High-precision practical calculation method for short-circuit current of high-resistance grounded ship power supply network

Abstract

The invention relates to a high-precision practical calculation method for short-circuit current of a high-resistance grounded ship power supply network, and the method comprises the steps: firstly, rapidly solving symmetric and asymmetric short-circuit current of a fault branch through a Thevenin equivalent method and an improved composite sequence network method under a reasonable assumed condition; and calculating a current distribution coefficient in the fault additional state network by adopting a superposition theorem, and further solving a protection branch current. According to the invention, the problem of complex protection branch current calculation under the multi-power-supply condition is solved. Through theoretical analysis and simulation verification of the method, it can be considered that under the assumed condition, the short-circuit current of a fault branch is mainly influenced by the number of generators put into operation, and the change of the fault position does not influence the magnitude of the short-circuit current of the fault branch.

Description

Technical Field

[0001] This invention belongs to the field of short-circuit current calculation technology for ship power supply systems, specifically involving a high-precision and practical calculation method for short-circuit current in high-resistance grounded ship power supply networks, applicable to short-circuit current calculation of various ship power station systems under multiple operating conditions. Background Technology With the significant increase in the capacity of ship power supply networks, to adapt to the multi-condition operation requirements of ships during cruising, operations, and berthing, ship power station configurations have gradually evolved from single or dual power stations to multi-power station systems. Simultaneously, the network structure has also upgraded from a simple radial structure to a complex radial or ring structure with multiple branches. This structural change improves the reliability of ship power supply networks but also increases operational complexity. Among various ship electrical faults, short-circuit faults not only occur most frequently but are also the most dangerous. Rapid and accurate short-circuit current calculations provide crucial information for relay protection settings and switch breaking capacity verification, which is essential for ensuring the safe operation of ships.

[0002] Currently, the calculation of short-circuit current for ship power supply networks typically combines the equivalent generator method with the star-network transformation method. However, when this existing method is directly applied to structurally complex systems, the calculation process is cumbersome and the error is relatively large, requiring targeted improvements.

[0003] Ship power supply networks widely adopt a neutral point grounding method with high resistance. This grounding method can effectively suppress power frequency overvoltage and transient overvoltage caused by single-phase grounding faults. However, due to the small amplitude of the fault current, fault detection and location technology faces challenges.

[0004] In existing related technologies, such as patent document (CN102255290A), a current adaptive protection method is disclosed. This method uses the fault network simplification method and fault steady-state component theory to obtain the relationship between the node voltage before the fault and the fault steady-state injection current. It uses the network system parameter matrix to construct a linear mapping relationship between the fault steady-state injection current and the fault steady-state branch current, thus constructing current adaptive protection. However, this method is mainly aimed at the relay protection of power systems (especially power grids with distributed power sources) and does not consider the special structure and high-resistance grounding characteristics of ship power supply networks, so it cannot be directly applied to the short-circuit current calculation of ship power supply networks. Patent document (CN105373834B) discloses a distribution network short-circuit current calculation method and system based on distributed computing. This method is built on a distributed computing platform, obtains distribution network parameters through SCADA system and CIM model, transforms the physical model into a computational model, determines the fault type and calculates the circuit parameters at the time of the fault, calculates the normal operating state using the forward and backward push method, and superimposes the normal operating state with the fault state to obtain the fault current. However, this method is primarily designed for power distribution networks and does not consider the high-resistance grounding characteristics of ship power supply networks, nor is it optimized for the multi-station structure of ship power supply networks. Power distribution networks and ship power supply networks differ significantly in structure and parameter characteristics. Ship power supply networks are typically smaller in scale, more complex in structure, and employ high-resistance grounding methods. This method does not take these special characteristics into account and cannot be directly applied to ship power supply networks. Summary of the Invention This invention proposes a high-precision and practical method for calculating short-circuit currents in complex ship power supply networks employing high-resistance grounding. First, under reasonable assumptions, the symmetrical and asymmetrical short-circuit currents of faulted branches are rapidly solved using the Thevenin equivalent method and an improved composite sequence network method to address the challenges posed by diverse power supply network topologies. Then, the superposition theorem is employed to calculate current distribution coefficients in the fault-addition state network, thereby determining the protection branch current and resolving the complexity of calculating protection branch currents under multi-source conditions.

[0005] To achieve the above objectives, the technical solution of the present invention is: a high-precision and practical calculation method for short-circuit current in a high-resistance grounded ship power supply network, comprising the following steps: Step 1: Input the calculation parameters, including the system rated line voltage, impedance of large and small capacity generators and the number of generators in operation, generator neutral point grounding resistance, and total system capacitive reactance; Step 2: Calculate the fault branch current. The equivalent potential of the system and the positive-sequence and zero-sequence equivalent impedances are calculated using Thevenin's equivalent theorem and the improved composite sequence network method. Then, the phase current and zero-sequence current of the fault branch are calculated. Step 3: Calculate the protection branch current. After determining the fault branch in the fault additional state network, calculate the distribution coefficient of each protection branch and multiply it by the phase current and zero-sequence current of the fault branch respectively to obtain the fault phase current and zero-sequence current of each protection branch. Furthermore, in step 1, the assumptions for the system parameters include: (1) The positive and negative sequence impedances of all components in the system are the same; the cable impedance is much smaller than its capacitive reactance and also much smaller than the generator impedance; the generator impedance is much smaller than the generator neutral point grounding resistance. (2) All generators in the system have the same phase; all loads are static loads, and the impact of loads on short-circuit calculations is not considered; (3) The generator subtransient and transient circuits are equivalent to a series model of a power source and an impedance.

[0006] Furthermore, in step 2, the improved composite sequence network method is optimized for the characteristics of high-resistance grounded ship power supply networks, specifically including: In the case of a three-phase short circuit, only the positive sequence network of the system needs to be considered. The formula for calculating the short-circuit current of the faulted branch is:

[0007] In the formula: The potential is positive sequence, and the amplitude is taken as the rated phase voltage of the generator; The positive sequence composite impedance; This refers to the three-phase short-circuit current of the faulty branch. This represents the positive sequence component of the three-phase short-circuit current. During a two-phase short circuit, the positive sequence current is half that of a three-phase short circuit, and the short-circuit current of phase B is equal to that of a three-phase short circuit. times; During a single-phase ground fault, because the neutral point is grounded through a high-resistance circuit, the zero-sequence combined impedance is much larger than the positive and negative-sequence combined impedance. Therefore, it can be simplified to only consider the zero-sequence network. The sequence currents of the faulted branch and the faulted branch current are as follows:

[0008]

[0009] In the formula: This refers to the single-phase ground fault current of the faulty branch. The zero-sequence composite impedance is equal to ;R n The neutral point grounding resistance of the system; It is a zero-order capacitive reactance; When a two-phase ground fault occurs, since the zero-sequence composite impedance is much greater than the positive and negative sequence composite impedance, it can be equivalent to a composite sequence network when a two-phase short circuit occurs, and the fault phase current is approximately equal to the phase-to-phase fault current. Furthermore, in step 2, when calculating the single-phase ground fault current in the ship power supply network with the neutral point grounded through a high-resistance ground, it is only necessary to consider the parallel value of the system's ground capacitance reactance and the neutral point grounding resistance of all operating generators in the zero-sequence network.

[0010] Furthermore, in step 2, the composite sequence network under a two-phase-to-ground short circuit can be equivalent to the composite sequence network under a single-phase-to-ground short circuit, but the equivalent zero-sequence potential is different. equal The zero-sequence current of the faulty branch is half that of the single-phase short circuit, and the calculation formula is as follows:

[0011] In the formula: The zero-sequence component of the two-phase-to-ground short-circuit current of the faulted branch. This is the zero-sequence component of the single-phase-to-ground short-circuit current in the faulted branch. It is the zero-sequence composite impedance, consistent with that of a single-phase ground fault.

[0012] Furthermore, in step 2, when calculating the fault branch current, considering that the cable impedance in the ship's power supply network is much smaller than its capacitive reactance and generator impedance, the cable impedance is ignored, and the generator is directly connected to the fault point, simplifying the calculation process. Furthermore, in step 3, the method for constructing the fault-addition state network is as follows: take the fault point as the root node, the power source that injects fault current into the point as the parent node, regard each protection branch as a new root node, and the generator impedance that receives the short-circuit current provided by it as a new parent node. Referring to the calculation method of the system equivalent impedance, the corresponding generator impedances can be connected in parallel to obtain the equivalent impedance of the protection branch. Furthermore, when the fault point is located at any position, the equivalent impedance of the system can be obtained by Thevenin's equivalent theorem, as follows:

[0013] In the formula: The equivalent impedance of the system; denoted as the generator impedance; n represents the total number of generators connected in parallel.

[0014] Furthermore, in step 3, the formula for calculating the distribution coefficient of the protection branch current is:

[0015] In the formula: To protect the equivalent impedance of the branch; is the system equivalent impedance; j is the fault type coefficient, where 1 represents positive sequence impedance and 0 represents zero sequence impedance.

[0016] Furthermore, when a short-circuit fault occurs in the system, based on the assumed conditions, a fault-additional state network is directly constructed according to the system topology, and the allocation coefficients of each protection branch are calculated without the need for three-sequence network analysis.

[0017] Compared with the prior art, the present invention has the following beneficial effects: (1) Under reasonable assumptions, this invention uses Thevenin equivalent method and improved composite sequence network method to quickly calculate the symmetrical and asymmetrical short-circuit current of fault branches, thus solving the problem of diversified power supply network topology.

[0018] (2) The proposed method uses the superposition theorem to calculate the current distribution coefficient in the fault additional state network, and then calculates the protection branch current, which solves the problem of complex calculation of protection branch current under multiple power supply conditions. Attached Figure Description

[0019] Figure 1 This is the equivalent circuit of the generator; Figure 2 System architecture diagram; Figure 3 This is a three-phase short-circuit positive sequence network; Figure 4 It is a two-phase short-circuit composite sequence network; Figure 5 It is a single-phase grounding composite sequence network; Figure 6 For a simplified single-phase grounding composite sequence network; Figure 7 For a two-phase-to-ground short-circuit composite sequence network, (a) is a composite sequence network and (b) is an approximate positive and negative sequence network; Figure 8 Add a state network for positive-sequence faults; Figure 9 Here is a flowchart for short-circuit current calculation; Figure 10 This is a simulation platform model. Detailed Implementation

[0020] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments. This invention employs Thevenin's equivalent method and an improved composite sequence network method to quickly solve for symmetrical and asymmetrical short-circuit currents in faulted branches. Then, using the superposition theorem, it calculates the current distribution coefficients in the fault-addition state network, thereby determining the current in the protected branch. This chapter explains the calculation principles of Thevenin's equivalent method, the improved composite sequence network method, and the superposition theorem, as well as the calculation process based on these principles.

[0021] Basic Assumptions: Based on the characteristics of ship power supply networks, the following assumptions are made: (1) The positive and negative sequence impedances of all components in the system are the same; the cable impedance is much smaller than its capacitive reactance and also much smaller than the generator impedance; the generator impedance is much smaller than the generator neutral point grounding resistance.

[0022] (2) All generators in the system have the same phase; all loads are static loads, and the influence of load on short-circuit calculation is not considered.

[0023] (3) The generator's subtransient and transient circuits are equivalent to a series model of a power source and an impedance. Taking the subtransient circuit as an example, the equivalent circuit is as follows: Figure 1 As shown in the figure. The power supply potential in the figure... This refers to the generator's rated phase voltage; R a For generator armature resistance, This is the generator's subtransient impedance.

[0024] Thevenin's Equivalent Theorem: When analyzing short-circuit faults in a radial power supply network, it can be viewed as a tree topology, with the fault point as the root node and the power source injecting fault current into that point as its parent node. In this structure, except for the root node, each node has one and only one parent node, and the paths between nodes are unique. When a short circuit occurs, the fault current is mainly concentrated on the unique path from the power source to the fault point, and the current in other branches is approximately zero. Therefore, during a short circuit, it is only necessary to analyze the paths from all operating generators to the short-circuit point. According to Thevenin's Equivalent Theorem, any complex linear network can be equivalent to a series connection of a power source and an impedance. By gradually merging the generators, the Thevenin equivalent circuit of the entire network is finally obtained. Since cable impedance is ignored, the order of generator merging does not affect the equivalent result, and changes in the fault location also do not affect the equivalent result. Figure 2 As shown in the figure, B1 to B3 are the corresponding power stations (busbars). Z L1 to Z L7 The impedance of the generator terminal cable; Z L8 to Z L11 Let be the impedance of the jumper cable between the two power stations. During system operation, only one jumper cable is connected between the two power stations. When the cable impedance is negligible, each generator can be considered directly connected to the fault point. That is, when the fault point is located at F1, F2, or any position in the diagram, the equivalent impedance of the system can be obtained using Thevenin's equivalent theorem as follows: (1) In the formula: The equivalent impedance of the system; The generator impedance; n This represents the total number of generators connected in parallel.

[0025] Since all generators have the same rated voltage, the equivalent voltage is easily obtained. This is the generator's rated phase voltage.

[0026] Three-phase short-circuit composite sequence network: When a three-phase short circuit occurs in a ship power supply network with a neutral point grounded through a high-resistance circuit, the short-circuit current calculation method is the same as that for a directly grounded system. After performing Thevenin equivalents on the system, only the positive sequence network needs to be considered, such as... Figure 3 As shown in the figure. The potential is positive sequence, and the amplitude is taken as the rated phase voltage of the generator; Given the positive-sequence composite impedance, and based on the assumptions, all cable impedances are negligible. It is approximately taken as the parallel value of the positive sequence impedance (transient or subtransient) of all the generators in operation.

[0027] At this time, the formula for calculating the short-circuit current of the faulty branch is: (2) In the formula: This refers to the three-phase short-circuit current of the faulty branch. This represents the positive sequence component of the three-phase short-circuit current.

[0028] Two-phase short-circuit composite sequence network: When a two-phase short circuit occurs in a ship power supply network with a neutral point grounded through a high-resistance circuit, assuming a short circuit between phases B and C, the composite sequence network is as follows: Figure 4 The diagram shows that the positive sequence current is half the value during a three-phase short circuit. (3) In the formula: This represents the positive sequence component of the two-phase short-circuit current in the faulted branch. , Positive and negative sequence combined impedance.

[0029] From the composite sequence network during a two-phase short circuit, it can be seen that the positive and negative sequence currents are equal in magnitude but opposite in direction. At this time, the short-circuit current of phase B is equal to that of a three-phase short circuit. times (4) In the formula: This refers to the two-phase short-circuit current of the faulty branch. , These are the positive and negative sequence components of the two-phase short-circuit current in the faulted branch. a For rotation factor, .

[0030] Single-phase ground fault composite sequence network: When a single-phase ground fault occurs in a ship power supply network with a neutral point grounded through a high-resistance circuit, the effect of distributed capacitance in the system cannot be ignored and can be expressed as capacitive reactance. Assume the neutral point grounding resistance of the system is... R n ; , , These are the positive, negative, and zero-sequence capacitive reactances of the cable line, respectively. , , These represent the positive, negative, and zero-sequence combined impedances of the generators within the system. Then, during a phase-A ground fault, the composite sequence network is as follows: Figure 5 As shown.

[0031] The sequence currents of the faulty branch can be obtained as follows: (5) In the formula: , , These are the positive, negative, and zero-sequence components of the single-phase ground fault current in the faulted branch.

[0032] In a ship power supply network with a neutral point grounded through a high-resistance ground, because Much larger and , Much larger Therefore, the single-phase grounding composite sequence network can be simplified to Figure 6 In the picture The equivalent zero-sequence potential is equal to It can be assumed that the zero-sequence voltage is equal at all points in the power supply network, and the amplitude is equal to the fault phase voltage.

[0033] At this time, the sequence currents and fault branch current of the faulted branch are respectively (6) (7) In the formula: This refers to the single-phase ground fault current of the faulty branch. The zero-sequence composite impedance is equal to .

[0034] In other words, when calculating the single-phase ground fault current in a ship power supply network with a neutral point grounded through a high-resistance ground, it is only necessary to consider the parallel value of the system's ground capacitive reactance and the neutral point grounding resistance of all operating generators in the zero-sequence network.

[0035] Two-phase-to-ground short circuit composite sequence network: When a two-phase-to-ground short circuit occurs in the ship's power supply network, assuming a short circuit between phases B and C, the composite sequence network is as follows: Figure 7 (a). In the figure , , These are the system's positive, negative, and zero-sequence combined impedances, respectively. , , These represent the positive, negative, and zero-sequence components of the fault voltage, respectively. Since the zero-sequence combined impedance is much larger than the positive and negative sequence combined impedance, the zero-sequence network can be considered an open circuit. The sequence network can be approximated as a composite sequence network under a two-phase short circuit, as shown below. Figure 7 (b). Therefore, the fault phase current is approximately equal to the interphase fault current, as shown in the following equation. (8) In the formula: This refers to the two-phase-to-ground short-circuit current of the faulty branch. This represents the two-phase short-circuit current of the faulty branch.

[0036] Depend on Figure 7 As shown in (b), the positive sequence voltage equals the negative sequence voltage equal to half the fault phase voltage, and the negative sequence voltage equals the zero sequence voltage. Therefore, the composite sequence network during a two-phase-to-ground short circuit in the system can also be equivalent to... Figure 6 The form is different; the equivalent zero-sequence potential is different at this time. equal That is, the zero-sequence current of the faulty branch is 1 / 2 times that of the single-phase short circuit, as shown in the following formula. (9) In the formula: The zero-sequence component of the two-phase-to-ground short-circuit current of the faulted branch. This is the zero-sequence component of the single-phase-to-ground short-circuit current in the faulted branch. It is the zero-sequence composite impedance, consistent with that of a single-phase ground fault.

[0037] Superposition Theorem: When a short-circuit fault occurs in a system, the system can be represented by a superposition of the normal operating state and the fault-added state, according to the superposition theorem. In the fault-added network, there is only one fault-added potential at the fault point, and the short-circuit current flows from the fault-added potential to the grounding point. This characteristic can be used to quickly solve for the distribution coefficients of each protection branch. Multiplying the corresponding distribution coefficients by the fault branch current allows for the calculation of the current in each protection branch during the fault.

[0038] Shipboard power supply networks operate in various modes. For a specific topology under a particular mode, a faulty branch can be denoted as... f c ( c (This can be represented by generator G, bus B, line L, and load H), and the protection branch is denoted as... c (B), where the busbar number in parentheses is used to distinguish the location of the measurement point. The current distribution coefficient can then be defined as: (12) In the formula: To protect the branch current; This refers to the fault branch current; jThis is the fault type coefficient. When it is 1, it represents the fault phase current and the fault phase current distribution coefficient. When it is 0, it represents the zero-sequence current and the zero-sequence current distribution coefficient. The function for calculating the allocation coefficients; s This indicates the system operating condition number; different numbers represent different operating conditions of the system.

[0039] When a short-circuit fault occurs in the system, based on the aforementioned assumptions, a fault-additional state network can be directly constructed according to the system topology, and the allocation coefficients of each protection branch can be calculated without performing traditional three-sequence network analysis. Figure 2 Taking the mid-topology as an example, when the power station jumper cable put into the system is Z L8 , Z L9 If a three-phase short circuit occurs at fault point F2, its fault-related network will be as follows: Figure 8 As shown in the figure. to L1 to L9 are the positive sequence impedance of the generator; L1 to L9 are cables. An additional power supply is provided for the fault, equal in magnitude but opposite in direction to the voltage at point F2 during normal operation. At this point, each protection branch can be considered a new root node, and the generator impedance receiving the short-circuit current provided by it can be considered a new parent node. Referring to the calculation method for the system's equivalent impedance, the corresponding generator impedances can be connected in parallel to obtain the equivalent impedance of the protection branch.

[0040] The distribution factor is calculated as the ratio of the system's equivalent impedance to the equivalent impedance of the protection branch, as shown in the following formula. (13) In the formula: To protect the equivalent impedance of the branch; The equivalent impedance of the system is calculated as shown in equation (1). j This is the fault type coefficient, where 1 represents positive sequence impedance and 0 represents zero sequence impedance.

[0041] The short-circuit current calculation process of this invention is as follows: Step 1: Input calculation parameters, including the system rated line voltage. Impedance and number of large and small capacity generators in operation , , n G, n g; Generator neutral point grounding resistance R gn Total system capacitance .

[0042] Step 2: Calculate the fault branch current. The equivalent potential of the system is calculated using Thevenin's equivalent theorem and the improved composite sequence network method. and positive-sequence and zero-sequence equivalent impedance , Then, the phase current of the faulty branch can be calculated. Zero-sequence current in faulty branches .

[0043] Step 3: Calculate the current in the protected branch. Identify the faulty branch in the fault supplementary state network. f c Then, the distribution coefficients of each protection branch were calculated. and respectively with , Multiplying these yields the fault phase current and zero-sequence current for each protected branch. The fault phase current and zero-sequence current of each protected branch can be used for relay protection setting calculations.

[0044] Overall process available Figure 9 express.

[0045] Example: Figure 2 The multi-power supply system shown is analyzed. System parameters are as follows: the rated voltage of all generators is 6.6kV; G3, G4, and G5 are large-capacity units with a rated power of 3MW and an impedance of 0.093+. j 1.434Ω; G1, G2, G6, and G7 are small-capacity units with a rated power of 1.6MW and an impedance of 0.23+. j 2.7Ω; the generator terminal cable length is 0.03km, and the power station bridging cable length is 0.18km; the positive and negative sequence impedances and zero sequence impedance of the cable are respectively: 0.083+ j 0.0342Ω / km, 0.292+ j 0.121Ω / km; the neutral point grounding resistance of each generator is 1270Ω, and the total capacitance per phase to ground of the system is 1.5μF.

[0046] Build a simulation model for this computational example in CloudPSS, such as... Figure 10 As shown. Settings Figure 2 The power station jumper cables deployed in the medium system are Z L8 , Z L9 Fault simulations were performed on fault points F1 and F2, and the simulation results were compared and verified with the calculation results of the method proposed in this invention and the traditional composite sequence network method.

[0047] The simulation and calculation results for three-phase short circuit, two-phase short circuit, two-phase ground fault, and single-phase ground fault at fault points F1 and F2 are shown in Table 1-4.

[0048] Table 1 Comparison of Three-Phase Short-Circuit Current Results (kA)

[0049] Table 2 Comparison of two-phase short-circuit current results (kA)

[0050] Table 3 Comparison of two-phase ground fault current results (kA)

[0051] Table 4 Comparison of Single-Phase Ground Fault Current Results (A)

[0052] As shown in Table 1-4, when the fault points are located at F1 and F2, the maximum errors of this method compared with the composite sequence network method and CloudPSS simulation values ​​are 2.2% and 2.56%, respectively, verifying the practicality of this method. The results indicate that this method has high calculation accuracy. Its error mainly stems from neglecting the impedance of the power station's jumper cables, but the resulting deviation is extremely small and fully meets the accuracy requirements for relay protection setting calculations in practical engineering.

[0053] by Figure 8 Taking a three-phase short circuit at point F2 as an example, the distribution coefficient of each protection branch is calculated and multiplied by the fault branch current to obtain the calculation result of the current of each protection branch. The simulation results are compared and verified with the calculation results, as shown in Table 5.

[0054] Table 5 Short-circuit current (kA) of each protection branch during three-phase short circuit of F2

[0055] As shown in Table 5, when a three-phase short circuit occurs at F2, the maximum errors of the protection branch current calculated by this method compared with the composite sequence network method and the CloudPSS simulation values ​​are 2.4% and 3.53%, respectively, verifying the correctness of this method. It meets the requirements of actual engineering for relay protection setting calculations.

Claims

1. A high-precision and practical calculation method for short-circuit current in a high-resistance grounded ship power supply network, characterized in that, Includes the following steps: Step 1: Input the calculation parameters, including the system rated line voltage, impedance of large and small capacity generators and the number of generators in operation, generator neutral point grounding resistance, and total system capacitive reactance; Step 2: Calculate the fault branch current. The equivalent potential of the system and the positive-sequence and zero-sequence equivalent impedances are calculated using Thevenin's equivalent theorem and the improved composite sequence network method. Then, the phase current and zero-sequence current of the fault branch are calculated. Step 3: Calculate the protection branch current. After determining the fault branch in the fault additional state network, calculate the distribution coefficient of each protection branch and multiply it by the phase current and zero-sequence current of the fault branch respectively to obtain the fault phase current and zero-sequence current of each protection branch.

2. The high-precision and practical calculation method for short-circuit current of high-resistance grounded ship power supply network according to claim 1, characterized in that: In step 1, the assumptions for the system parameters include: (1) The positive and negative sequence impedances of all components in the system are the same; the cable impedance is much smaller than its capacitive reactance and also much smaller than the generator impedance; the generator impedance is much smaller than the generator neutral point grounding resistance. (2) All generators in the system have the same phase; all loads are static loads, and the impact of loads on short-circuit calculations is not considered. (3) The generator subtransient and transient circuits are equivalent to a series model of a power source and an impedance.

3. The high-precision and practical calculation method for short-circuit current in a high-resistance grounded ship power supply network according to claim 1, characterized in that: In step 2, the improved composite sequence network method is optimized for the characteristics of high-resistance grounded ship power supply networks, specifically including: In the case of a three-phase short circuit, only the positive sequence network of the system needs to be considered. The formula for calculating the short-circuit current of the faulted branch is: In the formula: The positive sequence potential is taken as the rated phase voltage of the generator; Z 1Σ The positive sequence composite impedance; This refers to the three-phase short-circuit current of the faulty branch. This represents the positive sequence component of the three-phase short-circuit current. During a two-phase short circuit, the positive sequence current is half that of a three-phase short circuit, and the short-circuit current of phase B is equal to that of a three-phase short circuit. times; During a single-phase ground fault, because the neutral point is grounded through a high-resistance circuit, the zero-sequence combined impedance is much larger than the positive and negative-sequence combined impedance. Therefore, it can be simplified to only consider the zero-sequence network. The sequence currents of the faulted branch and the faulted branch current are as follows: In the formula: Z represents the single-phase ground fault current of the faulty branch. 0Σ The zero-sequence composite impedance is equal to R n X is the neutral point grounding resistance of the system; 0C It is a zero-order capacitive reactance; When a two-phase ground fault occurs, since the zero-sequence composite impedance is much greater than the positive and negative sequence composite impedance, it can be equivalent to a composite sequence network when a two-phase short circuit occurs, and the fault phase current is approximately equal to the phase-to-phase fault current.

4. The high-precision and practical calculation method for short-circuit current of high-resistance grounded ship power supply network according to claim 3, characterized in that: In step 2, when calculating the single-phase ground fault current in the ship power supply network with the neutral point grounded through a high resistance, it is only necessary to consider the parallel value of the system's ground capacitance reactance and the neutral point grounding resistance of all operating generators in the zero-sequence network.

5. The high-precision and practical calculation method for short-circuit current of high-resistance grounded ship power supply network according to claim 3, characterized in that: In step 2, the composite sequence network under a two-phase-to-ground short circuit can be equivalent to the composite sequence network under a single-phase-to-ground short circuit, but the equivalent zero-sequence potential is different. equal The zero-sequence current of the faulty branch is half that of the single-phase short circuit, and the calculation formula is as follows: In the formula: The zero-sequence component of the two-phase-to-ground short-circuit current of the faulted branch. Z represents the zero-sequence component of the single-phase-to-ground short-circuit current in the faulted branch. 0Σ It is the zero-sequence composite impedance, consistent with that of a single-phase ground fault.

6. The high-precision and practical calculation method for short-circuit current of high-resistance grounded ship power supply network according to claim 1, characterized in that: In step 2, when calculating the fault branch current, considering that the cable impedance in the ship's power supply network is much smaller than its capacitive reactance and generator impedance, the cable impedance is ignored, and the generator is directly connected to the fault point to simplify the calculation process.

7. The high-precision and practical calculation method for short-circuit current of high-resistance grounded ship power supply network according to claim 1, characterized in that: In step 3, the method for constructing the fault-addition state network is as follows: take the fault point as the root node, the power source that injects fault current into the point as the parent node, take each protection branch as a new root node, and take the generator impedance that receives the short-circuit current provided by it as a new parent node. Referring to the calculation method of the system equivalent impedance, the corresponding generator impedances can be connected in parallel to obtain the equivalent impedance of the protection branch.

8. The high-precision and practical calculation method for short-circuit current of high-resistance grounded ship power supply network according to claim 7, characterized in that: When the fault point is located at any location, the equivalent impedance of the system can be obtained by Thevenin's equivalent theorem, as follows: In the formula: Z eq The equivalent impedance of the system; denoted as the generator impedance; n represents the total number of generators connected in parallel.

9. The high-precision and practical calculation method for short-circuit current of a high-resistance grounded ship power supply network according to claim 1, characterized in that: In step 3, the formula for calculating the distribution coefficient of the protected branch current is: In the formula: z j,c(B) To protect the equivalent impedance of the branch; The equivalent impedance of the system; j is the fault type coefficient, where 1 represents positive sequence impedance and 0 represents zero sequence impedance.

10. The high-precision and practical calculation method for short-circuit current of a high-resistance grounded ship power supply network according to claim 9, characterized in that: When a short-circuit fault occurs in the system, based on the assumptions, the fault-addition state network is directly constructed according to the system topology, and the allocation coefficients of each protection branch are calculated without the need for three-sequence network analysis.

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