A kind of C 4 F 7 N / CO 2 / O 2 Early warning method for mixing ratio of ternary mixed gas

Abstract

The present invention relates to a warning method for the mixing ratio of a C4F7N / CO2 / O2 ternary mixed gas, belonging to the field of insulating gas monitoring. The method includes: sampling the temperature and pressure values of the ternary mixed gas in the device to be measured and sorting them from low to high according to the temperature; performing linear fitting on the sorted temperature and the corresponding pressure values to obtain the slope of the fitted straight line as the first slope; based on the sampled temperature and the corresponding pressure values, as well as the rated mixing ratio and temperature-pressure characteristic curve built in the density relay, obtaining the pressure value at 20°C corresponding to the sampled pressure value under the built-in rated mixing ratio; calculating the mean value of the slopes of each sampled pressure value and the pressure value at 20°C corresponding to it as the second slope; calculating the difference between the first slope and the second slope, and if the difference is greater than the threshold, a warning is issued. By comparing the difference between the first slope and the second slope and issuing a warning when the difference is greater than the threshold, this method solves the problem of how to issue a timely warning when the mixing ratio of the ternary mixed gas changes.

Description

Technical Field

[0001] The present invention relates to the field of insulating gas monitoring, and particularly to a warning method for the mixing ratio of a C 4 F 7 N / CO 2 / O 2 ternary mixed gas. Background Art

[0002] C 4 F 7 N / CO 2 / O 2 Due to its environmental friendliness and good insulating performance, the ternary mixed gas is increasingly widely used as an insulating medium for high-voltage electrical equipment in the power industry. The decrease in the density of the ternary mixed gas will directly affect the reduction of its insulating performance, resulting in a significant increase in the occurrence frequency of defects such as discharge and overheating in electrical equipment, threatening the safe and stable operation of the power system. Therefore, it is necessary to focus on monitoring its density change during the operation of electrical equipment. Currently, the existing method for monitoring the density of C 4 F 7 N / CO 2 / O 2 ternary mixed gas mainly measures the temperature and pressure values of the ternary mixed gas through the temperature sensor and pressure sensor in the density relay installed on the electrical equipment, and converts the measured pressure value into its corresponding pressure value at 20°C through the temperature-pressure characteristic curve built in the density relay, so as to obtain the density of the ternary mixed gas.

[0003] However, due to reasons such as non-standard gas filling and leakage of electrical equipment, the mixing ratio of C 4 F 7 N / CO 2 / O 2 in the ternary mixed gas in the electrical equipment may change. It can be understood that when the mixing ratio of the ternary mixed gas changes, its temperature-pressure characteristic curve will also change accordingly. At this time, if the temperature-pressure characteristic curve built in the density relay, that is, the temperature-pressure characteristic curve under the rated mixing ratio, is used to convert the corresponding pressure value at 20°C, a large error will occur, resulting in a large error in the density of the ternary mixed gas obtained, causing misjudgment of the density relay. Currently, the existing method for monitoring the mixing ratio of C 4 F 7 N / CO 2 / O 2 ternary mixed gas mainly relies on the method of regular manual inspection to judge whether the mixing ratio is qualified, and it is difficult to judge whether the mixing ratio is qualified in real time during the inspection interval.

[0004] Therefore, it is urgent to study in C 4F 7 N / CO 2 / O 2 A method for timely warning when the mixing ratio of a ternary mixed gas changes, so as to improve the quality and efficiency of power system operation and maintenance. Summary of the Invention

[0005] In view of the above analysis, the present invention aims to provide a C 4 F 7 N / CO 2 / O 2 Warning method for the mixing ratio of a ternary mixed gas, which is used to solve the problem of how to give a timely warning when the mixing ratio of the ternary mixed gas changes.

[0006] The present invention provides a C 4 F 7 N / CO 2 / O 2 Warning method for the mixing ratio of a ternary mixed gas, the method comprising the following steps:

[0007] Sampling the temperature and pressure values of the C 4 F 7 N / CO 2 / O 2 ternary mixed gas in the device to be measured for a period of time to obtain a number of temperature and corresponding pressure values, and sorting them from low to high according to temperature; constructing a coordinate system with temperature as the X-axis and pressure value as the Y-axis, and linearly fitting the sorted temperature and corresponding pressure values under this coordinate system, and taking the slope of the straight line after linear fitting as the first slope;

[0008] Based on the sampled temperature and corresponding pressure values and the rated mixing ratio and temperature-pressure characteristic curve built in the density relay, obtaining the pressure value at 20 °C corresponding to the sampled pressure value under the built-in rated mixing ratio; calculating the average value of the slopes of each sampled pressure value and its corresponding pressure value at 20 °C as the second slope;

[0009] Calculating the difference between the first slope and the second slope, and if the difference is greater than the threshold value, giving a warning.

[0010] Further, calculating the difference between the first slope and the second slope, if the difference is greater than the threshold value and the first slope is greater than the second slope, giving a warning of a high proportion of C 4 F 7 N gas; if the difference is greater than the threshold value and the first slope is less than the second slope, giving a warning of a low proportion of C 4 F 7 N gas.

[0011] Further, based on the sampled temperature and the corresponding pressure value, perform density interpolation on the temperature-pressure characteristic curve built in the density relay to obtain the pressure value at 20°C corresponding to each sampled pressure value of the ternary mixed gas under the built-in rated mixing ratio, and calculate the slope between each sampled pressure value and the pressure value at 20°C corresponding thereto.

[0012] Further, for each sampled pressure value:

[0013] Find the two temperature-pressure characteristic curves closest to it based on this pressure value and the corresponding temperature;

[0014] Calculate the proportionality coefficient based on the temperature corresponding to this pressure value and the temperature values corresponding to the two temperature-pressure characteristic curves closest to it;

[0015] Obtain the pressure value at 20°C corresponding to this sampled pressure value under the built-in rated mixing ratio based on the proportionality coefficient and the pressure values at 20°C corresponding to the two temperature-pressure characteristic curves closest to it;

[0016] Calculate the slope between this sampled pressure value and the pressure value at 20°C corresponding to it under the built-in rated mixing ratio.

[0017] Further, calculate the proportionality coefficient through the following formula:

[0018] , (1)

[0019] Wherein, represents the proportionality coefficient of the pressure value of the i-th sample, represents the temperature value of the i-th sample, 、 represent the temperature values corresponding to the two temperature-pressure characteristic curves closest to the pressure value of the i-th sample.

[0020] Further, obtain the pressure value at 20°C corresponding to the sampled pressure value of the ternary mixed gas under this built-in rated mixing ratio through the following formula:

[0021] , (2)

[0022] Wherein, represents the pressure value at 20°C corresponding to the pressure value of the i-th sample under the rated mixing ratio, 、 represent the pressure values at 20°C corresponding to the two temperature-pressure characteristic curves closest to the pressure value of the i-th sample.

[0023] Further, calculate the slope between the sampled pressure value and the pressure value at 20°C corresponding to it under the built-in rated mixing ratio through the following formula:

[0024] , (3)

[0025] wherein, represents the slope between the i-th sampled pressure value and the pressure value at 20°C corresponding to it under the built-in rated mixing ratio, represents the i-th sampled pressure value, represents the i-th sampled temperature.

[0026] Further, calculate the average value of the slopes of all sampled pressure values and their corresponding pressure values at 20°C through the following formula, and use this average value as the second slope:

[0027] , (4)

[0028] wherein, n represents the number of sampled pressure values, represents the second slope.

[0029] Further, the temperature-pressure characteristic curve of the ternary mixed gas under the rated mixing ratio is a series of temperature-pressure characteristic curve clusters with the temperature value as the X-axis and the pressure value as the Y-axis, representing the temperature-pressure curves corresponding to different gas densities under this rated mixing ratio.

[0030] Further, the density relay includes a temperature sensor and a pressure sensor. The temperature and pressure values of the ternary mixed gas are sampled at a fixed sampling frequency for a period of time through the temperature sensor and the pressure sensor, obtaining a number of temperatures and corresponding pressure values, and sorting the temperatures and corresponding pressure values from low to high; construct a coordinate system with the temperature as the X-axis and the pressure value as the Y-axis, obtain the coordinate points corresponding to the sorted temperatures and corresponding pressure values in this coordinate system, perform a linear fitting on all coordinate points to form a straight line, and use the slope of this straight line as the first slope.

[0031] Compared with the prior art, the present invention can at least achieve one of the following beneficial effects:

[0032] 1. By performing density interpolation on the temperature-pressure characteristic curve built in the density relay, the present invention obtains the pressure value of the C 4 F 7 N / CO 2 / O 2 ternary mixed gas under the built-in rated mixing ratio, improving the accuracy of the output result of the density relay.

[0033] 2. The present invention compares the difference between the first slope and the second slope, and issues a warning when the difference is greater than the threshold, solving the problem of how to timely issue a warning when the mixing ratio of the ternary mixed gas changes, which is of great significance for ensuring the safe and stable operation of electrical equipment. 4 F 7 N / CO 2 / O 2 When the mixing ratio of the ternary mixed gas changes, it is of great significance for ensuring the safe and stable operation of electrical equipment.

[0034] 3. When the mixing ratio of the ternary mixed gas in the device to be tested changes, the present invention can issue a real-time warning only through a density relay, without adding additional devices, reducing costs and improving work efficiency.

[0035] In the present invention, the above technical solutions can also be combined with each other to achieve more preferred combination schemes. Other features and advantages of the present invention will be described in the subsequent specification, and some advantages can be made obvious from the specification or understood by implementing the present invention. The objectives and other advantages of the present invention can be realized and obtained through the content specifically pointed out in the specification and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The drawings are only for the purpose of showing specific embodiments and are not considered to be a limitation of the present invention. Throughout the drawings, the same reference signs represent the same components;

[0037] Figure 1 For the embodiment C 4 F 7 N / CO 2 / O 2 Flowchart of the warning method for the mixing ratio of the ternary mixed gas;

[0038] Figure 2 For the embodiment C 4 F 7 N / CO 2 / O 2 Schematic diagram of the temperature-pressure characteristic curve of the ternary mixed gas at a certain rated mixing ratio. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0039] The following will specifically describe the preferred embodiments of the present invention with reference to the drawings, where the drawings form a part of this application and are used together with the embodiments of the present invention to explain the principles of the present invention, rather than to limit the scope of the present invention.

[0040] A specific embodiment of the present invention discloses a method for warning the mixing ratio of a ternary mixed gas of C 4 F 7 N / CO 2 / O 2 As Figure 1As shown, the method includes the following steps:

[0041] Step S1: Use a density relay to sample the temperature and pressure values of the C 4 F 7 N / CO 2 / O 2 ternary mixed gas over a period of time to obtain a number of temperature and corresponding pressure values, and sort them from low to high according to temperature; construct a coordinate system with temperature as the X-axis and pressure value as the Y-axis, and perform linear fitting on the sorted temperature and corresponding pressure values under this coordinate system, and take the slope of the straight line after linear fitting as the first slope;

[0042] Step S2: Based on the sampled temperature and corresponding pressure values, as well as the rated mixing ratio and temperature-pressure characteristic curve built in the density relay, obtain the pressure value at 20°C corresponding to the sampled pressure value under the built-in rated mixing ratio; calculate the average value of the slopes of each sampled pressure value and its corresponding pressure value at 20°C as the second slope;

[0043] Step S3: Calculate the difference between the first slope and the second slope, and if the difference is greater than the threshold, give an early warning.

[0044] Specifically, in step S1, the density relay includes a temperature sensor and a pressure sensor, and samples the temperature and pressure values of the ternary mixed gas over a period of time at a fixed sampling frequency through the temperature sensor and the pressure sensor to obtain a number of temperature and corresponding pressure values, and sort the temperature and corresponding pressure values from low to high according to temperature. Construct a coordinate system with temperature as the X-axis and pressure value as the Y-axis, and obtain the coordinate points corresponding to the sorted temperature and corresponding pressure values under this coordinate system, and perform a linear fitting on all the coordinate points to form a straight line, and take the slope of this straight line as the first slope.

[0045] Exemplarily, within 48 hours, sample the temperature and pressure values of the ternary mixed gas every 1 hour to obtain 48 temperature and corresponding pressure values. Sort them from low to high according to temperature, denoted as (T 1 , P 1 ), (T 2 , P 2 ),...... (T 48 , P 48 ), where T 1 represents the lowest temperature T min within these 48 hours, and T 48 represents the highest temperature T max within these 48 hours. Construct a coordinate system with temperature as the X-axis and pressure value as the Y-axis, and obtain (T 1 , P 1 ), (T2 , P 2 ),......(T 48 , P 48 The 48 coordinate points corresponding to), and perform a linear fitting on the 48 coordinate points to form a straight line L, calculate the slope k of the straight line L, and use k as the first slope.

[0046] It should be noted that operation and maintenance experience shows that the leakage of the ternary mixed gas in the electrical equipment is very slow. The time required for the pressure of the equipment with relatively serious leakage to drop by 0.05 MPa is in quarters. Therefore, the temperature and pressure values can be sampled within a suitable time period, such as within 48 hours. During this time period, it can be considered that the mixing ratio and density of the ternary mixed gas in the electrical equipment to be measured do not change; and, under the condition of equal density, the temperature and pressure of the ternary mixed gas are linearly related; therefore, the first slope obtained by the above method is the slope of the temperature-pressure characteristic curve corresponding to the ternary mixed gas in the equipment to be measured under the actual mixing ratio.

[0047] Specifically, in step S2, C 4 F 7 N / CO 2 / O 2 The temperature-pressure characteristic curve of the ternary mixed gas under a certain rated mixing ratio is as Figure 2 shown. It can be seen from Figure 2 that the temperature-pressure characteristic curve is a series of temperature-pressure characteristic curve clusters with the temperature value as the X-axis and the pressure value as the Y-axis, representing the temperature-pressure curves corresponding to different gas densities under this rated mixing ratio. Among them, each temperature-pressure characteristic curve corresponds to a gas density and is obtained through experiments.

[0048] It should be noted that when using C 4 F 7 N / CO 2 / O 2 ternary mixed gas as the insulating medium of high-voltage electrical equipment, the ternary mixed gas usually uses a specific rated mixing ratio, and the temperature-pressure characteristic curve of the ternary mixed gas under this rated mixing ratio is built in the density relay to improve the measurement accuracy.

[0049] Furthermore, based on the sampled temperature and the corresponding pressure value, perform density interpolation on the temperature-pressure characteristic curve built in the density relay to obtain the pressure value at 20 °C corresponding to the pressure value of each sample of the ternary mixed gas under the built-in rated mixing ratio, and calculate the slope between the pressure value of each sample and its corresponding pressure value at 20 °C.

[0050] Furthermore, for each sampled pressure value:

[0051] Find the two temperature-pressure characteristic curves closest to it based on the pressure value and the corresponding temperature;

[0052] Calculate the proportionality coefficient based on the temperature corresponding to the pressure value and the temperature values corresponding to the two temperature-pressure characteristic curves closest to it;

[0053] Based on the proportionality coefficient and the pressure values at 20°C corresponding to the two temperature-pressure characteristic curves closest to it, obtain the pressure value at 20°C corresponding to the sampled pressure value under the built-in rated mixing ratio;

[0054] Calculate the slope between the sampled pressure value and the pressure value at 20°C corresponding to it under the built-in rated mixing ratio.

[0055] It should be noted that in order to compensate for the gas pressure change caused by the ambient temperature change, the pressure values of the measured ternary mixed gas need to be converted to the corresponding pressure values at 20°C to ensure that when only the ambient temperature changes, the obtained pressure values and gas densities at 20°C remain unchanged.

[0056] Specifically, the finding of the two temperature-pressure characteristic curves closest to it based on the pressure value and the corresponding temperature includes:

[0057] Substitute the pressure value into all the temperature-pressure characteristic curves under the built-in rated mixing ratio to obtain the corresponding temperature values respectively;

[0058] Calculate the differences between the temperature value corresponding to the pressure value and the temperature values corresponding to all the temperature-pressure characteristic curves, and select the temperature-pressure characteristic curve corresponding to the maximum difference less than 0 and the temperature-pressure characteristic curve corresponding to the minimum difference greater than 0 as the two temperature-pressure characteristic curves closest to it.

[0059] Furthermore, the proportionality coefficient is calculated through the following formula:

[0060] , (1)

[0061] where represents the proportionality coefficient of the i-th sampled pressure value, represents the i-th sampled temperature value, , represent the temperature values corresponding to the two temperature-pressure characteristic curves closest to the i-th sampled pressure value.

[0062] Furthermore, the pressure value at 20°C corresponding to the sampled ternary mixed gas under the built-in rated mixing ratio is obtained through the following formula:

[0063] , (2)

[0064] Among them, represents the pressure value at 20°C corresponding to the pressure value of the i-th sampling under the rated mixing ratio, , represents the pressure values at 20°C corresponding to the two temperature-pressure characteristic curves closest to the pressure value of the i-th sampling.

[0065] Furthermore, the slope between the sampled pressure value and the pressure value at 20°C corresponding to it under the built-in rated mixing ratio is calculated by the following formula:

[0066] , (3)

[0067] Among them, represents the slope between the pressure value of the i-th sampling and the pressure value at 20°C corresponding to it under the built-in rated mixing ratio, represents the pressure value of the i-th sampling, represents the temperature of the i-th sampling.

[0068] Furthermore, the mean value of the slopes of all sampled pressure values and the pressure values at 20°C corresponding to them is calculated by the following formula, and this mean value is used as the second slope:

[0069] , (4)

[0070] Among them, n represents the number of sampled pressure values, represents the second slope.

[0071] It should be noted that since each built-in temperature-pressure characteristic curve is obtained through experiments, and the temperature-pressure characteristic curves corresponding to the ternary mixed gas at different densities are all different. In order to balance the time, manpower, and material resources consumed in actual experiments, the experiments are carried out at equal density intervals. Therefore, the built-in temperature-pressure characteristic curves cannot cover all densities, resulting in the possibility that the actually measured temperature and pressure values may not fall on the built-in temperature-pressure characteristic curves, and the corresponding pressure value at 20°C cannot be directly read according to the characteristic curves, so density interpolation is required.

[0072] Exemplarily, assume that the temperature-pressure characteristic curve of the ternary mixed gas under the rated mixing ratio is built in the density relay, denoted as k1, and the temperature-pressure characteristic curve is a cluster of temperature-pressure characteristic curves arranged in ascending order of gas density. Assume that the temperature and corresponding pressure values of the ternary mixed gas sampled by the temperature sensor are (T 1 , P 1 ), (T 2 , P 2 ),...... (T 48 , P 48), for the pressure value P 1 : Substitute P 1 into all the temperature-pressure characteristic curves under k1, and respectively obtain the corresponding temperature values, denoted as T 1,1 ,......, T 1,n , where n represents the number of all temperature-pressure characteristic curves under k1, 1 represents the temperature-pressure characteristic curve with the smallest density among them, and n represents the temperature-pressure characteristic curve with the largest density among them. Calculate △T 1,t = T 1 - T 1,t (t ∈ n). If △T 1,2 <0 and is the largest, then select the temperature-pressure characteristic curve 2 corresponding to T 1,2 and the temperature-pressure characteristic curve 3 corresponding to T 1,3 as the two temperature-pressure characteristic curves closest to the sampled P 1 . Calculate the proportionality coefficient 1 of P through formula (1). On the two temperature-pressure characteristic curves corresponding to T 1,2 and T 1,3 , respectively find the pressure values P 1 corresponding to P 1,2,20 at 20°C, P 1,3,20 . Obtain the pressure value of P 1 at 20°C corresponding to k1 through formula (2): . Similarly, obtain the pressure values of the remaining sampled pressure values P 2 ,......, P 48 at 20°C corresponding to k1 as P 2,20 ,......, P 48,20 . Calculate the slope 1 between P 1,20 and P through formula (3). Similarly, calculate the slopes k 2 ,......, P 48 between P 2,20 ,......, P 48,20 and P 2 ,......, k 48 through formula (3). Calculate the mean value of k 1 ,......, k 48 through formula (4) to obtain , and take as the second slope.

[0073] It should be noted that since the temperature-pressure characteristic curves under each rated mixing ratio in the above example are arranged in ascending order of gas density, when △T 1,2When it is less than 0 and corresponds to the maximum temperature-pressure characteristic curve 2, the adjacent temperature-pressure characteristic curve 3 is △T 1,3 >0 and corresponds to the temperature-pressure characteristic curve when it is the smallest.

[0074] It can be understood that the present invention obtains C by performing density interpolation on the temperature-pressure characteristic curves built in the density relay 4 F 7 N / CO 2 / O 2 The pressure value of the ternary mixed gas at the built-in rated mixing ratio improves the accuracy of the output result of the density relay.

[0075] Specifically, in step S3, calculate the difference between the first slope and the second slope. If the difference is greater than the threshold and the first slope is greater than the second slope, perform C 4 F 7 N gas ratio high warning; if the difference is greater than the threshold and the first slope is less than the second slope, perform C 4 F 7 N gas ratio low warning.

[0076] Specifically, regarding C 4 F 7 N, CO 2 , O 2 All three gases are regarded as ideal gases, and they all conform to the ideal gas state equation:

[0077] , (5)

[0078] Among them, is the pressure value, is the density of the gas, is the gas constant, is the Kelvin temperature, is the relative molecular mass.

[0079] It should be noted that when the mixing ratio of the ternary mixed gas in the device under test does not change, the first slope should be close to the second slope. Therefore, the difference between the first slope and the second slope should be less than the threshold, that is, |k - | ≤ a. It can be understood that during implementation, the threshold a should be set according to the monitoring accuracy requirements of the density relay. And when the mixing ratio of the ternary mixed gas in the device under test changes, the difference between the first slope and the second slope is greater than the threshold, and a warning should be issued in a timely manner, such as sending a warning message.

[0080] It can be understood that since CO 2 and O 2 gases are small molecules, while C 4 F7 The relative molecular mass of N gas is significantly higher than that of CO 2 and O 2 gas. When the proportion of C 4 F 7 N gas is on the high side, the average relative molecular mass M of the ternary mixed gas obtained by using the ideal gas state equation will increase significantly, resulting in an increase in the slope of the temperature and pressure characteristic curve at equal density. Therefore, when |k - | > a and k > , it can be judged that the proportion of C 4 F 7 N is on the high side; conversely, when the proportion of C 4 F 7 N gas is on the low side, the average relative molecular mass M of the ternary mixed gas will decrease significantly, resulting in a decrease in the slope of the temperature and pressure characteristic curve at equal density. Therefore, when |k - | > a and k < , it can be judged that the proportion of C 4 F 7 N is on the low side. By comparing the difference between the first slope and the second slope and giving an early warning when the difference is greater than the threshold, the present invention solves the problem of how to give an early warning in time when the mixing ratio of the C 4 F 7 N / CO 2 / O 2 ternary mixed gas changes, which is of great significance for ensuring the safe and stable operation of electrical equipment. When the mixing ratio of the ternary mixed gas in the device to be tested changes, the present invention can give a real-time early warning only through a density relay, without adding additional devices, reducing costs and improving work efficiency.

[0081] Compared with the prior art, the beneficial effects of the method for warning the mixing ratio of the C 4 F 7 N / CO 2 / O 2 ternary mixed gas provided by the present invention are as follows:

[0082] 1. By performing density interpolation on the temperature-pressure characteristic curve built in the density relay, the present invention obtains the pressure value of the C 4 F 7 N / CO 2 / O 2 ternary mixed gas at the built-in rated mixing ratio, improving the accuracy of the output result of the density relay.

[0083] 2. By comparing the difference between the first slope and the second slope and giving an early warning when the difference is greater than the threshold, the present invention solves the problem of how to give an early warning in time when the C 4 F 7 N / CO 2 / O2 The problem of how to give a timely warning when the mixing ratio of the ternary mixed gas changes is of great significance for ensuring the safe and stable operation of electrical equipment.

[0084] 3. When the mixing ratio of the ternary mixed gas in the device to be measured changes, the present invention can give a real-time warning only through a density relay, without adding additional devices, reducing the cost and improving the work efficiency.

[0085] Those skilled in the art can understand that all or part of the processes of implementing the methods of the above embodiments can be completed by instructing relevant hardware through a computer program, and the program can be stored in a computer-readable storage medium. Among them, the computer-readable storage medium is a magnetic disk, an optical disk, a read-only memory or a random access memory, etc.

[0086] The above is only the preferred specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for early warning of the mixing ratio of C4F7N / CO2 / O2 ternary mixed gas, characterized in that: The method comprises the following steps: The temperature and pressure values ​​of the C4F7N / CO2 / O2 ternary mixed gas in the device to be tested are sampled by a density relay over a period of time to obtain a number of temperatures and corresponding pressure values, and the temperatures are sorted from low to high. A coordinate system is constructed with the temperature as the X-axis and the pressure value as the Y-axis. In the coordinate system, the sorted temperatures and corresponding pressure values ​​are linearly fitted, and the slope of the straight line after the linear fitting is obtained as the first slope. Based on the sampled temperature and the corresponding pressure value and the temperature-pressure characteristic curve of the rated mixing ratio built into the density relay, the pressure value at 20°C corresponding to the sampled pressure value under the built-in rated mixing ratio is obtained; the average of the slopes of each sampled pressure value and its corresponding pressure value at 20°C is calculated as the second slope; Calculating a difference between the first slope and the second slope, and issuing an early warning if an absolute value of the difference is greater than a threshold; For each sampled pressure value: Find the two temperature-pressure characteristic curves closest to the pressure value and the corresponding temperature; Calculate the proportionality coefficient based on the temperature corresponding to the pressure value and the temperature values ​​corresponding to the two temperature-pressure characteristic curves closest to the pressure value; Based on the proportionality coefficient and the pressure values ​​at 20°C corresponding to the two temperature-pressure characteristic curves closest thereto, the pressure value at 20°C corresponding to the sampled pressure value under the built-in rated mixing ratio is obtained; Calculate the slope between the sampled pressure value and its corresponding pressure value at 20°C under the built-in rated mixing ratio; The proportionality factor is calculated using the following formula: ， （1） in, represents the proportionality coefficient of the pressure value of the ith sample, represents the temperature value of the i-th sample, , Indicates the temperature values ​​corresponding to the two temperature-pressure characteristic curves closest to the pressure value of the i-th sample; The pressure value of the ternary mixed gas sample at 20°C corresponding to the built-in rated mixing ratio is obtained by the following formula: ， （2） in, It represents the pressure value at 20℃ corresponding to the pressure value of the i-th sample at the rated mixing ratio. , Indicates the pressure values ​​corresponding to the two temperature-pressure characteristic curves at 20°C that are closest to the pressure value of the i-th sample; The slope between the sampled pressure value and the corresponding pressure value at 20°C at the built-in rated mixing ratio is calculated using the following formula: ， （3） in, It represents the slope between the pressure value of the ith sample and its corresponding pressure value at 20°C under the built-in rated mixing ratio. represents the pressure value of the ith sample, represents the temperature of the i-th sample.

2. The method for early warning of the mixing ratio of C4F7N / CO2 / O2 ternary mixed gas according to claim 1, characterized in that: Calculating the difference between the first slope and the second slope, and if the difference is greater than a threshold and the first slope is greater than the second slope, issuing a warning that the proportion of C4F7N gas is too high; If the difference is greater than the threshold and the first slope is less than the second slope, a warning of a low C4F7N gas proportion is issued.

3. The method for early warning of the mixture ratio of C4F7N / CO2 / O2 ternary mixed gas according to claim 1, characterized in that: Based on the sampled temperature and the corresponding pressure value, density interpolation is performed on the temperature-pressure characteristic curve built into the density relay to obtain the pressure value at 20°C corresponding to each sampled pressure value of the ternary mixed gas under the built-in rated mixing ratio, and the slope between each sampled pressure value and its corresponding pressure value at 20°C is calculated.

4. The method for early warning of the mixing ratio of C4F7N / CO2 / O2 ternary mixed gas according to claim 1, characterized in that: The slopes of all sampled pressure values ​​and their corresponding pressure values ​​at 20°C are averaged using the following formula, and the average is used as the second slope: ， （4） Where n represents the number of sampled pressure values. Represents the second slope.

5. The method for early warning of the mixing ratio of C4F7N / CO2 / O2 ternary mixed gas according to claim 1, characterized in that: The temperature-pressure characteristic curve of the ternary mixed gas at the rated mixing ratio is a series of temperature-pressure characteristic curve clusters with the temperature value as the X-axis and the pressure value as the Y-axis, representing the temperature-pressure curves corresponding to different gas densities at the rated mixing ratio.

6. The method for early warning of the mixture ratio of C4F7N / CO2 / O2 ternary mixed gas according to claim 1, characterized in that: The density relay includes a temperature sensor and a pressure sensor, through which the temperature and pressure values ​​of the ternary mixed gas are sampled at a fixed sampling frequency within a period of time to obtain a number of temperatures and corresponding pressure values, and the temperatures and corresponding pressure values ​​are sorted from low to high according to the temperature; A coordinate system is constructed with temperature as the X-axis and pressure as the Y-axis. In this coordinate system, coordinate points corresponding to the sorted temperatures and corresponding pressure values ​​are obtained. All coordinate points are linearly fitted to form a straight line, and the slope of the straight line is obtained as the first slope.

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