RU2522177C2 - Method for determining quality of compounding of electric machine windings - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to electrical measurement equipment and is intended to determine the quality of compounding of electric machine windings at test stages of winding insulation at manufacture and operation, and namely of stator windings of oil-filled submerged asynchronous electric motors. Essence of the invention is as follows: constant voltage U is supplied to a measurement object and resistance R(t) of the object is measured during the period of time, which is sufficient to achieve the resistance value of the practically set value. Then, values of transient current i(t)=U/R(t) are determined. A diagnostic feature of quality assessment of the winding compounding is determined as per the current curve in the form of a product of experimental values of the main characteristics of compound (ε_a·ρ_v)_exp - absolute dielectric permeability and specific volumetric resistance respectively; then, compounding quality criterion K_k is determined by comparing experimental characteristics of the compound to its nameplate data by the following formula: $$K_k=\frac{{\left({\epsilon }_a\cdot {\rho }_v\right)}_{\text{namep}}}{{\left({\epsilon }_a\cdot {\rho }_v\right)}_{\exp }}.$$

EFFECT: improving objectiveness of quality assessment of winding compounding.

4 dwg, 4 tbl

Description

The invention relates to techniques for electrical measurements and is intended to determine the quality of compounding of the windings of electrical machines at the stages of testing the insulation of the windings in the manufacture and operation, in particular the stator windings of oil-filled submersible asynchronous motors.

A number of practical methods for assessing the insulation state of windings of electric machines are based on measuring the insulation resistance using electrical measuring instruments (electrometers, megaohmmeters, etc.) for some time sufficient to fix the resistance values both in practically established modes and in transient charge processes a capacitor, the two plates of which are the winding of an electric machine and its body. Between these plates there is a multilayer insulation (wire insulation, slot insulation and winding compound).

There is a method of measuring the value of insulation resistance [1], which is aimed at reducing the measurement time when assessing the state of insulation and establishing standards for comparing measurement results. This method consists in measuring the resistance value, its first and second derivatives with respect to time, and for the steady-state value of the insulation resistance, take the double value of the insulation resistance at that moment in time when the first derivative has a maximum and the second derivative is zero.

The disadvantage of this method is the complexity of processing the measurement results, as well as the uncertainty of the assessment of the result, because a diagnostic symptom is not formulated (a criterion for assessing the quality of isolation). In this case, only the insulation resistance is measured, although the main property of the dielectric is polarization, which is characterized by its dielectric constant in addition to the specific volume resistance.

A known method that is used to control the moisture of the insulation [2].

In tests, the entire dependence of the insulation resistance R (t) to a steady state is not determined. An assessment of the insulation state is carried out according to the resistance values measured by a megohmmeter, after 15 and 60 s (R ₁₅ and R ₆₀ ) after switching on the voltage. According to these data, the absorption coefficient K _abc = R ₆₀ / R _{15 is} determined. At the same time, as noted in [2], it was experimentally established that with permissible wetting of the insulation, K _abs > 1.3. At K _abs <1.3, the insulation is unacceptably wet and the corresponding electrical equipment cannot be operated.

The disadvantage of this method is that, as in the previous case, only the transient value of the insulation resistance is measured, and the absorption coefficient can have different values depending on the sizes of the tested objects, i.e. the diagnostic feature does not allow to evaluate the quality of isolation reliably, because explicitly do not take into account the characteristics of the dielectric.

A known method for diagnosing the case insulation of an electric machine [3], which consists in the fact that a constant voltage U _{p is} applied to the measurement object. Upon reaching a steady-state value of the charging current, the object is short-circuited and, after discharging the geometric capacitance of the object, the value of the restored voltage at the time of its maximum U _{Bm is} measured.

Next, determine the value of the absorption coefficient k _a according to the formula

$$k_a=\frac{U_{Bm}}{U_p}\cdot \mathrm{one\; hundred}\%$$

and compare it with the average value of k _a characteristic for this type of isolation in the initial state. By the magnitude of the reduction measured k _a relative to the original judge the technical condition of the insulation. Moreover, a decrease in k _{a by} more than 50% indicates deterioration of the insulation.

The disadvantage of this method is that the measurement process is quite complex and requires a special electrometer. In addition, it is far from obvious that the diagnostic feature - “maximum value of the recovery voltage” determines the state of the dielectric, i.e. deterioration of its basic characteristics - ε _a and ρ _ν .

Closest to the invention, the technical solution is a method for measuring the steady-state value of the insulation resistance [4], in which the value of the resistance is taken as the steady-state value of the insulation resistance when it practically ceases to change.

The diagnostic feature in this case is the so-called polarization index

Ip = R ₁ / R ₁₀ ,

where R ₁ , R ₁₀ - insulation resistance, measured by a megohmmeter on the 1st and 10th minutes, respectively.

It was established by the technical conditions [5] for submersible induction motors that Ip should be at least 2. The undoubted advantage of this method, which is widely used in practice, is the simplicity and high reliability of measuring insulation resistance using a megohmmeter with high accuracy.

The disadvantage of this method, as shown by the tests of insulation of submersible induction motors, is the fact that the value of Ip can reach values of 3-9 or more, i.e. the degree of uncertainty of this criterion is very great. Measuring only the resistance does not provide an assessment of the main property of the insulation dielectric - polarization, which is characterized by dielectric constant, although the diagnostic feature is called the "polarization index". This does not allow to objectively evaluate the actual quality of compounding the windings of an electric machine in comparison with any standard.

An object of the invention is to improve the quality of manufacture of electric motors by controlling the compounding process based on the definition of an objective universal criterion (compounding quality criterion - K _k ), which allows to evaluate the quality of compounding of the windings of electric machines.

The solution of this problem is achieved by the fact that in the method for determining the quality of compounding of the windings of electric machines, which consists in the fact that a constant voltage U is applied to the measurement object, the resistance R (t) of the object is measured for a time sufficient to achieve the resistance value of a practically steady-state value, determine values of the transient current i (t) = U / R (t), then, according to the current curve, a diagnostic sign is determined for evaluating the quality of compounding of the winding in the form of a product of experimental values SIC characteristics of compound (ε _a · ρ _{v) exp} - absolute permittivity and volume resistivity, respectively, and then determine the compounding quality criterion K _k by comparing the experimental characteristics of the compound with its nameplate data using the formula:

$$K_k=\frac{{({\epsilon }_a\cdot {\rho }_v)}_{P\mathrm{but}\mathrm{from}P}}{{({\epsilon }_a\cdot {\rho }_v)}_{\mathrm{uh}\mathrm{to}\mathrm{from}P}}.$$

The main characteristics of the insulation include specific volumetric electrical resistance ρ _v and absolute dielectric constant ε _a . None of the indicated methods for assessing the quality of insulation [1-6] explicitly takes into account exactly the dielectric constant ε _a , which does not allow the formation of an objective evaluation criterion, on the basis of which it would be possible to compare the test sample with some standard characterizing the quality “ideal "Isolation.

In this case, the diagnostic feature is the product of the absolute dielectric constant ε _a and the specific volume electric resistance ρ _v - the main characteristics of the insulation dielectric of the windings.

The method for determining the value of the criterion K _k is implemented when testing the insulation of the windings of an electric machine and consists in calculating the time constant T according to the transient current schedule i (t), which characterizes the process of charging a capacitor connected to a constant voltage source. The capacitor plates are the stator winding wire and the body of the electric machine.

It was found that the time constant T, determined experimentally, depends on the main characteristics of the compound - the absolute dielectric constant ε _a , specific volumetric electrical resistance ρ _v and is proportional to the ratio of the leakage current i _ut equal to the steady-state value of the charge current i (t) to the absorption current i _abs (t) at t = 0-i _abs (0):

. $$T={({\epsilon }_a\cdot {\rho }_v)}_{E\mathrm{TO}\mathrm{FROM}P}\cdot \frac{i_{\mathrm{at}t}}{i_{\mathrm{but}b\mathrm{from}}(0)}$$

.

Obtaining from this expression the experimental value of the diagnostic feature (ε _a · ρ _v ) _exp

, $${({\epsilon }_a\cdot {\rho }_v)}_{E\mathrm{TO}\mathrm{FROM}P}=T\cdot \frac{i_{\mathrm{but}b\mathrm{from}}(0)}{i_{\mathrm{at}t}}$$

,

the value of the criterion K _k is determined through the passport ("ideal") values ε _a and ρ _{v of the} applied compound as follows:

. $$3\ge K_k=\frac{{\left({\epsilon }_a\cdot {\rho }_v\right)}_{P\mathrm{but}\mathrm{from}P}}{{\left({\epsilon }_a\cdot {\rho }_v\right)}_{\mathrm{uh}\mathrm{to}\mathrm{from}P}}\ge \mathrm{one}$$

.

From this expression it follows that the value of the criterion K _k , as shown by experimental studies, should be in a fairly narrow range from 1 to 3, i.e. the accuracy of diagnosis is increased and the invariance of the criterion for the size of the electric machine is ensured.

If the dielectric properties deteriorate (wetting of the insulation, contamination with impurities, the appearance of coarse concentrated defects, for example, cracking or punctures, etc.), the value of the criterion K _k may turn out to be more than 3, which is an objective sign of a deterioration in the quality of compounding of the windings of an electric machine.

The proposed method for determining the quality of compounding of the windings of electrical machines is illustrated in graphical and tabular materials.

Figure 1, 2, 3 presents the results of processing experimental data to determine the criterion for assessing the quality of compounding of the stator windings for a number of asynchronous submersible electric motors of various power and dimensions (values of transient current, constructed graphs of transient currents for the corresponding motors).

Figure 4 presents the comparison results of evaluating the quality criterion for compounding submersible electric motors of various power and equivalent circuit. For comparison, Table 4 also shows the corresponding values of the polarization indices and absorption coefficients.

The proposed method for determining the quality of compounding the windings of electrical machines is implemented as follows.

The tested insulation of the PEM (winding-casing) is connected to a high-voltage source of constant voltage U and a megaohmmeter is measured for insulation resistance R (t) for a time t during which the resistance value reaches a practically steady-state value, as is customary for exponential functions. As experiments show, this time is approximately 10 minutes. The measurements are carried out with increasing time intervals (approximately): 0, 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, 3.0, 5.0, 7.0, 10.0 minutes (10-15 measurements in total), as shown in Table 1-3 as examples for a number of asynchronous PEMs of various capacities and sizes.

Then, the values of the transient current i (t) = U / R (t) are calculated, the values are presented for the corresponding time instants in Table 1-3 of FIGS. 1-3.

Transition current graphs are being constructed; i (t) shown in figures 1-3 for the respective motors, and represented by approximating functions of the following form:

$$i(t)=i_{\mathrm{at}t}+i_{\mathrm{but}b\mathrm{from}}(0)\cdot e^{(-t/T)},\text{(one)}$$

where i _ut - leakage current, µA;

i _abs (0) is the absorption current at t = 0;

T = R ₀ · C is the time constant of the capacitor charge, C.

Here R ₀ · and C is the resistance and capacitance of the capacitor charge circuit corresponding to the equivalent circuit shown in Fig.4. The equivalent circuit corresponds to the charge process, which is described by formula (1).

Based on formula (1) and the equivalent circuit, the following quantities are written corresponding to the charge process:

$$R_{\mathrm{and}s}=\frac{U}{i_{\mathrm{at}t}}=\frac{{\rho }_v\cdot {\delta }_{\mathrm{uh}}}{S_{\mathrm{uh}}},\text{(2)}$$

where R _from is the insulation resistance, Ohm, expressed in terms of the specific volume resistance ρ _v , and the equivalent thickness δ _e and the surface area S _{e of the} insulation.

$$C={\epsilon }_a\cdot \frac{S_{\mathrm{uh}}}{{\delta }_{\mathrm{uh}}}.\text{(3)}$$

Note that in expressions (2) and (3), multilayer insulation is presented in the form of an equivalent single layer, since a more detailed analysis shows that the compound layer with equivalent and unknown values of δ _e and S _e is predominant. Such a representation does not contradict the experimental curves i (t), which take into account the polarization processes in the 3-layer insulation of the windings of the SEM.

$${\mathrm{<figure-callout\; id="0"\; label="R"\; filenames="00000019.png,00000020.png"\; state="\{\{state\}\}">R</figure-callout>}}_0=\frac{i_{\mathrm{at}t}}{i_{\mathrm{but}b\mathrm{from}}(0)}\cdot R_{\mathrm{and}s}-\text{(four)}$$

the charge circuit resistance unknown in advance, expressed in terms of the values known from the experiment.

Taking into account (2), (3), (4), the expression for the time constant T is determined:

$$T={\mathrm{<figure-callout\; id="0"\; label="R"\; filenames="00000019.png,00000020.png"\; state="\{\{state\}\}">R</figure-callout>}}_0\cdot C=\frac{i_{\mathrm{at}t}}{i_{\mathrm{but}b\mathrm{from}}(0)}\cdot R_{\mathrm{and}s}\cdot C=\frac{i_{\mathrm{at}t}}{i_{\mathrm{but}b\mathrm{from}}(0)}\cdot \frac{{\rho }_v\cdot {\delta }_{\mathrm{uh}}}{S_{\mathrm{uh}}}\cdot {\epsilon }_{\mathrm{but}}\cdot \frac{S_{\mathrm{uh}}}{{\delta }_{\mathrm{uh}}}=\frac{i_{\mathrm{at}t}}{i_{\mathrm{but}b\mathrm{from}}(0)}\cdot {\rho }_v\cdot {\epsilon }_a.\text{(5)}$$

The experimental value of the work is determined.

$${({\epsilon }_a\cdot {\epsilon }_a)}_{\mathrm{uh}\mathrm{to}\mathrm{from}P}=T\cdot \frac{i_{\mathrm{but}b\mathrm{from}}(0)}{i_{\mathrm{at}t}}\text{(6)}$$

Here, the value of the time constant T is determined, as is customary for exponential functions (Figs. 1-3): on the graph i (t) there is a point with the ordinate

$$0.37i_{\mathrm{but}b\mathrm{from}}(0)=0.37\cdot [i(t)-i_{\mathrm{at}t}].\text{(7)}$$

From this point, the perpendicular drops to the intersection with the time axis. The intersection point defines a segment equal to the time constant T of the capacitor charge in the selected scale along the time axis t.

The compounding quality criterion K _{k is} determined by comparing the experimental characteristics of the compound with their certified values

$$K_k=\frac{{\left({\epsilon }_a\cdot {\rho }_v\right)}_{P\mathrm{but}\mathrm{from}P}}{{\left({\epsilon }_a\cdot {\rho }_v\right)}_{\mathrm{uh}\mathrm{to}\mathrm{from}P}}\text{(8)}$$

Based on the experimental data, a range of admissible values of the criterion K _{k is established} for the respective products and the type of compound for insulation of the windings. The experiments with a series of asynchronous PEDs with the Elplast 220 ID compound showed (see Table 4) that the value of the criterion K _k is in the range from 1 to 3:

$$3\ge K_k\ge \mathrm{one.}\text{(9)}$$

Thus, the criterion K _k is an objective and universal criterion, practically independent of the size of the product, because determined experimentally taking into account only the basic properties of the dielectric ε _a and ρ _v .

List of references

1. RF patent 2101716. A method of measuring the steady-state value of the insulation resistance.

2. V.V. Bazutkin, V.P. Larionov, Yu.S. Pintal. High Voltage Technique: Insulation and Overvoltage in Electrical Systems: University Textbook, 3rd ed., Rev. and add. - M .: Energoatomizdat, 1986. - 464 p.: Ill.

3. RF patent 2229143. A method for diagnosing case insulation of an electric machine.

4. Operating manual MI 2077. http://www.astena.ru/mi2077_2.html

5. Submersible induction motors of the unified BPED series. Specifications TU 3381-030-00136679-2011. OJSC "Bugulma Electric Pump Plant", RT, Bugulma, 2011

6. Polarization Index Test. http://www.powertestasia.com/polarization-index/polarization-index-testing.html

Claims (1)

A method for determining the quality of compounding the windings of electric machines, which consists in applying a constant voltage U to the measurement object, measuring the resistance R (t) of the object for a time sufficient to achieve the resistance value of a practically steady-state value, and determining the values of the transient current i (t) = U / R (t), etc., determine the current curve for diagnostic sign assessing the quality of the winding in a product compounding experimental values of main characteristics of the compound (ε _a · ρ _{v) ekcp} - abso yutnoy dielectric constant and volume resistivity, respectively, and then determine the compounding quality criterion K _k by comparing the experimental characteristics of the compound with its nameplate data using the formula:

.

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