Insulation monitoring method and system for low-voltage direct-current system of thermal power plant

Abstract

The invention relates to the technical field of cable insulation monitoring, and discloses a thermal power plant low-voltage DC system insulation monitoring method and system, and the method comprises the steps: measuring the voltage values of a positive electrode and a negative electrode to the ground; based on the voltage values of the positive electrode and the negative electrode to the ground, calculating respective insulation resistances of the positive electrode and the negative electrode to the ground; and the fault branch is positioned by combining the insulation resistance and the leakage current data of the branch. According to the method, the numerical value of the resistance after reaching the stable state is calculated as the basis of insulation monitoring, the actual condition of the insulation resistance of the low-voltage direct-current cable can be reflected more accurately, and therefore the accuracy and reliability of insulation monitoring are improved. The insulation monitoring method for the low-voltage direct-current system of the thermal power plant has a certain engineering application value.

Description

Technical Field

[0001] The present invention relates to the technical field of cable insulation monitoring, and specifically to an insulation monitoring method and system for the low-voltage DC system in a thermal power plant. Background Art

[0002] In order to ensure the stable power supply of key secondary equipment in a thermal power plant, including relay protection devices, automatic control systems, electrical measuring instruments, and operating power supplies, etc., it is crucial to achieve continuous and accurate monitoring of the low-voltage DC system. Such monitoring measures ensure that the secondary equipment can operate without interference, thereby maintaining the safety, stability, and efficient operation of the entire power plant. A separate low-voltage DC power supply system is configured in the thermal power plant to supply power to these secondary equipment, ensuring reliable power supply for important loads such as protection, metering, monitoring, and lighting.

[0003] In the complex operation scenarios of a thermal power plant, the low-voltage DC system faces diverse fault challenges, and the root causes of these faults are numerous and complex. Due to the wide distribution of system equipment in different environmental conditions with significant temperature differences, the tolerance of equipment insulation materials is challenged, and it is prone to aging, deterioration, or even damage due to fluctuations in environmental temperature or current thermal effects, thereby triggering grounding faults. In addition, improper planning during system design and renovation, as well as mistakes in operation processes such as equipment installation and commissioning, also lead to grounding faults in the DC system. According to the specific causes of the faults, they can be refined into various types, including but not limited to grounding caused by water seepage in the junction box in rainy environments, accidental grounding caused by small animals straying into the equipment area, and grounding due to insulation layer breakage caused by physical contact between equipment.

[0004] In the actual operation scenario, if there are problems such as unreasonable design or inappropriate equipment selection in the low-voltage DC power supply system of a thermal power plant, it will significantly increase the risks of power facility damage, system faults, and even personal safety. Summary of the Invention

[0005] In view of the above existing problems, the present invention is proposed.

[0006] Therefore, the technical problem solved by the present invention is the problem of inaccurate calculation of the insulation monitoring resistance existing in the existing insulation monitoring device without considering the cable stray parameters.

[0007] To solve the above technical problem, the present invention provides the following technical solution: An insulation monitoring method for the low-voltage DC system in a thermal power plant, including: measuring the voltage values of the positive and negative poles to the ground; calculating the insulation resistances of the positive and negative poles to the ground respectively based on the voltage values of the positive and negative poles to the ground; and locating the faulty branch by combining the insulation resistance and the leakage current data of the branch.

[0008] As a preferred embodiment of the insulation monitoring method for the low-voltage DC system in a thermal power plant according to the present invention, before measuring the voltage values of the positive and negative poles to the ground, the insulation monitoring device periodically switches the positive and negative pole switches alternately, and simultaneously measures the voltage values of the positive and negative poles to the ground during the switching process.

[0009] As a preferred embodiment of the insulation monitoring method for the low-voltage DC system in a thermal power plant according to the present invention, where measuring the voltage values of the positive and negative poles to the ground includes, when the positive pole is closed and the negative pole is open, measuring the voltage of the positive pole line to the ground as U 1 , and the voltage of the negative pole line to the ground as U 2 ; when the positive pole is open and the negative pole is closed, measuring the voltage of the positive pole line to the ground as U' 1 , and measuring the voltage of the negative pole line to the ground as U' 2 .

[0010] As a preferred embodiment of the insulation monitoring method for the low-voltage DC system in a thermal power plant according to the present invention, where the voltage values of the positive and negative poles to the ground include, according to the principle of resistance voltage division, the following relationship exists between the voltage of the positive and negative pole lines to the ground:

[0011]

[0012] where R represents the equivalent insulation resistance of the cable to the ground.

[0013] As a preferred embodiment of the insulation monitoring method for the low-voltage DC system in a thermal power plant according to the present invention, before calculating the insulation resistance of the positive and negative poles to the ground respectively, it further includes analyzing the stray parameters of the DC cable and correcting the measurement error caused by the cable stray parameters. The DC resistance per unit length of the low-voltage DC cable is expressed as:

[0014]

[0015] where r 1 represents the resistance per unit length of the low-voltage DC cable; ρ represents the resistivity of the conductor; s represents the rated cross-sectional area of the low-voltage DC cable.

[0016] As a preferred embodiment of the insulation monitoring method for the low-voltage DC system in a thermal power plant according to the present invention, where calculating the insulation resistance of the positive and negative poles to the ground respectively includes, the equivalent insulation resistance of the positive and negative poles to the ground can be expressed as:

[0017]

[0018] By rotating and switching the resistance of the positive pole to the ground and the resistance of the negative pole to the ground in the unbalanced bridge, the insulation resistance of the low-voltage DC cable is obtained.

[0019] As a preferred embodiment of the insulation monitoring method for the low-voltage DC system in a thermal power plant according to the present invention, the following steps are included: An insulation monitoring device is configured for the low-voltage DC cables in the thermal power plant to detect the leakage current. The insulation monitoring device configured for the low-voltage DC cables in the power plant includes 24 branch loads, and each branch is equipped with an independent leakage current sensor. The current detected by the leakage current sensor is the algebraic sum of the currents on the positive pole, negative pole, and shielding layer.

[0020] An insulation monitoring system for the low-voltage DC system in a thermal power plant adopting any of the methods described in the present invention, which includes: a voltage detection module that periodically switches the positive and negative switches alternately through the insulation monitoring device, and simultaneously measures the voltage values of the positive and negative poles to the ground during the switching process; an insulation calculation module that calculates the insulation resistance of the positive and negative poles to the ground respectively based on the measured voltage values of the positive and negative poles to the ground, introducing miscellaneous parameter correction according to the principle of resistance voltage division; a positioning module that locates the faulty branch by combining the insulation resistance and the leakage current data of the branch.

[0021] A computer device includes: a memory and a processor; the memory stores a computer program, which includes: when the processor executes the computer program, the steps of any of the methods described in the present invention are realized.

[0022] A computer-readable storage medium stores a computer program thereon, which includes: when the computer program is executed by the processor, the steps of any of the methods described in the present invention are realized.

[0023] The beneficial effects of the present invention: The method of the present invention uses the value of the calculated resistance after reaching the stable state as the basis for insulation monitoring, which can more accurately reflect the actual condition of the insulation resistance of the low-voltage DC cable, thereby improving the accuracy and reliability of insulation monitoring. The insulation monitoring method for the low-voltage DC system in a thermal power plant proposed in this article has certain engineering application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following will briefly introduce the drawings required for the description of the embodiments. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings. Among them:

[0025] Figure 1 It is the overall flowchart of the insulation monitoring method for the low-voltage DC system in a thermal power plant provided by an embodiment of the present invention;

[0026] Figure 2 It is the cross-sectional view of a two-core cable of the insulation monitoring method for the low-voltage DC system in a thermal power plant provided by an embodiment of the present invention;

[0027] Figure 3The equivalent circuit diagram of the cable for the insulation monitoring method of the low-voltage DC system in a thermal power plant provided by an embodiment of the present invention;

[0028] Figure 4 The schematic diagram of the principle of the insulation resistance measurement method for the insulation monitoring method of the low-voltage DC system in a thermal power plant provided by an embodiment of the present invention;

[0029] Figure 5 The structure diagram of the insulation monitoring device configured for the low-voltage DC cable of the insulation monitoring method of the low-voltage DC system in a thermal power plant provided by an embodiment of the present invention;

[0030] Figure 6 The structure diagram of the insulation monitoring device configured for the low-voltage DC cable of the insulation monitoring method of the low-voltage DC system in a thermal power plant provided by an embodiment of the present invention;

[0031] Figure 7 The curve of the insulation resistance changing with time and its partial enlarged view for the insulation monitoring method of the low-voltage DC system in a thermal power plant provided by the second embodiment of the present invention. Specific embodiments

[0032] In order to make the above objects, features, and advantages of the present invention more obvious and understandable, the following will describe the specific embodiments of the present invention in detail with reference to the accompanying drawings of the specification. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the scope of protection of the present invention.

[0033] Embodiment 1, referring to Figure 1 , which is an embodiment of the present invention, provides an insulation monitoring method for the low-voltage DC system in a thermal power plant, including:

[0034] In S1: Measure the voltage values of the positive and negative poles to the ground.

[0035] In S2: Based on the voltage values of the positive and negative poles to the ground, calculate the insulation resistances of the positive and negative poles to the ground respectively.

[0036] In S3: Combine the insulation resistance and the leakage current data of the branch to locate the faulty branch.

[0037] Furthermore, in the DC power supply system of a thermal power plant, DC cables are the connecting lines between various electrical instruments and automation devices. DC cables mainly include conductor cores, insulation layers, metal shielding layers, protective layers, etc. According to different requirements of the working environment and location, the composition structure of DC cables has some differences. For example, cables laid in indoor cable trenches, pipelines or underground need to withstand greater mechanical external forces, and an armored metal layer needs to be added to prevent damage to the internal conductor cores, insulation layers, shielding layers, etc.; DC cables in flame-retardant situations also require a flame-retardant sheath to meet the fire prevention requirements.

[0038] In the circuit design of the DC system in a thermal power plant, due to its characteristics of light load and small rated current, a refined DC cable structure is often adopted, including various configurations such as 2-core, 4-core, and 5-core. The core conductor of the cable is woven from multi-strand copper or aluminum stranded wires with high conductivity. In terms of the insulation layer, advanced materials such as oil-impregnated paper, high-quality rubber, and high-density polyethylene are used to tightly wrap around the conductor to ensure the safety of electrical insulation and enhance the durability of the cable. At the same time, fillers are filled between the conductor cores to make the cable shape regular and the internal structure stable. To improve the anti-interference ability of the cable, a shielding layer composed of a finely woven copper mesh is designed to tightly wrap around the outside of the conductor cores, and through an effective grounding mechanism, effectively shield external electromagnetic interference and ensure the purity and stability of signal transmission. The protective layer is made of aluminum and lead materials, and its strong outer shell provides strong mechanical protection for the cable, resists physical damage during transportation and laying, and has moisture-proof performance to ensure that the cable maintains a good working state in a complex environment. In summary, the design of the DC circuit cables in a thermal power plant comprehensively considers factors such as high-efficiency conduction, safe insulation, stable structure, anti-interference, and durable protection to meet the high-demand operating environment of the power plant. The cross-section of a two-core DC cable is as Figure 2 shown.

[0039] The equivalent model of the DC cable is as Figure 3 shown, including a positive conductor core, a negative conductor core, and a grounding shielding layer.

[0040] Figure 3 where: R p , R n , R g are the stray resistances of the positive conductor core, negative conductor core, and shielding layer of the low-voltage DC cable per unit length, with the unit of Ω / m, reflecting the active power loss effect generated when the circuit passes current; L p , L n , L g are the stray inductances of the positive conductor core, negative conductor core, and shielding layer per unit length, with the unit of nH / m; M p-n , M p-g , M n-gStray mutual inductances between the positive and negative line cores per unit length, between the positive line core and the grounding shield, and between the negative line core and the grounding shield, in nH / m; G p-n , G p-g , G n-g Stray conductances between the positive and negative line cores per unit length, between the positive line core and the grounding shield, and between the negative line core and the grounding shield, in S / m; C p-n , C p-g , C n-g Stray capacitances between the positive and negative line cores per unit length, between the positive line core and the grounding shield, and between the negative line core and the grounding shield, in nF / m.

[0041] Furthermore, for the insulation resistance measurement by unbalanced bridge - branch leakage current detection, that is, the voltage signal is measured by the unbalanced bridge method, and relevant calculations are performed on the collected voltage to calculate the insulation resistance value. The working principle is as follows: The insulation monitoring device periodically (such as daily) switches the K+ and K- switches alternately, and at the same time, the voltage values of the positive and negative poles to the ground are measured synchronously during this process. Based on these measured values, the insulation resistances of the positive and negative poles to the ground are calculated respectively to achieve the monitoring of the insulation state. Once it is found that the insulation resistance value of a certain pole to the ground decreases, the system will further use the data provided by each leakage current sensor to accurately locate the faulty branch. The principle of the insulation resistance measurement method is as Figure 4 shown.

[0042] Figure 4 In the figure: R+ and R- are the equivalent insulation resistances of the positive and negative pole cables of the entire low - voltage DC system to the ground. Their magnitudes are closely related to the conductance per unit length of the DC cable and are easily affected by factors such as temperature, humidity, and mechanical damage. They are the monitoring targets of the insulation monitoring device.

[0043] When K+ (positive pole) is closed and K- (negative pole) is open, the measured voltage of the positive line to the ground is U 1 , and the measured voltage of the negative line to the ground is U 2 ; when K+ is open and K- is closed, the measured voltage of the positive line to the ground is U' 1 , and the measured voltage of the negative line to the ground is U' 2 . According to the principle of resistance voltage division, the following relationships exist for the voltages of the positive and negative lines to the ground:

[0044]

[0045] The equivalent insulation resistances of the positive and negative poles to the ground can be expressed as:

[0046]

[0047] The insulation resistance of the low-voltage DC cable can be obtained by rotating and switching the positive pole-to-ground resistance and the negative pole-to-ground resistance in the unbalanced bridge. The structure of the insulation monitoring device configured for the low-voltage DC cable in a thermal power plant is as Figure 5 shown.

[0048] The insulation monitoring device configured for the low-voltage DC cable in a thermal power plant mainly includes 24 branch loads. Each branch is equipped with a separate leakage current sensor. The current detected by the leakage current sensor is the algebraic sum of the currents on the positive pole, the negative pole, and the shielding layer. The same set of unbalanced bridges is shared for this section of the busbar.

[0049] Furthermore, in order to analyze the influence of the stray parameters of the low-voltage DC cable on the measurement performance of the insulation monitoring device, this paper uses finite element analysis to analyze the stray resistance, inductance, and their frequency-varying characteristics of the low-voltage DC cable.

[0050] The DC resistance per unit length of the low-voltage DC cable can be expressed as:

[0051]

[0052] where r 1 represents the resistance per unit length of the low-voltage DC cable; ρ represents the conductor resistivity; s represents the rated cross-sectional area of the low-voltage DC cable. Under the DC field, the current is evenly distributed on the conductor cross-section. The oscillation frequency during the transient switching process of the unbalanced bridge proposed in this paper is below 10 kHz. In this frequency band, the DC resistance per unit length of the low-voltage DC cable is almost unchanged, and the frequency-varying characteristics do not need to be considered. The DC resistance is directly used as the stray resistance parameter of the cable. The resistance per unit length of the cable is 0.0115 Ω / m, and the resistance per unit length of the shielding layer is 0.00097 Ω / m.

[0053] The stray inductance of the low-voltage DC cable is the magnetic flux linked with different conductors generated by the magnetic field under the excitation of unit current, and is expressed as:

[0054]

[0055] where Ψ p , Ψ g , Ψ n are the total magnetic fluxes linked with the positive core, the negative core, and the shielding layer respectively; L pp is the self-inductance of the positive core; L gg is the self-inductance of the shielding layer; L m is the self-inductance of the negative core; L p-g is the mutual inductance between the positive core and the shielding layer; L n-p is the mutual inductance between the positive core and the negative core; L n-g is the mutual inductance between the negative core and the shielding layer. The total magnetic flux of each DC cable includes I p , I g , In The magnetic flux corresponding to the magnetic field of the excitation source.

[0056] Specifically, taking Ψ p as an example, it includes Ψ p generated by I pp , Ψ g generated by I g-p , and Ψ n generated by I n-p . Similarly, it can be obtained that Ψ n includes Ψ nn , Ψ n-p and Ψ g-n ; Ψ g includes Ψ gg , Ψ n-g and Ψ p-g . The magnetic flux corresponding to the cable inductance is as Figure 6 shown.

[0057] In the frequency band of 0 - 10 kHz, the external magnetic flux (external inductance) of the low - voltage DC cable is hardly affected. The external magnetic flux (external inductance) dominates, so the uneven current distribution has a relatively small overall impact on the self - inductance. Therefore, the frequency - varying characteristics of the cable can be ignored.

[0058] This embodiment also provides an insulation monitoring system for the low - voltage DC system of a thermal power plant, including a voltage detection module that periodically rotates and switches the positive and negative switches through the insulation monitoring device, and simultaneously measures the voltage values of the positive and negative poles to the ground during the switching process; an insulation calculation module that calculates the insulation resistances of the positive and negative poles to the ground respectively based on the measured voltage values of the positive and negative poles to the ground, introducing miscellaneous parameter correction according to the resistance voltage - division principle; and a positioning module that locates the faulty branch by combining the insulation resistance and the leakage current data of the branch.

[0059] If the above functions are implemented in the form of software function units and sold or used as independent products, they can be stored in a computer - readable storage medium. Based on such an understanding, the technical solution of the present invention, in essence, or the part that contributes to the prior art or a part of this technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions for causing a computer device (which can be a personal computer, a server, or a network device, etc.) to execute all or part of the steps of the methods described in various embodiments of the present invention. And the aforementioned storage medium includes: USB flash drives, mobile hard disks, read - only memories (ROM, Read - Only Memory), random access memories (RAM, Random Access Memory), magnetic disks, or optical discs, etc., which can store program codes.

[0060] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as a definite sequence list of executable instructions for implementing logical functions, and can be specifically implemented in any computer-readable medium for use by an instruction execution system, apparatus, or device (such as a computer-based system, a system including a processor, or other systems that can fetch and execute instructions from the instruction execution system, apparatus, or device), or used in combination with these instruction execution systems, apparatuses, or devices. For the purposes of this specification, a "computer-readable medium" can be any device that can contain, store, communicate, propagate, or transmit a program for use by or in connection with an instruction execution system, apparatus, or device.

[0061] More specific examples (non-exhaustive list) of computer-readable media include the following: an electrical connection part with one or more wirings (electronic device), a portable computer disk cartridge (magnetic device), a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), an optical fiber device, and a portable compact disc read-only memory (CDROM). Additionally, the computer-readable medium can even be paper or other suitable media on which the program can be printed, because the program can be obtained electronically, for example, by optically scanning the paper or other media, followed by editing, interpretation, or other suitable processing when necessary, and then stored in a computer memory.

[0062] It should be understood that various parts of the present invention can be implemented using hardware, software, firmware, or a combination thereof. In the above-described embodiments, multiple steps or methods can be implemented using software or firmware stored in a memory and executed by a suitable instruction execution system. For example, if implemented using hardware, as in another embodiment, any one or a combination of the following techniques well known in the art can be used: discrete logic circuits having logic gate circuits for implementing logical functions on data signals, application-specific integrated circuits having suitable combinational logic gate circuits, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0063] Example 2, referring to Figure 5 and Figure 7 , which is an embodiment of the present invention, provides an insulation monitoring method for the low-voltage DC system of a thermal power plant. In order to verify the beneficial effects of the present invention, scientific demonstration is carried out through economic benefit calculation and simulation experiments.

[0064] In order to verify the effectiveness of the insulation monitoring method for the low-voltage DC system of a thermal power plant proposed herein, a simulation model is built on the Matlab / Simulink platform as Figure 5 shown. The simulation parameters of the low-voltage DC cable are shown in Table 1.

[0065] Table 1 Low-voltage DC cable simulation parameters

[0066]

[0067] The curve of insulation resistance changing with time and its partial enlarged view are respectively as Figure 7 shown below.

[0068] Based on Figure 7 data analysis: In the preliminary calculation of insulation resistance, the unbalanced bridge-branch leakage detection method may be lower than the actual value. If the calculated value that has not reached the steady state is adopted, it will increase the risk of misjudgment of the equipment. Therefore, to ensure the accuracy of insulation monitoring, the calculation result after the resistance value stabilizes should be used as the evaluation standard. For a detailed analysis Figure 7 In the left figure is the change curve. Before 0.01 seconds, both the positive and negative switches are off, and the circuit is open. The resistance cannot be directly calculated by voltage measurement, so the resistance value is shown as 0 at this time. At 0.01 seconds, the closing of any switch will cause a change in voltage measurement and start the resistance calculation process, accompanied by the charging and discharging process of the cable equivalent capacitance, marking the transition of the circuit from the non-steady state to the steady state. Taking the positive pole grounding fault as an example: Under normal conditions, the voltage of the positive pole to the ground is 110V, the voltage of the negative pole to the ground is -110V, and the equivalent capacitances between the positive and negative poles to the ground and each other are all charged. If the positive pole is grounded, the voltage of the positive pole to the ground drops suddenly, the voltage of the negative pole to the ground rises, while the voltage between the positive and negative poles remains unchanged. This change causes the equivalent capacitance of the positive pole to the ground to discharge, and the equivalent capacitance of the negative pole to the ground to charge, experiencing a transitional stage of charge redistribution and balance. As Figure 7 shown in the partial enlarged view, after the transitional stage is completed, the calculated resistance value finally stabilizes at 499.47Ω, and the insulation resistance value can be calculated more accurately.

[0069] The simulation results show that using the value of the calculated resistance after reaching the steady state as the basis for insulation monitoring can more accurately reflect the actual condition of the insulation resistance of the low-voltage DC cable, thus improving the accuracy and reliability of insulation monitoring. The insulation monitoring method for the low-voltage DC system of thermal power plants proposed in this paper has certain engineering application value.

[0070] 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 the preferred embodiments, those of ordinary skill in the art should understand that the technical solutions of the present invention can be modified or equivalently replaced without departing from the spirit and scope of the technical solutions of the present invention, and they should all be covered within the scope of the claims of the present invention.

Claims

1. A method for monitoring insulation of a low voltage DC system in a thermal power plant, characterized in that: include: Measure the voltage between the positive and negative electrodes and the ground; Based on the voltage values ​​of the positive electrode and the negative electrode to the ground, calculate the insulation resistance of the positive electrode and the negative electrode to the ground respectively; Combine the insulation resistance and branch leakage current data to locate the faulty branch.

2. The method for monitoring insulation of a low voltage DC system in a thermal power plant according to claim 1, characterized in that: Before measuring the voltage value of the positive electrode and the negative electrode to the ground, the method further includes: The insulation monitoring device periodically switches the positive and negative switches, and simultaneously measures the voltage values ​​of the positive and negative poles to the ground during the switching process.

3. The insulation monitoring method for a low voltage DC system in a thermal power plant according to claim 2, characterized in that: The measuring of the voltage value of the positive electrode and the negative electrode to the ground includes, when the positive electrode is closed and the negative electrode is disconnected, measuring the voltage of the positive electrode line to the ground as U1 and the voltage of the negative electrode line to the ground as U2; When the positive pole is disconnected and the negative pole is closed, the voltage between the positive line and the ground is measured as U'1, and the voltage between the negative line and the ground is measured as U'2.

4. The insulation monitoring method for a low voltage DC system in a thermal power plant according to claim 3, characterized in that: The voltage values ​​of the positive and negative electrodes to the ground include: according to the resistance voltage division principle, the voltages of the positive and negative electrodes to the ground have the following relationship: Where R represents the equivalent insulation resistance of the cable to ground.

5. The insulation monitoring method for a low voltage DC system in a thermal power plant according to claim 4, characterized in that: Before calculating the insulation resistance of the positive electrode and the negative electrode to the ground, the method further includes analyzing the stray parameters of the DC cable and correcting the measurement error caused by the stray parameters of the cable. The DC resistance per unit length of the low-voltage DC cable is expressed as: Among them, r1 represents the resistance per unit length of the low-voltage DC cable; ρ represents the conductor resistivity; s represents the rated cross-sectional area of ​​the low-voltage DC cable.

6. The insulation monitoring method for a low voltage DC system in a thermal power plant according to claim 5, characterized in that: The calculation of the insulation resistance of the positive electrode and the negative electrode to the ground includes that the equivalent insulation resistance of the positive electrode and the negative electrode to the ground can be expressed as: The insulation resistance of the low-voltage DC cable is obtained by rotating the positive electrode-to-ground resistance and the negative electrode-to-ground resistance in the unbalanced bridge.

7. The insulation monitoring method for a low voltage DC system in a thermal power plant according to claim 6, characterized in that: The leakage current is detected by configuring an insulation monitoring device for the low-voltage DC cable of the thermal power plant. The insulation monitoring device for the low-voltage DC cable of the power plant includes 24 branch loads, each branch is equipped with a separate leakage current sensor, and the current detected by the leakage current sensor is the algebraic sum of the currents on the positive electrode, the negative electrode and the shielding layer.

8. An insulation monitoring system for a low voltage DC system in a thermal power plant using the method according to any one of claims 1 to 7, characterized in that: include, The voltage detection module periodically switches the positive and negative switches through the insulation monitoring device, and synchronously measures the voltage values ​​of the positive and negative electrodes to the ground during the switching process; The insulation calculation module calculates the insulation resistance of the positive electrode and the negative electrode to the ground based on the measured voltage value of the positive electrode and the negative electrode to the ground and introduces miscellaneous parameter correction according to the resistance voltage division principle; The positioning module locates the faulty branch by combining the insulation resistance and branch leakage current data.

9. A computer device comprising: A memory and a processor; the memory stores a computer program, characterized in that: when the processor executes the computer program, the steps of the insulation monitoring method for a low-voltage DC system of a thermal power plant as described in any one of claims 1-7 are implemented.

10. A computer-readable storage medium having a computer program stored thereon, characterized in that: When the computer program is executed by a processor, the steps of the insulation monitoring method for a low-voltage DC system of a thermal power plant as described in any one of claims 1 to 7 are implemented.

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