Fault discrimination method between alternating current and direct current systems for hybrid overhead line

Abstract

The invention relates to the technical field of electrical automation, in particular to an alternating current and direct current system fault judgment method for a hybrid overhead line, which comprises the following steps of: acquiring three-phase current and voltage data of a single end of an alternating current line in a hybrid overhead line system; judging a fault judgment starting criterion between the alternating current system and the direct current system by detecting zero-sequence current abnormity; intercepting three-phase voltage data after the starting criterion is established, and locking a fault phase through voltage jump; intercepting three-phase current data after the starting criterion is established, and separating a direct current component from fault phase current; and distinguishing an AC grounding fault and an AC-DC system fault based on the DC component attenuation ratio. The method comprises the following steps of: extracting a direct-current component of transient current of an alternating-current line through mathematical morphology (MM), and judging whether a fault is between an alternating-current system and a direct-current system based on evaluation on a direct-current level; reliable judgment can be completed within 30 ms by only utilizing information of a single end of an alternating current line without depending on communication, and the method is high in practicability.

Description

Technical Field

[0001] The present invention relates to the technical field of electrical automation, and particularly to a method for discriminating faults between AC and DC systems for a hybrid overhead line. Background Art

[0002] With the rapid increase in power supply capacity, continuous growth in power consumption demand, as well as increasingly tight resources and energy and urgent environmental protection requirements, it has become an urgent task to improve the transmission capacity of key sections of the regional power grid. To achieve this goal, it is usually necessary to consider building a large number of new transmission lines or transforming existing transmission lines. However, affected by multiple factors such as terrain conditions and policy restrictions, new transmission channels often face challenges such as high implementation difficulty and high investment costs. An effective solution is to upgrade the existing key transmission channels within the regional power grid to high-voltage DC lines, thereby constructing an AC-DC hybrid overhead line system.

[0003] In the current power transmission system, both the high-voltage DC system and the high-voltage AC transmission system are equipped with dedicated protection systems to identify and handle various faults on the transmission line. The main line protection of the high-voltage DC system mainly includes traveling wave protection and sudden change protection, which can respond quickly within a few milliseconds after a fault occurs. At the same time, the system is also equipped with highly reliable pilot differential protection and low-voltage protection as backup protection. The line protection in the high-voltage AC system mainly adopts current protection, distance protection, and pilot differential protection, etc.

[0004] For the fault scenarios of a single DC or AC system, existing research has deeply analyzed the performance and principles of the respective protections of the AC-DC systems; however, with the emergence of the AC-DC hybrid overhead line system, the current discrimination method for faults between the AC-DC systems derived from the hybrid overhead line is not yet perfect. Summary of the Invention

[0005] The present invention provides a method for discriminating faults between AC and DC systems for a hybrid overhead line, which can effectively solve the problems in the background art.

[0006] To achieve the above object, the technical solution adopted by the present invention is:

[0007] A method for discriminating faults between AC and DC systems for a hybrid overhead line, comprising the following steps:

[0008] Obtain the three-phase current and voltage data at a single end of the AC line in the hybrid overhead line system;

[0009] Determine the starting criterion for discriminating faults between the AC and DC systems by detecting the abnormality of the zero-sequence current;

[0010] Intercept the three-phase voltage data after the starting criterion is established, and lock the fault phase through voltage mutation;

[0011] Intercept the three-phase current data after the starting criterion is established, and separate the DC component from the fault-phase current;

[0012] Distinguish between AC grounding faults and faults between AC and DC systems based on the DC component attenuation ratio.

[0013] Furthermore, calculate the zero-sequence current in real time, specifically expressed as:

[0014] I M(0) = I M(a) + I M(b) + I M(c) ;

[0015] Judge the starting criterion through integral operation, specifically expressed as:

[0016] ∫I M(0) dt > δ set ;

[0017] where, I M(a) , I M(b) and I M(c) respectively represent the measured values of the three-phase currents at the M end of the AC line, and δ set represents the threshold value of the starting criterion.

[0018] Furthermore, intercept the three-phase voltage data of the first cycle after the starting criterion is established, and calculate the sum of the change amounts within the short window of each phase voltage, specifically expressed as:

[0019]

[0020] where, ΔU i represents the change amount of the phase voltage, which is obtained by subtracting the data of one cycle before; N represents the number of data points within the short window, and T represents the number of data points corresponding to one cycle.

[0021] Furthermore, extract the DC component of the fault-phase current based on the mathematical morphology algorithm, including the following steps:

[0022] Set the structural element b as a smooth sequence with a length of half a power frequency cycle;

[0023] Perform signal preprocessing on the fault-phase current I S ;

[0024] Based on the morphological transformation output of smoothing the signal through opening operation and filling the gap through closing operation, specifically expressed as:

[0025] Y S =(I open + I close ) / 2;

[0026] where, Iopen 、I close respectively represent the opening operation to smooth the signal and the closing operation to fill the gaps, and Y S represents the transformed signal.

[0027] Furthermore, the fault-phase current I S is mirror-extended to eliminate the boundary effect.

[0028] Furthermore, the maximum value is obtained by sliding with the structuring element b to highlight the signal peak, specifically expressed as:

[0029]

[0030] The minimum value is obtained by sliding with the structuring element b to highlight the signal trough, specifically expressed as:

[0031] I S ⊙b;

[0032] The trough is smoothed by the opening operation to retain the peak, specifically expressed as:

[0033]

[0034] The peak is smoothed by the closing operation to fill the trough, specifically expressed as:

[0035]

[0036] The morphological transformation is specifically expressed as:

[0037]

[0038] Furthermore, the attenuation ratio of three consecutive DC components is calculated, specifically expressed as:

[0039] BS 1 = Y S2 / Y S1 , BS 2 = Y S3 / Y S2 ;

[0040] The judgment logic for AC grounding faults satisfies the following conditions:

[0041] BS 1 < 1 and BS 2 < 1;

[0042] The judgment logic for faults between AC and DC systems satisfies the following conditions:

[0043] BS 1 ≥ 1 or BS 2 ≥ 1.

[0044] An AC-DC system fault discrimination system for a hybrid overhead line, comprising:

[0045] A data acquisition module configured to acquire three-phase current and voltage signals of an AC line;

[0046] A startup criterion trigger module configured to calculate the integral value of the zero-sequence current in real time and trigger the fault discrimination process when the integral value exceeds a dynamically set threshold;

[0047] A fault phase identification module configured to intercept the three-phase voltage waveforms at the initial stage of a fault and lock the fault phase or determine it as an inter-system fault by calculating the voltage mutation of each phase;

[0048] A DC component extraction module configured to perform mathematical morphology processing on the fault phase current to separate the DC component in the current signal;

[0049] A fault type discrimination module configured to analyze the attenuation characteristics of the DC component and determine it as an inter-system fault between the AC-DC systems or an AC grounding fault according to whether the attenuation ratio continuously exceeds a set threshold.

[0050] A computer-readable storage medium having stored thereon a computer program, which when executed by a processor implements the steps of the above-mentioned method for discriminating faults between AC-DC systems of a hybrid overhead line.

[0051] An electronic device comprising a memory, a processor, and a computer program stored on the memory and running on the processor, wherein when the processor executes the program, it implements the steps of the above-mentioned method for discriminating faults between AC-DC systems of a hybrid overhead line.

[0052] The beneficial effects of the present invention are:

[0053] The present invention discloses a method and a system for discriminating faults between AC-DC systems of a hybrid overhead line. By extracting the DC component of the transient current of the AC line through mathematical morphology (MM) and based on the evaluation of the DC level, it is discriminated whether it is an inter-system fault between the AC-DC systems; only using the information at one end of the AC line and not relying on communication, reliable discrimination can be completed within 30 ms, and the practicability is relatively strong. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments recorded in the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0055] Figure 1 It is a schematic flowchart of the method of the present invention;

[0056] Figure 2 Schematic diagram of the hybrid overhead line system according to the embodiment of the method of the present invention;

[0057] Figure 3 Results of zero-sequence current under different fault scenarios in the embodiment of the method of the present invention;

[0058] Figure 4 Results of extracting DC components under different fault scenarios in the embodiment of the method of the present invention. Detailed implementation manners

[0059] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0060] It should be noted that when an element is referred to as being "fixed to" another element, it can be directly on the other element or there may also be a middle element. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there may be a middle element at the same time. The terms "vertical", "horizontal", "left", "right" and similar expressions used herein are only for the purpose of illustration and do not represent the only implementation manners.

[0061] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the description of the present invention herein are only for the purpose of describing specific embodiments and are not intended to limit the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items.

[0062] As Figure 1 shown, a method for discriminating faults between AC and DC systems for a hybrid overhead line disclosed by the present invention includes the following steps:

[0063] Obtain the three-phase current and voltage data at the single end of the AC line in the hybrid overhead line system; determine the start criterion for discriminating faults between AC and DC systems by detecting abnormal zero-sequence current; intercept the three-phase voltage data after the start criterion is established, and lock the fault phase through voltage mutation; intercept the three-phase current data after the start criterion is established, and separate the DC component from the fault-phase current; distinguish between AC ground faults and faults between AC and DC systems based on the DC component attenuation ratio.

[0064] Obtain high-precision three-phase current and voltage signals to provide basic data for subsequent analysis. High-precision current transformers (CTs) and voltage transformers (PTs) are not specifically deployed at the M end (the installation location of the protection device) of the AC line, and the sampling frequency needs to meet the requirements of power frequency signal analysis; the protection device is built-in with a high-speed AD conversion module to convert analog signals into digital signals.

[0065] Three-phase current (I M(a) , I M(b) , I M(c) ) and voltage (U M(a) , U M(b) , U M(c) ) need to be synchronously sampled to avoid phase errors; the original signal is low-pass filtered to filter out high-frequency noise (such as lightning interference), and the data cache stores historical data for at least one power frequency cycle for subsequent change quantity calculation.

[0066] The startup criterion determines whether there is a zero-sequence current path by detection and triggers the subsequent process; the zero-sequence current is calculated in real time, specifically expressed as:

[0067] I M(0) = I M(a) + I M(b) + I M(c) ;

[0068] The startup criterion is determined through integral operation, specifically expressed as:

[0069] ∫I M(0) dt > δ set ;

[0070] Among them, I M(a) , I M(b) and I M(c) respectively represent the measured values of the three-phase current at the M end of the AC line, and δ set represents the threshold value of the startup criterion.

[0071] Among them, δ set is usually set according to the maximum unbalanced zero-sequence current during normal system operation, and usually takes an empirical value such as the integral value of 0.1 - 0.3 times the rated current; if the integral value exceeds the threshold, it is determined as "possible fault" (including AC grounding fault or inter-system fault), and the subsequent process is started; otherwise, the discrimination is terminated.

[0072] Furthermore, based on the voltage change quantity, the fault phase is identified. The three-phase voltage data of the first cycle after the startup criterion is established is intercepted, and the sum of the change quantities within the short window of each phase voltage calculation is calculated, specifically expressed as:

[0073]

[0074] Among them, ΔU iIndicates the change in phase voltage, obtained by subtracting the data from one cycle before; N represents the number of data points in the short window, and T represents the number of data points corresponding to one cycle. The phase with the largest change is the faulty phase (e.g., when phase A is grounded, ΔU a increases significantly).

[0075] Further extract the DC component from the faulty phase current. Based on the mathematical morphology algorithm to extract the DC component of the faulty phase current, including the following steps:

[0076] Set the structuring element b as a smooth sequence with a length of half a power frequency cycle; the structuring element b needs to meet the conditions of smoothness, with the origin at its center, and a length of half a power frequency cycle (10 ms) in order to effectively extract the DC component in the signal.

[0077] Perform signal preprocessing on the faulty phase current I S ; perform mirror extension on the faulty phase current I S to eliminate the boundary effect. For the signal I S , mirror splice it forward and backward respectively according to the length of each quarter cycle to form data with a length of half a cycle. Apply the morphological transformation method to the spliced data to obtain the DC components of each segment, and then combine them in order to obtain the DC component in the entire signal.

[0078] Perform morphological transformation output based on opening operation to smooth the signal and closing operation to fill the gaps, specifically expressed as:

[0079] Y S =(I open +I close ) / 2;

[0080] Among them, I open , I close respectively represent the opening operation to smooth the signal and the closing operation to fill the gaps, and Y S represents the transformed signal.

[0081] Mathematical morphology (MM) is used to extract the DC component from the transient current. Different from the traditional Fourier transform or wavelet transform, it avoids the distortion at the front and back ends of the waveform (i.e., the stripe effect). The morphology algorithm uses the method of the interaction between the structuring element and the actual signal to analyze the signal. The structuring element is a set, which is determined according to the purpose of signal analysis. All operations of mathematical morphology are composed of the combination of the two most basic operations of dilation and erosion and set operations. Two important composite operations among them are morphological opening and closing. Specifically:

[0082] Take the maximum value by sliding the structuring element b to highlight the signal peak, specifically expressed as:

[0083]

[0084] Taking the minimum value by sliding through the structural element b to highlight the signal trough, specifically expressed as:

[0085] I S ⊙b;

[0086] Smoothing the trough by opening operation and retaining the peak, specifically expressed as:

[0087]

[0088] Smoothing the peak by closing operation and filling the trough, specifically expressed as:

[0089]

[0090] The morphological transformation is specifically expressed as:

[0091]

[0092] Filtering the power frequency AC component and retaining the DC trend by the mean value of opening and closing operations; processing the extended signal in segments and outputting a DC component Y every 10 ms S1 ,Y S2 ,Y S3 .

[0093] Furthermore, distinguishing the ground fault from the inter - system fault according to the attenuation characteristics of the DC component, calculating the attenuation ratio of three consecutive DC components, specifically expressed as:

[0094] BS 1 =Y S2 / Y S1 , BS 2 =Y S3 / Y S2 ;

[0095] The DC component decays due to system damping, and the judgment logic for AC ground fault satisfies the following conditions:

[0096] BS 1 <1 and BS 2 <1;

[0097] The DC system continuously injects current, and the DC component does not decay or even rises. The judgment logic for the inter - AC - DC system fault satisfies the following conditions:

[0098] BS 1 ≥1 or BS 2 ≥1.

[0099] The following further illustrates the method of the present invention in combination with an embodiment:

[0100] Such as Figure 2The figure shows a schematic diagram of the AC-DC hybrid overhead line system described in the present invention, and the system parameters are shown in Table 1 below.

[0101] Table 1

[0102]

[0103] The starting criterion of this method is constructed based on the integral of zero-sequence current, and the subsequent process is only activated when an AC line ground fault or an inter-system fault is recognized, which can avoid the long-term operation of this method. As is well known, when a single-phase ground fault occurs on an AC line, there is a zero-sequence current path between the fault grounding point and the system neutral point. Similarly, when an inter-system line fault occurs between the AC and DC systems, as shown by the red arrow in Figure 2 . At this time, the neutral point of the AC can form a zero-sequence current path with the neutral point of the DC, and there should also be an obvious zero-sequence current after the fault. For other faults, such as AC phase-to-phase faults, there is no zero-sequence current path. To test the sensitivity of the starting criterion, the zero-sequence current at side M is calculated respectively in the scenarios of high-resistance grounding fault of phase A of the AC and inter-system fault, and the results are shown in Figure 3 .

[0104] Figure 3 The results in show that when a high-resistance single-phase ground fault occurs in the AC, the amplitude of the zero-sequence current is also significantly increased compared with that before the fault. When an inter-system fault occurs, the zero-sequence current also changes significantly. It can be seen that single-pole grounding and inter-system faults can be identified based on the zero-sequence current. Next, it is necessary to further distinguish between the two to realize the discrimination of inter-system faults. After a fault occurs on the AC line, the transient signal of the current can be expressed as:

[0105]

[0106] where Ae -at is the decaying DC component, and I m (n), are the amplitude and initial phase of the nth harmonic respectively.

[0107] Theoretically, the DC component level in the current transient signal after the fault will decrease with time. After an inter-system fault, the transient signal of the current can be expressed as:

[0108]

[0109] Compared with the grounding fault, there is an additional I CD injected by the DC system in the formula, and the extracted DC component no longer shows a decaying trend, so the grounding fault and the inter-system fault can be distinguished. In different fault scenarios, the DC component is extracted by the MM algorithm respectively, and the results are as shown in Figure 4As shown. It can be seen that in the case of a single-phase grounding fault, the amplitude of the DC component is the highest at the initial stage of the fault and gradually decays; in the case of an inter-system fault, due to the injection of a DC current source, the amplitude of the DC component continues to increase.

[0110] The present invention also discloses an inter-system fault discrimination system for a hybrid overhead line, including:

[0111] A data acquisition module configured to acquire three-phase current and voltage signals of the AC line;

[0112] A starting criterion trigger module configured to calculate the integral value of the zero-sequence current in real time and trigger the fault discrimination process when the integral value exceeds the dynamically set threshold;

[0113] A fault phase identification module configured to intercept the three-phase voltage waveforms at the initial stage of the fault and lock the fault phase or determine it as an inter-system fault by calculating the voltage mutation of each phase;

[0114] A DC component extraction module configured to perform mathematical morphology processing on the fault-phase current to separate the DC component in the current signal;

[0115] A fault type discrimination module configured to analyze the attenuation characteristics of the DC component and determine it as an inter-system fault between AC and DC systems or an AC grounding fault according to whether the attenuation ratio continuously exceeds the set threshold.

[0116] The present invention also discloses a computer-readable storage medium, on which a computer program is stored, and when the program is executed by a processor, the steps of the above-mentioned inter-system fault discrimination method for a hybrid overhead line are implemented.

[0117] The present invention also discloses an electronic device, including a memory, a processor, and a computer program stored on the memory and running on the processor. When the processor executes the program, the steps of the above-mentioned inter-system fault discrimination method for a hybrid overhead line are implemented.

[0118] Those skilled in the art of this industry should understand that the present invention is not limited by the above embodiments. What is described in the above embodiments and the specification only illustrates the principle of the present invention. Without departing from the spirit and scope of the present invention, the present invention will have various changes and improvements, and these changes and improvements all fall within the scope of the present invention claimed. The scope of protection claimed by the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining faults between AC and DC systems of hybrid overhead lines, characterized in that: The following steps are involved: Acquire three-phase current and voltage data at a single end of an AC line in a hybrid overhead line system; By detecting the abnormal zero-sequence current, the fault identification and starting criteria between AC and DC systems are determined; Intercept the three-phase voltage data after the start-up criteria are met, and lock the fault phase through voltage mutation; Intercept the three-phase current data after the start-up criterion is established, and separate the DC component from the fault phase current; The AC ground fault and the AC / DC system fault are distinguished based on the DC component attenuation ratio.

2. The method for determining faults between AC and DC systems of hybrid overhead lines according to claim 1, characterized in that: Real-time calculation of zero-sequence current, specifically expressed as: I M(0) =I M(a) +I M(b) +I M(c) ; The startup criterion is determined by integral operation, which is specifically expressed as: ∫I M(0) dt>δ set ; Among them, I M(a) ,I M(b) and I M(c) They represent the three-phase current measurement values ​​at the M end of the AC line, δ set Indicates the threshold value for starting the criterion.

3. The method for determining faults between AC and DC systems of hybrid overhead lines according to claim 1, characterized in that: The three-phase voltage data of the first cycle after the start criterion is established is intercepted, and the sum of the changes in each phase voltage within the calculation short window is calculated, which is specifically expressed as: Among them, ΔU i It represents the change in phase voltage, which is obtained by subtracting the data one cycle ago; N represents the number of data points in the short window, and T represents the number of data points corresponding to one cycle.

4. The method for determining faults between AC and DC systems of hybrid overhead lines according to claim 1, characterized in that: The DC component of the fault phase current is extracted based on the mathematical morphology algorithm, including the following steps: Set the structural element b to a smooth sequence with a length of half a power frequency cycle; For the fault phase current I S Perform signal preprocessing; The morphological transformation output is based on the open operation to smooth the signal and the closed operation to fill the gap, which is specifically expressed as: Y S =(I open +I close ) / 2; Among them, I open ,I close They represent the open operation to smooth the signal and the closed operation to fill the gap, Y S represents the transformed signal.

5. The method for determining faults between AC and DC systems of hybrid overhead lines according to claim 4, characterized in that: For the fault phase current I S Perform mirror extension to eliminate boundary effects.

6. The method for determining faults between AC and DC systems of hybrid overhead lines according to claim 4, characterized in that: The maximum value is obtained by sliding the structural element b to highlight the signal peak, which is specifically expressed as: The minimum value is obtained by sliding the structural element b to highlight the signal trough, which is specifically expressed as: I S ⊙b; The troughs are smoothed by opening operation and the peaks are retained, which can be expressed as: The peaks are smoothed and the troughs are filled by closing operations, which can be specifically expressed as: The morphological transformation is specifically expressed as:

7. The method for determining faults between AC and DC systems of hybrid overhead lines according to claim 4, characterized in that: Calculate the attenuation ratio of three consecutive DC components, which can be expressed as: BS1=Y S2 / AND S1 BS2 and S3 / AND S2 ; The judgment logic of AC ground fault meets the following conditions: BS1<1 and BS2<1; The judgment logic of AC / DC system faults meets the following conditions: BS1≥1 or BS2≥1.

8. A fault identification system between AC and DC systems for hybrid overhead lines, characterized in that: include: A data acquisition module configured to acquire three-phase current and voltage signals of an AC line; A start-up judgment trigger module is configured to calculate the zero-sequence current integral value in real time, and trigger the fault judgment process when the integral value exceeds a dynamically set threshold; A fault phase identification module is configured to intercept the three-phase voltage waveform at the initial stage of the fault, and lock the fault phase or determine it as an inter-system fault by calculating the voltage mutation of each phase; a DC component extraction module configured to perform mathematical morphological processing on the fault phase current to separate the DC component in the current signal; The fault type identification module is configured to analyze the attenuation characteristics of the DC component and determine whether it is an AC / DC system fault or an AC grounding fault based on whether the attenuation ratio is continuously higher than a set threshold.

9. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the program is executed by a processor, the steps of the method for fault identification between AC and DC systems of hybrid overhead lines as described in any one of claims 1 to 7 are implemented.

10. An electronic device comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that: When the processor executes the program, the steps of the method for fault identification between AC and DC systems of hybrid overhead lines as described in any one of claims 1 to 7 are implemented.

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