Fault diagnosis method based on double thresholds of characteristic product concentration percentage and ratio

Abstract

The invention discloses a fault diagnosis method based on characteristic product concentration percentage and ratio dual threshold values, comprising the following steps: step 1, carrying out environment-friendly gas local overheating, spark discharge and corona discharge experiments, and detecting decomposition products; 2, screening characteristic decomposition products, and calculating the concentration percentage and the concentration ratio of the characteristic decomposition products; and step 3, constructing a fault diagnosis criterion based on double thresholds of the concentration percentage and the concentration ratio of the characteristic decomposition product, and diagnosing the fault. The problems that in the prior art, environment-friendly gas insulation medium decomposition products are very many in variety and similar in structure, and misjudgment is likely to be caused in fault diagnosis are solved, local overheating, spark discharge and corona discharge of environment-friendly gas insulation power equipment can be effectively diagnosed, misjudgment among various types of faults is avoided, and the fault diagnosis accuracy is improved. And the accuracy of fault diagnosis is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of insulation monitoring and fault diagnosis of electric power equipment, and in particular relates to a fault diagnosis method based on a dual threshold value of a characteristic product concentration percentage and a ratio. Background Art

[0002] The development of the global green and low-carbon power industry has put forward new requirements for the industrialization and upgrading of power equipment. SF6 gas, which is widely used in the field of high-voltage power equipment, has a strong greenhouse effect and a lifespan of up to 3,200 years, causing serious impact on the environment. At present, environmentally friendly gas insulation media such as C4F7N and its mixed gas, C5F 10 O and its mixed gas, C6F 12 O and its mixed gases have attracted widespread attention from scholars at home and abroad, and some power equipment using these environmentally friendly gases as insulating media has been put into trial operation. However, environmentally friendly gas-insulated power equipment inevitably has insulation defects during manufacturing, transportation, and installation. Under long-term operation, these insulation defects will cause high electric fields to be generated locally in the power equipment, resulting in local overheating or discharge, accompanied by the decomposition of the gas insulating medium. Although some decomposition products have certain insulating properties, the insulating properties of the decomposition products are lower than those of the gas insulating medium. With the development of faults and the accumulation of decomposition products, the insulating properties of the gas insulating medium decrease, threatening the safe and reliable operation of the power equipment.

[0003] The characteristics of decomposition products, such as type, concentration, generation rate, and ratio, are closely related to the type of fault. Power equipment fault diagnosis based on SF6 decomposition product analysis has been widely used in fault monitoring of SF6 gas-insulated power equipment due to its advantage of not being subject to electromagnetic interference. However, since the decomposition products of environmentally friendly gas insulation media are very diverse and have similar structures, the use of characteristic decomposition product analysis to diagnose faults is prone to misjudgment, resulting in difficulties in judging the type and severity of the fault, which limits the further development of environmentally friendly power equipment fault diagnosis based on decomposition product analysis. Summary of the invention

[0004] The purpose of the present invention is to provide a fault diagnosis method based on a dual threshold value of characteristic product concentration percentage and ratio, which solves the problem in the prior art that the decomposition products of environmentally friendly gas insulating media are of many types and similar in structure, and fault diagnosis is prone to misjudgment.

[0005] The technical solution adopted by the present invention is: a fault diagnosis method based on a dual threshold value of characteristic product concentration percentage and ratio, specifically comprising the following steps: Step 1, conducting environmentally friendly gas local overheating, spark discharge and corona discharge experiments, and detecting decomposition products; Step 2, screening characteristic decomposition products, and calculating the concentration percentage and concentration ratio of the characteristic decomposition products; Step 3: Construct a fault diagnosis criterion based on the dual thresholds of characteristic decomposition product concentration percentage and concentration ratio, and diagnose the fault.

[0006] The technical solution adopted by the present invention is also characterized in that: Furthermore, step 1 is specifically as follows: construct an experimental platform for local overheating, spark discharge and corona discharge of environmentally friendly gases, carry out local overheating, spark discharge and corona discharge experiments of environmentally friendly gases, and use gas chromatography-mass spectrometry and electron attachment mass spectrometry to detect decomposition products.

[0007] Furthermore, the above experimental platform is transformed from GIS, and its main body is composed of a high-voltage bushing and an experimental cavity, with an air chamber partition between the two.

[0008] Furthermore, the local overheating experimental platform is composed of a power supply, a heating element, a temperature control module, an experimental device and a decomposition product detection; the temperature control module adopts PID to perform temperature control.

[0009] Furthermore, the spark discharge experimental platform is composed of a control console, a voltage regulator, a filter, a transformer, a protective resistor, a capacitive voltage divider, a relay control circuit, an experimental chamber and a decomposition product detection system; a needle-plate electrode is installed in the experimental chamber, the needle electrode is connected to the high voltage end, and the plate electrode is grounded; spark discharge is generated by controlling the relay circuit.

[0010] Furthermore, the above-mentioned corona discharge experimental platform is composed of a control console, a voltage regulator, a filter, a transformer, a protective resistor, a capacitive voltage divider, an experimental cavity and a decomposition product detector; the voltage and current signals in the experiment are detected by a high-voltage probe and a Rogowski coil respectively; a needle-plate electrode is installed in the experimental cavity, and a stable corona discharge is generated by controlling the experimental voltage.

[0011] Furthermore, the screening of characteristic decomposition products in step 2 is specifically as follows: the characteristic decomposition products are screened according to the detected decomposition products, and the type, concentration, generation rate and ratio characteristics of the screened characteristic decomposition products can characterize the local overheating, spark discharge and corona discharge types of environmentally friendly gas.

[0012] Furthermore, the concentration percentage and concentration ratio of the characteristic decomposition products calculated in step 2 are as follows: According to the selected characteristic decomposition products, the concentration percentage and concentration ratio of the characteristic decomposition products are calculated. The concentration percentage is the percentage of the concentration of each characteristic decomposition product in the concentration of all the selected characteristic decomposition products. By calculating the concentration percentage of each characteristic decomposition product under local overheating, spark discharge and corona discharge, the concentration percentage threshold range of each characteristic decomposition product is obtained. According to the screened characteristic decomposition products, a set of independent characteristic decomposition product ratio types are constructed. According to the concentration characteristics of the characteristic decomposition products under local overheating, spark discharge and corona discharge, different ratio types are constructed. The characteristic decomposition product ratios constructed under local overheating, spark discharge and corona discharge are calculated respectively, and the threshold range of the characteristic decomposition product ratio type under local overheating, spark discharge and corona discharge is obtained.

[0013] Further, step 3 is specifically as follows: constructing an environmentally friendly power equipment fault diagnosis criterion according to the concentration percentage threshold and the concentration ratio threshold of the characteristic decomposition product calculated in step 2; Compare the characteristic decomposition product concentration percentage calculated in step 2 with the corona discharge concentration percentage threshold. If the corona discharge concentration percentage threshold is met but the spark discharge and local overheating concentration percentage thresholds are not met, it can be diagnosed as corona discharge. If the corona discharge concentration percentage threshold is met, and the spark discharge and local overheating concentration percentage threshold is also met, the calculated concentration ratio is compared with the corona discharge; If the corona discharge concentration ratio threshold is met, it is diagnosed as corona discharge, otherwise it continues to be compared with the concentration ratio thresholds of spark discharge and local overheating; The same method can be used to diagnose spark discharge and local overheating faults.

[0014] Furthermore, the types and concentrations of characteristic decomposition products of local overheating are greatly affected by the temperature range. Therefore, local overheating is divided into mild overheating, moderate overheating and severe overheating according to the temperature range, and the concentration percentage thresholds and concentration ratio thresholds of the characteristic decomposition products of these three cases are calculated respectively; when performing fault diagnosis, first combine the concentration percentage and concentration ratio of the characteristic decomposition products to diagnose whether it is mild overheating. If the mild overheating threshold is met, it is diagnosed as mild overheating; otherwise, continue to compare with the moderate overheating and severe overheating thresholds; in this way, the severity of local overheating is judged according to the concentration percentage and concentration ratio of the characteristic decomposition products.

[0015] The beneficial effects of the present invention are: (1) The present invention provides a fault diagnosis method based on dual thresholds of characteristic product concentration percentage and concentration ratio, which can effectively diagnose local overheating, spark discharge and corona discharge of environmentally friendly gas-insulated power equipment, avoid misjudgment between various types of faults, and improve the accuracy of fault diagnosis.

[0016] (2) The environmentally friendly gas insulating medium in the present invention can be C4F7N and its mixed gas, C5F 10 O and its mixed gas, C6F 12O and its mixed gas, etc., can apply the concentration percentage and concentration ratio of the characteristic decomposition products of these environmentally friendly gas insulating media to diagnose local overheating and discharge faults of environmentally friendly power equipment.

[0017] (3) The present invention provides a fault diagnosis method based on the decomposition products of environmentally friendly gas insulating media, which can judge the fault type and severity of environmentally friendly gas insulated power equipment in real time according to the characteristic decomposition products, thereby issuing early warnings, ensuring the safe and reliable operation of the power equipment, and saving the manpower, material and financial costs of fault inspection and repair. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a flow chart of the fault diagnosis method of the present invention. DETAILED DESCRIPTION

[0019] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0020] The present invention is based on the fault diagnosis method of characteristic product concentration percentage and ratio dual threshold value, such as Figure 1 As shown, the following steps are included: Step 1: Conduct environmentally friendly gas local overheating, spark discharge and corona discharge experiments, and detect the decomposition products.

[0021] In step 1, an environmentally friendly gas local overheating, spark discharge and corona discharge experimental platform is built to carry out local overheating, spark discharge and corona discharge experiments of environmentally friendly gas, and decomposition products are detected by gas chromatography-mass spectrometry and electron attachment mass spectrometry. The experimental platform in step 1 is transformed from GIS, and the main body consists of a high-voltage bushing and an experimental cavity, with a gas chamber partition between the two.

[0022] The local overheating experimental platform in step 1 is composed of a power supply, a heating element, a temperature control module, an experimental device and a decomposition product detection. The temperature control module uses PID for temperature control.

[0023] In step 1, the spark discharge experimental platform consists of a control console, a voltage regulator, a filter, a transformer, a protective resistor, a capacitive voltage divider, a relay control circuit, an experimental chamber, and a decomposition product detection. A needle-plate electrode is installed in the experimental chamber, the needle electrode is connected to the high voltage end, and the plate electrode is grounded. Spark discharge is generated by controlling the relay circuit.

[0024] In step 1, the corona discharge experimental platform consists of a control console, a voltage regulator, a filter, a transformer, a protective resistor, a capacitive voltage divider, an experimental chamber, and a decomposition product detection. The voltage and current signals in the experiment are detected by a high-voltage probe and a Rogowski coil, respectively. A needle-plate electrode is installed in the experimental chamber, and a stable corona discharge is generated by controlling the experimental voltage.

[0025] The decomposition product detection in step 1 includes gas chromatography-mass spectrometry and electron attachment mass spectrometry. Gas chromatography-mass spectrometry combines the advantages of chromatograph and mass spectrometer, separates the decomposition products and then detects them, and can qualitatively and quantitatively detect the concentration of decomposition products. Electron attachment mass spectrometry uses the negative ion mode in the mass spectrometer for detection, and detects the intensity of negative ions and traces its negative ionization path to qualitatively obtain the concentration of the decomposition product. Gas chromatography-mass spectrometry has a long detection time, mainly detects stable decomposition products, has a high detection limit, and requires a high degree of separation of decomposition products. Electron attachment mass spectrometry does not require gas separation, has a fast detection speed, and has a very low detection limit. Therefore, combining these two detection methods can enable a more comprehensive detection of decomposition products.

[0026] Step 2: Screen the characteristic decomposition products and calculate the concentration percentage and concentration ratio of the characteristic decomposition products.

[0027] In step 2, characteristic decomposition products are first screened based on the detected decomposition products. The characteristics of the screened characteristic decomposition products, such as type, concentration, generation rate, ratio, etc., can characterize the local overheating, spark discharge and corona discharge types of environmentally friendly gas.

[0028] Step 2: Calculate the concentration percentage and concentration ratio of the characteristic decomposition products based on the selected characteristic decomposition products. The concentration percentage refers to the percentage of the concentration of each characteristic decomposition product in the concentration of all the selected characteristic decomposition products. For example, if there are four characteristic decomposition products selected, they are recorded as A, B, C and D, and the concentration percentages of these four characteristic decomposition products are: A / (A+B+C+D)×100%, B / (A+B+C+D)×100%, C / (A+B+C+D)×100% and D / (A+B+C+D)×100%. By calculating the concentration percentages of these four characteristic decomposition products under local overheating, spark discharge and corona discharge, the concentration percentage threshold ranges of these four characteristic decomposition products can be obtained.

[0029] In step 2, a set of independent characteristic decomposition product ratio types are constructed based on the selected characteristic decomposition products. For example, if there are four characteristic decomposition products selected, they are recorded as A, B, C and D respectively. Different ratio types, such as A / B and C / D, are constructed based on the concentration characteristics of these four characteristic decomposition products under local overheating, spark discharge and corona discharge. The characteristic decomposition product ratios constructed under local overheating, spark discharge and corona discharge are calculated respectively, and the threshold range of the characteristic decomposition product ratio type under local overheating, spark discharge and corona discharge can be obtained.

[0030] Step 3: Construct a fault diagnosis criterion based on a dual threshold of characteristic decomposition product concentration percentage and concentration ratio.

[0031] In step 3, according to the concentration percentage threshold and concentration ratio threshold of the characteristic decomposition products calculated in step 2, a fault diagnosis criterion for environmentally friendly power equipment is constructed, as shown in the attached figure. Figure 1 As shown. First, the concentration percentage and concentration ratio of the characteristic decomposition products are calculated. Then, the calculated characteristic decomposition product concentration percentage is compared with the corona discharge concentration percentage threshold. If the corona discharge concentration percentage threshold is met, but the spark discharge and local overheating concentration percentage thresholds are not met, it can be diagnosed as corona discharge. If the corona discharge concentration percentage threshold is met, and the spark discharge and local overheating concentration percentage thresholds are also met, the calculated concentration ratio is compared with the corona discharge. If the corona discharge concentration ratio threshold is met, it is diagnosed as corona discharge, otherwise it continues to be compared with the spark discharge and local overheating concentration ratio thresholds. The same method can be used to diagnose spark discharge and local overheating faults.

[0032] The temperature range of local overheating in step 3 is relatively wide. Generally speaking, the types and concentrations of local overheating characteristic decomposition products are greatly affected by the temperature range. Therefore, local overheating can be divided into mild overheating, moderate overheating and severe overheating according to the temperature range. When performing fault diagnosis, first combine the concentration percentage and concentration ratio of the characteristic decomposition products to diagnose whether it is mild overheating. If the mild overheating threshold is met, it is diagnosed as mild overheating. Otherwise, continue to compare with the moderate overheating and severe overheating thresholds. In this way, the severity of local overheating can be judged based on the concentration percentage and concentration ratio of the characteristic decomposition products.

[0033] The technical solution of the present invention is further described below through the drawings and embodiments.

[0034] Example 1 The fault diagnosis method based on the characteristic product concentration percentage and ratio dual threshold value specifically includes the following steps: Step 1, conducting environmentally friendly gas local overheating, spark discharge and corona discharge experiments, and detecting decomposition products; Step 2, screening characteristic decomposition products, and calculating the concentration percentage and concentration ratio of the characteristic decomposition products; Step 3: Construct a fault diagnosis criterion based on the dual thresholds of characteristic decomposition product concentration percentage and concentration ratio, and diagnose the fault.

[0035] Example 2 The fault diagnosis method based on the characteristic product concentration percentage and ratio dual threshold value specifically includes the following steps: Step 1, conducting environmentally friendly gas local overheating, spark discharge and corona discharge experiments, and detecting decomposition products; Step 1 is as follows: build an experimental platform for local overheating, spark discharge and corona discharge of environmentally friendly gas, carry out local overheating, spark discharge and corona discharge experiments of environmentally friendly gas, and detect decomposition products by gas chromatography-mass spectrometry and electron attachment mass spectrometry; The experimental platform is transformed from GIS. The main body consists of a high-voltage bushing and an experimental cavity, with an air chamber between them. The local overheating experimental platform consists of a power supply, a heating element, a temperature control module, an experimental device, and a decomposition product detection system. The temperature control module uses PID for temperature control. The spark discharge experimental platform consists of a control console, a voltage regulator, a filter, a transformer, a protective resistor, a capacitive voltage divider, a relay control circuit, an experimental chamber, and a decomposition product detection system. A needle-plate electrode is installed in the experimental chamber, the needle electrode is connected to the high voltage end, and the plate electrode is grounded. Spark discharge is generated by controlling the relay circuit. The corona discharge experimental platform consists of a control console, a voltage regulator, a filter, a transformer, a protective resistor, a capacitive voltage divider, an experimental chamber, and a decomposition product detector. The voltage and current signals in the experiment are detected by a high-voltage probe and a Rogowski coil respectively. A needle-plate electrode is installed in the experimental chamber to generate a stable corona discharge by controlling the experimental voltage. Step 2, screening characteristic decomposition products, and calculating the concentration percentage and concentration ratio of the characteristic decomposition products; The specific steps of screening characteristic decomposition products in step 2 are as follows: screening characteristic decomposition products according to the detected decomposition products, wherein the type, concentration, generation rate and ratio characteristics of the screened characteristic decomposition products can characterize the types of local overheating, spark discharge and corona discharge of the environmentally friendly gas; The concentration percentage and concentration ratio of the characteristic decomposition products calculated in step 2 are as follows: According to the selected characteristic decomposition products, the concentration percentage and concentration ratio of the characteristic decomposition products are calculated. The concentration percentage is the percentage of the concentration of each characteristic decomposition product in the concentration of all the selected characteristic decomposition products. By calculating the concentration percentage of each characteristic decomposition product under local overheating, spark discharge and corona discharge, the concentration percentage threshold range of each characteristic decomposition product is obtained. According to the selected characteristic decomposition products, a set of independent characteristic decomposition product ratio types are constructed. According to the concentration characteristics of the characteristic decomposition products under local overheating, spark discharge and corona discharge, different ratio types are constructed. The characteristic decomposition product ratios constructed under local overheating, spark discharge and corona discharge are calculated respectively, and the threshold range of the characteristic decomposition product ratio type under local overheating, spark discharge and corona discharge is obtained. Step 3, constructing a fault diagnosis criterion based on a double threshold of characteristic decomposition product concentration percentage and concentration ratio, and diagnosing the fault; Step 3 is specifically as follows: constructing a fault diagnosis criterion for environmentally friendly power equipment according to the concentration percentage threshold and concentration ratio threshold of the characteristic decomposition products calculated in step 2; Compare the characteristic decomposition product concentration percentage calculated in step 2 with the corona discharge concentration percentage threshold. If the corona discharge concentration percentage threshold is met but the spark discharge and local overheating concentration percentage thresholds are not met, it can be diagnosed as corona discharge. If the corona discharge concentration percentage threshold is met, and the spark discharge and local overheating concentration percentage threshold is also met, the calculated concentration ratio is compared with the corona discharge; If the corona discharge concentration ratio threshold is met, it is diagnosed as corona discharge, otherwise it continues to be compared with the concentration ratio thresholds of spark discharge and local overheating; The same method can be used to diagnose spark discharge and local overheating faults; Local overheating is divided into mild overheating, moderate overheating and severe overheating according to the temperature range, and the concentration percentage threshold and concentration ratio threshold of the characteristic decomposition products of these three cases are calculated respectively; when performing fault diagnosis, the concentration percentage and concentration ratio of the characteristic decomposition products are first combined to diagnose whether it is mild overheating. If the mild overheating threshold is met, it is diagnosed as mild overheating; otherwise, it is compared with the moderate overheating and severe overheating thresholds; the severity of the local overheating is judged according to the concentration percentage and concentration ratio of the characteristic decomposition products.

[0036] Example 3 C5F 10 Taking spark discharge and corona discharge of O / CO2 mixed gas as examples, CF4O and C3F8 are selected as characteristic decomposition products, and the concentration percentages of these two characteristic decomposition products are calculated respectively. The concentration percentage of CF4O under spark discharge is 16.1%~36.9%, and the concentration percentage under corona discharge is 3.2%~7.4%. The concentration percentage of C3F8 under spark discharge is 63.1%~83.9%, and the concentration percentage under corona discharge is 92.6%~96.8%. Therefore, the concentration percentage threshold ranges of CF4O and C3F8 in spark discharge and corona discharge can be determined respectively. In addition, the ratio of CF4O concentration (c(CF4O)) to C3F8 concentration (c(C3F8)) is constructed, and the range of c(CF4O) / c(C3F8) under spark discharge is 0.2~0.6, and the concentration range of c(CF4O) / c(C3F8) under corona discharge is below 0.1. Therefore, it can be determined that the threshold ranges of c(CF4O) / c(C3F8) under spark discharge and corona discharge are 0.2~0.6 and below 0.1, respectively. Combining the concentration percentage method with the concentration ratio method, a dual threshold can be used to determine the fault type, thereby improving the accuracy of fault identification.

Claims

1. A fault diagnosis method based on a dual threshold of characteristic product concentration percentage and ratio, characterized in that: The specific steps include: Step 1, conducting environmentally friendly gas local overheating, spark discharge and corona discharge experiments, and detecting decomposition products; Step 2, screening characteristic decomposition products, and calculating the concentration percentage and concentration ratio of the characteristic decomposition products; Step 3: Construct a fault diagnosis criterion based on a dual threshold of characteristic decomposition product concentration percentage and concentration ratio, and diagnose the fault.

2. The fault diagnosis method based on characteristic product concentration percentage and ratio dual threshold value according to claim 1 is characterized in that: The step 1 is specifically as follows: building an environmentally friendly gas local overheating experimental platform, a spark discharge experimental platform and a corona discharge experimental platform, carrying out local overheating, spark discharge and corona discharge experiments of environmentally friendly gases, and using gas chromatography-mass spectrometry and electron attachment mass spectrometry to detect decomposition products.

3. The fault diagnosis method based on characteristic product concentration percentage and ratio dual threshold value according to claim 2 is characterized in that: The experimental platforms are all transformed from GIS, and the main body consists of a high-voltage bushing and an experimental cavity, with an air chamber between the two.

4. The fault diagnosis method based on characteristic product concentration percentage and ratio dual threshold value according to claim 3 is characterized in that: The local overheating experimental platform is composed of a power supply, a heating element, a temperature control module, an experimental device and a decomposition product detection; the temperature control module adopts PID to perform temperature control.

5. The fault diagnosis method based on characteristic product concentration percentage and ratio dual threshold value according to claim 3 is characterized in that: The spark discharge experimental platform is composed of a control console, a voltage regulator, a filter, a transformer, a protective resistor, a capacitive voltage divider, a relay control circuit, an experimental cavity and a decomposition product detection system; a needle-plate electrode is installed in the experimental cavity, the needle electrode is connected to the high voltage end, and the plate electrode is grounded; spark discharge is generated by controlling the relay circuit.

6. The fault diagnosis method based on characteristic product concentration percentage and ratio dual threshold value according to claim 3 is characterized in that: The corona discharge experimental platform is composed of a control console, a voltage regulator, a filter, a transformer, a protective resistor, a capacitive voltage divider, an experimental cavity and a decomposition product detector; the voltage and current signals in the experiment are detected by a high-voltage probe and a Rogowski coil respectively; a needle-plate electrode is installed in the experimental cavity, and a stable corona discharge is generated by controlling the experimental voltage.

7. The fault diagnosis method based on characteristic product concentration percentage and ratio dual threshold value according to claim 1, characterized in that: The specific method of screening characteristic decomposition products in step 2 is as follows: screening characteristic decomposition products based on the detected decomposition products, wherein the type, concentration, generation rate and ratio characteristics of the screened characteristic decomposition products can characterize the types of local overheating, spark discharge and corona discharge of environmentally friendly gas.

8. The fault diagnosis method based on characteristic product concentration percentage and ratio dual threshold value according to claim 1, characterized in that: The concentration percentage and concentration ratio of the characteristic decomposition products calculated in step 2 are specifically as follows: According to the selected characteristic decomposition products, the concentration percentage and concentration ratio of the characteristic decomposition products are calculated. The concentration percentage is the percentage of the concentration of each characteristic decomposition product in the concentration of all the selected characteristic decomposition products. By calculating the concentration percentage of each characteristic decomposition product under local overheating, spark discharge and corona discharge, the concentration percentage threshold range of each characteristic decomposition product is obtained. According to the screened characteristic decomposition products, a set of independent characteristic decomposition product ratio types are constructed. According to the concentration characteristics of the characteristic decomposition products under local overheating, spark discharge and corona discharge, different ratio types are constructed. The characteristic decomposition product ratios constructed under local overheating, spark discharge and corona discharge are calculated respectively, and the threshold range of the characteristic decomposition product ratio type under local overheating, spark discharge and corona discharge is obtained.

9. The fault diagnosis method based on characteristic product concentration percentage and ratio dual threshold value according to claim 1, characterized in that: The step 3 is specifically as follows: constructing a fault diagnosis criterion for environmentally friendly power equipment according to the concentration percentage threshold and the concentration ratio threshold of the characteristic decomposition product calculated in step 2; Compare the characteristic decomposition product concentration percentage calculated in step 2 with the corona discharge concentration percentage threshold. If the corona discharge concentration percentage threshold is met but the spark discharge and local overheating concentration percentage thresholds are not met, it can be diagnosed as corona discharge. If the corona discharge concentration percentage threshold is met, and the spark discharge and local overheating concentration percentage threshold is also met, the calculated concentration ratio is compared with the corona discharge; If the corona discharge concentration ratio threshold is met, it is diagnosed as corona discharge, otherwise it continues to be compared with the concentration ratio thresholds of spark discharge and local overheating; The same method can be used to diagnose spark discharge and local overheating faults.

10. The fault diagnosis method based on characteristic product concentration percentage and ratio dual threshold value according to claim 9, characterized in that: The local overheating is divided into mild overheating, moderate overheating and severe overheating according to the temperature range, and the concentration percentage threshold and concentration ratio threshold of the characteristic decomposition products of the three cases are calculated respectively; when performing fault diagnosis, firstly diagnose whether it is mild overheating by combining the concentration percentage and concentration ratio of the characteristic decomposition products, and if the mild overheating threshold is met, it is diagnosed as mild overheating; otherwise, continue to compare with the moderate overheating and severe overheating thresholds; The severity of local overheating is determined based on the concentration percentage and concentration ratio of the characteristic decomposition products.

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