Method and system for fault detection of hybrid dc transmission system

Abstract

The application relates to a fault detection method and system of a hybrid DC power transmission system. By obtaining pole current and pole voltage in fault data, the fault occurrence condition is judged by comparing the current amplitude. After the fault occurs, the pole voltage and the pole current are converted by a pole mode, the refraction coefficient phase is determined according to the obtained line mode voltage and line mode current, and the corresponding relationship between the refraction coefficient phase and the frequency is determined according to an equivalent circuit. The fault area is identified by the phase of the refraction coefficient, the fault resistance and the noise interference can be resisted, communication time is not needed, and technical support is provided for safe and reliable operation of the DC power transmission.

Description

Technical Field

[0001] This invention relates to the field of power transmission protection technology, specifically to a fault detection method and system for hybrid DC power transmission systems. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Hybrid DC transmission technology enables long-distance, high-capacity, multi-terminal power transmission. During operation, corresponding protection schemes are required to determine the location of faults. Protection schemes for DC transmission systems can be categorized into time-domain analysis-based and frequency-domain analysis-based schemes. Traveling wave protection schemes based on time-domain analysis identify faults by detecting voltage differentials, changes in voltage, and current gradients. However, this type of protection scheme suffers from difficulties in detecting near-distance faults, poor fault resistance, and poor interference resistance. Frequency-domain analysis-based protection schemes often utilize the frequency differences between fault information within and outside the fault zone to construct the protection principle. DC transmission systems contain boundary elements such as current-limiting reactors and DC filters. The attenuation of high-frequency signals and the smoothing of waveforms by these boundary elements cause significant differences in fault information between the inside and outside of the fault zone.

[0004] Based on the above problems, some protection schemes for hybrid DC transmission systems utilize fault detection methods established by initial traveling wave phase characteristics, and incorporate lightning interference criteria based on low-frequency traveling wave phase characteristics, thereby relying on traveling wave phase to identify faults. These schemes classify both lines as faults within the designated area, and the fault handling method of completely isolating any fault is detrimental to system stability. Some protection schemes establish dual-end protection schemes based on the measured wave impedance expression and fault characteristics of three-terminal hybrid DC transmission systems. However, due to the reliance on dual-end quantity information, while increasing the near-end fault identification rate, it also increases communication time. Summary of the Invention

[0005] To address the technical problems mentioned above, this invention provides a fault detection method and system for hybrid DC transmission systems. By identifying fault areas through the phase of the refractive index, it can withstand fault resistance and noise interference without requiring communication time, thus providing technical support for the safe and reliable operation of DC transmission.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] The first aspect of the present invention provides a fault detection method for a hybrid DC transmission system, comprising the following steps:

[0008] Obtain the pole current and pole voltage from the fault data, and determine the fault occurrence by comparing the current amplitude;

[0009] After a fault occurs, the polar voltage and polar current are converted into polar modes. Based on the obtained line mode voltage and line mode current, the phase of the refractive index is determined. The correspondence between the phase of the refractive index and the frequency is determined based on the equivalent circuit.

[0010] The phase of the refractive index corresponding to the first frequency is within the second threshold range, and a fault occurs in the positive region.

[0011] When the phase of the refractive index corresponding to the second frequency is less than the first threshold, a fault occurs outside the reverse region.

[0012] When the phase of the refractive index corresponding to the third frequency is less than the first threshold, a positive out-of-range fault occurs.

[0013] Furthermore, an action signal is issued when any of the following occurs: a fault within the forward zone, a fault outside the reverse zone, or a fault outside the forward zone.

[0014] Furthermore, the fault occurrence is determined by comparing the current amplitude. Specifically, a fault occurs when the rated current exceeds the product of the electrode current and the threshold, as shown in the following formula:

[0015] |i Δ |>k set I n ;

[0016] In the formula: i Δ I n and k set These are the polar current, rated current, and threshold current, respectively.

[0017] Furthermore, the electrode voltage and electrode current are converted to electrode modes, and the refractive index phase is determined based on the obtained line mode voltage and line mode current, as shown in the following equation:

[0018]

[0019] In the formula: α ph. Let u1 be the phase of the refractive index, i1 be the line-mode voltage, and Z be the line-mode current. c1 This is the line-mode impedance.

[0020] Furthermore, the correspondence between the phase and frequency of the refractive index is determined based on the equivalent circuit, specifically as follows:

[0021] Based on the topology of the hybrid DC transmission system, the converter section is divided into several regions, and the corresponding Peterson equivalent circuits are obtained respectively.

[0022] The phase expression for the refractive index of the corresponding region is determined based on the obtained Peterson equivalent circuit;

[0023] Based on the parameters in the Peterson equivalent circuit of each region, the phase characteristics of the corresponding refractive index are obtained, and the correspondence between the phase and frequency of the refractive index is determined based on the phase characteristics.

[0024] Furthermore, the first frequency is the frequency point corresponding to the measurement point selected within the set frequency range.

[0025] Furthermore, the second frequency is a frequency point higher than the set value.

[0026] Furthermore, the third frequency is a set fixed frequency point.

[0027] Furthermore, fault detection based on the refractive index phase is achieved using the following formula:

[0028]

[0029] Where, k set1 and k set2 These are the first threshold and the second threshold, respectively, α ph.(M1\M2\M3\M4\...\Mn) The first frequency is M1-Mn, where M1-Mn are all measurement points, and α ph.(＞x) For the second frequency, α ph.(y) It is the third frequency.

[0030] A second aspect of the present invention provides a system for implementing the above-described method, comprising:

[0031] The data acquisition module is configured to acquire the pole current and pole voltage from the fault data and determine the fault occurrence by comparing the current amplitude.

[0032] The data processing module is configured to: after a fault occurs, the polar voltage and polar current are converted into polar modes, and the phase of the refractive index is determined based on the obtained line mode voltage and line mode current; and the correspondence between the phase of the refractive index and the frequency is determined based on the equivalent circuit.

[0033] The fault identification module is configured such that: if the phase of the refractive index corresponding to the first frequency is within the second threshold range, a fault occurs in the positive region;

[0034] The fault identification module is also configured to: if the phase of the refractive index corresponding to the second frequency is less than the first threshold, a fault occurs outside the reverse region;

[0035] The fault identification module is also configured to detect faults outside the positive region when the phase of the refractive index corresponding to the third frequency is less than the first threshold.

[0036] Compared with existing technologies, one or more of the above technical solutions have the following beneficial effects:

[0037] Based on the phase-frequency characteristics of the refractive index in a hybrid DC system, where the phase is unaffected by fault resistance, fault regions can be identified through the phase of the refractive index. This method can withstand fault resistance and noise interference without requiring communication time, improving the ability to identify high-resistance faults and providing technical support for the safe and reliable operation of DC transmission. Furthermore, since the phase of the refractive index is only related to the boundary, it is unaffected by long lines and can improve the ability to identify near-end faults. Attached Figure Description

[0038] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0039] Figure 1 This is a schematic diagram of the fault detection process of a hybrid DC transmission system provided in one or more embodiments of the present invention;

[0040] Figure 2 This is a schematic diagram of the Peterson equivalent circuit of the G1 region provided in one or more embodiments of the present invention;

[0041] Figure 3 This is a schematic diagram of the Peterson equivalent circuit of the G2 region provided in one or more embodiments of the present invention;

[0042] Figure 4 This is a schematic diagram of the Peterson equivalent circuit of the G3 region provided in one or more embodiments of the present invention;

[0043] Figure 5 This is a schematic diagram of the phase characteristics of the refractive index provided by one or more embodiments of the present invention;

[0044] Figure 6 This is a schematic diagram of the phase characteristics of the refractive index provided by one or more embodiments of the present invention;

[0045] Figure 7 This is a schematic diagram of the phase characteristics of the refractive index provided by one or more embodiments of the present invention;

[0046] Figure 8 This is a schematic diagram of the topology of a hybrid DC transmission system provided in one or more embodiments of the present invention. Detailed Implementation

[0047] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0048] It should be noted that the following detailed descriptions are exemplary and intended to provide further illustration of the invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0049] As introduced in the background section, traditional DC system protection schemes can be divided into those based on time-domain analysis and those based on frequency-domain analysis. Traveling wave protection schemes based on time-domain analysis identify faults by detecting voltage differentials, changes in voltage, and current gradients. However, these schemes suffer from difficulties in detecting near-field faults, poor fault resistance tolerance, and low interference capability. To improve the performance of these protection schemes, some existing technologies have proposed adaptive traveling wave protection schemes and protection schemes based on the product of voltage and current changes, significantly enhancing the fault resistance tolerance of these schemes.

[0050] Frequency-domain-based protection schemes often utilize the frequency differences between fault information within and outside the protection zone to construct the protection principle. DC transmission systems contain boundary elements such as current-limiting reactors and DC filters. The attenuation of high-frequency signals and the smoothing of waveforms by these boundary elements cause significant differences in fault information between the inside and outside of the protection zone. Some inventions utilize the high-frequency energy on both sides of the DC filter and current-limiting reactor for fault identification, fully leveraging the characteristics of these boundary elements.

[0051] Regarding protection schemes for hybrid DC transmission systems, some existing technologies utilize fault detection methods based on initial traveling wave phase characteristics, or incorporate lightning interference criteria based on low-frequency traveling wave phase characteristics. These schemes rely on traveling wave phase for fault identification, making them highly susceptible to line parameters. Furthermore, these schemes classify both lines as faults within the designated area, and the complete isolation of any fault is detrimental to system stability. Some existing technologies have established dual-end protection schemes based on the measured wave impedance expression and fault characteristics of three-terminal hybrid DC transmission systems. However, these schemes rely on dual-end quantity information, increasing the near-end fault identification rate but also increasing communication time.

[0052] Therefore, the following embodiments provide a fault detection method and system for hybrid DC transmission systems. By identifying the fault area through the phase of the refractive index, it can withstand fault resistance and noise interference, and does not require communication time, thus providing technical support for the safe and reliable operation of DC transmission.

[0053] Example 1:

[0054] like Figures 1-8 As shown, the fault detection method for a hybrid DC transmission system includes the following steps:

[0055] Step 1: Collect pole current and pole voltage fault data (referring to line current and line voltage in DC system), input the pole current into the starting element program, and determine whether a fault has occurred by comparing the current amplitude. If the condition is met, proceed to the next step; otherwise, the program returns.

[0056] Step 2: Perform pole-mode conversion on the pole voltage and pole current to obtain the line-mode voltage and line-mode current, and calculate the refractive index and phase;

[0057] Step 3: Input the phase of the refractive index into the fault identification program. If the phase meets the condition, it is judged as an internal fault and an action signal is issued; otherwise, it is judged as an external fault and the program ends.

[0058] The specific steps are as follows:

[0059] 1) Start the component program

[0060] Considering the existence of disturbances and coupling phenomena in DC systems, disturbances mean short-term fluctuations in voltage or current, which can easily trigger malfunctions in protection systems. Coupling occurs when, after a fault occurs, the healthy electrode experiences voltage or current fluctuations due to coupling. Therefore, to reduce the risk of malfunctions in protection systems, the starting element determines whether a fault has occurred by comparing the current amplitude, based on the following criteria:

[0061] |i Δ |>k set I n

[0062] In the formula: i Δ I n and k set These represent the electrode current, rated current, and threshold current, respectively. set (In this embodiment, k) set (1.1V).

[0063] To enhance the reliability of the starting element, the starting element is set to operate only after the criterion is met three consecutive times.

[0064] The line-mode voltage is obtained by performing pole-mode conversion on the pole voltage and pole current, and the refractive index phase is calculated, as shown in the following formula:

[0065]

[0066] In the formula: α ph. The phase represents the refractive index, u1 represents the line-mode voltage, i1 represents the line-mode current, and Z represents the phase phase. c1 Represents line mode impedance

[0067] 2) Fault Identification

[0068] (1) Derive the Peterson equivalent circuit after the fault, such as Figures 2-4 As shown.

[0069] Will Figure 8 The three converter sections are divided into three regions: G1, G2, and G3. The equivalent Peterson circuits of each region are derived.

[0070] Region G1.

[0071] After the fault occurs, the incident wave u rs The wave travels through line l1 to measurement point M1, generating a reflected wave u. fs and refracted wave u zs At this time, the Peterson equivalent circuit is as follows: Figure 2 As shown. Based on the Peterson equivalent circuit, the expression for the phase of the refractive index can be obtained as follows:

[0072]

[0073] Where, r a l a g a and c a These represent the resistance, inductance, conductance, and capacitance of line l1, respectively; L1 represents the inductance of the current-limiting reactor L1.

[0074] Region G2.

[0075] After a fault occurs on line l1, the traveling wave reaches measurement point M2 via the line, and then undergoes reflection. At this time, the components on the back side of the measurement point include the converter MMC1 and line l2. Conversely, after a fault occurs on line l2, the traveling wave reaches measurement point M3 via the line, and then undergoes reflection. At this time, the components on the back side of the measurement point include the converter MMC1 and line l1. Based on this, the Peterson equivalent circuit of region G2 is obtained as follows: Figure 3 As shown, the phase expression for the refractive index is:

[0076]

[0077] Among them, Z l2 Z represents the wave impedance of l2. L3 Z represents the equivalent impedance of the current-limiting reactor L3. MMC2 Z represents the equivalent impedance of converter MMC2. MMC1 Including inductors (L cmmc1 ), capacitor (C) mmc1 ) and resistance (R) mmc1 ).

[0078] Region G3.

[0079] The traveling wave propagation mechanism in region G3 is the same as in region G1, but the composition differs. Incident wave u rs When the wave propagates from l2 to MMC2, the reflected wave u fs Return to l2, and the refracted wave u zs Entering the MMC2 converter. The equivalent circuit of region G3 is as follows: Figure 4 As shown. The expression for the phase of the refractive index at this time is:

[0080]

[0081] (2) Analyze the phase characteristics of the refractive index to obtain Figures 5-7 Where the horizontal axis represents Frequency / Hz and the vertical axis represents Phase / rad, the phase characteristics of the refractive index can be obtained as follows:

[0082] When a fault occurs outside the reverse region, only a traveling wave exists during the Peterson law operation time; therefore, the phase of the refractive index depends on the phase of the traveling wave. According to... Figures 5-7 It can be seen that the phase of the refractive index above 1kHz is 0°;

[0083] When a fault occurs within the zone, the phase of the refractive index at a specific frequency is a constant;

[0084] When there is a fault outside the positive zone, the phase of the refractive index at a specific frequency is 0°.

[0085] (4) The fault detection method based on the phase of the refractive index has the following criteria:

[0086] The phase of the refractive index corresponding to the set first frequency is at the threshold (0.9k). set2 1.1k set2 A positive fault occurred within the specified range.

[0087] The phase of the refractive index corresponding to the set second frequency is less than the threshold k. set1 A reverse region out-of-zone fault occurred;

[0088] The phase of the refractive index corresponding to the set third frequency is less than the threshold k. set1 A positive out-of-zone fault occurred.

[0089] In this embodiment, 0.9 and 1.1 are margins left for the judgment, since k set2 It can be calculated from system parameters; theoretically, k can be used directly here. set2 While this serves as a basis for judgment, considering practical applications, providing a range of 0.9 and 1.1 is sufficient to handle common systems.

[0090] This embodiment uses the following formula to achieve fault detection based on the phase of the refractive index:

[0091]

[0092] Where, k set1 and k set2 Representing the threshold, k in this embodiment set1 The value is 5°, k set2 The angles are 86.5°, 51.5°, and 28.6° respectively.

[0093] α ph.(M1\M2\M3\M4)The first set frequencies are (20-100)Hz (measurement point M1), (5-50)Hz (measurement point M2), (5-50)Hz (measurement point M3), and (10-1000)Hz (measurement point M4);

[0094] α ph.(＞1kHz) The second frequency is set, which in this embodiment is higher than 1kHz;

[0095] α ph.(9.8Hz) The third frequency is set to 9.8Hz.

[0096] Based on the characteristic that the phase of the refractive index in a hybrid DC system is unaffected by fault resistance, this embodiment can improve the ability to identify high-resistance faults. Since the phase of the refractive index is only related to the boundary, it is not affected by long lines and can improve the ability to identify near-end faults.

[0097] From α phG1 α phG2 α phG3 As can be seen from the calculation formula, its calculation process is completely unrelated to the fault resistance R, meaning that the phase calculation of the refractive index does not involve the fault resistance. Applying a higher fault resistance will not change the phase of the refractive index, that is, it will not change the reliability of the criterion.

[0098] Correspondingly, the calculation formula for the traditional scheme's criterion includes the fault resistance R. High-resistance faults will change the criterion, thus affecting its reliability. In addition, the calculation formula for the refractive index phase includes parameters for both the boundary and the line. The parameters for the boundary are much higher than those for the line, so the influence of the line parameters can be ignored in actual calculations.

[0099] Therefore, this criterion is suitable for long lines. Traditional traveling wave protection, on the other hand, is dominated by line parameters, and the reliability of the detection criterion is extremely poor for long lines.

[0100] Example 2:

[0101] A system for implementing the above method includes:

[0102] The data acquisition module is configured to acquire the pole current and pole voltage from the fault data and determine the fault occurrence by comparing the current amplitude.

[0103] The data processing module is configured to: after a fault occurs, the polar voltage and polar current are converted into polar modes, and the phase of the refractive index is determined based on the obtained line mode voltage and line mode current; and the correspondence between the phase of the refractive index and the frequency is determined based on the equivalent circuit.

[0104] The fault identification module is configured such that: if the phase of the refractive index corresponding to the first frequency is within the second threshold range, a fault occurs in the positive region;

[0105] The fault identification module is also configured to: if the phase of the refractive index corresponding to the second frequency is less than the first threshold, a fault occurs outside the reverse region;

[0106] The fault identification module is also configured to detect faults outside the positive region when the phase of the refractive index corresponding to the third frequency is less than the first threshold.

[0107] Example 3:

[0108] This embodiment provides a computer-readable storage medium storing a computer program thereon, which, when executed by a processor, implements the steps in the fault detection method for a hybrid DC transmission system as described in Embodiment 1 above.

[0109] Example 4:

[0110] This embodiment provides a computer device, including a memory, a processor, and a computer program stored in the memory and executable on the processor. When the processor executes the program, it implements the steps in the fault detection method for a hybrid DC transmission system as described in Embodiment 1 above.

[0111] The steps or networks involved in Embodiments 2 to 4 above correspond to those in Embodiment 1. For specific implementation details, please refer to the relevant description section of Embodiment 1. The term "computer-readable storage medium" should be understood as a single medium or multiple media including one or more instruction sets; it should also be understood as including any medium capable of storing, encoding, or carrying an instruction set for execution by a processor and enabling the processor to perform any of the methods in this invention.

[0112] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A fault detection method for a hybrid DC transmission system, characterized in that, Includes the following steps: Obtain the pole current and pole voltage from the fault data, and determine the fault occurrence by comparing the current amplitude; After a fault occurs, the polar voltage and polar current are converted into polar modes. Based on the obtained line mode voltage and line mode current, the phase of the refractive index is determined. The correspondence between the phase of the refractive index and the frequency is determined based on the equivalent circuit. The polar voltage and polar current are converted into polar modes. Based on the obtained line mode voltage and line mode current, the phase of the refractive index is determined, as shown in the following formula: ； In the formula, α ph. The phase of the refractive index, u 1 represents the line-mode voltage. i 1 represents the line-mode current. Z c1 Line-mode impedance; The phase of the refractive index corresponding to the first frequency is within the second threshold range, and a fault occurs in the positive region. When the phase of the refractive index corresponding to the second frequency is less than the first threshold, a fault occurs outside the reverse region. When the phase of the refractive index corresponding to the third frequency is less than the first threshold, a fault occurs outside the positive region. Fault detection based on the refractive index phase is achieved using the following formula: ； in, k set1 and k set2 These are the first threshold and the second threshold, respectively. The refractive index phase corresponds to the first frequency, and M1-Mn are all measurement points. The phase of the refractive index corresponding to the second frequency. The phase of the refractive index corresponds to the third frequency.

2. The fault detection method for a hybrid DC transmission system as described in claim 1, characterized in that, An action signal is issued when any of the following occurs: a fault within the forward zone, a fault outside the reverse zone, or a fault outside the forward zone.

3. The fault detection method for a hybrid DC transmission system as described in claim 2, characterized in that, The fault occurrence is determined by comparing the current amplitude. Specifically, a fault occurs when the rated current exceeds the product of the electrode current and the threshold, as shown in the following formula: ； In the formula: , I n and k set These are the polar current, rated current, and threshold current, respectively.

4. The fault detection method for a hybrid DC transmission system as described in claim 1, characterized in that, The relationship between the phase and frequency of the refractive index is determined based on the equivalent circuit, specifically as follows: Based on the topology of the hybrid DC transmission system, the converter section is divided into several regions, and the corresponding Peterson equivalent circuits are obtained respectively. The phase expression for the refractive index of the corresponding region is determined based on the obtained Peterson equivalent circuit. Based on the parameters in the Peterson equivalent circuit of each region, the phase characteristics of the corresponding refractive index are obtained, and the correspondence between the phase and frequency of the refractive index is determined based on the phase characteristics.

5. The fault detection method for a hybrid DC transmission system as described in claim 1, characterized in that, The first frequency is the frequency point corresponding to the measurement point selected within the set frequency range.

6. The fault detection method for a hybrid DC transmission system as described in claim 1, characterized in that, The second frequency is a frequency point that is higher than the set value.

7. The fault detection method for a hybrid DC transmission system as described in claim 1, characterized in that, The third frequency is a set fixed frequency point.

8. A fault detection system for a hybrid DC transmission system, employing the fault detection method for a hybrid DC transmission system as described in any one of claims 1-7, characterized in that, include: The data acquisition module is configured to acquire the pole current and pole voltage from the fault data and determine the fault occurrence by comparing the current amplitude. The data processing module is configured to: after a fault occurs, the polar voltage and polar current are converted into polar modes, and the phase of the refractive index is determined based on the obtained line mode voltage and line mode current; and the correspondence between the phase of the refractive index and the frequency is determined based on the equivalent circuit. The fault identification module is configured such that: if the phase of the refractive index corresponding to the first frequency is within the second threshold range, a fault occurs in the positive region; When the phase of the refractive index corresponding to the second frequency is less than the first threshold, a fault occurs outside the reverse region. When the phase of the refractive index corresponding to the third frequency is less than the first threshold, a positive out-of-range fault occurs.

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