A method for protecting against multiple lightning overvoltages at an overhead line-DC submarine cable conversion station

Abstract

This invention discloses a method for protecting against multiple lightning strike overvoltages at an overhead line-DC submarine cable switching station, belonging to the field of lightning overvoltage protection technology. The method includes: establishing an electromagnetic transient simulation model of the overhead line-DC submarine cable switching station; building a lightning current source model; selecting the most stringent waveform and building multiple lightning strike current sources; building a broadband equivalent circuit model of the reactor to simulate the condition of lightning striking a line of the same polarity, and initially determining the reactor inductance value; evaluating whether the reverse polarity lightning impulse overvoltage of the DC submarine cable exceeds the withstand range, and selecting the optimal inductance value that ensures the reverse polarity lightning impulse voltage of the DC submarine cable meets the requirements; optimizing the surge arrester configuration scheme, and selecting the optimal scheme that provides the best overvoltage suppression effect and meets the requirements for surge arrester electrical stress. This invention, using the above method, solves the problem that existing technologies cannot design lightning overvoltage protection schemes for switching stations and submarine cables based on the lightning impulse withstand voltage characteristics of DC submarine cables.

Description

Technical Field

[0001] This invention relates to the field of lightning overvoltage protection technology, and in particular to a method for protecting against multiple lightning overvoltages at an overhead line-DC submarine cable conversion station. Background Technology

[0002] The first domestic and international integrated ±500 kV flexible DC transmission project utilizes a hybrid structure of overhead lines and submarine cables for its DC transmission lines, connecting them via a switching station. While the submarine cable is not directly threatened by lightning strikes, lightning overvoltages generated by lightning strikes on the DC overhead line can propagate as intrusion waves to the switching station and the submarine cable, threatening the insulation safety of equipment within the switching station and the connected submarine cable. Current manufacturing capabilities for DC submarine cables are limited, with a lightning impulse withstand voltage of only 1175 kV for the same polarity and 630 kV for the opposite polarity, resulting in relatively limited insulation margins. Therefore, a scientifically sound and reasonable lightning overvoltage protection scheme is urgently needed for this system.

[0003] Current research on the impact of lightning overvoltage mainly focuses on single lightning strikes, using typical single lightning current waveforms such as 2.6 / 50μs, 2.0 / 50μs, and 1 / 70μs to conduct overvoltage simulations and propose corresponding protection schemes. However, long-term statistical data from a provincial lightning location system shows that multiple-stroke ground flashes account for more than 40% of all lightning-induced ground flashes, and several incidents in recent years have resulted in damage and failure of equipment such as surge arresters in substations due to multiple lightning strikes. Therefore, it is necessary to develop effective protection designs for lightning intrusion waves at substations under multiple lightning strikes. Furthermore, there are significant differences in the recommended waveforms for multiple lightning strikes in existing standards. For example, IEEE recommends 5.63 / 77.5μs and 0.75 / 30.2μs for the negative polarity first return stroke and subsequent return strokes, respectively, while IEC recommends 1 / 200μs and 0.25 / 100μs. Different lightning current waveform parameters can significantly affect the overvoltage level that the equipment can withstand, resulting in a large deviation in protection design. Waveform factors have not been fully incorporated into protection design considerations.

[0004] Although existing research has explored the propagation mechanism and protection strategies of lightning intrusion waves, current technologies still have the following shortcomings in the analysis and protection against overvoltage of DC overhead lines and submarine cables:

[0005] First, existing research on lightning intrusion waves is mostly limited to single lightning strike scenarios. The subsequent return wave waveforms of multiple lightning strikes have shorter wavefront times, which are quite different from those of single lightning strikes. Furthermore, multiple lightning strikes have a short-term energy accumulation effect, and the surge arrester absorbs far more energy than in the case of a single lightning strike, resulting in significant safety hazards in the protection design.

[0006] Secondly, existing research lacks a systematic analysis of the impact of lightning current waveform parameters on the overvoltage level of the inrush surge and the electrical stress of the protection device. Typical protection designs often use standard waveforms such as 2.6 / 50μs and 1 / 70μs for simulation analysis, without considering the differences between the first and subsequent return stroke waveforms recommended in standards such as IEEE or IEC. Furthermore, a method for evaluating the most stringent overvoltage and surge arrester electrical stress based on different waveform characteristics has not yet been developed.

[0007] Furthermore, current technologies have not fully considered the differences in insulation capabilities of DC submarine cables when optimizing protection strategies. When the DC overhead line at the substation's incoming section is struck by a reverse polarity lightning current, the DC operating voltage, superimposed with the reverse polarity lightning impulse voltage, may cause a polarity reversal of the overvoltage at the cable connection point. At this time, a high electric field exists between the space charge in the submarine cable and the electrodes, leading to strong electric field distortion in the cable insulation, which significantly reduces the cable's insulation level. Current protection schemes lack effective suppression measures for lightning intrusion wave overvoltages at submarine cables, especially reverse polarity lightning overvoltages.

[0008] In summary, existing technologies still lack a complete and effective modeling, analysis, and protection design system when faced with practical factors such as multiple lightning strikes, differences in current waveforms, and limited insulation levels of DC submarine cables under lightning impacts. This makes it difficult to meet the high reliability operation requirements of integrated land-sea DC transmission projects in complex lightning environments. Summary of the Invention

[0009] The purpose of this invention is to provide a method for protecting against multiple lightning overvoltages at an overhead line-DC submarine cable conversion station, in order to solve the problems mentioned in the background art.

[0010] To achieve the above objectives, the present invention provides a method for protecting against multiple lightning overvoltages at an overhead line-DC submarine cable conversion station, comprising the following steps:

[0011] S1. Based on the electrical wiring diagram of the conversion station, the towers of the incoming line section, and the relevant parameters of the equipment in the conversion station, establish an electromagnetic transient simulation model of the overhead line-DC submarine cable conversion station;

[0012] S2. Based on the statistical results of artificial lightning triggering and lightning location systems, as well as relevant standards, propose a lightning current waveform parameter model and build a lightning current source model.

[0013] S3. Study the influence of lightning current waveform parameters on the overvoltage amplitude of equipment in the station, the reverse polarity voltage of DC submarine cable and the electrical stress of surge arrester, select the most stringent waveform and build multiple lightning current sources.

[0014] S4. Construct a broadband equivalent circuit model of the reactor, simulate the working condition of lightning strike on the same polarity line, study the influence of the reactor inductance value on the overvoltage of equipment at various points in the station and DC submarine cable, and preliminarily determine the reactor inductance value.

[0015] S5. Based on the most stringent lightning current waveform and the reactor selected in step S4, simulate the lightning current winding around the reverse polarity line to evaluate whether the reverse polarity lightning impulse overvoltage of the DC submarine cable exceeds the tolerance range. If it does not meet the requirements, return to S4 to increase the inductance value of the reactor until the optimal inductance value that makes the reverse polarity lightning impulse voltage of the DC submarine cable meet the requirements is selected.

[0016] S6. Optimize the surge arrester configuration scheme based on the overvoltage distribution characteristics of the equipment after adding the reactor, and check whether the electrical stress of the surge arrester meets the requirements. If the electrical stress of the surge arrester exceeds its tolerance range, consider increasing the number of surge arresters in parallel, and select the optimal scheme with the best overvoltage suppression effect and the surge arrester electrical stress meeting the requirements.

[0017] Preferably, step S1 specifically includes:

[0018] S11. Based on the electrical wiring diagram of the conversion station, some equipment in the conversion station is simulated using impulse inlet capacitors;

[0019] S12. Set the scatter plot of the nonlinear resistance current-current characteristic in the simulation according to the technical parameters and current-current characteristics of the surge arrester;

[0020] S13. Construct a multi-wave impedance model for the tower based on the tower shape diagram of the incoming line segment.

[0021] Preferably, step S2 includes the following steps:

[0022] S21. Based on the different lightning current waveform parameters obtained by artificial lightning strike, calculate the average time of the lightning current wavefront and the average time of the lightning current wavetail under subsequent lightning strikes, propose an actual lightning current parameter model, and fit it using the Heidler lightning current waveform function.

[0023] S22. Based on the statistical parameters in step S21, propose multiple lightning current parameter models;

[0024] S23. Based on the statistical data of the lightning location system, calculate the time interval between two adjacent return strokes and the proportion of different return strokes to determine the number of return strokes and the time interval between two adjacent return strokes; determine the lightning current amplitude by comparing the lightning withstand level of the tower and the magnitude of the maximum backflash current; establish a lightning current source model in the simulation software.

[0025] Preferably, in step S21, the actual lightning strike current source model is fitted using the Heidler function, and the resulting formula is shown below:

[0026] ;

[0027] In the formula: Indicates lightning current. The peak value of the base current. This is the wavefront attenuation coefficient. This is the wave tail attenuation coefficient. η is a parameter describing the steepness of the lightning base current, where η is the current peak correction factor.

[0028] Preferably, step S23 specifically involves: combining statistical data from the lightning location system to calculate the time interval between two adjacent return strokes and the proportion of different return stroke counts, thereby determining the time interval and number of return strokes for multiple lightning strikes; determining the lightning current amplitude by comparing the tower's lightning withstand level and the maximum lightning current, calculating the maximum lightning current of the tower using an electrical geometric model, and then simulating the tower's lightning current conditions with different amplitudes to obtain the maximum lightning current amplitude that will not cause flashover of the insulator string, which is used as the tower's lightning withstand level; taking the smaller of the two values ​​as the lightning current amplitude for single and multiple lightning strikes; and establishing a single lightning strike current source model by connecting a controlled current source in parallel with the lightning channel impedance.

[0029] Preferably, step S3 specifically includes:

[0030] S31. Based on the lightning current source model built in step S2, simulate and analyze the overvoltage distribution characteristics of equipment in the station when the lightning current waveform parameters are different.

[0031] S32. Compare the stress generated on the surge arrester by different lightning current parameters, and select the stringent waveform based on the analysis results of S31.

[0032] Preferably, step S31 specifically involves: based on the lightning current source model built in step S2, simulating and analyzing the overvoltage distribution characteristics of equipment in the station when the lightning current waveform parameters are different, and further considering the working conditions of lightning strikes of different polarities, specifically including two working conditions: negative polarity lightning strikes the positive line and positive polarity lightning strikes the negative line. At this time, the submarine cable will be subjected to reverse polarity voltage.

[0033] Preferably, step S4 includes the following steps:

[0034] S41. Construct the broadband equivalent circuit of the reactor, and set the parameters of the reactor inductance, parallel capacitor at both ends and capacitance to ground at both ends.

[0035] S42. Simulate the lightning strike on a line of the same polarity to study the effect of reactor inductance on overvoltage suppression.

[0036] Preferably, step S5 specifically involves: based on the most stringent lightning current waveform selected in step S3 and the reactor selected in step S4, simulating the lightning current winding around the reverse polarity line, evaluating whether the reverse polarity lightning impulse overvoltage of the DC submarine cable exceeds the tolerance range. If the reverse polarity overvoltage has exceeded the tolerance level of the DC submarine cable, then return to step S4 to select a larger inductance value for re-verification until the optimal inductance value that makes the reverse polarity lightning impulse tolerance voltage of the DC submarine cable meet the requirements is selected.

[0037] Preferably, step S6 includes the following steps:

[0038] S61. Optimize the surge arrester configuration scheme based on the overvoltage distribution of equipment in the station after adding reactors;

[0039] S62. Simulate and analyze the overvoltage characteristics of equipment in the station under different surge arrester configuration schemes, check the insulation margin of equipment in the station, the same polarity lightning impulse withstand voltage of DC submarine cable and the electrical stress of each surge arrester, and select the best scheme.

[0040] Therefore, the present invention employs the above-mentioned method for protecting against multiple lightning overvoltages at an overhead line-DC submarine cable conversion station, which has the following beneficial effects:

[0041] (1) This invention proposes a lightning protection design method for overhead line-DC submarine cable conversion station. Combining lightning location system and artificial lightning data, it proposes multiple lightning current waveform parameter models. Using PSCAD software, it builds lightning current source model and conversion station model, simulates and analyzes the electrical stress of equipment under different lightning current waveform parameters, proposes the most stringent waveform for lightning protection design, and builds multiple lightning current sources. This solves the problem of the existing technology that only considers a single lightning strike, ignores the influence of lightning current waveform parameters, and designs protection schemes in a one-sided manner.

[0042] (2) By configuring reactors and surge arresters, the lightning protection method for DC submarine cables and switching station equipment is designed. At the same time, the situation of lightning current bypassing the reverse polarity line is considered, and the reverse polarity lightning impulse withstand capability of DC submarine cables is evaluated. This solves the problem that the existing technology cannot design lightning overvoltage protection schemes for switching stations and submarine cables based on the lightning impulse withstand voltage characteristics of DC submarine cables.

[0043] 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

[0044] Figure 1 This is a flowchart illustrating a method for protecting an overhead line-DC submarine cable conversion station from multiple lightning strikes according to an embodiment of the present invention.

[0045] Figure 2 This is a schematic diagram of the broadband equivalent circuit of the reactor after adding a surge arrester according to an embodiment of the present invention;

[0046] Figure 3 This is a wiring diagram of the switching station according to an embodiment of the present invention;

[0047] Figure 4 This is a diagram showing the overvoltage amplitude distribution of a device according to an embodiment of the present invention;

[0048] Figure 5 This invention relates to the effect of reactor inductance value on overvoltage amplitude in an embodiment of the invention.

[0049] Figure 6 This is a schematic diagram of Scheme 1 of the present invention;

[0050] Figure 7 This is a schematic diagram of Scheme 2 of the present invention;

[0051] Figure 8 This is a schematic diagram of Scheme 3 of the present invention. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present 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 present invention without inventive effort are within the scope of protection of the present invention.

[0053] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0054] Example

[0055] like Figure 1 As shown, this invention provides a method for protecting against multiple lightning overvoltages at an overhead line-DC submarine cable conversion station, comprising the following steps:

[0056] S1. Based on the electrical wiring diagram of the conversion station, the towers of the incoming line section, and the relevant parameters of the equipment in the conversion station, establish an electromagnetic transient simulation model of the overhead line-DC submarine cable conversion station.

[0057] S2. Based on the statistical results of artificial lightning triggering and lightning location systems, as well as relevant standards, propose a lightning current waveform parameter model and build a lightning current source model.

[0058] S3. Study the influence of lightning current waveform parameters on the overvoltage amplitude of equipment in the station, the reverse polarity voltage of DC submarine cable and the electrical stress of surge arrester, select the most stringent waveform and build multiple lightning current sources.

[0059] S4. Construct a broadband equivalent circuit model of the reactor, simulate the working condition of lightning strike on the same polarity line, study the influence of the reactor inductance value on the overvoltage of equipment at various points in the station and DC submarine cable, and preliminarily determine the reactor inductance value.

[0060] S5. Based on the most stringent lightning current waveform and the reactor selected in step S4, simulate the lightning current winding around the reverse polarity line to evaluate whether the reverse polarity lightning impulse overvoltage of the DC submarine cable exceeds the tolerance range. If it does not meet the requirements, return to S4 to increase the inductance value of the reactor until the optimal inductance value that makes the reverse polarity lightning impulse voltage of the DC submarine cable meet the requirements is selected.

[0061] S6. Optimize the surge arrester configuration scheme based on the overvoltage distribution characteristics of the equipment after adding the reactor, check whether the electrical stress of the surge arrester meets the requirements. If the electrical stress of the surge arrester exceeds its tolerance range, consider increasing the number of surge arresters in parallel, and select the optimal scheme with the best overvoltage suppression effect and the surge arrester electrical stress meeting the requirements.

[0062] In this embodiment, step S1 specifically includes:

[0063] S11. Based on the electrical wiring diagram of the converter station, some equipment in the converter station is simulated using impulse inlet capacitors. In this embodiment, the electrical wiring diagram of the converter station is as follows: Figure 3 As shown in Table 1, the inlet capacitors of the equipment are as follows.

[0064] Specifically, using PSCAD / EMTDC electromagnetic transient simulation software and a segmented modeling method, the substation mainly consists of switching equipment. Under the action of lightning waves, the equipment such as disconnecting switches, circuit breakers, and transformers in the substation can be equivalent to the impulse inlet capacitance. The volt-ampere characteristics of the surge arresters in the substation are simulated using nonlinear resistance, and the simulation of the equipment connection lines in the substation uses the Bergeron model.

[0065] Table 1. Inlet Capacitance of Different Equipment

[0066] ;

[0067] S12. Based on the surge arrester's technical parameters and volt-ampere characteristics, set the scatter plot of the nonlinear resistance volt-ampere characteristics in the simulation. In this embodiment, the coordination current of the surge arrester in the station is 20 kA, the residual voltage is 1050 kV, and the maximum allowable absorbed energy is 5000 kJ. The surge arrester's volt-ampere characteristics are shown in Table 2:

[0068] Table 2. I-V Characteristics of Surge Arresters

[0069] ;

[0070] S13. Based on the tower shape diagram of the incoming line section, build a multi-wave impedance model of the tower. In this embodiment, the tower models are JG372 and ZC375. The wave impedance values ​​at different positions of the tower are calculated as shown in Table 3.

[0071] Table 3. Wave Impedance of Tower Body and Crossarm (Unit: kV)

[0072] ;

[0073] In this embodiment, step S13 specifically involves: The simulation of the overhead line uses a frequency-dependent model. Based on the tower type, height, crossarm length, span, conductor height and splitting condition, and ground wire height and splitting condition, the wave impedance of the main frame, support, and tower crossarm is calculated, and a multi-wave impedance model of the tower is built. The specific formula is as follows:

[0074] Wave impedance of each part of the main frame The calculation formula is as follows:

[0075] ;

[0076] in, Indicates the first section of the tower body The equivalent radius of the segment is calculated using the following formula:

[0077] ;

[0078] in, For the tower Radius of some support pillars; The radius of the support column at the base of the tower; For the tower The distance between two adjacent pillars; This is the distance between two adjacent supports at the base of the tower.

[0079] Wave impedance of each part of the support Calculated using the following formula:

[0080] ;

[0081] The crossarm of the tower is also simulated using a distributed parameter line segment, and its wave impedance is... The calculation formula is as follows:

[0082] ;

[0083] in, For the tower Partial crossarm equivalent radius, Indicates the first Partial crossarm height. The DC submarine cable simulation uses a frequency-dependent model; the insulator flashover criterion model considers the physical mechanism of insulator flashover and adopts the leading development model recommended by CIGRE (International Conference on Large Electric Systems), as shown in the following equation:

[0084] ;

[0085] in, t For time; U ( t () represents the impulse voltage across the insulator. The leader length; d This refers to the dry arc distance of the insulator; E 0 represents the minimum field strength required to sustain stream development; This is the leading development speed coefficient. In this example... Take 0.8, E 0 is taken as 600.

[0086] In this embodiment, step S2 includes the following steps:

[0087] S21. Based on the lightning current waveform parameters obtained under different return strokes from artificial lightning strikes, calculate the average time value of the lightning current wavefront and the average time value of the lightning current wavetail under subsequent return strokes, propose a model of actual lightning current parameters, and fit it using the Heidler lightning current waveform function to obtain the Heidler lightning current waveform. The possible values ​​of the parameter. This is the wavefront attenuation coefficient. This is the wave tail attenuation coefficient. Parameters describing the steepness of lightning base current, This is the peak current correction factor.

[0088] Step S21 is as follows: Based on the results of the artificial lightning-inducing experiment, including the number of return strokes under different lightning-inducing times, the lightning current amplitude, wavefront time, wave tail time, and time interval parameters corresponding to each return stroke, calculate the average value of the wavefront and wave tail times to obtain the actual subsequent return stroke waveform parameter model.

[0089] The results of the artificial lightning-induced experiment in this embodiment are shown in Table 4. The waveform parameters of the subsequent return lightning current were obtained as 0.36 / 18μs. The subsequent return lightning current was obtained by fitting the waveform. , , .

[0090] Table 4 Results of Artificial Lightning Trigger Experiment

[0091] ;

[0092] S22. Based on the provisions of IEC 62305-1, IEEE Std 1410-2010 and GB / T 50064-2014, and combined with the statistical parameters in step S21, various lightning current parameter models are proposed.

[0093] In this embodiment, based on the subsequent return lightning current waveform parameters obtained in step S21 and relevant standards, the lightning current parameter models are established as 2.6 / 50μs, 5.72 / 77μs, 0.36 / 18μs, 1 / 200μs and 0.25 / 100μs.

[0094] Based on relevant standards, the lightning current waveform parameter models are supplemented. Specifically, the IEC 62305-1 standard specifies the waveform parameters for the first negative polarity short-duration lightning strike as 1 / 200 μs and subsequent short-duration lightning strikes as 0.25 / 100 μs; the IEEE Std1410-2010 standard specifies the waveform parameters for the first negative polarity return stroke as 5.63 / 77.5 μs; and GB / T 50064-2014 specifies the use of 2.6 / 50 μs as the single lightning strike current waveform parameter. Based on the above lightning current waveforms, several lightning current parameter models are proposed. The lightning current source model is fitted using the Heidler function, as shown in the following equation. The fitting results are shown in Table 5.

[0095] ;

[0096] In the formula: Indicates lightning current. The peak value of the base current. This is the wavefront attenuation coefficient. The attenuation coefficient is the wave tail. Parameters describing the steepness of lightning base current, This is the peak current correction factor.

[0097] Table 5 Heidler function fitting table

[0098] ;

[0099] S23. Based on the statistical data from the lightning location system, calculate the time interval between two adjacent return strokes and the proportion of different return stroke counts to determine the number of return strokes and the time interval between two adjacent return strokes; determine the lightning current amplitude by comparing the tower's lightning withstand level and the maximum inrush current. Establish a lightning current source model in PSCAD / EMTDC software.

[0100] Step S23 specifically involves: Calculating the time interval between two adjacent return strokes and the proportion of different return stroke counts based on statistical data from the lightning location system, thus determining the time interval and number of return strokes for multiple lightning strikes; determining the lightning current amplitude by comparing the tower's lightning withstand level and the maximum lightning current; calculating the tower's maximum lightning current using the Electrical Geometry Model (EGM); and simulating tower conditions with different lightning current amplitudes using PSCAD / EMTDC software to obtain the maximum lightning current amplitude that will not cause flashover of the insulator string, which is used as the tower's lightning withstand level. The smaller of the two values ​​is taken as the lightning current amplitude for single and multiple lightning strikes. A single lightning strike current source model is established by connecting a controlled current source in parallel with the lightning channel impedance.

[0101] In this embodiment, the average time interval between multiple lightning strikes was statistically determined to be 35-80 ms. 2-5 return strokes accounted for 79.83% of multi-stroke ground flashes. Therefore, the number of return strokes was set to 5, and the interval was set to 10 ms without affecting the simulation results. The maximum inrush current of the tower was calculated to be 20 kA using the electrical geometry model. Simulation showed that the tower's inrush current withstand level was greater than 20 kA, so 20 kA was selected as the lightning current amplitude, and the No. 1 tower closest to the switching station was selected as the lightning strike point. The lightning current was simulated in PSCAD / EMTDC software using a controlled current source and a parallel connection of the lightning path impedance. The Heidler function was simulated using components from the CSMF library, and the lightning path impedance was set to 800 Ω.

[0102] Step S3 is as follows:

[0103] S31. Based on the lightning current source model built in step S2, simulate and analyze the overvoltage distribution characteristics of the equipment in the station. Consider the working conditions of lightning strikes of different polarities. That is, when a negative polarity lightning strikes the positive line, the cable on the positive line will bear a negative polarity voltage. Similarly, when a positive polarity lightning strikes the negative line, the cable on the negative line will generate a positive polarity overvoltage.

[0104] In this embodiment, the overvoltage distribution within the station is as follows: Figure 4 As shown in the figure, for equipment closer to the lightning strike point, the shorter the wavefront time of the lightning current, the higher the overvoltage generated on the equipment within the station under the same lightning current amplitude. Therefore, the overvoltage generated by 0.25 / 100 μs is relatively high. However, due to the attenuation and reflection of the wave due to the distance between the submarine cable and the lightning strike point, the overvoltage amplitude appears in the middle and late stages of the lightning current propagation oscillation. The 0.36 / 18 μs wavetail is relatively short and the duration of the lightning current energy is limited. The overvoltage at the submarine cable access point is less than 1 / 200 μs. Furthermore, the reverse polarity voltages caused by 1 / 200 μs and 0.25 / 100 μs are relatively high, at 177 kV and 151 kV, respectively.

[0105] S32. Compare the stress generated on the surge arrester by different lightning current parameters, and select the most stringent waveform based on the analysis results of S31. Specifically, study the influence of different lightning current waveform parameters on the overvoltage in the station and the reverse polarity voltage of the DC submarine cable, compare the electrical stress of the surge arrester when the lightning current parameters are different, and select the waveform with the highest overvoltage of the equipment in the station and the highest reverse polarity voltage of the submarine cable and the largest energy absorbed by the surge arrester as the most stringent waveform used for lightning protection design. Combine the calculation data in S2 to build a multiple lightning current source.

[0106] In this embodiment, among the above-mentioned single lightning strike waveforms, the surge arrester absorbs the most energy during the 1 / 200 μs back strike, reaching 148 kJ. Since the lightning current amplitude is the same, the difference in the current amplitude flowing through the surge arrester is small. Considering the electrical stress of the surge arrester and the overvoltage amplitude, the first back strike of 1 / 200 μs and subsequent back strikes of 0.25 / 100 μs are selected as the most stringent waveform design protection scheme.

[0107] Step S4 includes the following steps:

[0108] S41. Construct the broadband equivalent circuit of the reactor, and set the parameters of the reactor inductance, parallel capacitance across both ends, and capacitance to ground across both ends. In actual engineering, a surge arrester needs to be connected in parallel across the reactor to protect it. After adding the surge arrester, the broadband equivalent circuit of the reactor is as follows: Figure 2 As shown.

[0109] In this embodiment, the capacitance to ground before and after the reactor is 300pF, and the parallel capacitance across the reactor is 400pF.

[0110] S42. Simulate the lightning strike on a line of the same polarity to study the effect of reactor inductance on overvoltage suppression. Specifically, when the line is struck by the severe waveform selected in S3, analyze the suppression effect of the series reactor on the overvoltage of the equipment after the reactor, while also paying attention to the changes in overvoltage of the equipment before the reactor; simulate the overvoltage of the equipment in the station under different inductance values ​​to study the suppression effect of different reactor inductance values ​​on the overvoltage of the equipment after the reactor and the influence on the overvoltage amplitude of the equipment before the reactor; and preliminarily determine the reactor inductance value based on the combined effect of the reactor on the overvoltage distribution in the station and economic efficiency.

[0111] In this embodiment, the reactor is located after CT3. The simulation results for reactor inductance values ​​of 2, 4, 6, and 8 mH are as follows: Figure 5 As shown in the figure, when the inductance value is 4mH, the overvoltage of the equipment after the reactor is reduced by 294 kV, and the reactor has a saturation effect in suppressing overvoltage. However, after adding the reactor, the overvoltage of the equipment before the reactor increases significantly. Furthermore, as the inductance value of the reactor increases, the overvoltage of the equipment before the reactor increases slightly. Preliminary analysis suggests that a 4mH reactor is used to suppress overvoltage.

[0112] Step S5 specifically involves: based on the most stringent lightning current waveform selected in step S3 and the reactor selected in step S4, simulating the lightning current winding around the reverse polarity line, evaluating whether the reverse polarity lightning impulse overvoltage of the DC submarine cable exceeds the tolerance range. If the reverse polarity overvoltage has exceeded the tolerance level of the DC submarine cable, then return to step S4 to select a larger inductance value for re-verification until the optimal inductance value that makes the reverse polarity lightning impulse tolerance voltage of the DC submarine cable meet the requirements is selected.

[0113] In this embodiment, when a 4mH reactor is added, the reverse polarity voltage is less than 630kV when the most stringent waveform is used, which meets the requirements. Therefore, in the comprehensive step S4, a 4mH reactor is selected to suppress overvoltage.

[0114] Step S6 includes the following steps:

[0115] S61. Based on the overvoltage distribution of equipment in the station after adding reactors, optimize the surge arrester configuration scheme. Specifically, based on the overvoltage distribution characteristics of equipment in the station obtained in step S4, various surge arrester configuration schemes for substations are proposed by adding surge arresters, adjusting surge arrester positions, and setting line surge arresters, taking into account reactor surge arresters, thereby optimizing the surge arrester configuration scheme.

[0116] like Figure 6 As shown, the overvoltage at point L1 is relatively high after adding the reactor in this embodiment. Therefore, the following three configuration schemes are considered: Scheme 1, move the DL2 surge arrester to L1; Scheme 2, add a surge arrester at L1 based on the original configuration scheme; Scheme 3, move the DL2 surge arrester to L1 and add a line surge arrester on the tower closest to the switching station.

[0117] S62. Simulate and analyze the overvoltage characteristics of equipment in the station under different surge arrester configurations, verify the insulation margin of the equipment, the same-polarity lightning impulse withstand voltage of the DC submarine cable, and the electrical stress of each surge arrester, and select the optimal scheme. Verify whether the electrical stress of the surge arresters meets the requirements. If the electrical stress of the surge arresters exceeds their withstand range, consider increasing the number of surge arresters in parallel, and select the optimal scheme that has the best overvoltage suppression effect and meets the requirements for surge arrester electrical stress.

[0118] In this embodiment, the simulation results obtained using the most stringent waveform are shown in Table 6. When using Scheme 1, the highest overvoltage at coupling capacitor C1 is 1303kV; when using Scheme 2, the highest overvoltage at the insulating bushing is 1101kV; when using Scheme 3, the highest overvoltage at the insulating bushing is 1003kV, and the overvoltage at the submarine cable is 767kV. When using Scheme 3, the stress of each surge arrester is within the bearing range, and there is no need to optimize the number of parallel surge arresters. At the same time, the equipment insulation margin is high. Therefore, Scheme 3 is selected as the best scheme.

[0119] Table 6 Overvoltage amplitude of equipment with different surge arrester configurations (unit: kV)

[0120] ;

[0121] On the one hand, this invention takes into account the short return wavefront time of multiple lightning strikes, studies the influence of different lightning current waveform parameters on overvoltage of equipment in the station, proposes a method to identify the most severe overvoltage conditions, and, considering the short-term energy accumulation effect of multiple lightning strikes, proposes an evaluation method for the electrical stress of surge arresters subjected to multiple lightning strikes.

[0122] On the other hand, considering the limited lightning impulse withstand voltage of DC submarine cables, this invention designs an overvoltage protection scheme by combining two methods: series reactors and optimized surge arrester configuration. It also takes into account the verification of the reverse polarity lightning impulse withstand voltage of DC submarine cables, providing reference and guidance for the multi-lightning protection design and simulation analysis of overhead line-DC submarine cable conversion stations.

[0123] Therefore, the present invention adopts the above-mentioned method for multiple lightning overvoltage protection of overhead line-DC submarine cable conversion station, which solves the problem that the existing technology cannot design lightning overvoltage protection schemes for conversion stations and submarine cables based on the characteristics of lightning impulse withstand voltage of DC submarine cables.

[0124] 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 protecting against multiple lightning strike overvoltages at an overhead line-DC submarine cable conversion station, characterized in that, Includes the following steps: S1. Based on the electrical wiring diagram of the conversion station, the towers of the incoming line section, and the relevant parameters of the equipment in the conversion station, establish an electromagnetic transient simulation model of the overhead line-DC submarine cable conversion station; S2. Based on the statistical results of artificial lightning triggering and lightning location systems, as well as relevant standards, propose a lightning current waveform parameter model and build a lightning current source model. S3. Study the influence of lightning current waveform parameters on the overvoltage amplitude of equipment in the station, the reverse polarity voltage of DC submarine cable and the electrical stress of surge arrester, select the most stringent waveform and build multiple lightning current sources. S4. Construct a broadband equivalent circuit model of the reactor, simulate the working condition of lightning strike on the same polarity line, study the influence of the reactor inductance value on the overvoltage of equipment at various points in the station and DC submarine cable, and preliminarily determine the reactor inductance value. S5. Based on the most stringent lightning current waveform and the reactor selected in step S4, simulate the lightning current winding around the reverse polarity line to evaluate whether the reverse polarity lightning impulse overvoltage of the DC submarine cable exceeds the tolerance range. If it does not meet the requirements, return to S4 to increase the inductance value of the reactor until the optimal inductance value that makes the reverse polarity lightning impulse voltage of the DC submarine cable meet the requirements is selected. S6. Optimize the surge arrester configuration scheme based on the overvoltage distribution characteristics of the equipment after adding the reactor, check whether the electrical stress of the surge arrester meets the requirements. If the electrical stress of the surge arrester exceeds its tolerance range, consider increasing the number of parallel surge arresters and select the optimal scheme with the best overvoltage suppression effect and the surge arrester electrical stress meeting the requirements. Step S2 includes the following steps: S21. Based on the different lightning current waveform parameters obtained by artificial lightning strike, calculate the average time of the lightning current wavefront and the average time of the lightning current wavetail under subsequent lightning strikes, propose an actual lightning current parameter model, and fit it using the Heidler lightning current waveform function. S22. Based on the statistical parameters in step S21, propose multiple lightning current parameter models; S23. Based on the statistical data of the lightning location system, calculate the time interval between two adjacent return strokes and the proportion of different return stroke counts to determine the number of return strokes and the time interval between two adjacent return strokes; determine the lightning current amplitude by comparing the lightning withstand level of the tower and the magnitude of the maximum lightning current; establish a lightning current source model in the simulation software. Step S23 specifically involves: combining statistical data from the lightning location system to calculate the time interval between two adjacent return strokes and the proportion of different return stroke counts, thus determining the time interval and number of return strokes for multiple lightning strikes; determining the lightning current amplitude by comparing the tower's lightning withstand level and the maximum lightning current; calculating the maximum lightning current of the tower using an electrical geometric model; simulating different amplitude lightning currents striking the tower using software to obtain the maximum lightning current amplitude that will not cause flashover of the insulator string, which is used as the tower's lightning withstand level; and taking the smaller of the two values ​​as the lightning current amplitude for single and multiple lightning strikes; and establishing a single lightning strike current source model by connecting a controlled current source in parallel with the lightning channel impedance.

2. The method for protecting an overhead line-DC submarine cable conversion station from multiple lightning strikes with overvoltage as described in claim 1, characterized in that, Step S1 specifically includes: S11. Based on the electrical wiring diagram of the conversion station, some equipment in the conversion station is simulated using impulse inlet capacitors; S12. Set the scatter plot of the nonlinear resistance current-current characteristic in the simulation according to the technical parameters and current-current characteristics of the surge arrester; S13. Construct a multi-wave impedance model for the tower based on the tower shape diagram of the incoming line segment.

3. The method for protecting an overhead line-DC submarine cable conversion station from multiple lightning strikes by overvoltage as described in claim 1, characterized in that, In step S21, the actual lightning strike current source model is fitted using the Heidler function, and the resulting formula is shown below: ； In the formula: Indicates lightning current. The peak value of the base current. This is the wavefront attenuation coefficient. This is the wave tail attenuation coefficient. n η is a parameter describing the steepness of the lightning base current, where η is the current peak correction factor.

4. The method for protecting an overhead line-DC submarine cable conversion station from multiple lightning strikes according to claim 1, characterized in that, Step S3 is as follows: S31. Based on the lightning current source model built in step S2, simulate and analyze the overvoltage distribution characteristics of equipment in the station when the lightning current waveform parameters are different. S32. Compare the stress generated on the surge arrester by different lightning current parameters, and select the most stringent waveform based on the analysis results of S31.

5. The method for protecting an overhead line-DC submarine cable conversion station from multiple lightning strikes by overvoltage as described in claim 4, characterized in that, Step S31 specifically involves: based on the lightning current source model built in step S2, simulating and analyzing the overvoltage distribution characteristics of equipment in the station when the lightning current waveform parameters are different, and further considering the working conditions of lightning strikes of different polarities, specifically including two working conditions: negative polarity lightning strikes the positive line and positive polarity lightning strikes the negative line. In this case, the submarine cable will be subjected to reverse polarity voltage.

6. The method for protecting an overhead line-DC submarine cable conversion station from multiple lightning strikes according to claim 1, characterized in that: Step S4 includes the following steps: S41. Construct the broadband equivalent circuit of the reactor, and set the parameters of the reactor inductance, parallel capacitor at both ends and capacitance to ground at both ends. S42. Simulate the lightning strike on a line of the same polarity to study the effect of reactor inductance on overvoltage suppression.

7. The method for protecting an overhead line-DC submarine cable conversion station from multiple lightning strikes by overvoltage as described in claim 1, characterized in that, Step S5 specifically involves: based on the most stringent lightning current waveform selected in step S3 and the reactor selected in step S4, simulating the lightning current winding around the reverse polarity line, evaluating whether the reverse polarity lightning impulse overvoltage of the DC submarine cable exceeds the tolerance range. If the reverse polarity overvoltage has exceeded the tolerance level of the DC submarine cable, then return to step S4 to select a larger inductance value for re-verification until the optimal inductance value that makes the reverse polarity lightning impulse tolerance voltage of the DC submarine cable meet the requirements is selected.

8. The method for protecting an overhead line-DC submarine cable conversion station from multiple lightning strikes by overvoltage as described in claim 1, characterized in that, Step S6 includes the following steps: S61. Optimize the surge arrester configuration scheme based on the overvoltage distribution of equipment in the station after adding reactors; S62. Simulate and analyze the overvoltage characteristics of equipment in the station under different surge arrester configuration schemes, check the insulation margin of equipment in the station, the same polarity lightning impulse withstand voltage of DC submarine cable and the electrical stress of each surge arrester, and select the best scheme.

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