A longitudinal electric reactance protection method based on time domain model identification

Abstract

The present application belongs to the field of new energy grid-connected relay protection, and particularly relates to a longitudinal electric reactance protection method based on time domain model identification. According to inherent impedance and capacitive reactance parameters of a power transmission line, a longitudinal electric reactance protection action area is preset; three-phase current and voltage sample values on both sides of the power transmission line are obtained, differential voltage and differential current data are synthesized, and the variation of adjacent two sample points of the differential current is calculated; whether to start the longitudinal electric reactance protection is determined according to the differential current variation; after the protection is started, waveform similarity of the currents on both sides is detected; when the waveform similarity detection result is greater than a threshold value, it is judged as an external fault, and the longitudinal electric reactance protection is blocked; otherwise, an R-L equivalent circuit model equation is established, parameters of short-circuit impedance are identified, when the vector position of the longitudinal electric resistance and the longitudinal electric reactance in the complex plane falls into the preset longitudinal electric reactance protection action area, the longitudinal electric reactance protection enters a section, and after confirmation, the longitudinal electric reactance protection acts.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of new energy grid-connected relay protection, and particularly relates to a longitudinal electric reactance protection method based on time domain model identification. BACKGROUND

[0002] With the construction of new power systems, new energy power generation represented by wind power, photovoltaic and energy storage is growing rapidly, and the grid connection scale is penetrating from low voltage levels to 220kV and above. The power supply structure, control and operation of new energy sources are significantly different from traditional energy sources. High proportion of power electronic equipment is connected to the grid, and the fault current is affected by the control strategy and parameters, which no longer follows the synchronous machine power supply characteristics. The traditional relay protection technology is facing severe challenges.

[0003] The existing power transmission line generally adopts longitudinal current differential protection, which is based on power frequency data, and uses Fourier algorithm to extract the fundamental phase to identify faults. Considering the short-circuit characteristics of new energy, the current amplitude is limited, the harmonic content is high, and the phase is controlled. When the current frequency deviates, the differential current and the braking current based on the power frequency phase are affected, the calculation error is large, and the amplitude and phase of the obtained short-circuit current have great uncertainty, which greatly reduces the fault identification ability and the sensitivity of the current differential protection. Therefore, the principle of current differential protection is difficult to adapt to the complex and variable application scenarios of the power grid.

[0004] At present, longitudinal protection based on time domain model identification has been proposed, but it does not consider the problem of poor external fault identification caused by the input of shunt reactors and measurement errors. At the same time, the least square method is not optimized, which has large calculation amount and large fluctuation of results, and has a great impact on the reliability of protection. Taking a 500kV 400kM line as an example, with shunt reactors on both sides and a compensation degree of 70%, when an external three-phase short circuit occurs, the longitudinal protection identified by the capacitance model or impedance model may malfunction, as shown in the accompanying Figure 5 、 Figure 6 In addition, even if the line is not equipped with reactors or the reactor current is offset by transient compensation, when a metallic short circuit occurs at the bus, the differential current and the differential voltage are both low values. At this time, the influence of environmental noise and transformer error will be significantly raised, causing a large deviation between the measured impedance and the actual value, and reducing the reliability of the longitudinal protection identified by the capacitance model. SUMMARY

[0005] In order to overcome the problems in the prior art, the application provides a longitudinal electric reactance protection method based on time domain model identification, which comprises:

[0006] According to the inherent impedance and capacitive impedance parameters of the power transmission line, the longitudinal electric reactance protection action area is preset;

[0007] Obtaining the sampling values of three-phase currents and voltages at the same time on both sides of the transmission line, and synthesizing the differential voltage and the differential current, and calculating the variation of the differential current at two adjacent sampling points;

[0008] According to the variation of the differential current, it is determined whether to start the pilot reactance protection, and when the variation of the differential current is greater than a preset pilot protection current threshold, the waveform similarity of the currents on both sides is detected to obtain a waveform similarity detection result;

[0009] According to the waveform similarity detection result, it is determined whether it is an external fault, and if it is an external fault, the pilot reactance protection is blocked;

[0010] Otherwise, a time-domain equation is established according to an R-L equivalent circuit model of an internal fault, and the differential resistance and reactance values are calculated; when the pilot resistance and the pilot reactance fall into a preset pilot reactance protection action area in the complex plane, the pilot reactance protection is inserted, and after confirmation, the pilot reactance protection is actuated.

[0011] Further, the waveform similarity of the currents on both sides is detected to obtain a waveform similarity detection result, including:

[0012] ;

[0013] In the formula, is the similarity detection result; , is the sampling data of the current transformers on both sides at the same time; is the cumulative value of the product of the continuous N current sampling values; , is the cumulative value of the square of the continuous N current sampling values.

[0014] Further, according to the waveform similarity detection result, it is determined whether it is an external fault, including: if the waveform similarity detection result is greater than or equal to a similarity recognition threshold of the external fault, it is determined to be an external fault.

[0015] Further, the values of the pilot resistance and the pilot reactance are calculated, including:

[0016] According to the R-L equivalent circuit model of the internal fault, a time-domain equation is established, the current variation at each sampling interval is replaced by the differential value, and the reactance value is replaced by the inductance value:

[0017] ;

[0018] In the formula, represents the pilot reactance; represents the pilot resistance; represents the number of sampling points of one power frequency cycle; The sampling value representing the differential voltage is the sum of the voltages on both sides; The sampling value representing the differential current is the sum of the currents on both sides; The sampling value representing the differential current is the sum of the currents on both sides;

[0019] Let the to-be-solved quantity , , ,

[0020] Make the value of variance Minimum, and use the recursive least square method to establish the solving equation:

[0021] ;

[0022] Wherein, Indicates the Kalman gain matrix; Indicates the k Recursive relationship of the covariance matrix at time k- 1 moment; Indicates the parameter estimation value at time k ; Indicates the error between the estimation result at time k And the actual result; Indicates the forgetting factor.

[0023] Compared with the prior art, the present application has the following technical effects:

[0024] (1) According to the inherent impedance and capacitive impedance parameters of the power transmission line, the longitudinal reactance protection action area is preset, and through the waveform similarity detection of the currents on both sides during the fault, the problem of capacitive characteristic failure of general time-domain longitudinal protection during external fault is effectively solved, the performance of fault identification is significantly improved, and the fault area can be more accurately judged.

[0025] (2) The recursive least square algorithm optimized through the base is adopted, compared with the ordinary least square method based on fixed data window, the calculation amount is greatly reduced, and the calculation resources are effectively saved. Moreover, through the conversion and optimization of the base, the large volatility problem caused by the order of magnitude difference in the parameter identification calculation process and the secondary error caused by the difference calculation are avoided, the order of magnitude is significantly reduced, and the optimization effect is significant. In addition, the forgetting factor is added in the recursive least square and a dynamic updating mechanism is adopted, which is beneficial to the rapid convergence of the result at low value, and is beneficial to resisting error jitter at high value, can eliminate the influence of transient characteristics in the initial stage of fault on parameter identification in time, and can also respond to external condition changes in time, further improving the effectiveness of the measurement result. BRIEF DESCRIPTION OF DRAWINGS

[0026] In order to more clearly illustrate the technical solutions and advantages of the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description only constitute some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0027] Figure 1 It is an equivalent circuit model schematic diagram of an external grounding fault of a power transmission line.

[0028] Figure 2 It is an equivalent circuit model schematic diagram of an internal grounding fault of a power transmission line.

[0029] Figure 3 It is a preset longitudinal reactance protection action area schematic diagram of the present application.

[0030] Figure 4 It is a flowchart schematic diagram of the present application.

[0031] Figure 5 It is a test result of identification based on a traditional capacitance model when a three-phase short circuit occurs outside a region.

[0032] Figure 6 It is a test result of identification based on a traditional impedance model when a three-phase short circuit occurs outside a region.

[0033] Figure 7 It is a measurement result of waveform similarity when an external fault occurs.

[0034] Figure 8 It is a measurement result of waveform similarity when an internal fault occurs.

[0035] Figure 9 It is a convergence comparison of a basic least square method and a recursive least square method. DETAILED DESCRIPTION

[0036] In order to further illustrate the technical means and effects adopted by the present application to achieve the predetermined application purposes, the specific implementation, structure, features and effects of the technical solutions proposed according to the present application will be described in detail below in combination with the drawings and preferred embodiments. The specific features, structures or characteristics in one or more embodiments can be combined in any suitable form. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as understood by those skilled in the art of the technology to which the present application belongs.

[0037] Taking an external grounding fault of a single-phase power transmission line as an example, a π-type equivalent circuit model is as follows: Figure 1 .

[0038] When an external fault occurs, the differential current of the power transmission line is a capacitive reactance current generated by the line to the ground, and the differential current is:

[0039] ;

[0040] Let the differential impedance be , then:

[0041]

[0042] In the above formula, , respectively represent the current on both sides of the transmission line; , respectively represent the equivalent capacitance current to ground on both sides of the line; , respectively represent the voltage on both sides of the transmission line; represents the differential current; represents the differential voltage; represents the impedance of the transmission line to ground; represents the resistance component; represents the reactance component of the transmission line.

[0043] The imaginary part of the impedance of the transmission line to ground is the capacitive reactance of the transmission line, which can be obtained by the type and length of the line cable or actual measurement. For a general line, the capacitive reactance value of the transmission line is several hundred times the impedance value, which is several thousand to ten thousand ohms, which is the same as the normal operation of the line.

[0044] When an internal fault occurs, the capacitive reactance value of the line to ground is much larger than the line impedance, so it can be ignored. The R-L equivalent circuit model is as follows Figure 2 . From Figure 2 , the impedance of the line to ground is in parallel with , and then in series with the transition resistance , which is represented as:

[0045] ;

[0046] where, is the transition resistance when short-circuiting; is the equivalent resistance component of the short circuit, is the equivalent reactance component of the short circuit. For in parallel with , the reactance value will never exceed the positive sequence inductance of the line , so when an internal fault occurs, the imaginary part of the differential impedance is always less than the positive sequence impedance value.

[0047] From the above analysis, it can be seen that the differential reactance has completely different characteristics when the fault is in the area or outside the area. According to this theory, the basic criterion of longitudinal reactance protection is formed, and whether the longitudinal reactance protection acts or not is determined by calculating the size of the line differential reactance.

[0048] In one embodiment of the present application, referring to Figures 1-9 , a longitudinal reactance protection method based on time domain model identification is provided, comprising:

[0049] According to the inherent impedance and capacitive reactance parameters of the transmission line, the action area of the longitudinal reactance protection is preset;

[0050] The sampling values of the three-phase currents and voltages at the same time on both sides of the transmission line are obtained, and the differential voltage and the differential current are synthesized, and the variation of the adjacent two sampling points of the differential current is calculated;

[0051] According to the differential current variation, it is determined whether to start the longitudinal reactance protection, and when the variation of the differential current is greater than the preset longitudinal protection current threshold, the waveform similarity of the currents on both sides is detected to obtain a waveform similarity detection result;

[0052] According to the waveform similarity detection result, it is determined whether it is an external fault, and if the external fault condition is met, the longitudinal reactance protection is blocked;

[0053] If the external fault condition is not met, a time domain equation is established according to the R-L equivalent circuit model of the internal fault, the parameter identification is carried out through the recursive least square algorithm, and the differential resistance and reactance values are obtained; when the longitudinal resistance and the longitudinal reactance fall into the preset longitudinal reactance protection action area in the complex plane, the longitudinal reactance protection is entered, and after confirmation, the longitudinal reactance protection acts.

[0054] The above steps will be described in detail as follows:

[0055] Step 100: According to the inherent impedance and capacitive reactance parameters of the transmission line, the action area of the longitudinal reactance protection is preset.

[0056] As shown in Figure 3 , the maximum reactance value under the theoretical short-circuit condition :

[0057] ;

[0058] In the above formula, represents the positive sequence impedance setting value of the line; represents the first reliability coefficient; represents the standard margin, which is preferably , CT secondary rated value.

[0059] The minimum reactance value under the theoretical short-circuit condition :

[0060] ;

[0061] In the above formula, represents the second reliability coefficient; is the positive sequence line reactance value.

[0062] represents the transition resistance tolerance capability, only the accurate working range of the CT and the sampling circuit needs to be considered, about 600 ohms can be taken; the value of is taken.

[0063] Step 200: Obtain the sampling values of three-phase currents and voltages at the same time on both sides of the transmission line, synthesize the differential voltage and the differential current, and calculate the variation of the differential current at adjacent two sampling points.

[0064] The differential voltage and the differential current are:

[0065] ;

[0066] ;

[0067] In the above formula, represents the phase differential voltage of the transmission line; , respectively represent the sampling values of the phase voltages at the same time on both sides of the transmission line; represents the phase differential current of the transmission line; , respectively represent the sampling values of the phase currents at the same time on both sides of the transmission line.

[0068] Calculate the variation of the differential current at adjacent two sampling points:

[0069] ;

[0070] In the above formula, represents the differential current variation; represents the sampling value of the differential current at the time k ; represents the sampling value of the differential current at the time k -1.

[0071] Step 300: Determine whether to start the pilot reactance protection according to the differential current variation, when the variation of the differential current is greater than the preset pilot protection current threshold, perform the waveform similarity detection on the currents on both sides of the transmission line, and determine whether it is an external fault; if it is an external fault, the pilot reactance protection is blocked.

[0072] The waveform similarity detection of the current on both sides of the power transmission line utilizes the characteristics that the phase of the current on both sides is opposite when the external fault occurs and the phase of the current on both sides is basically the same when the internal fault occurs, and constructs a criterion for identifying the external fault:

[0073]

[0074] In the formula, T is the sampling data of the current transformer on both sides at the same time; and the denominator , is the cumulative value of the product of the continuous N current sampling values. , is the cumulative value after squaring the continuous N current sampling values. is the similarity calculation result.

[0075] If the waveform similarity detection result is greater than or equal to the similarity identification threshold when the external fault occurs, it is determined that the external fault occurs:

[0076]

[0077] In the formula, T is the sampling data of the current transformer on both sides at the same time; and the denominator represents the similarity identification threshold when the external fault occurs.

[0078] When the external fault occurs, the current on both sides basically meets the characteristics that the size is equal and the direction is opposite, in the range, even if part of the sampling data exists abnormal transformation, the calculation result is still a negative value, see ; and when the internal fault occurs, even if the new energy control strategy has a great influence on the fault phase, the result is a value greater than 0, see Figure 7 . Therefore, when the internal fault and the external fault occur, the detection result has obvious difference, and preferably, Figure 8 .

[0079] Step 400: According to the R-L equivalent circuit model of the internal fault, a time domain equation is established, and the recursive least square method optimized by the base is used 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 section, and after confirmation, the longitudinal reactance protection acts.

[0080] As an example, the step 400 includes:

[0081] Step 410: According to the R-L equivalent circuit model of the internal fault, a time domain equation is established, and the recursive least square method optimized by the base is used to calculate the values of the longitudinal resistance and the longitudinal reactance, specifically including:

[0082] ​​​​The initial model equation of the R-L equivalent circuit model is:

[0083]

[0084] In the above formula, represents the differential voltage; represents the longitudinal resistance; represents the differential current; represents the longitudinal inductance.

[0085] The formula is changed into:

[0086]

[0087] In the above formula, represents the number of sampling points per power frequency cycle; represents the longitudinal reactance; represents the current sampling serial number; represents the system frequency; represents the current differential current value; represents the last sampling differential current value; represents the current differential voltage value.

[0088] Let the differential current change amount ; the above formula can be simplified as follows:

[0089]

[0090] In the formula, , , can be obtained by sampling the voltage and current on both sides. Generally, the sampling value is the data obtained by amplifying the actual value by a certain multiple. If the amplification multiples of voltage and current are of the same dimension, the above formula still holds, so there is no need to adjust it again.

[0091] Let the to-be-solved quantity , , , and make the variance minimum, and then use the recursive least square method to establish the solving equation group:

[0092]

[0093] Among them, represents the Kalman gain matrix; represents the recursive relationship of the covariance at time k and time k-1 ; represents the parameter estimation value at time k .​​​​ denotes k error between the time estimation result and the actual result; denotes a forgetting factor.

[0094] Further, the recursive least square solves the equation of the forgetting factor Adopt a dynamic updating mechanism: within 0-30 ms after protection starting Take a lower value (0.6~0.8), 30 ms later Raise to (0.85~0.95), in addition, when the phase has no flow, the jump action is also immediately restored to a low value.

[0095] Step 420: When the pilot resistance and the pilot reactance fall into the preset pilot reactance protection action area in the vector position of the complex plane, the pilot reactance protection enters the section, and after a short time confirmation, the pilot reactance protection acts. Preferably, the confirmation time is 5~20 ms.

[0096] The test results show that the method can accurately and quickly identify the line fault, improve the reliability of the pilot protection, has the advantages of high sensitivity, fast action speed, clear action area, no need to set, strong anti-interference ability and the like, at the same time, the method is different from the traditional power frequency algorithm, is not affected by the changes of frequency and phase, has a wide application range, and is more suitable for new energy grid connection or flexible AC transmission system (FACTS) AC transmission line.

[0097] The above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should be included in the protection scope of the present application.

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 time 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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