Flexible dc transmission line fault discrimination method based on transient voltage waveform comparison

Abstract

The application discloses a flexible direct current transmission line fault discrimination method based on transient voltage waveform comparison, and comprises the following steps: based on a modular multilevel converter flexible direct current transmission system, calculating the first-line wave change rate waveform of a typical area internal and external fault line module voltage; performing normalization processing on the first-line wave change rate waveform of the typical area internal and external fault line module voltage to obtain an internal and external fault reference waveform; measuring the positive and negative electrode voltage and current of a line at a first end measuring point; calculating the first-line wave change rate of the measured line module voltage at the first end measuring point; performing normalization processing on the first-line wave change rate waveform of the line module voltage to obtain a waveform to be compared; calculating the distance between the waveform to be compared and the internal and external fault reference waveform; if the distance between the waveform to be compared and the internal fault reference waveform is smaller, the fault is judged as an internal fault, otherwise, the fault is judged as an external fault. The method has good internal and external fault distinguishing degree, high sensitivity, and overcomes the defect that the existing internal and external fault tolerance transition resistance ability is poor.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power system DC transmission, in particular to a flexible DC transmission line fault discrimination method based on transient voltage waveform comparison. BACKGROUND

[0002] The flexible DC transmission technology based on modular multilevel converter (MMC) is flexible and controllable, has low harmonic content, and has no commutation failure problem, and can be used to large-scale consumption of new energy, and has been widely used in the world. However, the damping of the flexible DC power grid is small, and after a short-circuit fault occurs on the DC side, the sub-module capacitor of the converter station discharges rapidly to the fault point, and the fault current increases rapidly, and within a few ms, the fault current will reach several times or even dozens of times of the rated current of the power electronic device, which will cause overcurrent damage to the valve group and other primary equipment of the converter, which requires the main protection to act quickly to isolate the fault, and the rapid identification of internal and external faults is the basis for the protection of the DC line. Therefore, the rapid and reliable identification of internal and external faults of the flexible DC line is a key problem to be solved.

[0003] The existing method usually utilizes the difference between the electrical information of the two converter stations at the ends of the line in internal and external faults, such as the correlation characteristics of the calculated voltage and the measured voltage based on the current, the amplitude characteristics of the forward and reverse traveling waves at the two sides of the line, etc., but this method requires communication between the converter stations, which is not convenient.

[0004] In view of this, a flexible DC transmission line fault discrimination method based on transient voltage waveform comparison is needed. SUMMARY

[0005] In view of the problem in the prior art that the difference between the electrical information of the two converter stations at the ends of the line in internal and external faults requires communication between the converter stations, which is not convenient, the present application provides a flexible DC transmission line fault discrimination method based on transient voltage waveform comparison, which can only identify internal and external faults based on local information, does not require communication between the converter stations, has good speed and reliability, has strong resistance to transition resistance, can effectively distinguish between internal end high resistance faults and external faults, and the specific technical scheme is as follows:

[0006] A flexible DC transmission line fault discrimination method based on transient voltage waveform comparison, comprising the following steps:

[0007] Based on the simplified model of the flexible DC system, the line-mode voltage first traveling wave change rate expressions of typical internal faults and external faults are calculated respectively;

[0008] The calculated line-mode voltage first traveling wave change rate waveform is normalized, and the processed waveform is the reference waveform;

[0009] The positive and negative line voltages and currents at the protection measurement point at one end of the line are measured;

[0010] The line-mode components of the voltage and current at the measurement point are obtained using the phase-mode to line-mode transformation matrix;

[0011] The line-mode fault voltage traveling wave is calculated, the first traveling wave waveform is obtained, and after the change thereof is calculated, normalization processing is performed to obtain a waveform to be compared;

[0012] The Euclidean distance is used to calculate the distance between the waveform to be compared and the reference waveform of the internal and external faults, and if the distance between the waveform to be compared and the reference waveform of the internal faults is smaller, it is judged as an internal fault, and vice versa, as an external fault;

[0013] Preferably, taking the positive pole ground fault of the two-end flexible DC line as an example, the MMC converter station is equivalent to an RCL series model, the DC line adopts the Bergeron model, and the simplified model of the system when the midpoint fault of the line occurs is as shown in Figure 2 .

[0014] Taking the positive pole ground fault of the two-end flexible DC line as an example, the calculation method of the first traveling wave change rate waveform of the line-mode voltage of the internal fault is as follows:

[0015]

[0016]

[0017]

[0018]

[0019]

[0020]

[0021] wherein, U R1 , I R1 the line-mode voltage and the line-mode current at the protection measurement point, respectively, A 1( s ) is a line transfer function, U b1_in ( s ) is a complex frequency domain expression of the first traveling wave of the line-mode voltage at the protection measurement point, and the time domain expression obtained by Laplace inverse transformation is U b1_in ( t ), and after derivation, the change rate waveform U’ b1_in ( t ) is a typical internal fault waveform, Ldc is the line flat wave inductance, Z C1 is the DC line line-mode wave impedance, Z C0 is the DC line ground-mode wave impedance, B t12_1 , B f12_1 , B f21_1 , B t21_1 is the traveling wave equivalent line-mode voltage source, B t12_0 , B f12_0 , B f21_0 , B t21_0 is the traveling wave equivalent ground-mode voltage source, Z MMC1 and Z MMC2 is the equivalent impedance of the converter stations at both ends of the line, R f is the fault resistance, is the fault additional voltage source of the in-zone fault;

[0022] The calculation method of the first traveling wave variation rate waveform of the line-mode voltage of the out-of-zone fault is as follows:

[0023]

[0024]

[0025]

[0026]

[0027] wherein, U R1 , I R1 are the line-mode voltage and the line-mode current at the protection measuring point respectively, A 1( s ) is the line transfer function, U b1_out ( s ) is the complex frequency domain expression of the first traveling wave of the line-mode voltage at the protection measuring point, and the time domain expression is obtained by Laplace inverse transformation U b1_out ( t ), and the variation rate waveform is obtained after derivation U’ b1_out ( t ) is a typical out-of-zone fault waveform, An equivalent voltage source is added to the fault for out-of-zone faults.

[0028] Preferably, a linear function is used to normalize the typical in-zone and out-of-zone fault voltage first-wave waveforms, specifically:

[0029]

[0030] wherein, U’ b1_in ( t ) and U’ b1_out ( t ) are the typical in-zone and out-of-zone fault line-mode voltage first-waves calculated above, U b1_in_std ( t ) and U b1_out_std ( t ) are the normalized waveforms, i.e. the in-zone and out-of-zone fault reference waveforms.

[0031] Preferably, the line-mode voltage first-wave change rate waveform is calculated using the following method based on the positive and negative line voltages and currents measured at the line first end:

[0032]

[0033]

[0034]

[0035]

[0036]

[0037] wherein, U p , U n are the positive and negative line voltages measured at the measurement point, I p , I n are the positive and negative line currents measured at the measurement point, T is a phase-mode transformation matrix, Z C1 is the line-mode wave impedance of the DC line, U b1 ( t ) is the line-mode voltage first-wave, U’ b1 ( t ) is the line-mode voltage first-wave change rate waveform.

[0038] Preferably, the linear mode voltage measured at the above-mentioned measuring point is normalized to obtain a to-be-compared waveform, and the Euclidean distance between the to-be-compared waveform and the in-zone and out-zone reference waveform is calculated, as follows:

[0039]

[0040]

[0041] wherein D in is the distance between the to-be-compared waveform and the in-zone reference waveform, D out is the distance between the to-be-compared waveform and the out-zone reference waveform.

[0042] By comparing D in and D out , the in-zone and out-zone faults are determined.

[0043]

[0044] Compared with the prior art, the present application has the following advantages and beneficial effects:

[0045] 1) The smoothing reactor of the DC system is a natural physical boundary, which presents a band-stop characteristic to high-frequency components, and the first-wave distortion of the fault voltage received at the local end is serious, and the characteristic difference between the first-wave of the fault voltage and the in-zone fault voltage is significant.

[0046] 2) The waveform data is limited to [-1, 0] by using the normalization method, and the action threshold does not need to be reset for different voltage grade systems, and the principle is simple.

[0047] 3) Through simulation test, the effectiveness and reliability of the proposed protection scheme are verified. It can accurately identify different types of in-zone and out-zone faults, and has strong resistance to transition resistance.

[0048] 4) The discrimination of in-zone and out-zone faults is based only on local information, and does not require communication between converter stations, and has good speed and reliability. BRIEF DESCRIPTION OF DRAWINGS

[0049] In order to more clearly illustrate the technical solutions in the specific embodiments or the prior art, the drawings needed to be used in the specific embodiments or the prior art description will be briefly introduced below. In all the drawings, similar elements or parts are generally identified by similar reference numerals. In the drawings, the elements or parts are not necessarily drawn according to the actual proportions.

[0050] Figure 1 is a topological structure diagram of a symmetrical bipolar MMC flexible DC transmission system;

[0051] Figure 2 For f in System simplified model diagram in positive electrode ground fault;

[0052] Figure 3 For f out System simplified model diagram in positive electrode ground fault;

[0053] Figure 4 Flexible DC transmission line area internal and external fault discrimination method flow chart based on line mode voltage first traveling wave distance. DETAILED DESCRIPTION

[0054] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0055] It should be understood that the terms "comprising" and "including" as used in the specification and the appended claims indicate the presence of the described features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof.

[0056] It should also be understood that the terms used in the present application specification are only for the purpose of describing specific embodiments and are not intended to limit the present application. As used in the present application specification and the appended claims, the singular forms "a", "an" and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0057] It should be further understood that the term "and / or" as used in the present application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations thereof, and includes these combinations.

[0058] The present application is a flexible DC line area internal and external fault discrimination method, which aims to solve the problem of discriminating between internal and external faults of a flexible DC transmission line. By solving the line mode voltage first traveling wave change rate expression at the measurement point when the DC line is faulty, normalizing the expression, and calculating the distance between the expression and the internal and external fault reference waveforms respectively, the internal and external faults are accurately discriminated by comparing the two distances.

[0059] Embodiment:

[0060] Take the symmetric bipolar MMC flexible DC transmission system as an example, the analysis of fault voltage traveling wave is carried out, and the system topological structure is as shown in Figure 1 The DC transmission line is a bipolar overhead line, L dc The current limiting reactor is used to suppress the fault current rise rate, and M and N are the protection installation positions at both ends of the DC line. Figure 1 The fault point of the reference waveform of the fault in the calculation area and the fault outside the area is given, wherein, f in The fault point in the area is, f out The MMC1 side fault point outside the area is.

[0061] A flexible DC transmission line fault discrimination method based on transient voltage waveform comparison specifically includes the following steps:

[0062] S1, draw f in and f out The simplified model of the fault system at the time of fault is calculated, and the first traveling wave change rate waveform of the line mode voltage is calculated.

[0063] In one embodiment, f in The simplified model of the system with positive ground fault at is as shown in Figure 2 In the figure, L dc is the line flat wave inductance, Z C1 is the DC line mode wave impedance, Z C0 is the DC line ground mode wave impedance, B t12_1 , B f12_1 , B f21_1 , B t21_1 is the traveling wave equivalent line mode voltage source, B t12_0 , B f12_0 , B f21_0 , B t21_0 is the traveling wave equivalent ground mode voltage source, Z MMC1 and Z MMC2 is the equivalent impedance of the converter station at both ends of the line, R f is the fault resistance, is the fault additional voltage source of the fault in the area. The calculation process of the first traveling wave change rate of the line mode voltage is as follows:

[0064]

[0065]

[0066]

[0067]

[0068]

[0069]

[0070] where, U R1 , I R1 are the line mode voltage and line mode current at the protection point respectively, A 1( s ) is the line transfer function, U b1_in ( s ) is the complex frequency domain expression of the first traveling wave of the line mode voltage at the protection point, and the time domain expression is obtained by Laplace inverse transform U b1_in ( t ), and the derivative is obtained U’ b1_in ( t ), that is, the fault waveform in the typical area.

[0071] In one embodiment, f out The system simplified model of the positive pole ground fault at Figure 3 is shown in the figure, in which L dc is the line flat wave inductance, Z C1 is the DC line mode wave impedance, Z C0 is the DC line ground mode wave impedance, B t12_1 , B t21_0 are the traveling wave equivalent line mode voltage source and the traveling wave equivalent ground mode voltage source respectively, Z MMC1 and Z MMC2 are the equivalent impedance of the converter station at both ends of the line, R f is the fault resistance, is the fault additional equivalent voltage source of the external fault. The calculation process of the first traveling wave rate of change of the line mode voltage is as follows:

[0072]

[0073]

[0074]

[0075]

[0076]

[0077]

[0078] wherein, U R1 , I R1 are the line-mode voltage and line-mode current at the protection point respectively, A 1( s ) is the line transfer function, U b1_out ( s ) is the complex frequency domain expression of the first traveling wave of the line-mode voltage at the protection point, and the time domain expression is obtained by inverse Laplace transform U b1_out ( t ), and the derivative is obtained to obtain the line-mode voltage first traveling wave rate waveform U’ b1_out ( t ), that is, the typical in-zone fault waveform.

[0079] S2, the linear function normalization is used to normalize the typical in-zone and out-zone fault waveforms calculated above.

[0080] In one embodiment, the linear function normalization will be used to normalize the in-zone line-mode voltage first traveling wave rate waveform U b1_in ( t ) and the out-zone line-mode voltage first traveling wave rate waveform U b1_out ( t ):

[0081]

[0082] wherein, U’ b1_in ( t ) and U’ b1_out ( t ) are the typical in-zone and out-zone fault waveforms calculated above, U b1_in_std ( t ), Ub1_out_std t ) is the normalized waveform, i.e. the in-zone and out-zone fault reference waveform.

[0083] S3, according to the positive and negative line voltage and current measured at the protection measurement point M, the normalized line-mode voltage first-wave change rate waveform is calculated.

[0084] In an embodiment, taking the MMC converter station measurement point M as an example, the positive and negative line voltages measured are U p U n , and the positive and negative line currents are I p I n After phase-mode transformation, the line-mode voltage U 1 and the line-mode current I 1 are obtained, and the line-mode voltage first-wave U b1 t is calculated, and the line-mode voltage first-wave change rate waveform U’ b1 t is obtained by derivation.

[0085]

[0086]

[0087]

[0088]

[0089]

[0090] In the formula, Z C1 is the line-mode wave impedance of the DC line, U b1 t is the fault voltage first-wave, U’ b1 t is the line-mode voltage first-wave change rate waveform.

[0091] Normalization processing is performed on it:

[0092]

[0093] S4, the Euclidean distance is used to calculate the distance between the to-be-compared and in-zone and out-zone reference waveforms.

[0094] ​​​​​​​

[0095] wherein D in is the distance between the first traveling wave of the fault voltage and the reference waveform in the area, D out is the distance between the first traveling wave of the fault voltage and the reference waveform outside the area.

[0096] S5, according to the waveform distance D in , D out determine whether the fault is an internal fault or an external fault:

[0097]

[0098] So far, the application proposes a flexible DC transmission line fault discrimination method based on transient voltage waveform comparison, and the specific process is as shown in Figure 4 , which comprises:

[0099] (1) calculating the first traveling wave change rate waveform of the line mode voltage of typical internal and external faults;

[0100] (2) normalizing the first traveling wave change rate waveform of the line mode voltage of typical internal and external faults to obtain the reference waveform of internal and external faults;

[0101] (3) calculating the first traveling wave change rate of the line mode voltage according to the measured positive and negative line voltages and currents at the measurement point, and obtaining the waveform to be compared after normalization processing;

[0102] (4) using the Euclidean distance to calculate the distance between the waveform to be compared and the reference waveform of internal and external faults;

[0103] (5) if the distance between the waveform to be compared and the reference waveform of internal fault is smaller, it is judged as an internal fault; otherwise, it is an external fault.

[0104] The application will be further described below through a specific simulation example.

[0105] This embodiment builds a double-ended flexible DC transmission system model as shown in Figure 1 for simulation test in PSCAD / EMTDC. The converter station adopts MMC model, and the specific model parameters are shown in Table 1. The DC transmission line adopts frequency variable parameter model, and the system sampling frequency is selected as 10 kHz.

[0106] Table 1 Model parameters

[0107]

[0108] A protection algorithm was developed on the MATLAB platform, and fault simulation data from the established PSCAD model was imported to verify the ability to distinguish between faults inside and outside the fault zone. Different fault types and different transition resistances were set at different locations on the DC transmission line TLine for both inside and outside fault zones. The simulation results are shown in Table 2.

[0109] Table 2 Fault identification results inside and outside the zone

[0110]

[0111]

[0112]

[0113] Table 2 shows that, for different fault distances and different transition resistances under fault conditions within the zone, the distance between the first traveling wave of the line-mode voltage received at line measurement point M and the reference waveform within the zone is... D in All are less than their distance from the reference waveform outside the region. D out All faults can be accurately identified as faults within the zone; for faults outside the zone located after the smoothing reactor at the end of the DC line, the distance between the first traveling wave of the line-mode voltage received at line measurement point M and the reference waveform within the zone is... D in All are greater than their distance from the reference waveform outside the region. D out All of these methods can accurately identify faults occurring outside the designated area. Therefore, the proposed protection scheme is largely unaffected by transition resistance and fault distance.

[0114] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention, and they should all be covered within the scope of the claims and specification of the present invention.

Claims

1. A method for HVDC line fault detection based on comparison of transient voltage waveforms, characterized in that, The method comprises the following steps: Based on the simplified model of the flexible DC system, the expression of the first traveling wave change rate of the line-mode voltage under the typical in-zone fault and out-zone fault is calculated respectively; The calculated line-mode voltage first traveling wave change rate waveform is normalized, and the processed waveform is the reference waveform; The positive and negative line voltages and currents at the protection measurement point of one end of the line are measured; The line-mode components of the voltage and current at the measurement point are obtained by using the phase-mode transformation matrix; The line-mode fault voltage traveling wave is calculated, the first traveling wave data is extracted, the first traveling wave change rate information is solved, and then normalized processing is performed to obtain the waveform to be compared; The distance between the waveform to be compared and the reference waveform is calculated by using the Euclidean distance, if the distance between the waveform to be compared and the in-zone reference waveform is smaller than the distance between the waveform to be compared and the out-zone reference waveform, it is judged as an in-zone fault, otherwise, it is judged as an out-zone fault; Taking the positive ground fault of the two-end flexible DC line as an example, the calculation method of the first traveling wave change rate waveform of the line-mode voltage under the in-zone fault is as follows: wherein, U R1 , I R1 are the line mode voltage and line mode current at the protection point, respectively, A 1( s ) is the line transfer function, U b1_in ( s ) is the complex frequency domain expression of the first arriving wave of the line mode voltage at the protection point, and the time domain expression is obtained by inverse Laplace transform U b1_in ( t ), and the rate of change waveform is obtained after derivation U’ b1_in ( t ) is the typical fault waveform, L dc is the line flat wave inductance, Z C1 is the DC line mode wave impedance, Z C0 is the DC line ground mode wave impedance, B t12_1 , B f12_1 , B f21_1 , B t21_1 is the equivalent line mode voltage source of the traveling wave, B t12_0 , B f12_0 , B f21_0 , B t21_0 is the equivalent ground mode voltage source of the traveling wave, Z MMC1 and Z MMC2 are the equivalent impedances of the converter stations at both ends of the line, R f is the fault resistance, is the fault additional voltage source of the in-zone fault; The calculation method of the first traveling wave change rate waveform of the line-mode voltage under the out-zone fault is as follows: wherein, U R1 , I R1 are the line-mode voltage and line-mode current at the protection point, respectively, U b1_out ( s ) is the complex frequency domain expression of the first wave of the line-mode voltage at the protection point, and the time domain expression is obtained by inverse Laplace transform U b1_out ( t ), and the rate of change waveform is obtained after derivation U’ b1_out ( t ) is the typical out-of-zone fault waveform, is the fault additional equivalent voltage source of the out-of-zone fault.

2. A method for HVDC line fault detection based on transient voltage waveform comparison as claimed in claim 1, wherein, The linear function normalization is used to normalize the first traveling wave change rate waveform of the typical in-zone and out-zone fault voltage, and the normalization is specifically as follows: wherein, U’ b1_in ( t ) and U’ b1_out ( t ) are the first wave change rate waveforms of the typical in-zone and out-of-zone fault line-mode voltages calculated above, U b1_in_std ( t ), U b1_out_std ( t ) are the normalized waveforms, i.e. the in-zone and out-of-zone fault reference waveforms.

3. A method for HVDC line fault detection based on transient voltage waveform comparison as claimed in claim 1, wherein, According to the positive and negative line voltages and currents measured at the first end of the line, the line-mode voltage first traveling wave change rate waveform is calculated by using the following method: in, U p , U n The positive and negative line voltages measured at the measurement point. I p , I n The positive and negative line currents measured at the measurement point. T The phase mode transformation matrix, Z C1 This is the line-mode impedance of the DC line. U b1 ( t () represents the first traveling wave of the line-mode voltage. U’ b1 ( t The waveform represents the rate of change of the first traveling wave of the line-mode voltage.

4. A method of HVDC line fault detection based on comparison of transient voltage waveforms according to claim 3, characterized in that, The measured line-mode voltage first traveling wave change rate waveform is normalized to obtain the waveform to be compared, and the normalization is specifically as follows: 。 5. A method of HVDC line fault detection based on comparison of transient voltage waveforms according to claim 4, characterized in that, The distance between the waveform to be compared and the in-zone and out-zone reference waveform is calculated by using the Euclidean distance, and the calculation is specifically as follows: wherein D in is the distance of the waveform to be compared to the reference waveform in the region, D out is the distance of the waveform to be compared to the reference waveform outside the region.

6. A method of HVDC line fault detection based on comparison of transient voltage waveforms according to claim 5, characterized in that, By comparing D in and D out the size of the area outside the fault, the specific as follows: 。

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