Decision-making method for neutral point grounding of offshore wind farms considering submarine cable life

Abstract

The present invention discloses a method for determining the neutral grounding method for offshore wind farms that takes into account the lifespan of submarine cables. By establishing evaluation indicators for offshore wind power demand compliance, grid fault risk, and relay protection correct operation rate, this method comprehensively reflects the impact of neutral grounding methods on the safety and reliability of offshore wind power grid-connected systems. A neutral grounding optimization model for offshore wind farms is then constructed. The method ultimately selects the neutral grounding method with the best economic and reliability among the following methods: ungrounded neutral point, grounded neutral point via a low resistance, grounded neutral point via a high resistance, grounded neutral point via an arc suppression coil, and directly grounded neutral point. This method has excellent practicality and provides guidance for the safe operation of offshore wind power systems and the planning of neutral grounding methods.

Description

Technical Field

[0001] The present invention belongs to the field of power system planning and operation, and in particular relates to a method for deciding a neutral point grounding mode for an offshore wind farm taking into account the life of a submarine cable. Background Art

[0002] Neutral point grounding is closely related to the safety, economy, and reliability of power system operation. Existing neutral point grounding methods primarily include ungrounded neutral point, neutral point grounding via a resistor, and neutral point grounding via an arc suppression coil. In the ungrounded neutral point method, when a single-phase grounding fault occurs in the system, the voltage magnitude and phase between the phases remain unchanged, and the balance of the three-phase system is not disrupted, allowing continued operation for a short period of time. However, to prevent the fault from expanding and causing a phase-to-phase short circuit, or single-phase arc grounding, which could cause system resonance and overvoltage, leading to system failure, the operating time with the fault point must not exceed 2 hours.

[0003] If a resistor is connected in series with the neutral point of a transformer or a grounding transformer to dissipate the electromagnetic energy from intermittent arc overvoltages, the neutral point potential is lowered, slowing the rise in voltage on the faulted phase. This reduces the likelihood of arc reignition, suppresses the magnitude of grid overvoltage, and enables selective ground fault protection. Furthermore, with a resistor grounding system, the electrical signature of a single-phase ground fault is more distinct, generally ensuring proper operation and isolation of the fault by relay protection devices. However, relay protection operation may interrupt power to some loads, impacting power supply reliability. In a system with a neutral point grounded via an arc suppression coil, the ground current is independent of the fault location. Because the residual current is minimal, the ground arc is extinguished instantaneously, effectively limiting the potential damage from arc overvoltages. Furthermore, arc suppression coils prevent most single-phase ground faults from developing into phase-to-phase short circuits, eliminating the need for relay protection and automatic devices to operate, thus improving system reliability.

[0004] As offshore wind farms continue to expand and wind power penetration rates increase, system topologies become increasingly complex and multi-sourced, posing significant challenges to their safe and stable operation. Wind turbines and offshore substations are often unmanned, and any incidents require significant time for maintenance personnel to arrive and address them. Therefore, electrical equipment selection must prioritize efficiency and reliability, minimizing failures. If a fault does occur, relay protection must operate quickly and accurately to eliminate the fault and prevent it from spreading. Furthermore, as wind farm installed capacity continues to increase, the length and number of on-site collector lines continue to increase, placing higher demands on the sustained and stable operation of these lines. Increasingly, wind farm collector lines are being constructed using power cables, which carry significant capacitive currents to ground during operation, making them susceptible to intermittent arcing overvoltages that can damage electrical equipment. The neutral point grounding method of offshore wind farms directly affects the operation mode of large-scale wind power grid-connected systems, the insulation level of equipment, etc. Therefore, establishing a scientific and reasonable decision-making method for selecting the neutral point grounding method of offshore wind farms is of great significance to the safe operation and planning decision-making of offshore wind power grid-connected systems.

[0005] Currently, offshore wind farms generally use an ungrounded neutral point. Due to a lack of technology to determine the neutral point grounding method for wind power systems, selecting the optimal neutral point grounding method among various options is difficult. In particular, as a crucial component of offshore wind farms, the reliability of submarine cables has a direct impact on the safety and economic efficiency of offshore wind farms. Therefore, the selection of a neutral point grounding method should fully consider its impact on cable lifespan. However, no relevant technology has been reported. Therefore, there is still a gap in the technology for determining the neutral point grounding method for offshore wind farms. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a decision-making method for the neutral point grounding mode of an offshore wind farm that takes into account the life of the submarine cable. The method quantifies the safety and reliability of the long-term operation of the offshore wind farm under different neutral point grounding modes by measuring the degree of compliance of offshore wind power demand, the risk of grid faults, and the correct operation rate of relay protection, thereby evaluating the impact of different neutral point grounding modes on the offshore wind power grid-connected system and achieving the purpose of optimizing the neutral point grounding mode of the offshore wind farm.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The present invention discloses a method for determining a neutral point grounding mode for an offshore wind farm taking into account the life of a submarine cable, comprising the following steps:

[0009] S101. Calculate the offshore wind farm demand compliance index M based on the wind farm operation status and planning expectations;

[0010] S102. Establish an impact function for the factory life of the submarine cable and calculate the power grid failure risk index R;

[0011] S103. Calculate the relay protection correct operation rate index X of the offshore wind farm transmission line based on wind power system faults and relay protection operation conditions;

[0012] S104. Build an optimization model for neutral point grounding of offshore wind farms using the demand compliance index M, the grid fault risk index R, and the relay protection correct operation rate index X.

[0013] S105. Utilize an optimization algorithm to solve a neutral point grounding optimization model of the wind power system and determine the neutral point grounding method.

[0014] In summary, the present invention solves the problem of existing technologies neglecting the impact of submarine cable life, attaches importance to the measurement of subsequent power grid operation reliability before and after a fault, and is of great significance to the safe operation of offshore wind power systems and the planning and decision-making of neutral point grounding methods. The present invention is applicable to the selection and decision-making of various neutral point grounding methods and has good practicality.

[0015] Compared with the prior art, the present invention has the following beneficial effects:

[0016] 1. Existing technologies are suitable for determining the neutral point grounding method for distribution networks. However, due to differences in the structure and operating mode of offshore wind farms, existing technologies cannot be directly applied to selecting the neutral point grounding method for offshore wind farms. This invention considers key factors affecting the safety, reliability, and economic efficiency of offshore wind farms and can determine the optimal neutral point grounding method for offshore wind farms.

[0017] 2. Existing technologies focus on short-circuit current and economic costs when selecting neutral-point grounding methods, with relatively simple consideration of overvoltage impacts, using insulation as the only constraint. This invention considers the impact of overcurrent and overvoltage on submarine cable insulation under different grounding methods. Because submarine cables are a critical component and a weak link in offshore wind farms, this invention maximizes the safety and reliability of different neutral-point grounding methods.

[0018] 3. Existing techniques often prioritize the correct operation of relay protection when selecting a neutral grounding method, but ignore the impact of wind turbines on relay protection, potentially leading to significant errors. This invention considers the correct operation of system relay protection under fault conditions for wind turbine grid-connected systems under different grounding methods, resulting in more accurate neutral grounding method decisions.

[0019] 4. The existing technology does not consider the impact of changes in wind power output. The present invention takes into account the wind speed distribution in the wind farm area and the impact of faults on wind turbine output, more accurately quantifies the reliability and safety under different neutral point grounding methods, and ensures the universality of the decided neutral point grounding method under various working conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to make the purpose, technical solutions and advantages of the invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings, in which:

[0021] Figure 1 The present invention discloses a flowchart of a specific implementation method of a method for deciding the neutral point grounding mode of an offshore wind farm taking into account the life of a submarine cable. DETAILED DESCRIPTION

[0022] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0023] like Figure 1 As shown, the present invention discloses a method for deciding the neutral point grounding mode of an offshore wind farm taking into account the life of the submarine cable, comprising the following steps:

[0024] S101. Calculate the offshore wind farm demand compliance index M based on the wind farm operation status and planning expectations;

[0025] S102. Establish an impact function for the factory life of the submarine cable and calculate the power grid failure risk index R;

[0026] S103. Calculate the relay protection correct operation rate index X of the offshore wind farm transmission line based on wind power system faults and relay protection operation conditions;

[0027] S104. Build an optimization model for neutral point grounding of offshore wind farms using the demand compliance index M, the grid fault risk index R, and the relay protection correct operation rate index X.

[0028] S105. Utilize an optimization algorithm to solve a neutral point grounding optimization model of the wind power system and determine the neutral point grounding method.

[0029] Among them, the neutral point grounding methods include ungrounded neutral point, grounded neutral point through a small resistor, grounded neutral point through a high resistance, grounded neutral point through an arc suppression coil and direct grounding of neutral point.

[0030] To further optimize the above technical solution, in step S101, the offshore wind power demand compliance index M is used to characterize the impact of random faults in the offshore wind farm collection system and wind speed randomness on the wind farm output. Therefore, the embodiment of the present invention can calculate the offshore wind power demand compliance index using the offshore wind power output demand and the output expectation of the offshore wind farm. The specific calculation method is:

[0031]

[0032] Where, P N is the demand for offshore wind power output; P WF.REL Output expectations for offshore wind farms.

[0033] To further optimize the above technical solution, it is necessary to determine the scale of the wind farm in advance, that is, the number of wind turbines involved in power generation and the output of each wind turbine under normal circumstances; fully consider the impact of random failures in the wind farm's power collection system and determine the failure probability of each wind turbine. Therefore, the following steps are also included:

[0034] The expected output of offshore wind farm P WF.REL Calculated as follows:

[0035]

[0036] Where p is the probability of wind turbine failure; a is the number of wind turbines in the offshore wind farm; T is the statistical time length; P G (t) is the output of a single wind turbine at time t, and P G It can be expressed as the output of a single wind turbine at any moment.

[0037] In order to further optimize the above technical solutions, the uncertainty of wind speed needs to be fully considered. Therefore, the randomness of wind speed affects the output of wind farms. The wind speed distribution is a long-term statistical data, which is obtained before the planning and design of wind farms. The output P of a single wind turbine at any time should be calculated according to the following formula: G :

[0038]

[0039] Where V ci is the cut-in wind speed; V co is the cut-out wind speed; V R is the rated wind speed; P R is the rated output power; u(v) is the power characteristic function of the wind turbine; v(t) is the wind speed distribution function at time t; u(v) is the power characteristic function of the wind turbine.

[0040] To further optimize the above technical solution, in some embodiments of the present invention, the wind turbine power characteristic function can be expressed as:

[0041]

[0042] To further optimize the above technical solution, in step S102, the impact of thermal aging and electrical aging of the submarine cable caused by overcurrent and overvoltage on the life of the submarine cable under different neutral point grounding methods is fully considered. Therefore, the submarine cable power grid fault risk index R is calculated according to the following method:

[0043] R=max{ΔL u ,ΔL i}

[0044] Where ΔL u ΔL is the reduction in the life of the submarine cable due to electrical aging; i It is the reduction of thermal aging life of submarine cable.

[0045] To further optimize the above technical solution, the impact of electrical aging on the life of submarine cables requires determining the fault duration and the voltage value of the submarine cable at each fault, so the following steps are also included:

[0046] Electrical aging life reduction ΔL u Calculated as follows:

[0047]

[0048] Where U fj U represents the voltage of the non-fault submarine cable relative to the ground when the jth system fault occurs; b Indicates the rated voltage when the system is operating normally; T j represents the fault duration of the jth system fault; n is the first technical parameter of the submarine cable, which can be obtained through the step breakdown test; N is the number of faults in the statistical time period.

[0049] In order to further optimize the above technical solution, the reduction in thermal aging life of submarine cables ΔL is calculated according to the following method: i :

[0050]

[0051] Where ΔL i.j is the reduction in the thermal aging life of the submarine cable when the jth system failure occurs; D is the second technical parameter of the submarine cable, which can be obtained through thermal aging experiments; m is the mass of the submarine cable conductor; C0 is the specific heat of the conductor at 0℃; R0 is the resistance of the conductor at 0℃; α is the temperature coefficient of resistivity; β is the temperature coefficient of specific heat; θ e I is the temperature of the cable during long-term normal operation; kj is the magnitude of the fault current during the j-th system fault.

[0052] To further optimize the above technical solution, in step S103, the relay protection correct operation rate index X is calculated according to the following method:

[0053]

[0054] Where η and λ are the number of correct protection actions and the number of faults during the statistical period, respectively.

[0055] To further optimize the above technical solution, in step S104, the neutral point grounding optimization model of the offshore wind farm includes an objective function and corresponding constraints, wherein:

[0056] The objective function of the neutral point grounding optimization model for offshore wind farms is:

[0057]

[0058] The constraints of the optimization model for neutral point grounding of offshore wind farms are:

[0059] I ij ≤I ij.xu

[0060] U i.min ≤U i ≤U i.max

[0061] |P j |≤P j.max

[0062] θ d ≤θ d.xu

[0063] U f ≤U ins

[0064] Where, O represents the optimization parameter of the neutral point grounding method of the offshore wind farm, which is used to represent the target affecting the safety and reliability of the offshore wind farm; I ij and I ij.xu The line l is composed of the i-th load node and the j-th load node respectively. ij Current and maximum allowable current; U i.min and U i.max are the minimum and maximum limits of the voltage at the i-th load node respectively; P j and P j.max are the power delivered to the jth load node and the maximum allowable power respectively; θ d and θ d.xu They are the maximum temperature and the maximum allowable temperature of the line when short-circuited; U f The non-fault relative ground voltage at the short-circuit point; U ins It is the maximum withstand voltage of transmission line insulation.

[0065] It can be understood that in the embodiment of the present invention, the optimization algorithm used to solve the optimization model of the neutral point grounding method of the wind power system can adopt a particle swarm algorithm, an ant colony algorithm, a simulated annealing algorithm, etc. The present invention does not limit the specific optimization algorithm. Those skilled in the art can directly use the existing optimization algorithm to solve the optimization model of the neutral point grounding method of the wind power system, and can select the optimal neutral point grounding method among the neutral point not grounded, the neutral point grounded through a small resistor, the neutral point grounded through a high resistance, the neutral point grounded through an arc suppression coil, and the neutral point directly grounded.

[0066] In some embodiments, the present embodiment may use a particle swarm algorithm to solve the wind power system neutral point grounding mode optimization model, and the solution process may include:

[0067] S1, randomly initialize the speed and position of each particle in the population;

[0068] S2. Calculate the fitness value of all particles based on the fitness function, i.e., the objective function of the neutral point grounding optimization model for offshore wind farms, and initialize the local optimal value and location of each particle;

[0069] S3, initialize the global optimal value of the population and its location;

[0070] S4. Iteratively update the fitness value of each particle, and update the local optimal value and position of each particle, update the global optimal value of the population and its position, the local optimal values of all particles and the positions corresponding to the particles;

[0071] S5. Update the global optimal value of the population and the position corresponding to the particle;

[0072] S6. Obtain the final global optimal value of the population and its corresponding position;

[0073] S7. Based on the positions of the particles, a set of feasible solutions to the neutral point grounding problem of the wind power system is obtained.

[0074] In an embodiment of the present invention, this decision-making method considers the impact of different neutral-point grounding methods on the safety and reliability of offshore wind farms and utilizes various indicators to evaluate the safety and reliability of offshore wind farms under different neutral-point grounding methods. The objective function is the product of the offshore wind farm demand compliance index M and the protection correct operation rate index X, divided by the grid fault risk index R (value O). This objective function, maxO, represents the highest grid operational reliability. For offshore wind farm operation planning, the objectives should be high offshore wind power demand compliance, high relay protection correct operation rate, and low grid fault risk. Unlike existing distribution network neutral-point grounding method decision-making optimization models, this method prioritizes offshore wind farm operational reliability, fully considering the electrothermal aging of submarine cables, wind power randomness, and relay protection reliability under different neutral-point grounding methods, thereby establishing a neutral-point grounding method decision-making model for offshore wind farm collection systems.

[0075] In summary, the present invention discloses a decision method for the neutral point grounding mode of an offshore wind farm taking into account the life of the submarine cable. By establishing evaluation indicators of offshore wind power demand compliance, grid fault risk, and correct operation rate of relay protection, it comprehensively reflects the impact of the neutral point grounding mode on the offshore wind power grid-connected system. Based on the electrothermal aging law of the submarine cable life, an influence function of the submarine cable factory life is constructed, thereby constructing an optimization model for the neutral point grounding mode of an offshore wind farm. By comparing and selecting, the neutral point is finally selected from the following modes: no grounding at the neutral point, grounding at the neutral point through a small resistor, grounding at the neutral point through a high resistance, grounding at the neutral point through an arc suppression coil, and direct grounding at the neutral point. The neutral point grounding method with the best economy and reliability is selected. That is, according to the impact of random faults of the offshore wind farm collection system and wind speed randomness on the wind farm output, the life of the submarine cable, and the number of correct protection operations and faults when the offshore wind farm collection system fails under different neutral point grounding methods, the demand compliance index M, the power grid fault risk index R, and the relay protection correct operation rate index X under different neutral point grounding methods are obtained. The objective function values under different neutral point grounding methods are calculated. Under the premise of satisfying all constraints, the neutral point grounding method with the largest objective function value is selected as the optimal neutral point grounding method.

[0076] In some embodiments of the present invention, wind speed data from an offshore wind farm in eastern China was used as a sample. After fitting the wind speed data for this offshore wind farm region, the basic wind speed, harmonic order, harmonic amplitude, and wind speed fundamental frequency were obtained, as shown in Table 1. By consulting the submarine cable manufacturer's manual, wind turbine manufacturer's manual, and offshore wind farm planning requirements, the technical parameters of the wind turbine and submarine cable were obtained, as shown in Table 2. Through long-term monitoring, the probability of wind turbine and line failure under different neutral point grounding methods for offshore wind farm collection systems is shown in Table 3. Statistics on offshore wind farm collection system failures and relay protection actions, as well as statistical data on fault information, are shown in Tables 4 and 5, respectively.

[0077] Table 1 Wind speed model parameters

[0078]

[0079] Table 2 Technical parameters of wind turbines and submarine cables

[0080]

[0081]

[0082] Table 3 Failure probability of wind turbines and lines

[0083]

[0084] Table 4 Statistical data of faults and relay protection actions

[0085]

[0086] Table 5 Fault information statistics

[0087]

[0088] The above data are analyzed using the offshore wind farm neutral point grounding method decision method that takes into account the life of the submarine cable. It can be determined that the optimal offshore wind farm collector line neutral point grounding method is grounding through a small resistance.

[0089] Specifically, the wind power demand compliance index M for different neutral point grounding methods for offshore wind farm collection systems is calculated as shown in Table 6. The wind power demand compliance is highest when the neutral point of the offshore wind farm collection system is grounded via a low resistance, lowest when it is directly grounded, and similar for ungrounded, high-resistance, and arc suppression coil-grounded systems.

[0090] Table 6 Calculation results of wind power demand compliance index M

[0091]

[0092] Table 7 shows the calculated relay protection correct operation rate (X) for different neutral point grounding methods in offshore wind farm collection systems. The correct operation rate is higher for direct neutral point grounding and low-resistance grounding, while lower for ungrounded and high-resistance grounding. Relay protection operates most reliably when the neutral point is grounded with low resistance.

[0093] Table 7 Calculation results of relay protection correct operation rate index X

[0094]

[0095] Table 8 shows the calculated grid fault risk index, R, for different neutral point grounding methods for offshore wind farm collection systems. Low-resistance grounding of the neutral point in the offshore wind farm collection system has the lowest grid fault risk and higher grid operation continuity. Direct grounding has the highest grid fault risk and the lowest grid operation continuity. The grid fault risk is similar when the neutral point is ungrounded, high-resistance grounded, or grounded with an arc suppression coil.

[0096] Table 8 Calculation results of power grid fault risk index R

[0097]

[0098]

[0099] The objective functions of different neutral point grounding methods for offshore wind farm collection systems were calculated, and the results are shown in Table 9. The objective function for the offshore wind farm collection system neutral point grounded via a low resistance is larger, while the objective function for the neutral point directly grounded is smaller. The objective functions for the neutral point ungrounded, grounded via an arc suppression coil, and high-resistance grounding are similar.

[0100] Table 9 Calculation results of power grid safe operation reliability O

[0101]

[0102] Safety indicators and cable insulation safety parameters are shown in Table 10. For the neutral point directly grounded method, during a single-phase ground fault, the non-fault-phase voltage relative to ground is low, but the conductor short-circuit temperature is excessive, exceeding the maximum allowable short-circuit temperature of the cable, resulting in a failure in safety indicators. When the neutral point is grounded via a low resistance, the conductor short-circuit temperature is high, but does not exceed the maximum allowable short-circuit temperature of the cable, thus meeting safety requirements. For the ungrounded neutral point, high-resistance grounding, and arc suppression coil grounding methods, the cable short-circuit temperature and non-fault-phase voltage relative to ground are similar, all satisfying safety constraints. The low-resistance neutral point grounding method meets safety requirements, and the objective function is the optimal of the five offshore wind farm collector system grounding schemes, validating the effectiveness and practicality of the decision-making model.

[0103] Table 10 Safety indicators and submarine cable insulation safety parameters

[0104]

[0105] The above analysis shows that this method can be applied to the selection and decision-making of various neutral point grounding methods, has good practicality, and is of great significance to the safe operation of offshore wind power systems and the planning and decision-making of neutral point grounding methods. It solves the problem that existing technologies ignore the impact of submarine cable life, and attaches importance to the measurement of subsequent power grid operation reliability before and after faults.

[0106] In the description of the present invention, it should be understood that the terms "coaxial", "bottom", "one end", "top", "middle", "the other end", "upper", "one side", "top", "inside", "outside", "front", "center", "both ends", etc., indicating the orientation or position relationship, are based on the orientation or position relationship shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operate in a specific orientation, and therefore cannot be understood as limiting the present invention.

[0107] In the present invention, unless otherwise clearly stipulated and limited, the terms "installation", "setting", "connection", "fixation", "rotation" and the like should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be the internal connection of two elements or the interaction relationship between two elements. Unless otherwise clearly defined, ordinary technicians in this field can understand the specific meanings of the above terms in the present invention according to the specific circumstances.

[0108] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for determining the neutral point grounding mode of an offshore wind farm considering the life of the submarine cable, characterized in that: The method includes calculating an offshore wind farm demand compliance index M based on the operation status and planning expectations of the offshore wind farm; establishing an influence function of the factory life of the submarine cable and calculating a power grid fault risk index R; calculating a relay protection correct operation rate index X of the offshore wind farm transmission line based on wind power system faults and relay protection operation conditions; constructing an offshore wind farm neutral point grounding mode optimization model using the demand compliance index M, the power grid fault risk index R, and the relay protection correct operation rate index X; and solving the wind power system neutral point grounding mode optimization model using an optimization algorithm to determine the neutral point grounding mode. The neutral point grounding optimization model for offshore wind farms includes an objective function and corresponding constraints, where: The objective function of the neutral point grounding optimization model for offshore wind farms is: The constraints of the optimization model for neutral point grounding of offshore wind farms are: I ij ≤I ij.xu IN i.min ≤U i ≤U i.max |P j |≤P j.max i d ≤θ d.xu IN f ≤U ins Where, O represents the optimization parameter of neutral point grounding mode of offshore wind farm; I ij and I ij.xu The line l is composed of the i-th load node and the j-th load node respectively. ij Current and maximum allowable current; U i.min and U i.max are the minimum and maximum limits of the voltage at the i-th load node respectively; P j and P j.max are the power delivered to the jth load node and the maximum allowable power respectively; θ d and θ d.xu They are the maximum temperature and the maximum allowable temperature of the line when short-circuited; U f The non-fault relative ground voltage at the short-circuit point; U ins It is the maximum withstand voltage of transmission line insulation.

2. The offshore wind farm neutral point grounding mode decision method considering the life of the submarine cable according to claim 1 is characterized in that: The offshore wind farm demand compliance index M is used to characterize the impact of random faults in the offshore wind farm collection system and wind speed randomness on the wind farm output. The specific calculation method is: Where, P N is the demand for offshore wind power output; P WF.REL Output expectations for offshore wind farms.

3. The offshore wind farm neutral point grounding mode decision method considering the life of the submarine cable according to claim 2 is characterized in that: The output expectation of an offshore wind farm is calculated as follows: Where p is the probability of wind turbine failure; a is the number of wind turbines in the offshore wind farm; T is the statistical time length; P G (t) is the output of a single wind turbine at time t, expressed as: Where V ci is the cut-in wind speed; V co is the cut-out wind speed; V R is the rated wind speed; P R is the rated output power; v(t) is the wind speed distribution function at time t; u(v) is the power characteristic function of the wind turbine.

4. The method for determining the neutral point grounding mode of an offshore wind farm considering the life of the submarine cable according to claim 1, characterized in that: The grid failure risk index R is calculated as follows: R=max{ΔL u ,ΔL i } Where ΔL u ΔL is the reduction in the life of the submarine cable due to electrical aging; i It is the reduction of thermal aging life of submarine cable.

5. The offshore wind farm neutral point grounding mode decision method considering the life of the submarine cable according to claim 4 is characterized by: The reduction in the electrical aging life of the submarine cable is calculated as follows: Where U fj U represents the voltage of the non-fault submarine cable relative to the ground when the jth system fault occurs; b Indicates the rated voltage when the system is operating normally; T j represents the fault duration of the jth system fault; n is the first technical parameter of the submarine cable, obtained through the step breakdown test; N is the number of faults in the statistical time period.

6. The offshore wind farm neutral point grounding mode decision method considering the life of the submarine cable according to claim 4 is characterized by: The reduction in the thermal aging life of the submarine cable is calculated as follows: Where N is the number of failures in the statistical time period; ΔL i.j is the reduction in the thermal aging life of the submarine cable when the jth system failure occurs; D is the second technical parameter of the submarine cable, obtained through thermal aging experiments; m is the mass of the submarine cable conductor; C0 is the specific heat of the conductor at 0℃; R0 is the resistance of the conductor at 0℃; α is the temperature coefficient of resistivity; β is the temperature coefficient of specific heat; θ e I is the temperature of the cable during long-term normal operation; kj is the magnitude of the fault current when the system fails for the jth time; T j Indicates the duration of the j-th system failure.

7. The method for determining the neutral point grounding mode of an offshore wind farm considering the life of the submarine cable according to claim 1, characterized in that: The relay protection correct operation rate index X is calculated as follows: Where η and λ are the number of correct protection actions and the number of faults during the statistical period, respectively.

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