CN109991513B - Method and system for calculating theoretical value of time constant of short-circuit current direct-current component - Google Patents
Method and system for calculating theoretical value of time constant of short-circuit current direct-current component Download PDFInfo
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- CN109991513B CN109991513B CN201910153205.6A CN201910153205A CN109991513B CN 109991513 B CN109991513 B CN 109991513B CN 201910153205 A CN201910153205 A CN 201910153205A CN 109991513 B CN109991513 B CN 109991513B
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- G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
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Abstract
The invention discloses a method and a system for calculating a theoretical value of a time constant of a direct current component of a short circuit current, wherein the method comprises the following steps: determining a peak value of an alternating component of the short-circuit current; fitting an upper envelope curve and a lower envelope curve of the short-circuit current waveform to obtain a first direct-current component attenuation curve; analyzing the short-circuit current waveform by using a harmonic analysis method to obtain a second direct current component attenuation curve; determining a first direct current component waveform and a second direct current component waveform, and respectively performing exponential fitting to obtain a first fitting formula and a second fitting formula respectively; and selecting a larger time constant as a theoretical value of the time constant according to the first fitting formula and the second fitting formula. According to the method, the time constant of the short-circuit current direct-current component is determined according to the set rule of curve fitting of the short-circuit current direct-current component, so that blindness of fitting formula selection can be avoided; by checking the accuracy of the calculated time constant, the accuracy of the calculation result is ensured.
Description
Technical Field
The invention relates to the technical field of power transmission, in particular to a method and a system for calculating a theoretical value of a time constant of a short-circuit current direct-current component.
Background
Compared with the working current of the power transmission line, the short-circuit current of the system is large, when a short-circuit fault occurs in the line or the bus, the magnetic flux of an inductance element in the system cannot be suddenly changed at the moment of the short-circuit, so that a direct-current component can occur in the short-circuit current, and the maximum value of the direct-current component is necessarily equal to the absolute value of the change amount of the periodic component of the short-circuit current. The system is at t 0 When a short-circuit fault occurs at a moment, the dc component IDC in the short-circuit current IDC can be expressed in the form of a simple exponential function:
wherein I is DC At the fault time t for the fault current 0 A direct current component at that time; t is the time after the start of the short circuit fault; τ is the dc time constant of the grid.
The existing electromagnetic transient state is adopted to calculate the short-circuit current under the corresponding fault, and the time constant obtained by carrying out corresponding mathematical treatment on the short-circuit current is more accurate. The following two methods are commonly used:
the method 1 comprises the steps of fitting an envelope curve of a current waveform, wherein the average value of an upper envelope curve and a lower envelope curve is a direct current component attenuation curve, and obtaining a time constant of a direct current component through exponential curve fitting, as shown in figure 1; where (∈) represents the upper envelope and (Δ) represents the lower envelope and (×) represents the fitted dc component.
Method 2, adopting a harmonic analysis method (usually adopting fast Fourier transform) to obtain a DC component attenuation curve, and adopting exponential curve fitting to obtain a time constant of the DC component, as shown in FIG. 2; wherein (x) represents an ac component and (Δ) represents a dc component.
However, some irregular problems still exist in the fitting process, so that the fitting curve is inaccurate, the calculated time constant and the actual error are large, and the circuit breaker is not beneficial to breaking short-circuit current.
Therefore, a method capable of accurately determining the time constant of the short-circuit current direct current component is required.
Disclosure of Invention
The invention provides a method and a system for calculating a theoretical value of a time constant of a short-circuit current direct-current component, which are used for solving the problem of how to accurately determine the time constant of the short-circuit current direct-current component.
In order to solve the above-mentioned problems, according to an aspect of the present invention, there is provided a method of calculating a theoretical value of a time constant of a direct current component of a short-circuit current, the method comprising:
fourier analyzing the short circuit current waveform to determine a peak value of an alternating current component of the short circuit current;
fitting an upper envelope curve and a lower envelope curve of the short-circuit current waveform to obtain a first direct-current component attenuation curve;
analyzing the short-circuit current waveform by using a harmonic analysis method to obtain a second direct current component attenuation curve;
respectively selecting curves in a preset time threshold period as a first direct current component waveform and a second direct current component waveform when the first direct current component attenuation curve and the second direct current component attenuation curve start to fail;
respectively performing exponential fitting on the first direct current component waveform and the second direct current component waveform to respectively obtain a first fitting formula and a second fitting formula which meet a preset format of a fitting principle;
and when the relative error of the first time constant and the second time constant in the first fitting formula and the second fitting formula is smaller than or equal to a first preset error threshold value, selecting a larger time constant as a theoretical value of the time constant.
Preferably, the value range of the preset time threshold is [60,150 ], and the unit is ms.
Preferably, the preset format formula is:
the fitting principle is as follows: i DC And I 0 The relative error of (2) is less than or equal to a second preset error threshold;
wherein I is DC For the moment of failure t 0 A direct current component of the fault current at the moment; t is the time after the fault begins; τ is a time constant; i 0 Is the peak value of the alternating component of the short-circuit current.
Preferably, wherein the method further comprises:
superposing the direct current component of the short-circuit current with the first direct current component waveform and the second direct current component waveform respectively to obtain a first superposition short-circuit current waveform and a second superposition short-circuit current waveform;
comparing the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform with the short-circuit current waveform respectively to obtain a comparison result; and if the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform and the short-circuit current waveform is greater than or equal to a preset coincidence threshold, the determined theoretical value of the time constant is accurate.
Preferably, wherein the method further comprises:
and when the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform or the second superimposed short-circuit current waveform and the short-circuit current waveform is smaller than a preset coincidence threshold value, indicating that the determined time constant theoretical value is inaccurate, and reselecting a new preset time threshold value for calculation.
Preferably, wherein the method further comprises:
when the acquired first fitting formula and second fitting formula do not meet the fitting principle; or when the relative error of the first time constant and the second time constant in the first fitting formula and the second fitting formula is larger than a first preset error threshold value, a new preset time threshold value is selected again for calculation.
According to another aspect of the present invention, there is provided a system for calculating a theoretical value of a time constant of a direct current component of a short-circuit current, the system comprising:
a peak value determining unit of alternating current component, which is used for carrying out Fourier analysis on the short circuit current waveform so as to determine the peak value of the alternating current component of the short circuit current;
the first direct current component attenuation curve acquisition unit is used for fitting the upper envelope curve and the lower envelope curve of the short-circuit current waveform to acquire a first direct current component attenuation curve;
the second direct current component attenuation curve acquisition unit is used for analyzing the short circuit current waveform by utilizing a harmonic analysis method so as to acquire a second direct current component attenuation curve;
the direct current component waveform determining unit is used for respectively selecting curves in a preset time threshold period as a first direct current component waveform and a second direct current component waveform when the first direct current component attenuation curve and the second direct current component attenuation curve start to fail;
the fitting formula obtaining unit is used for respectively carrying out exponential fitting on the first direct current component waveform and the second direct current component waveform so as to respectively obtain a first fitting formula and a second fitting formula which meet a preset format of a fitting principle;
and the time constant theoretical value determining unit is used for selecting a larger time constant as the time constant theoretical value when the relative error of the first time constant and the second time constant in the first fitting formula and the second fitting formula is smaller than or equal to a first preset error threshold value.
Preferably, the value range of the preset time threshold is [60,150 ], and the unit is ms.
Preferably, itThe preset format formula is as follows:
the fitting principle is as follows: i DC And I 0 The relative error of (2) is less than or equal to a second preset error threshold;
wherein I is DC For the moment of failure t 0 A direct current component of the fault current at the moment; t is the time after the fault begins; τ is a time constant; i 0 Is the peak value of the alternating component of the short-circuit current.
Preferably, wherein the system further comprises:
the verification unit is used for superposing the direct current component of the short-circuit current with the first direct current component waveform and the second direct current component waveform respectively so as to obtain a first superposition short-circuit current waveform and a second superposition short-circuit current waveform;
the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform are respectively compared with the short-circuit current waveform to obtain a comparison result; and if the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform and the short-circuit current waveform is greater than or equal to a preset coincidence threshold, the determined theoretical value of the time constant is accurate.
Preferably, the system further comprises:
the preset time threshold resetting unit is used for indicating that the determined time constant theoretical value is inaccurate when the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform or the second superimposed short-circuit current waveform and the short-circuit current waveform is smaller than a preset coincidence threshold value, and re-selecting a new preset time threshold value for calculation; or when the acquired first fitting formula and second fitting formula do not meet the fitting principle; or when the relative error of the first time constant and the second time constant in the first fitting formula and the second fitting formula is larger than a first preset error threshold, a new preset time threshold is selected again, and the direct current component waveform determining unit is returned.
The invention provides a method and a system for calculating a theoretical value of a time constant of a direct current component of a short circuit current, wherein the method comprises the following steps: determining a peak value of an alternating component of the short-circuit current; fitting an upper envelope curve and a lower envelope curve of the short-circuit current waveform to obtain a first direct-current component attenuation curve; analyzing the short-circuit current waveform by using a harmonic analysis method to obtain a second direct current component attenuation curve; determining a first direct current component waveform and a second direct current component waveform, and respectively performing exponential fitting to obtain a first fitting formula and a second fitting formula respectively; and selecting a larger time constant as a theoretical value of the time constant according to the first fitting formula and the second fitting formula. The invention determines the direct current component waveform to be fitted according to the preset time threshold value, and determines the time constant of the direct current component of the short circuit current according to the set rule of curve fitting of the direct current component of the short circuit current, thereby avoiding blindness of fitting formula selection; meanwhile, a method for checking whether the time constant calculation result is correct or not is provided, and the accuracy of the calculation result is ensured.
Drawings
Exemplary embodiments of the present invention may be more completely understood in consideration of the following drawings:
FIG. 1 is a schematic diagram of a method for determining a time constant of a DC component by fitting an upper envelope and a lower envelope of a short circuit current waveform;
FIG. 2 is a schematic diagram of a method for determining a DC component attenuation curve using a harmonic analysis method and determining a time constant of the DC component by DC curve fitting;
FIG. 3 is a flow chart of a method 300 of calculating a theoretical value of a time constant of a DC component of a short circuit current in accordance with an embodiment of the present invention;
FIG. 4 is a waveform diagram of a short circuit current according to an embodiment of the present invention;
FIG. 5a is a schematic diagram of a direct current component waveform fitting at 60ms failure time according to an embodiment of the present invention;
FIG. 5b is a schematic diagram of a direct current component waveform fitting at 120ms failure time according to an embodiment of the present invention;
FIG. 5c is a schematic diagram of a DC component waveform fitting at a failure time of 160ms according to an embodiment of the present invention;
fig. 6 is a graph comparing superimposed short circuit waveforms with initial short circuit current waveforms for a circuit breaker according to an embodiment of the present invention; and
fig. 7 is a schematic diagram of a system 700 for calculating a theoretical value of a time constant of a direct current component of a short circuit current according to an embodiment of the present invention.
Detailed Description
The exemplary embodiments of the present invention will now be described with reference to the accompanying drawings, however, the present invention may be embodied in many different forms and is not limited to the examples described herein, which are provided to fully and completely disclose the present invention and fully convey the scope of the invention to those skilled in the art. The terminology used in the exemplary embodiments illustrated in the accompanying drawings is not intended to be limiting of the invention. In the drawings, like elements/components are referred to by like reference numerals.
Unless otherwise indicated, terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art. In addition, it will be understood that terms defined in commonly used dictionaries should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense.
Fig. 3 is a flow chart of a method 300 of calculating a theoretical value of a time constant of a direct current component of a short circuit current according to an embodiment of the present invention. As shown in fig. 3, in the method for calculating the theoretical value of the time constant of the dc component of the short-circuit current according to the embodiment of the present invention, the waveform of the dc component to be fitted is determined according to the preset time threshold, and the time constant of the dc component of the short-circuit current is determined according to the set rule of curve fitting of the dc component of the short-circuit current, so that blindness of selecting a fitting formula can be avoided; meanwhile, a method for checking whether the time constant calculation result is correct or not is provided, and the accuracy of the calculation result is ensured. The method 300 of calculating a theoretical value of a time constant of a direct current component of a short circuit current provided by an embodiment of the present invention starts at step 301, where a fourier analysis is performed on a short circuit current waveform to determine a peak value of an alternating current component of the short circuit current at step 301.
In step 302, the upper envelope and the lower envelope of the short circuit current waveform are fitted to obtain a first dc component decay curve.
In the embodiment of the present invention, the method of acquiring the first dc component attenuation curve is the same as the principle of the method shown in fig. 1.
In step 303, the short-circuit current waveform is analyzed by using a harmonic analysis method to obtain a second dc component attenuation curve.
In the embodiment of the present invention, the method of acquiring the second dc component attenuation curve is the same as the principle of the method shown in fig. 2.
In step 304, curves within a preset time threshold period are selected as the first dc component waveform and the second dc component waveform when the first dc component attenuation curve and the second dc component attenuation curve start to fail. Preferably, the value range of the preset time threshold is [60,150 ], and the unit is ms.
For a preset time threshold, it may be determined according to a specific short circuit current waveform.
In the embodiment of the invention, in order to ensure the accuracy of the calculated time constant, the value range of the selected preset time threshold is [60,150 ], namely the value range is selected from the beginning of the fault, is not less than 60ms, is not more than 150ms and is selected between 80 and 120ms as much as possible.
For example, if the preset time threshold is selected to be 100, selecting a curve which is 100ms from the time of the fault moment to the time of 100ms from the first direct current component attenuation curve, and taking the curve as a first direct current component waveform; and selecting a curve which is 100ms from the second direct current component attenuation curve from the beginning of the fault time to the time of 100ms as a second direct current component waveform.
In step 305, exponential fitting is performed on the first dc component waveform and the second dc component waveform, so as to obtain a first fitting formula and a second fitting formula in a preset format that satisfy a fitting rule, respectively.
Preferably, the preset format formula is:
the fitting principle is as follows: i DC And I 0 The relative error of (2) is less than or equal to a second preset error threshold;
wherein I is DC For the moment of failure t 0 A direct current component of the fault current at the moment; t is the time after the fault begins; τ is a time constant; i 0 Is the peak value of the alternating component of the short-circuit current.
Preferably, wherein the method further comprises:
when the acquired first fitting formula and second fitting formula do not meet the fitting principle; or when the relative error between the first time constant and the second time constant in the first fitting formula and the second fitting formula is greater than the first preset error threshold, re-selecting a new preset time threshold, and returning to step 303 for calculation.
In an embodiment of the present invention, the second preset error threshold may be 5%, and the fitting principle is: i DC And I 0 The relative error of (2) is less than or equal to a second preset error threshold, i.e. guarantee I DC The value of which is close to the peak value I of the alternating current component 0 The difference between the two is approximately equal to the line operating current peak before the fault.
If I DC And I 0 If the relative error of (2) is greater than the second predetermined error threshold by 5%, then this indicates that the accuracy of the resulting time constant is low, and therefore the predetermined time threshold needs to be re-selected and re-calculated in step 303.
The value of the second preset error threshold value may be replaced by any other value, for example, 1%,6%,10%, etc.
In step 306, when the relative error between the first time constant and the second time constant in the first fitting formula and the second fitting formula is less than or equal to the first preset error threshold, a larger time constant is selected as the theoretical value of the time constant.
In the embodiment of the invention, the first preset error threshold value can be selected to be 10%, if the relative error between the first time constant and the second time constant is less than or equal to 10% of the first preset error threshold value, the accuracy of the calculated time constant meets the requirement, and the theoretical value of the time constant can be determined; otherwise, it indicates that the accuracy of the theoretical value of the time constant determined by this method is low, and a new preset time threshold needs to be selected again, and the calculation is performed again by returning to step 303.
The value of the first preset error threshold value 10% may be replaced by any other value, for example, 2%,5%, etc.
Preferably, wherein the method further comprises:
superposing the direct current component of the short-circuit current with the first direct current component waveform and the second direct current component waveform respectively to obtain a first superposition short-circuit current waveform and a second superposition short-circuit current waveform;
comparing the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform with the short-circuit current waveform respectively to obtain a comparison result; and if the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform and the short-circuit current waveform is greater than or equal to a preset coincidence threshold, the determined theoretical value of the time constant is accurate.
Preferably, wherein the method further comprises:
and when the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform or the second superimposed short-circuit current waveform and the short-circuit current waveform is smaller than a preset coincidence threshold value, indicating that the determined time constant theoretical value is inaccurate, and re-selecting a new preset time threshold value to return to the step 303 for calculation.
In the embodiment of the invention, the accuracy of the determined time constant theoretical value can be checked according to the comparison result of the superimposed short-circuit current waveform and the original short-circuit current waveform, so that the accuracy of the calculation result is ensured from two aspects.
If the two short-circuit current waveforms are coincident or similar, the calculation result is accurate, and the theoretical time constant value is the larger one of the first time constant and the second time constant; otherwise, a correction is performed by changing the fitted time axis range, i.e. re-selecting a new preset time threshold value, and returning to step 303 for calculation.
For example, as shown in fig. 4, a short-circuit current waveform of a certain system is shown, the fault occurs at 0.6s, and the steady-state value thereof is 72kA (peak value). And analyzing the short-circuit current waveform by using a harmonic analysis method by adopting the method shown in the step 303, and obtaining a direct current component attenuation curve. Then, the dc component waveforms are obtained and fitted by 60ms, 120m, and 160ms, respectively, from the start of the fault, and the fitting curves and fitting formulas shown in fig. 5a, 5b, and 5c are obtained, respectively.
As can be seen by comparing fig. 5a, 5b and 5c, the time axis ranges are different (i.e. the selected preset time thresholds are different), the obtained fitting formulas are also different, the largest direct current component is 84.495kA in fig. 5a, and the smallest direct current component is 70.57kA in fig. 5 c; the largest time constant is 1/12.09=83 ms in fig. 5c, and the smallest time constant is 1/16.76=60 ms in fig. 5 a. It can be seen that the time ranges are different, and the resulting time differences are very different.
From theoretical analysis, the maximum value of the DC component is necessarily equal to the absolute value of the change of the short-circuit current period component, thus I DC The value of (2) should be close to the peak value I of the AC component 0 (72 kA). Therefore, it is reasonable to take 120ms for the time frame of fig. 5 b.
When the obtained time constant is verified, firstly, an alternating current component and a direct current component of the short-circuit current are overlapped to form an overlapped short-circuit current waveform, and then the overlapped short-circuit current waveform is compared with the original short-circuit current in fig. 2, and the comparison result is shown in fig. 6. As can be seen from fig. 6, the superimposed short-circuit current waveform is relatively close to the calculated short-circuit current waveform, and therefore, the obtained time constant can be determined accurately.
Fig. 7 is a schematic diagram of a system 700 for calculating a theoretical value of a time constant of a direct current component of a short circuit current according to an embodiment of the present invention. As shown in fig. 7, a system 700 for calculating a theoretical value of a time constant of a dc component of a short-circuit current according to an embodiment of the present invention includes: a peak value determination unit 701 of an alternating current component, a first direct current component attenuation curve acquisition unit 702, a second direct current component attenuation curve acquisition unit 703, a direct current component waveform determination unit 704, a fitting formula acquisition unit 705, and a time constant theoretical value determination unit 706.
Preferably, the peak value determining unit 701 is configured to perform fourier analysis on the short-circuit current waveform to determine a peak value of the alternating-current component of the short-circuit current.
Preferably, the first dc component attenuation curve obtaining unit 702 is configured to fit an upper envelope and a lower envelope of the short-circuit current waveform to obtain a first dc component attenuation curve.
Preferably, the second dc component attenuation curve obtaining unit 703 is configured to analyze the short-circuit current waveform by using a harmonic analysis method, so as to obtain a second dc component attenuation curve.
Preferably, the dc component waveform determining unit 704 is configured to select, as the first dc component waveform and the second dc component waveform, curves of the first dc component attenuation curve and the second dc component attenuation curve within a preset time threshold period when the fault starts, respectively.
Preferably, the value range of the preset time threshold is [60,150 ], and the unit is ms.
Preferably, the fitting formula obtaining unit 705 is configured to perform exponential fitting on the first dc component waveform and the second dc component waveform, so as to obtain a first fitting formula and a second fitting formula in a preset format that satisfy a fitting rule, respectively.
Preferably, the preset format formula is:
the fitting principle is as follows: i DC And I 0 The relative error of (2) is less than or equal to a second preset error threshold;
wherein I is DC For the moment of failure t 0 A direct current component of the fault current at the moment; t is tThe time after the fault starts; τ is a time constant; i 0 Is the peak value of the alternating component of the short-circuit current.
Preferably, the theoretical time constant value determining unit 706 is configured to select a larger time constant as the theoretical time constant value when a relative error between the first time constant and the second time constant in the first fitting formula and the second fitting formula is less than or equal to a first preset error threshold.
Preferably, wherein the system further comprises: the verification unit is used for superposing the direct current component of the short-circuit current with the first direct current component waveform and the second direct current component waveform respectively so as to obtain a first superposition short-circuit current waveform and a second superposition short-circuit current waveform; the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform are respectively compared with the short-circuit current waveform to obtain a comparison result; and if the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform and the short-circuit current waveform is greater than or equal to a preset coincidence threshold, the determined theoretical value of the time constant is accurate.
Preferably, the system further comprises: the preset time threshold resetting unit is used for indicating that the determined time constant theoretical value is inaccurate when the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform or the second superimposed short-circuit current waveform and the short-circuit current waveform is smaller than a preset coincidence threshold value, and re-selecting a new preset time threshold value for calculation; or when the acquired first fitting formula and second fitting formula do not meet the fitting principle; or when the relative error of the first time constant and the second time constant in the first fitting formula and the second fitting formula is larger than a first preset error threshold, a new preset time threshold is selected again, and the direct current component waveform determining unit is returned.
The system 700 for calculating the theoretical value of the time constant of the dc component of the short-circuit current according to the embodiment of the present invention corresponds to the method 300 for calculating the theoretical value of the time constant of the dc component of the short-circuit current according to another embodiment of the present invention, and will not be described herein.
The invention has been described with reference to a few embodiments. However, as is well known to those skilled in the art, other embodiments than the above disclosed invention are equally possible within the scope of the invention, as defined by the appended patent claims.
Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise therein. All references to "a/an/the [ means, component, etc. ]" are to be interpreted openly as referring to at least one instance of said means, component, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.
Claims (8)
1. A method of calculating a theoretical value of a time constant of a direct current component of a short circuit current, the method comprising:
fourier analyzing the short circuit current waveform to determine a peak value of an alternating current component of the short circuit current;
fitting an upper envelope curve and a lower envelope curve of the short-circuit current waveform to obtain a first direct-current component attenuation curve;
analyzing the short-circuit current waveform by using a harmonic analysis method to obtain a second direct current component attenuation curve;
respectively selecting curves in a preset time threshold period as a first direct current component waveform and a second direct current component waveform when the first direct current component attenuation curve and the second direct current component attenuation curve start to fail;
respectively performing exponential fitting on the first direct current component waveform and the second direct current component waveform to respectively obtain a first fitting formula and a second fitting formula which meet a preset format of a fitting principle;
when the relative error of the first time constant and the second time constant in the first fitting formula and the second fitting formula is smaller than or equal to a first preset error threshold value, a larger time constant is selected as a theoretical value of the time constant;
wherein the method further comprises:
superposing the direct current component of the short-circuit current with the first direct current component waveform and the second direct current component waveform respectively to obtain a first superposition short-circuit current waveform and a second superposition short-circuit current waveform;
comparing the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform with the short-circuit current waveform respectively to obtain a comparison result; if the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform with the short-circuit current waveform is greater than or equal to a preset coincidence threshold value, the determined theoretical value of the time constant is accurate;
and when the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform or the second superimposed short-circuit current waveform and the short-circuit current waveform is smaller than a preset coincidence threshold value, indicating that the determined time constant theoretical value is inaccurate, and reselecting a new preset time threshold value for calculation.
2. The method of claim 1, wherein the preset time threshold has a value in ms in a range of [60,150 ].
3. The method of claim 1, wherein the pre-set format formula is:
the fitting principle is as follows: i DC And I 0 The relative error of (2) is less than or equal to a second preset error threshold;
wherein I is DC For the moment of failure t 0 A direct current component of the fault current at the moment; t is the time after the fault begins; τ is a time constant; i 0 Is the peak value of the alternating component of the short-circuit current.
4. The method according to claim 1, wherein the method further comprises:
when the acquired first fitting formula and second fitting formula do not meet the fitting principle; or when the relative error of the first time constant and the second time constant in the first fitting formula and the second fitting formula is larger than a first preset error threshold value, a new preset time threshold value is selected again for calculation.
5. A system for calculating a theoretical value of a time constant of a direct current component of a short circuit current, the system comprising:
a peak value determining unit of alternating current component, which is used for carrying out Fourier analysis on the short circuit current waveform so as to determine the peak value of the alternating current component of the short circuit current;
the first direct current component attenuation curve acquisition unit is used for fitting the upper envelope curve and the lower envelope curve of the short-circuit current waveform to acquire a first direct current component attenuation curve;
the second direct current component attenuation curve acquisition unit is used for analyzing the short circuit current waveform by utilizing a harmonic analysis method so as to acquire a second direct current component attenuation curve;
the direct current component waveform determining unit is used for respectively selecting curves in a preset time threshold period as a first direct current component waveform and a second direct current component waveform when the first direct current component attenuation curve and the second direct current component attenuation curve start to fail;
the fitting formula obtaining unit is used for respectively carrying out exponential fitting on the first direct current component waveform and the second direct current component waveform so as to respectively obtain a first fitting formula and a second fitting formula which meet a preset format of a fitting principle;
the time constant theoretical value determining unit is used for selecting a larger time constant as a time constant theoretical value when the relative error of the first time constant and the second time constant in the first fitting formula and the second fitting formula is smaller than or equal to a first preset error threshold value;
wherein the system further comprises:
the verification unit is used for superposing the direct current component of the short-circuit current with the first direct current component waveform and the second direct current component waveform respectively so as to obtain a first superposition short-circuit current waveform and a second superposition short-circuit current waveform; comparing the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform with the short-circuit current waveform respectively to obtain a comparison result; if the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform and the second superimposed short-circuit current waveform with the short-circuit current waveform is greater than or equal to a preset coincidence threshold value, the determined theoretical value of the time constant is accurate;
and the preset time threshold resetting unit is used for indicating that the determined theoretical value of the time constant is inaccurate when the comparison result indicates that the coincidence rate of the first superimposed short-circuit current waveform or the second superimposed short-circuit current waveform and the short-circuit current waveform is smaller than a preset coincidence threshold value, and re-selecting a new preset time threshold value for calculation.
6. The system of claim 5, wherein the predetermined time threshold has a value in ms in the range of [60,150 ].
7. The system of claim 5, wherein the pre-set format formula is:
the fitting principle is as follows: i DC And I 0 The relative error of (2) is less than or equal to a second preset error threshold;
wherein I is DC For the moment of failure t 0 A direct current component of the fault current at the moment; t is the time after the fault begins; τ is a time constant; i 0 Is the peak value of the alternating component of the short-circuit current.
8. The system of claim 5, wherein the system further comprises:
the preset time threshold resetting unit is used for judging whether the acquired first fitting formula and second fitting formula do not meet the fitting principle; or when the relative error of the first time constant and the second time constant in the first fitting formula and the second fitting formula is larger than a first preset error threshold, a new preset time threshold is selected again, and the direct current component waveform determining unit is returned.
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