CN108387824B - Oiled paper insulation state evaluation method for improving return voltage measurement mode - 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

Oiled paper insulation state evaluation method for improving return voltage measurement mode

Technical Field

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

Background

The oil-immersed power transformer has the advantages of high insulation strength, long service life and the like, and is widely applied to a power system, however, the transformer is inevitably influenced by electricity, heat, mechanical stress and other environmental factors in the operation process, so that the insulation performance of a medium is continuously deteriorated, and therefore, the diagnosis method for the oil-paper insulation state of the transformer is researched, the insulation state of the transformer is correctly evaluated, and the method has great theoretical and practical significance for ensuring the safe, stable and reliable operation of the whole power grid.

At present, the diagnosis method for the insulation state of transformer oil paper is a nondestructive diagnosis method based on dielectric response theory, wherein the recovery voltage method benefits from high sensitivity, large information quantity, strong anti-interference capability and convenient field test, thereby gaining wide attention. However, the long measurement time of the traditional return voltage method is a big defect, a large amount of time is consumed for repeated charging and discharging, the peak value of the return voltage needs to be measured in each cycle, the measurement time is further prolonged, the whole experiment process needs to consume 3-5 hours, and the efficiency is low; most of the existing improvement methods are carried out on the basis of charging and discharging once and repeatedly measuring a plurality of periods, only the influence rule of the method on the characteristic quantity is researched by changing the charging and discharging time, and the improvement on the measurement mode is not considered from the perspective of shortening the experimental time; in addition, the conventional recovery voltage measurement method directly evaluates the insulation state of the transformer oil paper by using the measured characteristic quantity, the experimental error has a great influence on the result, the main time constant is the most commonly used insulation diagnosis characteristic quantity in the recovery voltage measurement method, and the determination of the main time constant directly influences the final diagnosis result, so that the time corresponding to the peak value in the polarization spectrum needs to be determined as accurately as possible.

Disclosure of Invention

In order to solve the problems, the invention aims to provide an oiled paper insulation state evaluation method for improving a recovery voltage measurement mode, and the experimental time is shortened to be less than one third of that of the traditional method under the same polarization time. Meanwhile, an equivalent circuit with the identified parameters is simulated, and the corresponding relation exists between the peak time of the restored return voltage polarization spectrum and the moisture content in the insulating paper, so that the insulating state of the oil paper can be evaluated.

The technical scheme adopted by the invention is as follows:

an oil paper insulation state evaluation method for improving a return voltage measurement mode comprises the following steps:

step 1: exposing the dried insulating paper in the air in a laboratory to naturally absorb moisture, and then fully soaking the insulating paper in the new transformer insulating oil to obtain experimental samples with different moisture contents;

step 2: the automatic collection system for the insulation recovery voltage of the transformer oilpaper mainly writes a VB program into a 6517B electrostatic instrument to control the automatic switching of a high-voltage direct-current power supply module and an ammeter measurement module, so that the cyclic control of charging and discharging and measurement processes is realized, and the automatic collection of the insulation recovery voltage of the transformer oilpaper is realized;

and step 3: placing a sample in a three-electrode device and accessing the three-electrode device into a system, starting a return voltage measurement system, setting charging voltage, charging time, discharging cycle times and each discharging time on a parameter setting interface, starting an electrostatic instrument, initializing the electrostatic instrument, automatically executing a charging link by the system after collection is started, and then performing cyclic discharging and measuring processes according to gradually increasing step lengths, wherein the increment of the return voltage in a sampling interval at the initial moment of each measurement link is the initial slope of the link;

and 4, step 4: identifying the parameters of each branch in the extended Debye equivalent circuit by the obtained initial slope of each discharging link in the continuous discharging process;

and 5: after the parameters of each branch are identified, analyzing the equivalent circuit through a network function, and calculating the return voltage and the initial slope of each discharge link in the multi-discharge process by using the superposition theorem of the linear circuit; and comparing the initial slope calculation value with the measured value, and adjusting the parameters of the optimization algorithm to ensure that the overlap ratio of the initial slope calculation value and the measured value of each discharging link in the continuous discharging process is as high as possible, namely the identified parameters are as accurate as possible.

Step 6: the equivalent circuit with the identified parameters simulates a traditional return voltage measurement mode of repeated charging and discharging in matlab, a return voltage polarization spectrum is programmed and simulated, the law presented by the return voltage polarization spectrum restored by samples with different moisture contents is observed, the insulation state of the oil paper is evaluated according to the corresponding relation between the peak time and the moisture content in the polarization spectrum, and the larger the peak time is, the more the moisture content in the insulation paper is, and the worse the insulation state is.

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

the capacitor voltage on the branch i after the jth discharge is as follows:

wherein i represents each branch, j represents each discharge link, U₀For charging set before measurementVoltage, t_cFor the charging time, t_dcjFor each discharge time, τ_iThe time constant representing each branch is represented by the product of the RC parameters of the corresponding branch.

The equivalent circuit is analyzed according to the circuit characteristics, and RC parameters of each branch in the equivalent circuit and the geometric capacitance C of the transformer can be constructed by using the measured initial slope in combination with the circuit switching theorem_gThe optimal solution is obtained through an optimization algorithm and is brought into the formula

In the process of solving the insulation resistance R_gAt this time, U_rj(t) is a second recovery voltage value measured after the jth discharge stage,

indicating the slope of the corresponding sample point.

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

after the j discharge, the general formula of the recovery voltage generated when the capacitor on the branch i acts alone is as follows:

wherein u is_cpij(0_-) For measuring instantaneous capacitance voltage values for open circuits, polynomial N_ij(s) and D_ij(s) is obtained by analyzing the operation form of the equivalent circuit equation, and the coefficient a_0,ij、a_1,ij、…、a_n-1,ijAnd b_0,ij、b_1,ij、…、b_n+1,ijRelated to a parameter in the equivalent circuit is R_pi、C_pi、C_g、R_gThe combined result and the analysis process are complex, and can be obtained by matlab programming analysis and the zero point z is solved₁、z₂、…、z_n-1And pole p₁、p₂、…、p_n+1。

Through Laplace inverse transformation, after the jth discharge, the recovery voltage generated by the independent action of the i capacitor voltage of the branch circuit is as follows:

wherein, K_r,ijIs represented in the pair N_ij(s)/D_ij(s) factorizing the denominator polynomial in the order of coefficients, number of coefficients and D_ij(s) the order of the polynomial being related and K being determined by limiting_r,ijThe formula is as follows:

D_ij'(s) is D_ijFirst derivative of(s), p_r,ijIs the pole.

According to the superposition theorem, the recovery voltage and the initial slope thereof generated at the two ends of the circuit after the jth discharge and when the capacitor voltages on the n branches act together can be obtained.

The invention relates to an oiled paper insulation state evaluation method for improving a return voltage measurement mode, which has the advantages that:

the improved method makes breakthrough improvement on the measuring mode, greatly shortens the experimental time, can also evaluate the insulation state of the transformer oilpaper insulation samples with different moisture contents according to the recovery voltage polarization spectrum which is reduced by the equivalent circuit and is identified by the data measured by the method, and the polarization spectrum is calculated by the accurate equivalent circuit, so that the error introduced in the measuring process is greatly reduced, and the extracted peak time characteristic quantity evaluation result is more reliable.

Drawings

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

FIG. 2 is a diagram of a three electrode system;

wherein: v: the method comprises the following steps of voltmeter, 1-transformer oil, 2-upper polar plate, 3-oil-immersed paperboard and 4-lower polar plate.

Fig. 3 is a wiring diagram of the principle of return voltage measurement.

Fig. 4 is a diagram of a conventional recovery voltage method.

FIG. 5 is a graph of the measurement process of the improved recovery voltage method.

FIG. 6 is an expanded Debye equivalent circuit diagram.

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

FIG. 8 is a graph of calculated recovery voltages after 0.1s, 1s, 10s, 100s discharge time for samples with 0.7% moisture content.

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

Fig. 10 is a recovery voltage polarization spectrum of each reduced moisture content sample.

Detailed Description

An oil paper insulation state evaluation method for improving a return voltage measurement mode,

1) in the aspect of measurement mode: the method is realized through a long-time charging and a plurality of short-time continuous discharging processes, compared with the traditional method for measuring the recovery voltage by repeating a plurality of cycles according to the charging-discharging time ratio of 2:1, the experimental time under the same polarization time is shortened to be less than one third of the original time, and the experimental efficiency is improved;

2) in the aspect of branch capacitor voltage description: after the measuring mode is changed, the branch capacitor voltage after each discharging process is described in a cumulative attenuation index mode, and each discharging process is gradually carried out by multiplying one attenuation index on the basis of the previous capacitor voltage;

3) in the aspect of parameter identification: identifying the parameters of the extended Debye equivalent circuit by using the measured initial slope of each discharge link in the whole continuous discharge process and the slope of the second sampling point;

4) in terms of insulation evaluation characteristic quantity: the peak time of the return voltage polarization spectrum is calculated by utilizing the identified equivalent circuit parameters and is used as the characteristic quantity for evaluating the oil paper insulation state of the transformer, and the main time constant characteristic quantity in the polarization spectrum obtained by directly measuring data is not evaluated.

In the 1) above: the process of once charging many times short time continuous discharge for a long time originally can shorten the experimental time to the third of traditional approach under same polarization time, simultaneously, at every turn measure the link only need to record three preceding reply voltage value can, need not to measure the reply voltage peak value of this link and just end, improved experimental efficiency more.

In the 2) above: because the discharging process of the improved method is continuously performed for a plurality of times, the voltage of each branch capacitor after each discharging process is described as formula (1) in a form of multiplication and decay index.

In the step 3): according to the initial slope measured after each discharge link in the whole continuous discharge process, the RC parameters of each branch in the equivalent circuit and the geometric capacitance of the transformer are solved, and then the RC parameters and the geometric capacitance of the transformer are brought into the formula (2) to solve the insulation resistance R_gAt this time, U_rj(t) is a second recovery voltage value measured after the jth discharge stage,

indicating the slope of the corresponding sample point.

In the 4) above: the experimental result shows that the peak time in the recovery voltage polarization spectrum reduced by the improved method is increased along with the increase of the moisture content in the insulating paper, namely, the larger the peak time of the recovery voltage polarization spectrum reduced by the method is, the poorer the insulating state of the oil paper is.

As shown in figure 1, the invention shortens the measurement time to below one third of the traditional method under the same polarization time by a measurement mode of one-time long-time charging and multiple continuous discharging, improves the experimental efficiency, can accurately identify the parameters in the extended Debye equivalent circuit by the measured data, can calculate the recovery voltage and the initial slope thereof after improving the measurement mode according to the equivalent circuit, and simultaneously simulates the conventional recovery voltage measurement method to simulate the recovery voltage polarization spectrum on the equivalent circuit, and can evaluate the insulation state of the oil paper insulation sample by utilizing the peak time of the polarization spectrum. The specific implementation steps are as follows:

1) preparing experimental samples with the moisture contents of 0.7%, 1.3% and 3.4% in the insulating paper in a laboratory, and placing the samples in a three-electrode device for simulating an experimental transformer shown in fig. 2;

2) according to a principle wiring diagram of the measuring system shown in fig. 3, an experimental transformer is connected into the system, the return voltage measuring system is opened, a parameter setting interface is used for setting charging voltage 1000V, charging time 2000s, discharging cycle times 13 times, and discharging time to be 0.1s, 0.2s, 0.5s, 1s, 2s, 5s, 10s, 20s, 50s, 100s, 200s, 500s, and 1000s, respectively, an electrometer is started and initialized, the system automatically executes a charging link after starting acquisition by clicking, and performs a discharging and measuring cycle process according to the set discharging time, wherein a schematic diagram is shown in fig. 5, and a schematic diagram of a conventional return voltage measuring process is shown in fig. 4.

The initial slope value of each measuring link can be obtained according to the ratio of the return voltage increment at the initial moment of the link to the sampling interval, and the initial slope data of different moisture content samples measured after each discharging link are as follows:

TABLE 1 initial slope measured after each discharge step for samples with different moisture contents

	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 formed by connecting a plurality of RC branches in parallel, as shown in fig. 6, the medium relaxation processes of different degrees can be described intuitively in theory, and generally, the real state can be reflected more accurately by adopting 6 branches, that is, n is 6. The parameters of each branch in the equivalent circuit can be identified by using the initial slope and an optimization algorithm, and the specific analysis process is as follows:

after the jth discharge, the capacitor voltage on the branch i is as shown in the formula (1), the equivalent circuit is analyzed according to the circuit characteristics, and by combining the circuit changing theorem, the RC parameters of each branch in the equivalent circuit and the geometric capacitor C of the transformer can be constructed by utilizing the measured initial slope_gThe optimal solution is obtained through an optimization algorithm and is brought into the formula (2) to solve the insulated powerResistance R_gAt this time, U_rj(t) is a second recovery voltage value measured after the jth discharge stage,

indicating the slope of the corresponding sample point.

Parameters in the expanded Debye equivalent circuit are identified by a mixed frog-leaping algorithm, and the identification result is as follows, taking the water content in the insulating paper as 0.7 percent as an example:

TABLE 2 identification results at 0.7% moisture content

4) After identifying each branch parameter, analyzing the equivalent circuit through a network function, and calculating the return voltage and the initial slope thereof:

when the capacitor voltage of the branch 1 acts alone, the corresponding operational circuit is as shown in fig. 7, and when other branches act alone, only the voltage source needs to be moved to the corresponding branch.

When n is 6, the recovery voltage generated when the capacitor on branch i acts alone after the j-th discharge can be simplified into the following form:

wherein u is_cpij(0_-) For measuring instantaneous capacitance voltage values for open circuits, polynomial N_ij(s) and D_ij(s) is obtained by analyzing the operation form of the equivalent circuit equation, and the coefficient a_0,ij、a_1,ij、…、a_5,ijAnd b_0,ij、b_1,ij、…、b_7,ijRelated to a parameter in the equivalent circuit is R_pi、C_pi、C_g、R_gThe combined result can be used to find the zero z and the pole p using matlab.

Through Laplace inverse transformation, after the jth discharge, the recovery voltage generated by the independent action of the i capacitor voltage of the branch circuit is as follows:

wherein, K_r,ijIs represented in the pair N_ij(s)/D_ij(s) factorizing the denominator polynomial in the order of coefficients, number of coefficients and D_ijThe order of the(s) polynomial is related. Determination of K by limiting method_r,ijThe formula is as follows:

D_ij'(s) is D_ijFirst derivative of(s), p_r,ijIs the pole.

According to the superposition theorem, the recovery voltage and the initial slope thereof generated at the two ends of the circuit after the jth discharge when the capacitor voltages on the 6 branches act together can be obtained.

The calculated recovery voltage curve of the sample with the water content of 0.7% after the discharge time of 0.1s, 1s, 10s and 100s is shown in fig. 8, the coincidence degree of the initial slope of each discharge link in the calculated continuous discharge process and the initial slope obtained by measurement is 92.52% in fig. 9, which shows that the parameters of the equivalent circuit identified by the recovery voltage initial slope value of each discharge link measured by the improved method are effective, the polarization and depolarization processes in the insulating medium in the multiple discharge processes can be better reflected by the equivalent circuit, and the feasibility of the experimental method is theoretically verified. The parameters of the algorithm may be further adjusted so that the identified parameters are as accurate as possible.

A conventional return voltage measurement method is simulated for many times according to the charge-discharge ratio of 2:1 on an equivalent circuit with identified parameters, a return voltage polarization spectrum of each moisture content sample can be simulated through matlab as shown in figure 10, and it can be seen that the peak time of the return voltage polarization spectrum reduced by the equivalent circuit is increased along with the increase of the moisture content in the insulating paper, and a good corresponding relation exists between the moisture content and the insulating state of the oil paper, so that the validity of data obtained by the improved measurement method is verified, and the feasibility of subsequent evaluation of the insulating state of the oil paper is realized by the measurement method.

In conclusion, the improved method has the advantages that breakthrough improvement is made on the measurement mode, the experimental time is greatly shortened, the recovery voltage polarization spectrum of the equivalent circuit reduction identified according to the data measured by the method can be used for evaluating the insulation state of the transformer oilpaper insulation samples with different moisture contents, the polarization spectrum is calculated by the accurate equivalent circuit, errors introduced in the measurement process are greatly reduced, and the extracted peak time characteristic quantity evaluation result is more reliable.

The invention has been described with reference to specific embodiments, but is not limited to the foregoing embodiments, and all changes and modifications that come within the meaning and range of equivalency of the claims are intended to be embraced therein.

Claims (3)

1. An oil paper insulation state evaluation method for improving a return voltage measurement mode is characterized by comprising the following steps:

step 1: exposing the dried insulating paper in the air to absorb moisture naturally, and then fully soaking the paper in the insulating oil of the new transformer to obtain experimental samples with different moisture contents;

step 2: the automatic collection system of the insulation return voltage of the transformer oil paper controls the automatic switching of the high-voltage direct-current power supply module and the ammeter measuring module in the automatic collection system by writing a program into the electrostatic instrument, realizes the cyclic control of the charging and discharging and measuring processes, and automatically collects the insulation return voltage of the transformer oil paper;

and step 3: placing a sample in a three-electrode device and connecting the sample into an automatic recovery voltage acquisition system, starting the automatic recovery voltage acquisition system, setting charging voltage, charging time, discharging cycle times and discharging time on a parameter setting interface, starting an electrostatic instrument, initializing the electrostatic instrument, automatically executing a charging link by the automatic recovery voltage acquisition system after acquisition is started, and then performing cyclic discharge measurement according to gradually increasing step length, wherein the increment of the recovery voltage in a sampling interval at the initial moment is the initial slope of the link;

and 4, step 4: identifying the parameters of each branch in the extended Debye equivalent circuit by the initial slope of each discharge link in the obtained cyclic discharge measurement process;

and 5: after the parameters of each branch are identified, the equivalent circuit is analyzed through a network function, and the return voltage and the initial slope of each discharge link in the cyclic discharge measurement process can be calculated by utilizing the superposition theorem of the linear circuit; comparing the initial slope calculation value with the measured value, and adjusting the parameters of an optimization algorithm to ensure that the overlap ratio of the initial slope calculation value and the measured value of each discharging link in the continuous discharging process is as high as possible, namely the identified parameters are as accurate as possible;

step 6: the equivalent circuit with the identified parameters simulates a traditional return voltage measurement mode of repeated charging and discharging, a return voltage polarization spectrum is programmed and simulated, the law presented by the return voltage polarization spectrum restored by samples with different moisture contents is observed, the insulation state of the oil paper is evaluated according to the corresponding relation between the peak time and the moisture content in the polarization spectrum, and the larger the peak time is, the more the moisture content in the insulation paper is, and the worse the insulation state is.

2. The method for evaluating the insulation state of the oiled paper by improving the recovery voltage measurement mode according to claim 1, wherein the method comprises the following steps: in the step 4, the parameter identification process is as follows:

the equivalent circuit is analyzed according to the circuit characteristics, and RC parameters of each branch in the equivalent circuit and the geometric capacitance C of the transformer can be constructed by using the measured initial slope in combination with the circuit switching theorem_gThe optimal solution is obtained through an optimization algorithm and is brought into the formula

In the process of solving the insulation resistance R_gAt this time, U_rj(t) is a second recovery voltage value measured after the jth discharge stage,

representing the slope of the corresponding sampling point;

wherein i represents each branch, j represents each discharge link, U₀For the charging voltage set before measurement, t_cFor the charging time, t_dckFor each discharge time, τ_iThe time constant representing each branch is represented by the product of the RC parameters of the corresponding branch.

3. The method for evaluating the insulation state of the oiled paper by improving the recovery voltage measurement mode according to claim 2, wherein the method comprises the following steps: in step 5, the recovery voltage and the initial slope thereof are calculated as follows:

after the j discharge, the recovery voltage generated when the capacitor on the branch i acts alone can be simplified into the following form:

wherein u is_cpij(0_-) For measuring instantaneous capacitance voltage values for open circuits, polynomial N_ij(s) and D_ij(s) is obtained by analyzing the operation form of the equivalent circuit equation, and the coefficient a_0,ij、a_1,ij、…、a_n-1,ijAnd b_0,ij、b_1,ij、…、b_n+1,ijRelated to parameters in the equivalent circuit, the parameters are parameters R of each branch of the equivalent circuit_pi、C_piAnd parameter C_g、R_gThe zero point z can be found by matlab as a result of the combination₁、z₂、…、z_n-1And pole p₁、p₂、…、p_n+1；

Through the inverse Rayleigh transform, after the jth discharge, the recovery voltage generated by the independent action of the capacitor voltage on the branch i is as follows:

wherein, K_r,ijIs represented in the pair N_ij(s)/D_ij(s) factorizing the denominator polynomial in the order of coefficients, number of coefficients and D_ij(s) the order of the polynomial being related and K being determined by limiting_r,ijThe formula is as follows:

D_ij'(s) is D_ijFirst derivative of(s), p_r,ijIs a pole;

according to the superposition theorem, the recovery voltage and the initial slope thereof generated at the two ends of the circuit after the jth discharge and when the capacitor voltages on the n branches act together can be obtained.

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