CN108387824B - A method for evaluating the state of oil-paper insulation with improved recovery voltage measurement - Google Patents

Abstract

An improved recovery voltage measurement method for improving a recovery voltage measurement mode is an improved recovery voltage measurement method for one-time long-time charging and multiple short-time continuous discharging, automatic collection is realized by programming an electrometer for collecting recovery voltage data, samples with different moisture degrees are tested, and parameters of each branch of an extended Debye equivalent circuit are identified according to an initial slope measured in each discharging process; finally, the recovery voltage and the initial slope of each discharging link are calculated by combining network function analysis; the analysis finds that the coincidence degree of the initial slope calculation value and the measured value curve is higher, which indicates that the parameters in the equivalent circuit identified by the initial slope obtained by the improved return voltage measurement method are effective; meanwhile, the matlab is utilized to simulate the conventional recovery voltage cycle multiple charging and discharging processes on the equivalent circuit. The invention shortens the experimental time to be less than one third of the traditional method, and the restored return voltage polarization spectrum can also evaluate the insulation state of the transformer oilpaper insulation sample.

Description

A method for evaluating the state of oil-paper insulation with improved recovery voltage measurement

technical field

The invention relates to the technical field of transformer oil-paper insulation aging and life prediction, in particular to an oil-paper insulation state evaluation method based on an improved recovery voltage measurement method.

Background technique

Oil-immersed power transformers have the advantages of high dielectric strength and long service life, so they are widely used in power systems. However, during the operation of transformers, they will inevitably be subjected to electrical, thermal, mechanical stress and other environmental factors. Therefore, it is of great theoretical and practical significance to study the diagnosis method of the transformer oil-paper insulation state and correctly evaluate the insulation state of the transformer to ensure the safe, stable and reliable operation of the entire power grid.

At present, the diagnostic methods for the transformer oil-paper insulation state are all non-destructive diagnostic methods based on the dielectric response. Among them, the recovery voltage method benefits from its high sensitivity, large amount of information, strong anti-interference ability, and easy on-site testing. , and thus received widespread attention. However, the long measurement time of the traditional recovery voltage method is a major drawback. Repeated charging and discharging many times takes a lot of time. In each cycle, the peak recovery voltage needs to be measured, which further prolongs the measurement time. 3 to 5 hours, the efficiency is low; some of the existing improvement methods are mostly carried out on the basis of one charge and one discharge, and repeated measurement for multiple cycles, just to study its influence on the characteristic quantity by changing the charge and discharge time. In addition, the traditional recovery voltage measurement method uses the measured characteristic quantities to directly evaluate the transformer oil-paper insulation state, and the experimental error has a great influence on the results, while the main The time constant is the most commonly used insulation diagnostic feature in the recovery voltage measurement method. The determination of the main time constant directly affects the final diagnosis result. Therefore, it is necessary to determine the time corresponding to the peak value in the polarization spectrum as accurately as possible.

SUMMARY OF THE INVENTION

In order to solve these problems, the purpose of the present invention is to provide an oil-paper insulation state evaluation method with an improved recovery voltage measurement method, and under the same polarization time, the experiment time is shortened to less than one third of the traditional method. At the same time, the equivalent circuit of the identified parameters is simulated, and there is a corresponding relationship between the peak time of the restored voltage polarization spectrum and the moisture content in the insulating paper, which can evaluate the state of the oil-paper insulation.

The technical scheme adopted in the present invention is:

A method for evaluating the state of oil-paper insulation with an improved recovery voltage measurement method, comprising the following steps:

Step 1: Expose the dry insulating paper to the air to absorb moisture naturally in the laboratory, and then fully immerse it in the new transformer insulating oil to obtain experimental samples with different moisture contents;

Step 2: The transformer oil-paper insulation recovery voltage automatic acquisition system mainly controls the automatic switching of the high-voltage DC power supply module and the meter measurement module by writing the VB program to the 6517B electrostatic meter, realizing the cyclic control of the charging and discharging and measurement process, as well as the Automatic collection of transformer oil-paper insulation recovery voltage;

Step 3: Place the sample in the three-electrode device and connect it to the system, open the recovery voltage measurement system, set the charging voltage, charging time, number of discharge cycles, and each discharge time on the parameter setting interface, start the electrostatic meter, and initialize it , after the collection starts, the system will automatically perform the charging link, and then perform the process of cyclic discharge and measurement according to the gradually increasing step size. The increment of the recovery voltage at the initial moment of each measurement link within the sampling interval is the initial slope of the link;

Step 4: Identify the parameters of each branch in the extended Debye equivalent circuit with the obtained initial slope of each discharge link in the continuous discharge process;

Step 5: After identifying the parameters of each branch, analyze the equivalent circuit through the network function, and use the superposition theorem of linear circuits to calculate the recovery voltage and initial slope of each discharge link in the multi-discharge process; calculate the initial slope. Compare the value with the measured value, and adjust the parameters of the optimization algorithm so that the calculated value of the initial slope of each discharge link in the continuous discharge process coincides with the measured value as high as possible, that is, the identified parameters are as accurate as possible.

Step 6: The equivalent circuit of the identified parameters, simulate the traditional recovery voltage measurement method of repeated charging and discharging in matlab, program and simulate the recovery voltage polarization spectrum, and observe the recovery voltage polarization restored by samples with different moisture contents According to the law presented by the spectrum, the insulation state of the oil-paper is evaluated according to the corresponding relationship between the peak time and the moisture content in the polarization spectrum. The greater the peak time, the more moisture content in the insulating paper and the worse the insulation state.

In the step 4, the identification process is as follows:

After the jth discharge, the capacitor voltage on branch i is:

Among them, i represents each branch, j represents each discharge link, U ₀ is the charging voltage set before measurement, t _c is the charging time, t _dcj is each discharging time, τ _i represents the time constant of each branch, and is determined by the corresponding branch The product representation of the road RC parameters.

The equivalent circuit is analyzed according to the circuit characteristics, combined with the switching theorem, the objective function about the RC parameters of each branch in the equivalent circuit and the geometric capacitance C _g of the transformer can be constructed by using the measured initial slope, and the optimal value can be obtained through the optimization algorithm. solution, bring into the formula

表示对应采样点的斜率。Insulation resistance R _g can be solved in the middle, at this time, U _rj (t) is the second recovery voltage value measured after the jth discharge link,

Indicates the slope of the corresponding sampling point.

In the step 5, the recovery voltage and its initial slope are calculated as follows:

After the jth discharge, the recovery voltage generated when the capacitor on branch i acts alone is as follows:

Among them, u _cpij (0 _- ) is the instantaneous capacitor voltage value of the open circuit measurement, the polynomials N _ij (s) and D _ij (s) are obtained by analyzing the operation form of the equivalent circuit equation, the coefficients a _0,ij , a _1,ij , ..., a _n-1,ij and b _0,ij , b _1,ij , ..., b _n+1,ij are related to the parameters in the equivalent circuit, and are combinations of R _pi , C _pi , C _g , and R _g . The result of , the analysis process is complicated, which can be obtained by using matlab programming analysis, and find out the zero points z ₁ , z ₂ , ..., z _n-1 and the poles p ₁ , p ₂ , ..., _pn+1 .

Through the inverse Laplace transform, it can be obtained that after the jth discharge, the recovery voltage generated by the independent action of the branch i capacitor voltage is:

Among them, K _r,ij represents each coefficient corresponding to the factorization of the denominator polynomial in N _ij (s)/D _ij (s), and the number is related to the order of the D _ij (s) polynomial. The limit method determines the value of K _r,ij , the formula is as follows:

_{Di ij} '(s) is the first derivative of _{Di ij} (s), _{and pr,ij} is the pole.

According to the superposition theorem, after the jth discharge, when the capacitor voltages on the n branches act together, the recovery voltage and its initial slope are generated at both ends of the circuit.

An oil-paper insulation state evaluation method for improving the recovery voltage measurement method of the present invention has the advantages of:

In addition to making breakthrough improvements in the measurement method, the improved method greatly shortens the experimental time. The recovery voltage polarization spectrum restored by the equivalent circuit identified according to the data measured by this method can also be used for different measurements. The insulation state of the transformer oil-paper insulation sample with moisture content is evaluated, and the polarization spectrum is calculated by a relatively accurate equivalent circuit, the error introduced in the measurement process is greatly reduced, and the extracted peak time characteristic value is evaluated. Also more reliable.

Description of drawings

FIG. 1 is a schematic flow chart of the experiment of the present invention.

Figure 2 is a structural diagram of a three-electrode system;

Among them: V: voltmeter, 1-transformer oil, 2-upper plate, 3-oil-impregnated cardboard, 4-lower plate.

Figure 3 is a wiring diagram of the principle of measuring the recovery voltage.

FIG. 4 is a measurement process diagram of the conventional recovery voltage method.

Figure 5 is a measurement process diagram of the improved recovery voltage method.

Figure 6 is an extended Debye equivalent circuit diagram.

FIG. 7 is an operation circuit corresponding to the independent action of the capacitor voltage of branch 1 of the jth discharge link.

Fig. 8 is a graph showing the calculated recovery voltage of samples with 0.7% moisture content after 0.1s, 1s, 10s, and 100s of discharge time.

FIG. 9 is a comparison diagram of the measured value and the calculated value of the initial slope of each discharge link during the continuous discharge process.

FIG. 10 is the recovery voltage polarization spectra of the reduced samples with various moisture contents.

Detailed ways

A method for evaluating the state of oil-paper insulation with an improved recovery voltage measurement method,

1) In terms of measurement methods: it is realized through a long-term charging and multiple short-term continuous discharging processes. Compared with the traditional recovery voltage measurement method that repeats multiple cycles with a charge-discharge time ratio of 2:1, in the same Under the polarization time, the experimental time is shortened to less than one third of the original, which improves the experimental efficiency;

2) In the description of the branch capacitor voltage: after the measurement method is changed, the branch capacitor voltage after each discharge process is described in the form of a cumulative attenuation index. Each discharge is based on the previous capacitor voltage. Multiply by a decay exponent, progressive layer by layer;

3) In the aspect of parameter identification: use the measured initial slope of each discharge link in the entire continuous discharge process and the slope of the second sampling point to identify the parameters of the extended Debye equivalent circuit;

4) In terms of insulation evaluation characteristics: use the identified equivalent circuit parameters to calculate the peak time of the polarization spectrum of the recovery voltage as a characteristic quantity for evaluating the insulation state of the transformer oil-paper, rather than the polarization spectrum obtained directly from the measurement data. The main time constant characteristic quantity is evaluated.

In the above 1): the process of charging for a long time and discharging continuously for a short period of time can reduce the experimental time to one third of the traditional method under the same polarization time. At the same time, each measurement link only needs to measure It is enough to obtain the first three recovery voltage values, and it is not necessary to measure the recovery voltage peak value of this link until the end, which further improves the experimental efficiency.

In the above 2): since the discharge process of the improved method is carried out continuously for many times, the voltage of each branch capacitor after each discharge process is described as formula (1) in the form of cumulative decay index.

表示对应采样点的斜率。In the above 3): According to the measured initial slope after each discharge link in the whole continuous discharge process, after obtaining the RC parameters of each branch in the equivalent circuit and the geometric capacitance of the transformer, bring it into the formula (2) ) to solve the insulation resistance R _g , at this time, U _rj (t) is the second recovery voltage value measured after the jth discharge link,

Indicates the slope of the corresponding sampling point.

In the above 4): the experimental results show that the peak time in the recovery voltage polarization spectrum reduced by the improved method increases with the increase of the moisture content in the insulating paper, that is, the peak value of the recovery voltage polarization spectrum reduced by this method. The longer the time, the worse the oil-paper insulation state.

As shown in Figure 1, the present invention shortens the measurement time to less than one third of the traditional method under the same polarization time through the measurement method of one long charge and multiple continuous discharges, which improves the experimental efficiency, and the measured The data can accurately identify the parameters in the extended Debye equivalent circuit. According to the equivalent circuit, the recovery voltage and its initial slope after the improved measurement method can be calculated. At the same time, the conventional recovery voltage measurement is simulated on the equivalent circuit. The method simulates the polarization spectrum of the recovery voltage, and uses the peak time of the polarization spectrum to evaluate the insulation state of the oil-paper insulation sample. The specific implementation steps are as follows:

1) Prepare experimental samples with moisture contents of 0.7%, 1.3%, and 3.4% in the insulating paper in the laboratory, and place the samples in the three-electrode device for simulating the experimental transformer shown in Figure 2;

2) Connect the experimental transformer to the system according to the principle wiring diagram of the measurement system in Figure 3, open the recovery voltage measurement system, and set the charging voltage 1000V, charging time 2000s, discharge cycle times 13 times, and each discharge time in the parameter setting interface as 0.1s, 0.2s, 0.5s, 1s, 2s, 5s, 10s, 20s, 50s, 100s, 200s, 500s, 1000s, start the electrometer and initialize it. The cycle process of discharge and measurement at the set discharge time is shown in Figure 5, and the schematic diagram of the conventional recovery voltage measurement process is shown in Figure 4.

According to the ratio of the recovery voltage increment and the sampling interval at the initial moment of each measurement link, the initial slope value of the link can be obtained. The initial slope data of samples with different moisture contents measured after each discharge link are as follows:

Table 1 Initial slopes of samples with different moisture contents measured after each discharge cycle

0.1s 0.2s 0.5s 1s 2s 5s 10s 20s 50s 100s 200s 500s 1000s 0.7% 38.82 27.18 20.76 16.94 15.45 16.05 14.20 12.80 7.462 5.594 3.688 1.909 0.890 1.3% 37.46 29.78 27.64 25.68 23.42 20.14 16.10 21.78 15.69 6.29 3.91 1.92 0.38 3.4% 41.69 29.38 23.69 20.30 17.60 15.42 13.60 20.59 16.87 7.35 4.74 2.42 1.03

3) The extended Debye equivalent circuit is composed of multiple RC branches in parallel, as shown in Figure 6, which can theoretically describe different degrees of medium relaxation process intuitively. Generally, 6 branches can be used to more accurately Reflect the real state, ie n=6. The parameters of each branch in the equivalent circuit can be identified by using the above initial slope combined with the optimization algorithm. The specific analysis process is as follows:

表示对应采样点的斜率。After the jth discharge, the capacitor voltage on branch i is shown in formula (1). According to the circuit characteristics, the equivalent circuit is analyzed. Combined with the switching theorem, the measured initial slope can be used to construct the RC of each branch in the equivalent circuit. parameters and the objective function of the geometric capacitance C _g of the transformer, the optimal solution is obtained through the optimization algorithm, and is brought into the formula (2) to solve the insulation resistance R _g , at this time, U _rj (t) is the jth discharge link after The second measured recovery voltage value,

Indicates the slope of the corresponding sampling point.

The parameters in the extended Debye equivalent circuit are identified by the hybrid leapfrog algorithm. Taking the moisture content of the insulating paper as an example of 0.7%, the identification results are as follows:

Table 2 Identification results when the moisture content is 0.7%

4) After identifying the parameters of each branch, analyze the equivalent circuit through the network function, and calculate the recovery voltage and its initial slope:

When the capacitor voltage of branch 1 acts alone, the corresponding operation circuit is shown in Figure 7, and when other branches act alone, just move the voltage source to the corresponding branch.

When n=6, after the jth discharge, the recovery voltage generated when the capacitor on branch i acts alone can be simplified to the following form:

Among them, u _cpij (0 _- ) is the instantaneous capacitor voltage value of the open circuit measurement, the polynomials N _ij (s) and D _ij (s) are obtained by analyzing the operation form of the equivalent circuit equation, the coefficients a _0,ij , a _1,ij , ..., a _5,ij and b _0,ij , b _1,ij , ..., b _7,ij are related to the parameters in the equivalent circuit and are the result of the combination of R _pi , C _pi , C _g , and R _g , which can be Use matlab to find the zero point z and the pole point p.

Through the inverse Laplace transform, it can be obtained that after the jth discharge, the recovery voltage generated by the independent action of the branch i capacitor voltage is:

Among them, K _r,ij represents each coefficient corresponding to the factorization of the denominator polynomial in N _ij (s)/D _ij (s), and the number is related to the order of the _{Di ij} (s) polynomial. Using the limit method to determine the value of K _r,ij , the formula is as follows:

_{Di ij} '(s) is the first derivative of _{Di ij} (s), _{and pr,ij} is the pole.

According to the superposition theorem, after the jth discharge, when the capacitor voltages on the six branches act together, the recovery voltage and its initial slope are generated at both ends of the circuit.

The calculated recovery voltage curves of samples with 0.7% moisture content after 0.1s, 1s, 10s, and 100s of discharge time are shown in Figure 8. The calculated initial slope of each discharge link during the continuous discharge process is compared with the initial slope obtained by measurement. Figure 9 , the coincidence degree of the two is 92.52%, indicating that the parameters of the equivalent circuit identified by the initial slope value of the recovery voltage of each discharge link measured by the improved method are effective, and the equivalent circuit can better reflect the The polarization and depolarization processes inside the insulating medium during multiple discharges have theoretically verified the feasibility of the experimental method. The parameters of the algorithm can be further adjusted to make the identified parameters as accurate as possible.

On the equivalent circuit of the identified parameters, the conventional recovery voltage measurement method is simulated for many times according to the charge-discharge ratio of 2:1, and the recovery voltage polarization spectrum of each moisture content sample can be simulated by matlab as shown in Figure 10. It can be seen that by The peak time of the recovery voltage polarization spectrum restored by the equivalent circuit increases with the increase of the moisture content in the insulating paper, and there is a good correspondence between the moisture content and the insulation state of the oil-paper, which further verifies the improved measurement method. The validity of the data obtained and the feasibility of implementing subsequent oil-paper insulation condition assessments with this measurement method.

To sum up, the improved method not only made a breakthrough improvement in the measurement method and greatly shortened the experiment time, but also identified the recovery voltage polarization spectrum restored by the equivalent circuit according to the data measured by this method. , the insulation state of transformer oil-paper insulation samples with different moisture content can also be evaluated, and the polarization spectrum is calculated by a more accurate equivalent circuit, the error introduced in the measurement process is greatly reduced, and the peak time extracted The result of feature quantity evaluation is also more reliable.

The application of the present invention is only used to illustrate the present invention, but is not limited to the above examples, and improvements and equivalent transformations made on this basis shall fall within the protection scope of the appended claims of the present invention.

Claims (3)

1. a method for evaluating the oil-paper insulation state of an improved recovery voltage measurement mode, is characterized in that comprising the following steps: Step 1: Expose the dry insulating paper to the air to absorb moisture naturally, and then fully immerse it in the new transformer insulating oil to obtain experimental samples with different moisture contents; Step 2: The transformer oil-paper insulation recovery voltage automatic acquisition system, by writing a program to the electrostatic meter, controls the automatic switching of the high-voltage DC power supply module and the meter measurement module, realizes the cyclic control of the charging and discharging and measurement process, and controls the transformer oil-paper Automatic collection of insulation recovery voltage; Step 3: Place the sample in the three-electrode device and connect it to the automatic recovery voltage collection system, turn on the automatic recovery voltage collection system, set the charging voltage, charging time, number of discharge cycles, and each discharge time on the parameter setting interface, and start the static electricity After starting the acquisition, the automatic recovery voltage acquisition system will automatically perform the charging link, and then perform cyclic discharge measurement according to the gradually increasing step size. In each measurement link, the initial moment of the recovery voltage is incremented within the sampling interval. is the initial slope of the link; Step 4: identify the parameters of each branch in the extended Debye equivalent circuit with the initial slope of each discharge link obtained in the cyclic discharge measurement process; Step 5: After identifying the parameters of each branch, analyze the equivalent circuit through the network function, and use the superposition theorem of linear circuits to calculate the recovery voltage and initial slope of each discharge link during the cyclic discharge measurement process; The calculated slope value is compared with the measured value, and the parameters of the optimization algorithm are adjusted, so that the initial slope calculated value and measured value of each discharge link in the continuous discharge process coincide as high as possible, that is, the identified parameters are as accurate as possible; Step 6: The equivalent circuit of the identified parameters is simulated, the traditional recovery voltage measurement method of repeated charging and discharging is simulated, the recovery voltage polarization spectrum is programmed and simulated, and the recovery voltage polarization spectrum restored by the samples with different moisture contents is observed. According to the corresponding relationship between the peak time and the moisture content in the polarization spectrum, the insulation state of the oil-paper is evaluated. The greater the peak time, the more moisture content in the insulating paper and the worse the insulation state. 2. the oil-paper insulation state assessment method of a kind of improved recovery voltage measurement mode according to claim 1, is characterized in that: in described step 4, parameter identification process is as follows: The equivalent circuit is analyzed according to the circuit characteristics, combined with the switching theorem, the objective function about the RC parameters of each branch in the equivalent circuit and the geometric capacitance C _g of the transformer can be constructed by using the measured initial slope, and the optimal value can be obtained through the optimization algorithm. solution, bring it into the formula

Insulation resistance R _g can be solved in the middle, at this time, U _rj (t) is the second recovery voltage value measured after the jth discharge link,

represents the slope of the corresponding sampling point; Among them, i represents each branch, j represents each discharge link, U ₀ is the charging voltage set before the measurement, t _c is the charging time, t _dck is each discharging time, τ _i represents the time constant of each branch, and is determined by the corresponding branch The product representation of the road RC parameters. 3. the oil-paper insulation state assessment method of a kind of improved recovery voltage measurement mode according to claim 2, is characterized in that: in described step 5, recovery voltage and initial slope thereof are calculated as follows: After the jth discharge, the recovery voltage generated when the capacitor on branch i acts alone can be simplified into the following form:

Among them, u _cpij (0 _- ) is the instantaneous capacitor voltage value of the open circuit measurement, the polynomials N _ij (s) and D _ij (s) are obtained by analyzing the operation form of the equivalent circuit equation, the coefficients a _0,ij , a _1,ij , ..., a _n-1,ij and b _0,ij , b _1,ij , ..., _bn+1,ij are related to the parameters in the equivalent circuit, and are the parameters of each branch of the equivalent circuit R _pi , C _pi , and the result of the combination of parameters C _g , R _g , the zero points z ₁ , z ₂ , ..., z _n-1 and the poles p ₁ , p ₂ , ..., p _n+1 can be obtained by using matlab; Through the inverse Laplace transform, it can be obtained that after the jth discharge, the recovery voltage generated by the capacitor voltage on the branch i acting alone is:

Among them, K _r,ij represents each coefficient corresponding to the factorization of the denominator polynomial in N _ij (s)/D _ij (s), and the number is related to the order of the D _ij (s) polynomial. The limit method determines the value of K _r,ij , the formula is as follows:

_{Di ij} '(s) is the first derivative of _{Di ij} (s), and p _r,ij is the pole; According to the superposition theorem, after the jth discharge, when the capacitor voltages on the n branches act together, the recovery voltage and its initial slope are generated at both ends of the circuit.

Images