly from those of traditional energy sources. With a high proportion of power electronic equipment connected to the grid, fault currents are influenced by control strategies and parameters and no longer follow the characteristics of synchronous generator power sources, posing severe challenges to traditional relay protection technologies.

[0003] Existing transmission lines generally use longitudinal current differential protection, which is based on power frequency data and uses the Fourier algorithm to extract fundamental phasors for fault identification. Considering the fault characteristics of renewable energy short-circuits, such as limited current amplitude, high harmonic content, and controlled phase, when the current frequency shifts, the differential current and restraining current based on the power frequency phasors are affected, resulting in large calculation errors and significant uncertainty in the obtained short-circuit current amplitude and phase. This significantly reduces fault identification capabilities and the sensitivity of current differential protection. Therefore, the current differential protection principle is difficult to adapt to the complex and changing application scenarios of power grids.

[0004] At present, a longitudinal protection based on the time domain model identification concept has been proposed, but it does not consider the problem of poor recognition of out-of-zone faults caused by the input of shunt reactors and measurement errors. At the same time, the least squares method has not been optimized, resulting in problems such as large calculation amount and large fluctuation of results, which has a great impact on the reliability of protection. Taking a 500kV400kM line as an example, with shunt reactors on both sides and a compensation degree of 70%, when an out-of-zone three-phase short circuit occurs, the longitudinal protection using capacitance model or impedance model identification may malfunction. See the attached Figure 5 、 Figure 6 In addition, even if the line is not equipped with a reactor, or the reactor current is offset by transient compensation, when a metallic short circuit occurs at the busbar, the differential current and differential voltage Both are relatively low values. At this time, the influence of environmental noise and transformer transmission error will be significantly increased, resulting in a large deviation between the measured impedance and the actual value, and the reliability of the longitudinal protection identified by the capacitance model is reduced. Summary of the Invention

[0005] In order to overcome the problems in the prior art, the present invention proposes a longitudinal reactance protection method based on time domain model identification, comprising: Preset the pilot reactance protection action area according to the inherent impedance and capacitive reactance parameters of the transmission line; Obtain the sampling values of the three-phase current and voltage on both sides of the transmission line at the same time, synthesize the differential voltage and differential current, and calculate the change of the differential current between two adjacent sampling points; Determining whether to activate the pilot reactance protection according to the differential current change; and performing a waveform similarity test on the currents on both sides to obtain a waveform similarity test result when the differential current change is greater than a preset pilot protection current threshold; According to the waveform similarity detection results, determine whether it is an out-of-zone fault; if it is an out-of-zone fault, lock the longitudinal reactance protection; Otherwise, a time domain equation is established based on the RL equivalent circuit model of the fault in the area to calculate the differential resistance and reactance values; when the vector position of the longitudinal resistance and longitudinal reactance in the complex plane falls into the preset longitudinal reactance protection action area, the longitudinal reactance protection enters the section, and after confirmation, the longitudinal reactance protection is activated.

[0006] Furthermore, waveform similarity detection is performed on the currents on both sides to obtain waveform similarity detection results, including: ; Where, is the similarity detection result; 、 It is the sampling data of the current transformers on both sides at the same moment; It is the accumulated value of the product of N consecutive current sampling values; 、 It is the accumulated value of the square of N consecutive current sampling values.

[0007] Furthermore, judging whether it is an out-of-zone fault based on the waveform similarity detection result includes: judging it as an out-of-zone fault if the waveform similarity detection result is greater than or equal to a similarity recognition threshold for an out-of-zone fault.

[0008] Furthermore, the values of the longitudinal resistance and the longitudinal reactance are calculated, including: The time domain equation is established based on the RL equivalent circuit model of the fault in the area, with the current change in each sampling interval replacing the differential value and the reactance value replacing the inductance value: ; In the above formula, represents longitudinal reactance; Indicates the longitudinal resistance; Indicates the number of sampling points of one power frequency cycle; Indicates the sampling value of the differential voltage, which is the sum of the voltages on both sides; Indicates the sampling value of the differential current, which is the sum of the currents on both sides; Indicates the change in differential current; Order to be determined , , , Make the variance The value of is the smallest, and the recursive least squares method is used to establish the solution equation: ; in, represents the Kalman gain matrix; express k Moment and k- The recursive relation of the covariance matrix at moment 1; express k Parameter estimates at time t; express k The error between the estimated result and the actual result at each moment; Represents the forgetting factor.

[0009] Compared with the prior art, the present invention has the following technical effects: (1) The present invention pre-sets the pilot reactance protection action zone based on the inherent impedance and capacitive reactance parameters of the transmission line. By detecting the waveform similarity of the currents on both sides during a fault, the present invention effectively solves the problem of the capacitance characteristics failing in the event of an out-of-zone fault in the general time-domain pilot protection. This significantly improves the performance of fault identification and enables a more accurate determination of the fault area.

[0010] (2) The present invention adopts a recursive least squares algorithm optimized by the basis. Compared with the ordinary least squares method based on a fixed data window, the amount of calculation is greatly reduced, effectively saving computing resources. In addition, by converting and optimizing the basis, the large volatility problem caused by the order of magnitude difference in the parameter identification calculation process and the quadratic error caused by the differential calculation are avoided, which significantly reduces the order of magnitude and has a significant optimization effect. In addition, the forgetting factor is added to the recursive least squares and a dynamic update mechanism is adopted. Low values are conducive to rapid convergence of the results, and high values are conducive to resisting error jitter. It can timely eliminate the influence of the transient characteristics of the initial fault on parameter identification, and can also respond promptly to changes in external conditions, further improving the effectiveness of the measurement results. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the technical solutions and advantages of the embodiments of the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the prior art descriptions. 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 paying any creative work.

[0012] Figure 1 Schematic diagram of the equivalent circuit model of ground fault outside the transmission line area; Figure 2 Schematic diagram of the equivalent circuit model of ground fault in the transmission line area; Figure 3 This is a schematic diagram of the preset longitudinal reactance protection action area of the present invention; Figure 4 It is a schematic diagram of the process of the present invention; Figure 5 This is the test result based on the traditional capacitor model identification when there is a three-phase short circuit outside the zone; Figure 6 This is the test result based on the traditional impedance model identification when there is a three-phase short circuit outside the zone; Figure 7 is the measurement result of waveform similarity when fault occurs outside the zone; Figure 8 is the measurement result of waveform similarity during faults within the area; Figure 9 Convergence comparison of basic least squares and recursive least squares. DETAILED DESCRIPTION

[0013] To further illustrate the technical means and effects adopted by the present invention to achieve the intended purpose of the invention, the following, in conjunction with the accompanying drawings and preferred embodiments, describes in detail the specific implementation methods, structures, features, and effects of the technical solutions proposed by the present invention. Specific features, structures, or characteristics in one or more embodiments may be combined in any suitable form. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0014] Taking the out-of-area ground fault of a single-phase transmission line as an example, the π-type equivalent circuit model is as follows: Figure 1 .

[0015] When an out-of-area fault occurs, the differential current of the transmission line is the capacitive reactance current generated by the line to the ground. The differential current is: ; Let the differential impedance be , then:

[0016] In the above formula, 、 Respectively represent the current on both sides of the transmission line; 、 Respectively represent the equivalent capacitive current on both sides of the line to the ground; 、 Respectively represent the voltages on both sides of the transmission line; represents the differential current; represents the differential voltage; Indicates the impedance of the transmission line to ground; Represents the resistance component; Represents the reactance component of the transmission line.

[0017] Transmission line impedance to ground The imaginary part is the capacitive reactance of the transmission line, which can be obtained through the line cable model length or actual measurement. For general lines, the capacitive reactance value of the transmission line is hundreds of times the impedance value, ranging from several thousand to tens of thousands of ohms, which is the same as the situation when the line is operating normally.

[0018] When an internal fault occurs, the line-to-ground capacitance is much greater than the line impedance and can be ignored. The RL equivalent circuit model is as follows: Figure 2 .Depend on Figure 2 It can be seen that the impedance of the two ends of the line to the ground is for and After parallel connection, and then with the transition resistor Concatenation, expressed as: ; in, is the transition resistance during short circuit; is the equivalent resistance component of the short circuit, is the equivalent reactance component of the short circuit. and The reactance value after parallel connection must not exceed the line positive sequence inductance Therefore, when an intra-zone fault occurs, the imaginary part of the differential impedance must be smaller than the positive sequence impedance value.

[0019] The above analysis shows that the differential reactance has completely different characteristics when faults occur inside and outside the zone. Based on this theory, the basic criterion for longitudinal reactance protection is formed. By calculating the magnitude of the line differential reactance, the action of longitudinal reactance protection is determined.

[0020] In one embodiment of the present invention, referring to Figures 1-9 , provides a longitudinal reactance protection method based on time domain model identification, including: Preset the pilot reactance protection action area according to the inherent impedance and capacitive reactance parameters of the transmission line; Obtain the sampling values of the three-phase current and voltage on both sides of the transmission line at the same time, synthesize the differential voltage and differential current, and calculate the change of the differential current between two adjacent sampling points; Determining whether to activate the pilot reactance protection according to the differential current change; and performing a waveform similarity test on the currents on both sides to obtain a waveform similarity test result when the differential current change is greater than a preset pilot protection current threshold; According to the waveform similarity detection results, determine whether it is an out-of-zone fault. If the out-of-zone fault conditions are met, the longitudinal reactance protection is locked; If the out-of-zone fault conditions are not met, a time-domain equation is established based on the RL equivalent circuit model of the in-zone fault, and parameter identification is performed using the recursive least squares algorithm to obtain the differential resistance and reactance values. When the vector positions of the pilot resistance and pilot reactance in the complex plane fall into the preset pilot reactance protection action zone, the pilot reactance protection enters the section, and after confirmation, the pilot reactance protection is activated.

[0021] The following is a detailed explanation of each of the above steps: Step 100: Preset the pilot reactance protection action area according to the inherent impedance and capacitive reactance parameters of the transmission line.

[0022] like Figure 3 As shown, the maximum reactance value under theoretical short-circuit conditions : ; In the above formula, Indicates the line positive sequence impedance constant; represents the first reliability coefficient; Indicates the standard margin, which can be taken , is the secondary rated value of CT.

[0023] Theoretical minimum reactance under short-circuit conditions : ; In the above formula, represents the second reliability coefficient; is the line positive sequence capacitive reactance value.

[0024] Indicates the ability to withstand transition resistance, only the precise operating range of CT and sampling circuit needs to be considered, which is approximately , you can take 600 euros; Pick The value of .

[0025] Step 200: Acquire sampling values of three-phase current and voltage on both sides of the transmission line at the same time, synthesize the differential voltage and differential current, and calculate the change of two adjacent sampling points of the differential current.

[0026] The differential voltage and differential current are: ; ; In the above formula, Indicates the phase differential voltage of the transmission line; 、 Respectively represent the sampling values of the phase voltages on both sides of the transmission line at the same time; Indicates the phase differential current of the transmission line; 、 They respectively represent the sampling values of the phase currents on both sides of the transmission line at the same time.

[0027] Calculate the change of the differential current between two adjacent sampling points: ; In the above formula, Indicates the change in differential current; express k The differential current sampling value at the moment; express k The differential current sampling value at time -1.

[0028] Step 300: Determine whether to activate the pilot reactance protection based on the differential current change. When the differential current change is greater than a preset pilot protection current threshold, perform waveform similarity detection on the currents on both sides of the transmission line to determine whether it is an out-of-zone fault. If it is an out-of-zone fault, lock the pilot reactance protection.

[0029] The waveform similarity detection of the currents on both sides of the transmission line is performed, and the characteristics that the phases of the currents on both sides are opposite when there is an out-of-area fault and the phases of the currents on both sides are basically in the same direction when there is an in-area fault are used to construct the criterion for identifying out-of-area faults: ; Where, 、 is the sampling data of the current transformers on both sides at the same time; the denominator It is the accumulated value of the product of N consecutive current sampling values; 、 It is the accumulated value of the square of N consecutive current sampling values; is the similarity calculation result.

[0030] If the waveform similarity detection result is greater than or equal to the similarity recognition threshold for out-of-zone faults, it is identified as an out-of-zone fault: ; Where, Indicates the similarity recognition threshold for out-of-zone faults.

[0031] When an out-of-zone fault occurs, the currents on both sides are basically equal in magnitude and opposite in direction. exist Even if some of the sampled data have abnormal transmission, the calculation result is still negative. Figure 7 When an intra-zone fault occurs, even if the new energy control strategy has a greater impact on the fault phase, the result will be a value greater than 0, see Figure 8 Therefore, when a fault occurs inside the area or outside the area, the detection results are significantly different. .

[0032] Step 400: Establish a time domain equation based on the RL equivalent circuit model of the fault in the area, and use the recursive least squares method optimized by the basis to obtain the differential resistance and reactance values; when the vector position of the longitudinal resistance and the longitudinal reactance in the complex plane falls into the preset longitudinal reactance protection action area, the longitudinal reactance protection enters the stage, and after confirmation, the longitudinal reactance protection is activated.

[0033] As an example, this step 400 includes: Step 410: Establish a time-domain equation based on the RL equivalent circuit model of the fault in the area, and use the recursive least squares method optimized by the basis to calculate the values of the pilot resistance and the pilot reactance, specifically including: The initial model equation for establishing the RL equivalent circuit model is: ; In the above formula, represents the differential voltage; Indicates the longitudinal resistance; represents the differential current; Represents longitudinal inductance.

[0034] By replacing the differential value with the current change at each sampling interval and the inductance value with the reactance value, the formula becomes: ; In the above formula, Indicates the number of sampling points of one power frequency cycle; represents longitudinal reactance; Indicates the current sampling sequence number; Indicates the system frequency; Indicates the current differential current value; Indicates the differential current value of the last sampling; Indicates the current differential voltage value.

[0035] Let the differential current change ; The above formula can be simplified to the following form: ; Where, 、 、 Both can be obtained by sampling the voltage and current on both sides. Generally, the sampling value is the data after the actual value is magnified by a certain factor. If the magnification factor of voltage and current is of the same dimension, the above formula still holds true, so no further adjustment is required.

[0036] Order to be determined , , , so that the variance The value of is the smallest, and then the recursive least squares method is used to establish the solution of the equation system: ; in, represents the Kalman gain matrix; express k Moment and k-1 The recursive relation of moment covariance; express k Parameter estimates at time t; express k The error between the estimated result and the actual result at each moment; Represents the forgetting factor.

[0037] Furthermore, the recursive least squares method solves the forgetting factor in the equation Adopt dynamic update mechanism: protection within 0-30ms after startup Take a lower value (0.6~0.8), after 30ms Raise to (0.85~0.95), and immediately return to a low value when there is no flow in the phase or the jump occurs.

[0038] Step 420: When the vector position of the pilot resistance and the pilot reactance in the complex plane falls into the preset pilot reactance protection action area, the pilot reactance protection enters the stage, and after a short confirmation time, the pilot reactance protection is activated. Preferably, the confirmation time is 5-20ms.

[0039] Test results show that the method of the present invention can accurately and quickly identify line faults, improve the reliability of longitudinal protection, and has the advantages of high sensitivity, fast action speed, clear action area without setting, and strong anti-interference ability. At the same time, unlike traditional power frequency algorithms, this method is not affected by changes in frequency, phase, etc., has a wide range of applications, and is more suitable for new energy grid connection or flexible direct current system AC transmission lines.

[0040] The above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit the same. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention, and should all be included in the scope of protection of the present invention.

Claims

1. A longitudinal reactance protection method based on time domain model identification, characterized in that: include: Preset the pilot reactance protection action area according to the inherent impedance and capacitive reactance parameters of the transmission line; Obtain the sampling values of the three-phase current and voltage on both sides of the transmission line at the same time, synthesize the differential voltage and differential current, and calculate the change of the differential current between two adjacent sampling points; Determining whether to activate the pilot reactance protection according to the differential current change; and performing a waveform similarity test on the currents on both sides to obtain a waveform similarity test result when the differential current change is greater than a preset pilot protection current threshold; According to the waveform similarity detection results, determine whether it is an out-of-zone fault; If the fault is outside the zone, the longitudinal reactance protection will be locked; Otherwise, calculate the values of the pilot resistance and pilot reactance; when the vector position of the pilot resistance and pilot reactance in the complex plane falls into the preset pilot reactance protection action area, the pilot reactance protection enters the stage, and after confirmation, the pilot reactance protection is activated.

2. The longitudinal reactance protection method based on time domain model identification according to claim 1 is characterized in that: Perform waveform similarity detection on the currents on both sides to obtain waveform similarity detection results, including: ； Where, is the similarity detection result; 、 It is the sampling data of the current transformers on both sides at the same moment; It is the accumulated value of the product of N consecutive current sampling values; 、 It is the accumulated value of the square of N consecutive current sampling values.

3. The longitudinal reactance protection method based on time domain model identification according to claim 2 is characterized in that: The determining whether it is an out-of-zone fault according to the waveform similarity detection result includes: if the waveform similarity detection result is greater than or equal to a similarity recognition threshold for an out-of-zone fault, determining it is an out-of-zone fault.

4. The longitudinal reactance protection method based on time domain model identification according to claim 1 is characterized in that: Calculate the values of pilot resistance and pilot reactance, including: The time domain equation is established based on the RL equivalent circuit model of the fault in the area, with the current change in each sampling interval replacing the differential value and the reactance value replacing the inductance value: ； In the above formula, represents longitudinal reactance; Indicates the longitudinal resistance; Indicates the number of sampling points of one power frequency cycle; Indicates the differential voltage sampling value, which is the sum of the voltages on both sides; Indicates the differential current sampling value, which is the sum of the currents on both sides; Indicates the change in differential current; Order to be determined , , , Make the variance The value of is the smallest, and the recursive least squares method is used to establish the solution equation: ； in, represents the Kalman gain matrix; express k Moment and k- The recursive relation of the covariance matrix at moment 1; express k Parameter estimates at time t; express k The error between the estimated result and the actual result at each moment; Represents the forgetting factor.

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