CN111289804B - Method for evaluating correlation between transformer oil paper insulation dielectric response and paraffin content - Google Patents
Method for evaluating correlation between transformer oil paper insulation dielectric response and paraffin content Download PDFInfo
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Abstract
The invention discloses a method for evaluating the correlation between dielectric response and paraffin content of oil paper insulation of a transformer. The method comprises the steps of testing the frequency domain dielectric spectrum of the oil-immersed transformer on site under the severe cold condition, recording the environmental temperature, obtaining a corresponding test curve, combining a mathematical model obtained by a laboratory, extracting a site paraffin content evaluation index, and comparing the site paraffin content evaluation index with a paraffin content evaluation index expert library, so that the paraffin content of the oil-immersed transformer under the severe cold condition can be effectively evaluated, and a guidance method is provided for accurately evaluating the insulation state of the oil paper and removing faults in the actual engineering.
Description
Technical Field
The invention belongs to the field of insulation assessment and diagnosis of oil-immersed transformers, and particularly relates to a method for evaluating correlation between oil paper insulation dielectric response and paraffin content of a transformer.
Background
With the gradual maturity of the ultra-high voltage transmission technology, a plurality of ultra-high voltage projects are built on the ground in the northeast, inner Mongolia and Xinjiang cold areas. In addition, with the proposal of the concept of global energy Internet, the electric power of clean energy bases such as 'one-pole-one-way' and the like is possible to be transmitted by the ultra-high voltage transmission technology, and the historical lowest temperature of the north pole reaches-68 ℃. The large oil-immersed power transformer is a core part of a power system, whether a power grid can safely run is determined by the quality of the insulation state of the large oil-immersed power transformer, and paraffin precipitation of the oil-immersed power transformer is mostly found in Xinjiang, Jilin, Harbin and the like in severe cold weather, and is often accompanied with gas action, so that the transformer cannot be normally switched on; meanwhile, the paraffin is separated out, so that the insulation and heat dissipation are uneven, and the internal insulation of the transformer is damaged. Under the severe cold condition, paraffin becomes an important factor threatening the safe operation of the oil-immersed transformer.
At present, the frequency domain dielectric spectroscopy is widely applied to field insulation state evaluation and diagnosis due to the advantages of nondestructive testing, rich carried insulation information and strong anti-interference performance. However, when the dielectric spectrum curve of the oil-immersed transformer is tested on site under the severe cold condition, the relation between the spectrogram and the paraffin content is not established, a paraffin content evaluation method in the oil-immersed transformer under the severe cold condition is not available, and the accurate evaluation of the insulation state of the oil-immersed transformer cannot be realized. Therefore, an evaluation method for correlation between the insulation dielectric response of the oil paper of the transformer and the paraffin content is urgently needed to realize accurate evaluation of the insulation state of the oil-immersed transformer.
Disclosure of Invention
In order to effectively evaluate the paraffin content of the oil-immersed transformer under the high and cold conditions, the invention provides a method for evaluating the correlation between the insulation dielectric response of oil paper of the transformer and the paraffin content, which comprises the following steps:
the first step is as follows: dielectric spectrum testing platform for paraffin content frequency domain of built low-temperature oil-immersed transformer
The method comprises the following steps of constructing a low-temperature oil-immersed transformer paraffin content frequency domain dielectric spectrum test platform which mainly comprises a temperature control system and a frequency domain dielectric spectrum test system, wherein the temperature control box (1), a heating loop (7), a refrigerating loop (8), a first temperature sensor (9), a second temperature sensor (17), a temperature sensor loop (10), a temperature control box test control unit (11) and a first power supply lead (18) jointly form the temperature control system; the frequency domain dielectric spectrum testing system comprises a three-electrode outer wall (2), a high-voltage electrode (3), a test sample (4), insulating oil (19), a low-voltage electrode (5), a protective electrode (6), a grounding wire (16), a dielectric response testing loop (15), a dielectric response tester (14), a notebook computer (12), a testing data transmission line (20) and a second power supply lead (13); the temperature control box test control unit (11) sets a test temperature, the heating loop (7) or the refrigerating loop (8) acts, the first temperature sensor (9) and the second temperature sensor (17) feed the temperature in the temperature control box (1) back to the temperature control box test control unit (11), when the set temperature is reached, the heating loop (7) or the refrigerating loop (8) is controlled to be disconnected, and the frequency domain dielectric spectrum test system starts to test; the first temperature sensor (9) is fixed at the bottom of the inner layer of the temperature control box (1), and the second temperature sensor (17) is fixed outside the outer wall (2) of the three electrodes; the first power supply lead (18) supplies power to the low-temperature oil-immersed transformer paraffin content frequency domain dielectric spectrum test platform, and the second power supply lead (13) supplies power to the dielectric response tester (14);
the second step is that: preparing oil-immersed transformer XY model samples with different paraffin content and different voltage levels
The method comprises the following steps of preparing XY model samples of the oil-immersed transformer with the voltage grades of 110kV, 220kV and 500kV according to the volume ratio of 1:10, 1:50 and 1:100, wherein the paraffin content of the XY model samples under all the voltage grades is 0-5%, and the paraffin content gradient increment in the XY model samples is 0.01% of the total mass of the XY model samples;
the third step: testing the dielectric response frequency domain spectrum from 0.001Hz to 1kHz under the set low-temperature environment
The testing temperature is-60 ℃ to-20 ℃, the low-temperature testing temperature is set, the temperature control box testing control unit (11) is switched on, the refrigeration loop (8) is switched on, the temperature control box (1) starts to refrigerate, the first temperature sensor (9) and the second temperature sensor (17) jointly monitor the temperature in the temperature control box (1), the temperature is transmitted to the temperature control box testing control unit (11) in real time, when the temperature in the temperature control box (1) is consistent with a set value, the refrigeration loop (8) is switched off, the dielectric response tester (14) is started, the dielectric loss factor tan delta of 0.001Hz to 1kH under the set temperature is tested, wherein the testing characteristic frequency points are 0.001Hz, 0.00215Hz, 0.00464Hz, 0.01Hz, 0.0215Hz, 0.0464Hz, 0.1Hz, 0.215Hz, 0.464Hz, 1Hz, 2.15Hz, 4.64Hz, 10Hz, 20Hz, 40Hz, 70Hz, 110, 222.22Hz, 446.68Hz and 1000Hz, obtaining a frequency domain dielectric spectrum at a set temperature;
when testing the frequency domain dielectric spectrum at a temperature higher than the initial set value, the temperature control box test control unit (11) is connected with the heating loop (7) to heat the temperature control box (1), meanwhile, the first temperature sensor (9) and the second temperature sensor (17) jointly monitor the temperature in the temperature control box (1), the temperature is transmitted to the temperature control box test control unit (11) in real time, when the temperature in the temperature control box (1) is consistent with the set value, the heating loop (7) is disconnected, the dielectric response tester (14) is started, the dielectric loss factor tan delta of 0.001Hz to 1kH at the set temperature is tested, the frequency domain dielectric spectrum at the newly set temperature is obtained, and the fourth step is executed after the test is finished;
the fourth step: mathematical model establishment and paraffin content evaluation index extraction
Firstly, selecting a characteristic frequency point f from a frequency domain dielectric spectrum graph under a test set temperatureiCorresponding dielectric loss factor tan deltaiIn sequence of (f)1,tanδ1)、(f2,tanδ2)、(f3,tanδ3)、(f4,tanδ4)、(f5,tanδ5)……(f19,tanδ19)、(f20,tanδ20)，f20Is the highest characteristic frequency point, f1For the lowest characteristic frequency point, the relationship between the frequency f and the dielectric loss tangent tan delta at the set temperature is constructed by combining the formula (1):
wherein i is 1,2,3, …, n, k is 1,2,3, …, n, n is 20, i and k cannot be equal at the same time;
the characteristic frequency of the nearest point at the inflection point of a certain test curve from high frequency to low frequency is fiTake fi、fi-1And fi-2The slope tan delta' (f) of the curvei)、tanδ′(fi-1) And tan δ' (f)i-2) Recording the test temperature T, constructing the frequency average value at the 'turning point' by the formula (2), and constructing the upper limit W of the evaluation index by the formula (3)maxThe lower limit W of the structural evaluation index of formula (4)min：
Wherein, i is 1,2,3, …, n, n is 20; lambda is an XY model sample differentiation coefficient, and lambda is 1-3;
the fifth step: establishing an expert database for paraffin content evaluation indexes
Respectively carrying out low-temperature dielectric response tests on XY model samples of oil-immersed transformers with different voltage levels, wherein the test temperature is-60 ℃ to-20 ℃, the initial temperature is-60 ℃, and the temperature gradient is 2 ℃, wherein the paraffin content in the XY model samples under each voltage level is 0-5%, and the paraffin content gradient increment is 0.01% of the total mass of the XY model samples;
calculating a paraffin content evaluation index according to test curves of different contents, different test samples and different temperatures by combining the formula (1), the formula (2), the formula (3) and the formula (4), wherein the paraffin content evaluation index obtained by each test is WminAnd WmaxForming a section, wherein a plurality of groups of sample determination and evaluation sections form an expert database of paraffin content evaluation indexes at different voltage levels and different temperatures;
and a sixth step: evaluation of paraffin content in field test oil-immersed transformer
When the oil-immersed transformer is tested on site under the severe cold condition, the testing frequency, the testing temperature and the testing data are recorded, and (f) is sequentially carried out according to the testing result in the frequency domain dielectric spectrum graph under the site testing temperaturecs1,tanδcs1)、(fcs2,tanδcs2)、(fcs3,tanδcs3)、(fcs4,tanδcs4)、(fcs5,tanδcs5)……(fcs19,tanδcs19)、(fcs20,tanδcs20)，fcs20To the highest test eigenfrequencyPoint, fcs1For the lowest test characteristic frequency point, combining the formula (1) to construct the frequency f and the dielectric loss factor tan delta in the field testcsSatisfies the formula (5):
wherein i is 1,2,3, …, n, k is 1,2,3, …, n, n is 20, i and k cannot be equal at the same time;
taking the characteristic frequency of the inflection point closest to the high frequency of the test curve to the low frequency as fcsiTake fcsi、fcsi-1And fcsi-2The slope tan delta of the curvec′s(fcsi)、tanδc′s(fcsi-1) And tan deltac′s(fcsi-2) Recording the test temperature TcsThe formula (6) constructs the frequency average value at the 'turning point', and the formula (7) constructs the upper limit W of the field test evaluation indexcsmaxThe lower limit W of the evaluation index of the field test of the structure of formula (8)csmin：
Wherein, i is 1,2,3, …, n, n is 20; lambda is an XY model sample differentiation coefficient, and lambda is 1-3;
according to field test data, combining a formula (5), a formula (6), a formula (7) and a formula (8), drawing a curve, calculating upper and lower limit parameters of a paraffin content evaluation index, comparing the calculated value of the paraffin content evaluation index with a paraffin content evaluation index expert library, determining that the field test paraffin falls into an interval of the paraffin content evaluation index expert library, finding a corresponding paraffin content range, namely completing paraffin content evaluation in the oil-immersed transformer, and bringing the field test data of each time into the paraffin content evaluation index expert library.
The method can effectively realize the evaluation of the paraffin content of the oil-immersed transformer under the high and cold conditions, and provides a method for accurately evaluating the insulation performance and fault elimination of the field transformer in the actual engineering.
Drawings
FIG. 1 is a method for evaluating the correlation between the dielectric response of the oil paper insulation of a transformer and the content of paraffin.
Fig. 2 shows a paraffin-containing test curve of an oil-immersed transformer.
FIG. 3 is a schematic diagram of a testing platform of a method for evaluating the correlation between the dielectric response of the oil paper insulation of the transformer and the paraffin content.
Detailed Description
The invention will be further described with reference to the accompanying drawings in which:
the method provided by the invention comprises the following steps:
the first step is as follows: dielectric spectrum testing platform for paraffin content frequency domain of built low-temperature oil-immersed transformer
The method comprises the following steps of constructing a low-temperature oil-immersed transformer paraffin content frequency domain dielectric spectrum test platform which mainly comprises a temperature control system and a frequency domain dielectric spectrum test system, wherein the temperature control box (1), a heating loop (7), a refrigerating loop (8), a first temperature sensor (9), a second temperature sensor (17), a temperature sensor loop (10), a temperature control box test control unit (11) and a first power supply lead (18) jointly form the temperature control system; the frequency domain dielectric spectrum testing system comprises a three-electrode outer wall (2), a high-voltage electrode (3), a test sample (4), insulating oil (19), a low-voltage electrode (5), a protective electrode (6), a grounding wire (16), a dielectric response testing loop (15), a dielectric response tester (14), a notebook computer (12), a testing data transmission line (20) and a second power supply lead (13); the temperature control box test control unit (11) sets a test temperature, the heating loop (7) or the refrigerating loop (8) acts, the first temperature sensor (9) and the second temperature sensor (17) feed the temperature in the temperature control box (1) back to the temperature control box test control unit (11), when the set temperature is reached, the heating loop (7) or the refrigerating loop (8) is controlled to be disconnected, and the frequency domain dielectric spectrum test system starts to test; the first temperature sensor (9) is fixed at the bottom of the inner layer of the temperature control box (1), and the second temperature sensor (17) is fixed outside the outer wall (2) of the three electrodes; the first power supply lead (18) supplies power to the low-temperature oil-immersed transformer paraffin content frequency domain dielectric spectrum test platform, and the second power supply lead (13) supplies power to the dielectric response tester (14);
the second step is that: preparing oil-immersed transformer XY model samples with different paraffin content and different voltage levels
The method comprises the following steps of preparing XY model samples of the oil-immersed transformer with the voltage grades of 110kV, 220kV and 500kV according to the volume ratio of 1:10, 1:50 and 1:100, wherein the paraffin content of the XY model samples under all the voltage grades is 0-5%, and the paraffin content gradient increment in the XY model samples is 0.01% of the total mass of the XY model samples;
the third step: testing the dielectric response frequency domain spectrum from 0.001Hz to 1kHz under the set low-temperature environment
The testing temperature is-60 ℃ to-20 ℃, the low-temperature testing temperature is set, the temperature control box testing control unit (11) is switched on, the refrigeration loop (8) is switched on, the temperature control box (1) starts to refrigerate, the first temperature sensor (9) and the second temperature sensor (17) jointly monitor the temperature in the temperature control box (1), the temperature is transmitted to the temperature control box testing control unit (11) in real time, when the temperature in the temperature control box (1) is consistent with a set value, the refrigeration loop (8) is switched off, the dielectric response tester (14) is started, the dielectric loss factor tan delta of 0.001Hz to 1kH under the set temperature is tested, wherein the testing characteristic frequency points are 0.001Hz, 0.00215Hz, 0.00464Hz, 0.01Hz, 0.0215Hz, 0.0464Hz, 0.1Hz, 0.215Hz, 0.464Hz, 1Hz, 2.15Hz, 4.64Hz, 10Hz, 20Hz, 40Hz, 70Hz, 110, 222.22Hz, 446.68Hz and 1000Hz, obtaining a frequency domain dielectric spectrum at a set temperature;
when testing the frequency domain dielectric spectrum at a temperature higher than the initial set value, the temperature control box test control unit (11) is connected with the heating loop (7) to heat the temperature control box (1), meanwhile, the first temperature sensor (9) and the second temperature sensor (17) jointly monitor the temperature in the temperature control box (1), the temperature is transmitted to the temperature control box test control unit (11) in real time, when the temperature in the temperature control box (1) is consistent with the set value, the heating loop (7) is disconnected, the dielectric response tester (14) is started, the dielectric loss factor tan delta of 0.001Hz to 1kH at the set temperature is tested, the frequency domain dielectric spectrum at the newly set temperature is obtained, and the fourth step is executed after the test is finished;
the fourth step: mathematical model establishment and paraffin content evaluation index extraction
Firstly, selecting a characteristic frequency point f from a frequency domain dielectric spectrum graph under a test set temperatureiCorresponding dielectric loss factor tan deltaiIn sequence of (f)1,tanδ1)、(f2,tanδ2)、(f3,tanδ3)、(f4,tanδ4)、(f5,tanδ5)……(f19,tanδ19)、(f20,tanδ20)，f20Is the highest characteristic frequency point, f1For the lowest characteristic frequency point, the relationship between the frequency f and the dielectric loss tangent tan delta at the set temperature is constructed by combining the formula (1):
wherein i is 1,2,3, …, n, k is 1,2,3, …, n, n is 20, i and k cannot be equal at the same time;
the characteristic frequency of the nearest point at the inflection point of a certain test curve from high frequency to low frequency is fiTake fi、fi-1And fi-2The slope tan delta' (f) of the curvei)、tanδ′(fi-1) And tan δ' (f)i-2) Recording the test temperature T, constructing the frequency average value at the 'turning point' by the formula (2), and constructing the upper limit W of the evaluation index by the formula (3)maxThe lower limit W of the structural evaluation index of formula (4)min：
Wherein, i is 1,2,3, …, n, n is 20; lambda is an XY model sample differentiation coefficient, and lambda is 1-3;
the fifth step: establishing an expert database for paraffin content evaluation indexes
Respectively carrying out low-temperature dielectric response tests on XY model samples of oil-immersed transformers with different voltage levels, wherein the test temperature is-60 ℃ to-20 ℃, the initial temperature is-60 ℃, and the temperature gradient is 2 ℃, wherein the paraffin content in the XY model samples under each voltage level is 0-5%, and the paraffin content gradient increment is 0.01% of the total mass of the XY model samples;
calculating a paraffin content evaluation index according to test curves of different contents, different test samples and different temperatures by combining the formula (1), the formula (2), the formula (3) and the formula (4), wherein the paraffin content evaluation index obtained by each test is WminAnd WmaxForming a section, wherein a plurality of groups of sample determination and evaluation sections form an expert database of paraffin content evaluation indexes at different voltage levels and different temperatures;
and a sixth step: evaluation of paraffin content in field test oil-immersed transformer
When the oil-immersed transformer is tested on site under the severe cold condition, the testing frequency, the testing temperature and the testing data are recorded, and (f) is sequentially carried out according to the testing result in the frequency domain dielectric spectrum graph under the site testing temperaturecs1,tanδcs1)、(fcs2,tanδcs2)、(fcs3,tanδcs3)、(fcs4,tanδcs4)、(fcs5,tanδcs5)……(fcs19,tanδcs19)、(fcs20,tanδcs20)，fcs20Is the highest test characteristic frequency point, fcs1For the lowest test characteristic frequency point, combining the formula (1) to construct the frequency f and the dielectric loss factor tan delta in the field testcsSatisfies the formula (5):
wherein i is 1,2,3, …, n, k is 1,2,3, …, n, n is 20, i and k cannot be equal at the same time;
taking the characteristic frequency of the inflection point closest to the high frequency of the test curve to the low frequency as fcsiTake fcsi、fcsi-1And fcsi-2The slope tan delta of the curvec′s(fcsi)、tanδc′s(fcsi-1) And tan deltac′s(fcsi-2) Recording the test temperature TcsThe formula (6) constructs the frequency average value at the 'turning point', and the formula (7) constructs the upper limit W of the field test evaluation indexcsmaxThe lower limit W of the evaluation index of the field test of the structure of formula (8)csmin：
Wherein, i is 1,2,3, …, n, n is 20; lambda is an XY model sample differentiation coefficient, and lambda is 1-3;
according to field test data, combining a formula (5), a formula (6), a formula (7) and a formula (8), drawing a curve, calculating upper and lower limit parameters of a paraffin content evaluation index, comparing the calculated value of the paraffin content evaluation index with a paraffin content evaluation index expert library, determining that the field test paraffin falls into an interval of the paraffin content evaluation index expert library, finding a corresponding paraffin content range, namely completing paraffin content evaluation in the oil-immersed transformer, and bringing the field test data of each time into the paraffin content evaluation index expert library.
Claims (1)
1. The method for evaluating the relevance between the insulation dielectric response and the paraffin content of the transformer oil paper is characterized by comprising the following steps of:
the first step is as follows: dielectric spectrum testing platform for paraffin content frequency domain of built low-temperature oil-immersed transformer
The method comprises the following steps of constructing a low-temperature oil-immersed transformer paraffin content frequency domain dielectric spectrum test platform which mainly comprises a temperature control system and a frequency domain dielectric spectrum test system, wherein the temperature control box (1), a heating loop (7), a refrigerating loop (8), a first temperature sensor (9), a second temperature sensor (17), a temperature sensor loop (10), a temperature control box test control unit (11) and a first power supply lead (18) jointly form the temperature control system; the frequency domain dielectric spectrum testing system comprises a three-electrode outer wall (2), a high-voltage electrode (3), a test sample (4), insulating oil (19), a low-voltage electrode (5), a protective electrode (6), a grounding wire (16), a dielectric response testing loop (15), a dielectric response tester (14), a notebook computer (12), a testing data transmission line (20) and a second power supply lead (13); the temperature control box test control unit (11) sets a test temperature, the heating loop (7) or the refrigerating loop (8) acts, the first temperature sensor (9) and the second temperature sensor (17) feed the temperature in the temperature control box (1) back to the temperature control box test control unit (11), when the set temperature is reached, the heating loop (7) or the refrigerating loop (8) is controlled to be disconnected, and the frequency domain dielectric spectrum test system starts to test; the first temperature sensor (9) is fixed at the bottom of the inner layer of the temperature control box (1), and the second temperature sensor (17) is fixed outside the outer wall (2) of the three electrodes; the first power supply lead (18) supplies power to the low-temperature oil-immersed transformer paraffin content frequency domain dielectric spectrum test platform, and the second power supply lead (13) supplies power to the dielectric response tester (14);
the second step is that: preparing oil-immersed transformer XY model samples with different paraffin content and different voltage levels
The method comprises the following steps of preparing XY model samples of the oil-immersed transformer with the voltage grades of 110kV, 220kV and 500kV according to the volume ratio of 1:10, 1:50 and 1:100, wherein the paraffin content of the XY model samples under all the voltage grades is 0-5%, and the paraffin content gradient increment in the XY model samples is 0.01% of the total mass of the XY model samples;
the third step: testing the dielectric response frequency domain spectrum from 0.001Hz to 1kHz under the set low-temperature environment
The testing temperature is-60 ℃ to-20 ℃, the low-temperature testing temperature is set, the temperature control box testing control unit (11) is switched on, the refrigeration loop (8) is switched on, the temperature control box (1) starts to refrigerate, the first temperature sensor (9) and the second temperature sensor (17) jointly monitor the temperature in the temperature control box (1), the temperature is transmitted to the temperature control box testing control unit (11) in real time, when the temperature in the temperature control box (1) is consistent with a set value, the refrigeration loop (8) is switched off, the dielectric response tester (14) is started, the dielectric loss factor tan delta of 0.001Hz to 1kH under the set temperature is tested, wherein the testing characteristic frequency points are 0.001Hz, 0.00215Hz, 0.00464Hz, 0.01Hz, 0.0215Hz, 0.0464Hz, 0.1Hz, 0.215Hz, 0.464Hz, 1Hz, 2.15Hz, 4.64Hz, 10Hz, 20Hz, 40Hz, 70Hz, 110, 222.22Hz, 446.68Hz and 1000Hz, obtaining a frequency domain dielectric spectrum at a set temperature;
when testing the frequency domain dielectric spectrum at a temperature higher than the initial set value, the temperature control box test control unit (11) is connected with the heating loop (7) to heat the temperature control box (1), meanwhile, the first temperature sensor (9) and the second temperature sensor (17) jointly monitor the temperature in the temperature control box (1), the temperature is transmitted to the temperature control box test control unit (11) in real time, when the temperature in the temperature control box (1) is consistent with the set value, the heating loop (7) is disconnected, the dielectric response tester (14) is started, the dielectric loss factor tan delta of 0.001Hz to 1kH at the set temperature is tested, the frequency domain dielectric spectrum at the newly set temperature is obtained, and the fourth step is executed after the test is finished;
the fourth step: mathematical model establishment and paraffin content evaluation index extraction
Firstly, selecting a characteristic frequency point f from a frequency domain dielectric spectrum graph under a test set temperatureiCorresponding dielectric loss factor tan deltaiIn sequence of (f)1,tanδ1)、(f2,tanδ2)、(f3,tanδ3)、(f4,tanδ4)、(f5,tanδ5)……(f19,tanδ19)、(f20,tanδ20)，f20Is the highest characteristic frequency point, f1For the lowest characteristic frequency point, the relationship between the frequency f and the dielectric loss tangent tan delta at the set temperature is constructed by combining the formula (1):
wherein i is 1,2,3, …, n, k is 1,2,3, …, n, n is 20, i and k cannot be equal at the same time;
the characteristic frequency of the nearest point at the inflection point of a certain test curve from high frequency to low frequency is fiTake fi、fi-1And fi-2The slope tan delta' (f) of the curvei)、tanδ′(fi-1) And tan δ' (f)i-2) Recording the test temperature T, constructing the frequency average value at the 'turning point' by the formula (2), and constructing the upper limit W of the evaluation index by the formula (3)maxThe lower limit W of the structural evaluation index of formula (4)min：
Wherein, i is 1,2,3, …, n, n is 20; lambda is an XY model sample differentiation coefficient, and lambda is 1-3;
the fifth step: establishing an expert database for paraffin content evaluation indexes
Respectively carrying out low-temperature dielectric response tests on XY model samples of oil-immersed transformers with different voltage levels, wherein the test temperature is-60 ℃ to-20 ℃, the initial temperature is-60 ℃, and the temperature gradient is 2 ℃, wherein the paraffin content in the XY model samples under each voltage level is 0-5%, and the paraffin content gradient increment is 0.01% of the total mass of the XY model samples;
calculating a paraffin content evaluation index according to test curves of different contents, different test samples and different temperatures by combining the formula (1), the formula (2), the formula (3) and the formula (4), wherein the paraffin content evaluation index obtained by each test is WminAnd WmaxForming a section, wherein a plurality of groups of sample determination and evaluation sections form an expert database of paraffin content evaluation indexes at different voltage levels and different temperatures;
and a sixth step: evaluation of paraffin content in field test oil-immersed transformer
When the oil-immersed transformer is tested on site under the severe cold condition, the testing frequency, the testing temperature and the testing data are recorded, and (f) is sequentially carried out according to the testing result in the frequency domain dielectric spectrum graph under the site testing temperaturecs1,tanδcs1)、(fcs2,tanδcs2)、(fcs3,tanδcs3)、(fcs4,tanδcs4)、(fcs5,tanδcs5)……(fcs19,tanδcs19)、(fcs20,tanδcs20)，fcs20Is the highest test characteristic frequency point, fcs1For the lowest test characteristic frequency point, combining the formula (1) to construct the frequency f and the dielectric loss factor tan delta in the field testcsSatisfies the formula (5):
wherein i is 1,2,3, …, n, k is 1,2,3, …, n, n is 20, i and k cannot be equal at the same time;
taking the characteristic frequency of the inflection point closest to the high frequency of the test curve to the low frequency as fcsiTake fcsi、fcsi-1And fcsi-2The slope tan delta of the curve'cs(fcsi)、tanδ′cs(fcsi-1) And tan delta'cs(fcsi-2) Recording the test temperature TcsThe formula (6) is used for constructing the frequency average value at the turning point, and the formula (7) is used for constructing the upper part of the field test evaluation indexLimit of WcsmaxThe lower limit W of the evaluation index of the field test of the structure of formula (8)csmin：
Wherein, i is 1,2,3, …, n, n is 20; lambda is an XY model sample differentiation coefficient, and lambda is 1-3;
according to field test data, combining a formula (5), a formula (6), a formula (7) and a formula (8), drawing a curve, calculating upper and lower limit parameters of a paraffin content evaluation index, comparing the calculated value of the paraffin content evaluation index with a paraffin content evaluation index expert library, determining that the field test paraffin falls into an interval of the paraffin content evaluation index expert library, finding a corresponding paraffin content range, namely completing paraffin content evaluation in the oil-immersed transformer, and bringing the field test data of each time into the paraffin content evaluation index expert library.
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