An evaluation method for commutation failure caused by excitation inrush of an extra-high voltage direct current system

Abstract

The application discloses a kind of UHVDC system excitation inrush current caused by commutation failure evaluation method, it is related to UHVDC transmission technical field, comprising: S1, real-time data acquisition, data after commutation transformer empty load closing is extracted by FFT;S2, calculate commutation area and sum, obtain the commutation missing area generated by harmonic;S3, calculate the commutation missing area generated by the decrease of fundamental voltage amplitude;S4, when DC current rises, calculate the commutation demand area increment and total commutation evaluation area;S5, calculate total commutation area margin;S6, judge commutation result;The application provides a kind of UHVDC system excitation inrush current caused by commutation failure evaluation method, solves the existing UHVDC layered access system when commutation transformer empty load closing, because of ignoring harmonic phase angle, fundamental voltage drop and DC current rise and other multi-factor comprehensive influence, cannot accurately evaluate the commutation failure risk of same and different voltage grade commutator.

Description

Technical Field

[0001] This invention relates to the field of ultra-high voltage direct current (UHVDC) transmission technology, and in particular to an evaluation method for commutation failure caused by excitation inrush current in an UHVDC system. Background Technology

[0002] Ultra-High Voltage Direct Current (UHVDC) transmission, with its advantages of large transmission capacity, long transmission distance, low line loss, and low construction cost, has become one of the important methods for my country to realize the "West-to-East Power Transmission" strategy. Switching converter transformers is a common operating condition during the operation, commissioning, and maintenance of UHVDC transmission systems. However, during the no-load charging process of the converter transformer, the residual magnetism of the iron core and the bias magnetism generated by the transformer's energization can easily overlap, causing the iron core to saturate under the influence of the DC component, forming a high-peak inrush current. This inrush current contains a large number of harmonic components, which can lead to a reduction in commutation voltage amplitude and waveform distortion, potentially causing commutation failure in the DC system and seriously jeopardizing the safe and stable operation of the high-voltage DC system.

[0003] For UHVDC receiving-end tiered access systems, the inrush current generated by the transformer's no-load closing may first cause commutation failure in converters of the same voltage level. The commutation voltage distortion caused by harmonics in the inrush current is the main cause of commutation failure. Existing methods comprehensively consider the effects of voltage waveform distortion and amplitude reduction, proposing an inrush current commutation failure assessment factor to evaluate the commutation failure risk of converters of the same voltage level. However, they neglect the phase angle when calculating the harmonic commutation area, affecting the accuracy of the assessment. Current research mainly focuses on the impact of inrush current on the commutation process; the assessment methods do not consider all factors and lack a comprehensive commutation failure assessment method that comprehensively considers the impact of fundamental amplitude reduction and harmonic amplitude phase angle on the commutation area.

[0004] Because of the coupling relationship between the high-end and low-end AC grids in the tiered UHVDC system, the inrush current generated by the no-load closing of the transformer not only threatens the safe operation of converters of the same voltage level, but may also cause commutation failure in converters of different voltage levels. When the inrush current is too large, converters of different voltage levels may also experience commutation failure one after another. The main reason is not the direct transmission of harmonics through the AC interconnection channel, but the rise in DC current caused by the commutation failure of the converter of the same voltage level as the closed converter. However, existing research has not provided an assessment method for commutation failure of converters of different voltage levels. Summary of the Invention

[0005] The purpose of this invention is to provide an assessment method for commutation failure caused by inrush current in an ultra-high voltage direct current (UHVDC) system. This method addresses the problem that existing UHVDC hierarchical access systems cannot accurately assess the risk of commutation failure in converters of the same and different voltage levels when the converter transformer is closed under no-load conditions, due to the neglect of the combined effects of multiple factors such as harmonic phase angle, fundamental voltage drop, and DC current rise.

[0006] To achieve the above objectives, this invention provides an evaluation method for commutation failure caused by inrush current in an ultra-high voltage direct current (UHVDC) system, comprising the following steps: S1. Real-time acquisition of three-phase voltage and DC current data of the inverter-side converter bus, and extraction of data after no-load closing of the converter transformer through Fast Fourier Transform (FFT). S2. Based on the data extracted by FFT, calculate the commutation area and sum it to obtain the commutation missing area caused by harmonics; S3. Calculate the commutation loss area caused by the decrease in fundamental voltage amplitude during stable operation of the UHVDC hierarchical access system. S4. When the DC current rise is detected, calculate the increase in the commutation area requirement and combine it with the missing commutation area to obtain the total commutation evaluation area. S5. Calculate the reduction in commutation area margin caused by harmonics and the reduction in commutation area margin caused by the decrease in fundamental voltage amplitude, and obtain the total commutation area margin. S6. Standardize the total commutation evaluation area and the total commutation area margin, and determine whether the converter has experienced commutation failure based on the standardization results.

[0007] Preferably, the data extracted in S1 includes the fundamental voltage amplitude, the voltage amplitudes of the 2nd to 8th harmonics, and the voltage phase angle.

[0008] Preferably, the expression for the data extraction process using FFT in S1 is: ; In the formula, Indicates the commutation voltage; This indicates the amplitude of the fundamental voltage after being affected by the inrush current; Indicates the first Second harmonic voltage amplitude; Indicates the first Phase of the subharmonic voltage; Indicates harmonic index, ; Indicates the harmonic order; Indicates the power frequency angular frequency; Indicates time.

[0009] Preferably, the process of obtaining the commutation loss area caused by harmonics in S2 is as follows: S21. Calculate the commutation area based on the voltage amplitude and phase angle of the harmonics. The expression is: ; In the formula, Indicates the first The commutation area generated by subharmonics; Indicates the commutation overlap angle; Indicates the trigger angle; Indicates the shut-off angle of the commutation valve; Represents a constant; S22. Summing the commutation areas of the 2nd to 8th harmonics yields the commutation loss area caused by the harmonics, expressed as: ; In the formula, This represents the area of ​​commutation loss caused by harmonics.

[0010] Preferably, in S3, the commutation loss area caused by the decrease in the fundamental voltage amplitude is calculated. The expression is: ; In the formula, This represents the fundamental voltage amplitude during stable operation of the UHVDC hierarchical access system.

[0011] Preferably, the process of obtaining the total commutation evaluation area in S4 is as follows: S41. Calculate the increase in commutation area due to the increase in DC current, using the commutation area before and after the DC current increase. The expression is: ; In the formula, This indicates the commutation area required before the DC current rises. This indicates the commutation area required after the DC current increases. S42. Based on the commutation loss area caused by harmonics, the commutation loss area caused by the decrease in fundamental voltage amplitude, and the increase in commutation demand area caused by the increase in DC current, calculate the total commutation evaluation area, expressed as follows: .

[0012] Preferably, the process of obtaining the total commutation area margin in S5 is as follows: S51. Calculate the commutation area margin during stable operation of the UHVDC hierarchical access system. The expression is: ; In the formula, This indicates the minimum shut-off angle for commutation of the converter valve; S52. Calculate the reduction in commutation area margin due to harmonics. The expression is: ; S53. Calculate the reduction in commutation area margin due to the decrease in fundamental voltage amplitude. The expression is: ; In the formula, This indicates the decrease in the amplitude of the fundamental voltage. S54. Based on the results of S51-S53, calculate the total commutation area margin considering the reduction in harmonic and fundamental voltage amplitudes. The expression is: .

[0013] Preferably, the process of determining whether the converter has experienced commutation failure in S6 is as follows: S61. Obtain the total commutation evaluation area factor through per-unit processing. and total commutation area margin factor The expressions are as follows: ; ; S62. Make a judgment based on the result of the standardization process. If the condition is met, the converter is determined to have experienced a commutation failure; otherwise, the converter is determined to have not experienced a commutation failure and is in a safe operating state.

[0014] Therefore, the present invention employs the above-mentioned evaluation method for commutation failure caused by inrush current in an ultra-high voltage direct current system, which has the following beneficial effects: (1) Improved the calculation accuracy of the commutation loss area caused by harmonics: When calculating the commutation loss area caused by harmonics, the amplitude and phase angle of the 2nd to 8th harmonics were extracted by fast Fourier transform, and the harmonic phase angle was included in the calculation of the commutation area; compared with the existing method of ignoring the harmonic phase angle, the present invention considers the harmonic phase angle to make the calculation of the harmonic commutation loss area more accurate, thereby improving the accuracy of the commutation failure risk judgment. (2) A dynamic commutation area margin is constructed to reduce misjudgment: When calculating the total commutation area margin, the effects of two factors, namely the reduction of the fundamental voltage amplitude and the waveform distortion caused by harmonics, are considered at the same time. In the prior art, the total commutation area margin usually adopts a fixed value, while the present invention enables the total commutation area margin to be dynamically adjusted with the actual changes of the fundamental voltage and harmonics, which is closer to the actual engineering practice and therefore less likely to cause misjudgment. (3) It can effectively assess the commutation failure risk of converters at different voltage levels: For UHVDC hierarchical access systems, when the inrush current generated by the no-load closing of the converter transformer is too large, the DC current will rise after the commutation failure of the converter at the same voltage level, which may lead to the successive commutation failure of the converter at another voltage level; This invention calculates the increase in the commutation demand area caused by the increase in DC current and includes this increase in the total commutation evaluation area, thereby effectively assessing the commutation failure phenomenon of converters at different voltage levels; In actual engineering, this method can be applied to the commutation failure judgment of the receiving end of the entire hierarchical access system, and has a wider range of applications; (4) Higher accuracy in determining the commutation failure under conditions of large inrush current: Simulation verification shows that under conditions of large inrush current and actual commutation failure, such as closing angles of 15°~40°, 130°~145°, 195°~220°, and 310°~325°, the method of the present invention can correctly determine the commutation failure, while the existing method will misjudge that no commutation failure has occurred due to ignoring factors such as the reduction of the fundamental voltage. Therefore, when the inrush current is large, the present invention has higher accuracy in determining the commutation failure than the existing method.

[0015] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0016] Figure 1 This is an overall flowchart of an evaluation method for commutation failure caused by inrush current in an ultra-high voltage direct current system according to the present invention. Figure 2 This refers to the changes in commutation angle and turn-off angle after voltage drop in Embodiment 1 of the present invention. Figure 3 This refers to the changes in commutation angle and turn-off angle after voltage distortion in Embodiment 1 of the present invention. Figure 4 This is a schematic diagram of the commutation evaluation area criterion in Embodiment 1 of the present invention; Figure 5 This is the receiving-end topology of the UHVDC receiving-end hierarchical access system in Embodiment 2 of the present invention; Figure 6 This is the curve showing the change of the total commutation evaluation area factor with the closing angle in Embodiment 2 of the present invention; Figure 7 This is a comparison diagram of the two evaluation methods in Embodiment 2 of the present invention; Figure 8 The curves showing the change in the turn-off angle of the high- and low-end converters of Pole II in Embodiment 2 of the present invention; Figure 9 This is the DC current curve of pole II in Embodiment 2 of the present invention. Detailed Implementation

[0017] The following detailed description of embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0018] Example 1 A method for assessing commutation failure caused by inrush current in an ultra-high voltage direct current (UHVDC) system is disclosed, including a UHVDC hierarchical access system. In the receiving-end topology of the UHVDC hierarchical access system, each pole on the inverter side consists of two 12-pulse converters connected in series. The valve group closest to the DC transmission line is the high-end valve group, connected to the receiving-end 500kV converter bus; the valve group closest to the grounding pole is the low-end valve group, connected to the receiving-end 1000kV converter bus. The two AC buses are connected by a tie impedance. The term "connection" (or "coupling") represents the electrical connection strength between two AC power grids of different voltage levels at the receiving end. In practical engineering, the electrical distance between two power grids of different voltage levels is relatively large, constituting extremely weak coupling. The value is relatively large.

[0019] The closing operation of a converter transformer generates a high-amplitude inrush current. This current waveform exhibits a distinctly non-sinusoidal characteristic and contains a large number of low-order harmonic components, primarily second and third harmonics. When these harmonic currents are injected into the AC system, they cause severe distortion of the connected commutation voltage, directly affecting the commutation process of converters at the same voltage level.

[0020] Please see Figure 1 A method for evaluating commutation failure caused by inrush current in an ultra-high voltage direct current system includes the following steps: S1. Real-time acquisition of three-phase voltage and DC current data of the inverter-side converter bus. In order to analyze the highest harmonic content caused by voltage waveform distortion leading to commutation failure, the data after the converter transformer is closed under no-load conditions is extracted by Fast Fourier Transform (FFT). The extracted data includes the fundamental voltage amplitude, the voltage amplitude of the 2nd to 8th harmonics, and the voltage phase angle.

[0021] The expression for the commutation voltage affected by inrush current, obtained through FFT data extraction, is as follows: ; In the formula, Indicates the commutation voltage; This indicates the amplitude of the fundamental voltage after being affected by the inrush current; Indicates the first Second harmonic voltage amplitude; Indicates the first Phase of the subharmonic voltage; Indicates harmonic index, ; Indicates the harmonic order; Indicates the power frequency angular frequency; Indicates time.

[0022] S2. Based on the data extracted by FFT, calculate the commutation area and sum it to obtain the commutation missing area caused by harmonics.

[0023] S21. Calculate the commutation area based on the voltage amplitude and phase angle of the harmonics. The expression is: ; In the formula, Indicates the first The commutation area generated by subharmonics; Indicates the commutation overlap angle; Indicates the trigger angle; Indicates the shut-off angle of the commutation valve; This represents a constant; during stable operation of the UHVDC hierarchical access system, , .

[0024] S22. Summing the commutation areas of the 2nd to 8th harmonics yields the commutation loss area caused by the harmonics, expressed as: ; In the formula, This represents the area of ​​commutation loss caused by harmonics.

[0025] S3. Besides harmonics causing commutation voltage distortion and thus commutation area loss, a decrease in the fundamental voltage amplitude at the converter bus can also cause commutation area loss. Based on the fundamental voltage amplitude during stable operation of the UHVDC hierarchical access system, and considering harmonic phase, calculate the commutation area loss caused by the decrease in fundamental voltage amplitude. To make the result more accurate, the expression is: ; In the formula, This represents the fundamental voltage amplitude during stable operation of the UHVDC hierarchical access system.

[0026] S4. When an increase in DC current is detected, calculate the increase in commutation area required and combine it with the missing commutation area to obtain the total commutation evaluation area.

[0027] S41. Calculate the increase in commutation area due to the increase in DC current, using the commutation area before and after the DC current increase. The expression is: ; In the formula, This indicates the commutation area required before the DC current rises. This indicates the commutation area required after the DC current increases.

[0028] pass and Calculated The expression is: .

[0029] In a UHVDC tiered access system, when the inrush current generated by the no-load closing of the converter transformer is large, a commutation failure in one voltage level converter may lead to subsequent commutation failures in another voltage level converter. The main reason is that after a commutation failure occurs in a converter of the same voltage level as the closing converter transformer, the DC current increases, resulting in a larger commutation area requirement. Therefore, when determining commutation failure for converters of different voltage levels, the increase in commutation area requirement due to the increased DC current must also be considered. The expression is: ; In the formula, Indicates the commutation inductance; This represents the DC current after the rise, which can be obtained through measurement; S42. Based on the commutation loss area caused by harmonics, the commutation loss area caused by the decrease in fundamental voltage amplitude, and the increase in commutation demand area caused by the increase in DC current, calculate the total commutation evaluation area, expressed as follows: .

[0030] S5. Calculate the reduction in commutation area margin caused by harmonics and the reduction in commutation area margin caused by the decrease in fundamental voltage amplitude to obtain the total commutation area margin.

[0031] S51. Calculate the commutation area margin during stable operation of the UHVDC hierarchical access system. The expression is: ; In the formula, This represents the minimum shut-off angle for commutation of the converter valve, taken as... ; S52, such as Figures 2-3 As shown, both a decrease in the fundamental voltage amplitude and waveform distortion will lead to a reduction in the commutation area margin. Figure 2 The area of ​​region ABCD in the middle represents the loss of commutation area caused by the reduction in voltage amplitude. To compensate for this lost area, the commutation overlap angle is... It will increase because the commutation overlap angle increases after the voltage amplitude decreases. The area of ​​region CDEF represents the amount of reduction in commutation area margin after the voltage amplitude decreases. Figure 3 The area of ​​the HIGK region represents the missing commutation area caused by waveform distortion. To compensate for this missing area, the commutation overlap angle is... It will increase because the commutation overlap angle increases due to waveform distortion. The area of ​​region GKLM represents the reduction in commutation area margin after waveform distortion; the reduction in commutation area margin due to harmonics. The expression is: .

[0032] S53. Calculate the reduction in commutation area margin due to the decrease in fundamental voltage amplitude. The expression is: ; In the formula, This indicates the decrease in the amplitude of the fundamental voltage.

[0033] S54. Based on the results of S51-S53, calculate the total commutation area margin considering the reduction in harmonic and fundamental voltage amplitudes. The expression is: ; The UHVDC hierarchical access system operates stably, the fundamental amplitude decreases, and the commutation voltage after the converter transformer is closed under no-load conditions is as follows: Figure 4 As shown, after the converter transformer is closed under no-load, the increase in DC current increases the required commutation area, and at the same time, the commutation overlap angle increases. The increase in commutation overlap angle due to the increase in DC current is... .

[0034] S6. Standardize the total commutation evaluation area and the total commutation area margin, and determine whether the converter has experienced commutation failure based on the standardization results.

[0035] S61. Obtain the total commutation evaluation area factor through per-unit processing. and total commutation area margin factor The expressions are as follows: ; .

[0036] S62. Make a judgment based on the result of the standardization process. If the missing area of ​​the commutation exceeds the carrying capacity of the commutation margin, it is determined that the converter is at risk of commutation failure; otherwise, it is determined that the converter has not experienced commutation failure and is in a safe operating state.

[0037] Example 2: Simulation Verification In PSCAD / EMTDC, build as follows Figure 5The UHVDC hierarchical access system model shown was simulated and verified under the conditions of three-valve group operation and single-valve group commissioning, with the high-end converter transformer of Pole I and the low-end converter transformer of Pole I respectively put into operation. The following simulation takes the commissioning of the high-end converter transformer of Pole I as an example.

[0038] Verification 1: Commutation failure of the Pole II high-end converter.

[0039] The commissioning of the Pole I high-side converter transformer may cause commutation failure in the Pole II high-side converter, which is also at the 500kV voltage level. Before the commutation failure in the Pole II high-side converter, the DC current hardly increases, which is considered... Therefore, when calculating the total commutation evaluation area factor, only the reduction in fundamental amplitude and harmonic waveform distortion need to be considered. Simulations were performed on the high-end converter transformer of pole I with a closing angle of 0-360° at 2.0s. The results are as follows: Figure 6 .

[0040] from Figure 6 As can be seen, the total commutation evaluation area changes periodically with the closing angle. When the closing angle is 0°, 180°, or 360°, i.e., when closing at the voltage zero crossing point, the total commutation evaluation area is the largest, the commutation area margin considering the reduction of the fundamental amplitude is the smallest, and the impact on the commutation process is the most severe. When the closing angle is 90° or 270°, i.e., when closing at the voltage maximum value, the total commutation evaluation area is the smallest, the commutation area margin considering the reduction of the fundamental amplitude is the largest, and the risk of commutation failure is the lowest.

[0041] Under these simulation conditions, the total commutation evaluation area is greater than the maximum commutation area margin when the closing angle is 0°~40°, 130°~220°, and 310°~360°, indicating that the commutator valve fails to commutate; when the closing angle is 40°~130° and 220°~310°, the total commutation evaluation area is less than the commutation area margin, indicating that the commutator valve does not fail to commutate, and the evaluation results are consistent with the actual situation.

[0042] When the closing angle is 15°~40°, 130°~145°, 195°~220°, or 310°~325°, commutation failure actually occurs in all cases. The total commutation evaluation area factor and total commutation area margin factor calculated by the method of this invention and existing methods are as follows: Figure 7 As shown, the total commutation evaluation area factor calculated by the existing method can be seen. The value is too small; the method of this invention takes into account the reduction in fundamental amplitude and waveform distortion when calculating the total commutation area margin factor, and the total commutation area margin factor calculated by the method of this invention is obtained. The total commutation evaluation area factor obtained by the method of this invention under four closing angles. All are greater than the commutation area margin factor The condition is determined to be a commutation failure; existing methods calculate the total commutation area margin factor. The calculation did not take into account the reduction in fundamental amplitude. , which is a fixed value, represents the total commutation missing area factor under four closing angles. If the values ​​are all less than the commutation area margin, it will be incorrectly judged as no commutation failure has occurred. That is, when the closing angle is 15°~40°, 130°~145°, 195°~220°, or 310°~325°, and the inrush current is large and commutation failure occurs, the method of the present invention has higher accuracy.

[0043] Verification 2: Commutation failure of the low-end converter of Pole II.

[0044] When the inrush current is too high, the energization of the high-end converter transformer of Pole I may also cause commutation failure of the low-end converter of Pole II. In this case, because the coupling between the two AC systems is very weak, the inrush current generated by the closing of the high-end converter transformer of Pole I has little impact on the low-end 1000kV converter bus. The main reason for the commutation failure of the low-end converter of Pole II is that the DC current rises after the commutation failure of the high-end converter of Pole II, causing an increase in the commutation area requirement. At this time, the increase in the commutation area requirement needs to be considered. The impact.

[0045] Taking the closing angle of 0°, where the DC current rise is the largest, as an example, the high-end converter transformer of pole I closes under no-load conditions at 2.0s. The DC current of pole II and the turn-off angle of the high-end and low-end converters of pole II are as follows: Figures 8-9 As shown, commutation failure occurred in both the high-end and low-end converters of pole II. The commutation failure time of the low-end converter of pole II occurred 3ms later than that of the high-end converter of pole II, and the DC current increased significantly. The total commutation evaluation area factor under this condition was calculated. Among them, the incremental factor of commutation demand area The proportion is relatively large, and the commutation area margin factor is relatively high. , The determination that a commutation failure had occurred is consistent with the actual situation.

[0046] Simulation results show that the commutation failure evaluation index It can accurately determine the commutation failure of the high-end and low-end converters of pole II when the pole I high-end and low-end converter transformers are closed under no-load conditions at different closing angles, verifying the applicability and reliability of this indicator in different operating scenarios.

[0047] Therefore, this invention adopts the aforementioned assessment method for commutation failure caused by inrush current in an ultra-high voltage direct current (UHVDC) system, improving the calculation accuracy of the commutation loss area caused by harmonics. When calculating the commutation loss area caused by harmonics, the amplitude and phase angle of the 2nd to 8th harmonics are extracted using Fast Fourier Transform, and the harmonic phase angle is incorporated into the commutation area calculation. Compared to existing methods that ignore the harmonic phase angle, this invention considers the harmonic phase angle for a more accurate calculation of the harmonic commutation loss area, thereby improving the accuracy of commutation failure risk assessment. A dynamic commutation area margin is constructed to reduce misjudgments. When calculating the total commutation area margin, the influence of both the fundamental voltage amplitude reduction and the waveform distortion caused by harmonics is considered simultaneously. In existing technologies, the total commutation area margin is usually a fixed value, while this invention allows the total commutation area margin to be dynamically adjusted according to the actual changes in the fundamental voltage and harmonics, which is closer to engineering practice and therefore less prone to misjudgments. It can effectively assess the commutation failure risk of converters at different voltage levels. For UHVDC hierarchical access systems, when the commutation area is... When the inrush current generated by the no-load closing of the converter is too large, the commutation failure of a converter of the same voltage level will cause the DC current to rise, which may lead to the successive commutation failure of a converter of another voltage level. This invention calculates the increase in commutation area required due to the increase in DC current and incorporates this increase into the total commutation evaluation area, thereby effectively evaluating the commutation failure phenomenon of converters of different voltage levels. In practical engineering, this method can be applied to the commutation failure judgment of the receiving end of the entire hierarchical access system, with a wider range of applications. It has higher judgment accuracy under the condition of large inrush current: Simulation verification shows that under the condition of large inrush current and actual commutation failure, such as closing angles of 15°~40°, 130°~145°, 195°~220°, and 310°~325°, the method of this invention can correctly judge the commutation failure, while the existing method will misjudge that no commutation failure has occurred because it ignores factors such as the reduction of fundamental voltage. Therefore, when the inrush current is large, the method of this invention has higher judgment accuracy than the existing method.

[0048] 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 preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for evaluating commutation failure caused by inrush current in an ultra-high voltage direct current system, characterized in that, Includes the following steps: S1. Real-time acquisition of three-phase voltage and DC current data of the inverter-side converter bus, and extraction of data after no-load closing of the converter transformer through Fast Fourier Transform (FFT). S2. Based on the data extracted by FFT, calculate the commutation area and sum it to obtain the commutation missing area caused by harmonics; S3. Calculate the commutation loss area caused by the decrease in fundamental voltage amplitude during stable operation of the UHVDC hierarchical access system. S4. When the DC current rise is detected, calculate the increase in the commutation area requirement and combine it with the missing commutation area to obtain the total commutation evaluation area. S5. Calculate the reduction in commutation area margin caused by harmonics and the reduction in commutation area margin caused by the decrease in fundamental voltage amplitude, and obtain the total commutation area margin. S6. Standardize the total commutation evaluation area and the total commutation area margin, and determine whether the converter has experienced commutation failure based on the standardization results.

2. The evaluation method for commutation failure caused by inrush current in an ultra-high voltage direct current system according to claim 1, characterized in that: The data extracted from S1 includes the fundamental voltage amplitude, the voltage amplitudes of the 2nd to 8th harmonics, and the voltage phase angle.

3. The evaluation method for commutation failure caused by inrush current in an ultra-high voltage direct current system according to claim 2, characterized in that, The expression for the data extraction process using FFT in S1 is: ； In the formula, Indicates the commutation voltage; This indicates the amplitude of the fundamental voltage after being affected by the inrush current; Indicates the first Second harmonic voltage amplitude; Indicates the first Phase of the subharmonic voltage; Indicates harmonic index, ; Indicates the harmonic order; Indicates the power frequency angular frequency; Indicates time.

4. The evaluation method for commutation failure caused by inrush current in an ultra-high voltage direct current system according to claim 3, characterized in that, The process of obtaining the commutation loss area caused by harmonics in S2 is as follows: S21. Calculate the commutation area based on the voltage amplitude and phase angle of the harmonics. The expression is: ； In the formula, Indicates the first The commutation area generated by subharmonics; Indicates the commutation overlap angle; Indicates the trigger angle; Indicates the shut-off angle of the commutation valve; Represents a constant; S22. Summing the commutation areas of the 2nd to 8th harmonics yields the commutation loss area caused by the harmonics, expressed as: ； In the formula, This represents the area of ​​commutation loss caused by harmonics.

5. The evaluation method for commutation failure caused by inrush current in an ultra-high voltage direct current system according to claim 4, characterized in that, In S3, calculate the commutation loss area caused by the decrease in fundamental voltage amplitude. The expression is: ； In the formula, This represents the fundamental voltage amplitude during stable operation of the UHVDC hierarchical access system.

6. The evaluation method for commutation failure caused by inrush current in an ultra-high voltage direct current system according to claim 5, characterized in that, The process of obtaining the total commutation evaluation area in S4 is as follows: S41. Calculate the increase in commutation area due to the increase in DC current, using the commutation area before and after the DC current increase. The expression is: ； In the formula, This indicates the commutation area required before the DC current rises. This indicates the commutation area required after the DC current increases. S42. Calculate the total commutation evaluation area based on the commutation loss area caused by harmonics, the commutation loss area caused by the decrease in fundamental voltage amplitude, and the increase in commutation demand area caused by the increase in DC current. The expression is: 。 7. The evaluation method for commutation failure caused by inrush current in an ultra-high voltage direct current system according to claim 6, characterized in that, The process of obtaining the total commutation area margin in S5 is as follows: S51. Calculate the commutation area margin during stable operation of the UHVDC hierarchical access system. The expression is: ； In the formula, This indicates the minimum shut-off angle for commutation of the converter valve; S52. Calculate the reduction in commutation area margin due to harmonics. The expression is: ； S53. Calculate the reduction in commutation area margin due to the decrease in fundamental voltage amplitude. The expression is: ； In the formula, This indicates the decrease in the amplitude of the fundamental voltage. S54. Based on the results of S51-S53, calculate the total commutation area margin considering the reduction in harmonic and fundamental voltage amplitudes. The expression is: 。 8. The evaluation method for commutation failure caused by inrush current in an ultra-high voltage direct current system according to claim 7, characterized in that, The process for determining whether a converter has experienced commutation failure in S6 is as follows: S61. Obtain the total commutation evaluation area factor through per-unit processing. and total commutation area margin factor The expressions are as follows: ； ； S62. Make a judgment based on the result of the standardization process. If the condition is met, the converter is determined to have failed to commutate; otherwise, the converter is determined not to have failed to commutate.

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