On the Selection of Test Voltages for Monolith-2 Turn Insulation of Stator Windings of Large Electric Machines Prior to Impregnation
Abstract Generated abstract
The paper addresses the selection of preventive test voltages for Monolith-2 turn insulation in stator windings of large electric machines before impregnation. It reports power-frequency and impulse breakdown tests on coil specimens with several glass-mica and glass-tape insulation constructions, then applies statistical processing to estimate breakdown distributions for whole coils and stator windings. The authors argue that test voltages should detect gross defects likely to cause interturn short circuits while avoiding damage to groundwall insulation, and they derive recommended voltage levels using defect, air-gap, epoxy-compound strength, and overvoltage considerations. The study concludes that higher standards than the existing 750 V per turn for coils and 500 V per turn for stator windings are justified for machines up to 6300 kW and 10 kV.
Full Text
ON THE SELECTION OF TEST VOLTAGES FOR MONOLITH-2 TURN INSULATION OF STATOR WINDINGS OF LARGE ELECTRICAL MACHINES BEFORE IMPREGNATION
P. L. LYSENKO, V. G. SYAKOV, G. G. SHALIN
(Presented by the scientific seminar of the Departments of Electrical Machines and Apparatus and General Electrical Engineering)
In the production process, turn insulation is subjected to numerous mechanical effects, often damaging the insulation. During operation, turn insulation experiences prolonged thermomechanical loads; it is affected by overvoltages tens of times higher than the operating voltage.
The correct selection of test voltages facilitates the detection of defects in the insulation during manufacture and their timely elimination, thereby increasing operational reliability.
For large electrical machines developed by scientific-research institutes and manufactured by the Sibelektrotyazhmash plant, for operating voltages from 380 to 10000 V, three designs of turn insulation have been adopted, depending on the rated voltages and powers.
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PSD wire, reinforced with a glass-mica tape 0.09 mm thick with half-lap wrapping, or PETVSD wire.
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PSD wire, reinforced with a glass-mica tape 0.13 mm thick with half-lap wrapping.
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PSD wire, reinforced with a glass-mica tape 0.13 mm thick with half-lap wrapping and with a glass tape 0.1 mm thick with half-lap wrapping.
As specimens for testing turn insulation according to variant No. 1, coils of the stator winding of a high-voltage electric motor AZ-290 (290 kW, 6 kV) were selected; and for variants 2 and 3, those of a low-voltage electric motor AZ-290 (290 kW, 660 V).
In variant No. 1 the effective turn contained one elementary conductor, which is characteristic of high-voltage machines with power up to 1000 kW; and in variants 2 and 3, six elementary conductors, which is characteristic of low-voltage machines and high-voltage machines with power above 1000 kW.
The coils were manufactured according to the technology adopted for stator windings with Monolith-2 insulation. A feature of the technological process is the application of the turn and ground insulation of the coils with dry glass-mica tapes, followed by impregnation of the coils after their insertion into the stator core with an epoxy compound by the vacuum-pressure method.
After winding the loop sections, pressing them, stretching and straightening them, and carrying out the insertion operations, the short-term dielectric strength of the turn insulation was determined. For the tests, the coils were cut in the middle of the slot portions into two equal parts. Each half-coil-
the coil included the entire one-sided end-winding part, which is very important, since damage to the turn insulation during its manufacture is most likely in the end-winding parts. The ends at the cut locations were spread apart. To avoid the influence of an adjacent breakdown on the value of the breakdown voltage, the voltage was applied to the tested pair of conductors through a turn.
The tests were carried out both with power-frequency voltage and with impulse voltage; for this purpose a 2K8 high-voltage installation was used (an installation analogous to 2K7 in [1]).
Figure 1 shows integral curves of the breakdown voltages of turn insulation (made of PSD wire insulation, one layer of glass-mica tape 0.13 mm thick, and glass tape 0.1 mm thick, half-lapped) for specimens, coils, and stator windings.
Fig. 1. Curves 1, 2 are rectified integral curves of the breakdown voltages of turn insulation of specimens (electrode length 950 mm) after stretching, pressing, and pressure testing (1), and after insertion into the stator core (2). Curves 3, 4, 5, 6 are calculated curves of the breakdown voltages of turn insulation of coils of machines with power from 2500 to 5000 kW: \(U = 6\ \mathrm{kV}\), \(m = 20.8\) (3), \(U = 10\ \mathrm{kV}\), \(m = 26.4\) (4), of stator windings of the same machines at the corresponding voltages: \(m = 1010\) (5), \(m = 1270\) (6).
The recalculation of curves 3, 4, 5, 6 was carried out using the distribution function for the probability of the smallest member of a variational series (sample) [2]:
\[ F_{tu}=1-\left[1-F_u\right]^m, \]
where
\(F_u\) is the probability distribution of breakdown voltages of individual specimens;
\(F_{tu}\) is the probability distribution of breakdown voltages of products (coil, stator winding);
the coefficient \(m\) is found as the ratio of the surface area of the turn insulation of the whole coil (stator winding) to the surface area of the specimen.
For coils of the machine nomenclature of the “Sibelektrotyazhmash” plant, the coefficient \(m\) ranges from 21 to 40; for stator windings of machines, from 1000 to 1900. The voltage distribution over the turns of a coil was assumed to be uniform. The test results for other types of turn insulation were processed in an analogous manner.
The data from processing the breakdown voltages (Table 1) show that the electrical strength of the turn insulation of coils and of machine stator windings as a whole is at a sufficiently high level. This also applies to the minimum values, which mainly characterize the serviceability of the insulation. Mathematical processing of the test data made it possible to substantiate the selection of test voltages for turn insulation.
The main purpose of preventive tests is to reject weakened points of the insulation before its impregnation and thereby to eliminate cases of turn-to-turn short circuits in windings after impregnation and baking of the insulation.
It is most expedient to check the turn insulation with high voltage after manufacture of the coil with the uninsulated
Table 1
Breakdown voltages of turn insulation
| Construction of turn insulation | After stretching, straightening, and pressing of specimens — AC voltage, kV amplitude: $U_{50\%}$ | After stretching, straightening, and pressing of specimens — AC voltage, kV amplitude: $S^*$ | After stretching, straightening, and pressing of specimens — AC voltage, kV amplitude: $\vartheta^*$ | After stretching, straightening, and pressing of specimens — AC voltage, kV amplitude: $U_{1\%}$ | After stretching, straightening, and pressing of specimens — impulse voltage: $U_{50\%}$ | After stretching, straightening, and pressing of specimens — impulse voltage: $S$ | After stretching, straightening, and pressing of specimens — impulse voltage: $\vartheta$ | After stretching, straightening, and pressing of specimens — impulse voltage: $U_{1\%}$ | For coils after stretching, straightening, and pressing — $U_{\mathrm{per}}$: $U_{50\%}$ | For coils after stretching, straightening, and pressing — $U_{\mathrm{per}}$: $U_{1\%}$ | For coils after stretching, straightening, and pressing — $U_{\mathrm{imp}}$: $U_{50\%}$ | For coils after stretching, straightening, and pressing — $U_{\mathrm{imp}}$: $U_{1\%}$ | Stator windings after laying — $U_{\mathrm{per}}/U_{50,1\%}$ | Stator windings after laying — $U_{\mathrm{imp}}/U_{50,1\%}$ |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PETVSD | 5.3 | 1.2 | 0.23 | 2.0 | 8.1 | 1.6 | 0.19 | 3.5 | 2.8 | 1.8 | 5.0 | 3.5 | 1.6 | 3.5 |
| PSD + glass-mica tape, thickness 0.09 mm | 4.6 | 0.56 | 0.13 | 3.25 | 3.5 | 3.0 | ||||||||
| PSD + glass-mica tape, thickness 0.13 mm | 4.7 | 0.7 | 0.15 | 3.0 | 8.0 | 0.97 | 0.12 | 5.7 | 3.3 | 2.7 | 6.25 | 5.25 | 2.4 | 4.3 |
| PSD + glass-mica tape, thickness 0.13 mm + glass tape, thickness 0.1 mm | 5.1 | 0.48 | 0.09 | 3.7 | 8.4 | 0.62 | 0.07 | 6.7 | 4.2 | 3.7 | 7.3 | 6.7 | 3.1 | 3.0 |
* $S$ — standard deviation; $\vartheta$ — coefficient of variation.
(before applying the groundwall insulation) and after placing the coils in the stator core, before connecting the coil jumpers.
In the adopted designs of turn insulation (Table 2), the distance between the copper of the conductors of adjacent turns is 0.6, 0.8, and 1.2 mm.
Let us take the limiting case. The insulation between turns has a through defect of the type of cracks, punctures, or destruction of the insulation, while the distances between turns are preserved.
Using the dependence of the discharge voltage in air over the surface of dry insulation [3], one can determine the breakdown voltages. Their values are entered in Table 2 (columns 3, 4) and are 1.3, 1.7, and 2 kV.
A comparison of the breakdown voltages of through defects and the minimum values of the electric strength of the turn insulation shows that such defects of the turn insulation are absent in the samples.
There is a natural desire to reject through defects, but, as is evident from Table 2 (columns 8, 9), for this a test voltage per coil would be required, depending on the number of turns (Table 2, column 7), of up to 36 kV. It is difficult to create test installations for such voltages, and there is no need for this.
When selecting test voltages, it is evidently necessary to proceed from the following basic considerations: the test voltages must reject coils with gross defects capable of causing interturn short circuits in operation, and measures must be taken so that, during the testing of the turn insulation, the groundwall insulation is not damaged.
Proceeding from the first requirement, the turn insulation after impregnation and baking must have an electric strength that ensures reliable operation of the insulation and is higher than the operating overvoltages.
Slits, voids, and other defects in the insulation are filled during impregnation with an epoxy compound, which has a sufficiently high level of electric strength after polymerization.
Let us take the worst case, when the conductors approach one another at small distances and the insulation between them has a through defect. In this case the electric strength of the turn insulation will be determined by the thickness of the layer of epoxy compound that has filled the through defect. One can find the smallest distance \(\delta\) between turns that can still provide, by means of the compound layer, the required electric strength.
The values of \(\delta\) can be determined from the expression
\[ \delta \leq \frac{k U_n}{\sqrt{2}\,E_k}\;(\text{mm}), \]
where
\(k\) is a coefficient determining the multiple of the minimum electric strength of the turn insulation relative to overvoltages. By analogy with groundwall insulation, with a certain margin, one may take \(k = 2.5\).
\(U_n\) is the overvoltage of the turn insulation (kV/turn), calculated according to [5] as
\[ U_n = 0.122\, U_H l_n \;(\text{kV ampl}), \]
where
\(U_H\) is the line voltage of the machine, kV;
\(l_n\) is the length of the slot part of the coil, m;
\(E_k\) is the minimum value of the electric strength of the epoxy compound. It may be taken as equal to 20 kV/mm.
Table 2
| Winding insulation construction | Turn insulation thickness, mm | Air-proof test voltage, kV (rms) | Air-proof test voltage, kV (ampl.) | Machine power, kW | Nominal voltage, kV | Number of turns in the coil, pcs. | Voltage per turn of the coil during testing, kV (rms) | Voltage per turn of the coil during testing, kV (ampl.) | Minimum permissible air gap between turns, \(\delta\), mm | Breakdown voltage of the air gap, \(U_0\), ampl. | Recommended test voltages \((\theta/\theta)\) before application of body insulation, ampl. | Recommended test voltages \((\theta/\theta)\) after laying the coils in the stator, ampl. |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| PETVSD or PSD + glass-mica tape 0.09 mm | 0.6 | 1.3 | 1.8 | 320–800 | up to 1 | 4–2 | 5–2.6 | 7–3.6 | 0.045 | 550 | 1000 | 750 |
| PETVSD or PSD + glass-mica tape 0.09 mm | 0.6 | 1.3 | 1.8 | 315–1000 | 6; 10 | 11–17 | 14–22 | 19–31 | 0.07 | 670 | 1000 | 750 |
| PSD + glass-mica tape 0.13 mm | 0.8 | 1.7 | 2.4 | 1000–2500 | 6; 10 | 5–9; 8–15 | 8.5–15; 13.5–25 | 12–22; 19–36 | 0.06; 0.11 | 635; 850 | 1250 | 1000 |
| PSD + glass-mica tape 0.13 mm + glass tape 0.1 mm | 1.2 | 2 | 2.8 | 2500–6300 | 6; 10 | 2–4; 3–7 | 4–8; 6–14 | 5.6–11; 8–20 | 0.09; 0.16 | 780; 1500 | 1500 | 1250 |
Knowing $\delta$ (using the dependence of the discharge voltage on dry insulation), one can determine the voltage $U_\delta$ required to detect through damage of depth $\delta$ in unimpregnated turn insulation shunted by an air layer.
Similar results can be obtained if one considers the case in which, as a result of mechanical effects, the insulation between turns is thinned to the value $\delta$; in this case, with some approximation, the minimum electric strength of the insulation may be taken as equal to $E_k$, and the breakdown voltages of the unimpregnated insulation in thin layers as equal to $U_\delta$.
Table 2 (columns 10, 11) gives the values of $\delta$ and $U_\delta$. In the same table (columns 12, 13) recommended values are given for the test voltages of the turn insulation of coils of stator windings of large electrical machines of the ATD-2 and VAO series with power up to 6300 kW and voltage up to 10 kV. The test voltages were selected not lower than the values of $U_\delta$, taking into account the technical capabilities of installations and instruments for testing turn insulation.
Testing the turn insulation of individual coils (before the application of the groundwall insulation) does not present any particular difficulties. The high-voltage source may be a high-frequency installation generating damped high-frequency oscillations [4]. The installation is simple and convenient. It can provide a test voltage of up to 1500–2000 V/turn, regardless of the number of turns in the coil under test. As has been investigated, the distribution of voltage over the turns of the coil is practically uniform. Testing turn insulation during winding operations can be carried out by means of a 2K8 installation and other devices, more often called “turn-to-turn short-circuit detectors.” In order not to damage the groundwall insulation when testing the turn insulation, it is necessary to insulate the stator core with the coils under test from ground.
The value of the impulse coefficient $K_U$ is of interest. Its values were calculated from the relation:
\[ K_U = \frac{U\ \text{impulse}}{U\ \text{alternating amplitude}}, \]
where $U_{\mathrm{imp}}$ is the breakdown voltage from the 2K8 installation. For the adopted turn-insulation designs, $K_U$ may be taken as equal to unity.
In conclusion, it should be said that the work carried out has made it possible to raise the standards for test voltages of turn insulation relative to the existing standards (750 V/turn for coils and 500 V/turn for stator windings), thereby making them more effective.
REFERENCES
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Z. G. Kaganov, N. V. Krutov. Installations for preventive tests of turn insulation of high-voltage electric motors with preliminary charging of the winding. In: Tests of Turn Insulation of Electrical Machines. Moscow, 1959.
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N. V. Smirnov, I. V. Dunin-Barkovskii. A Course in Probability Theory and Mathematical Statistics. Moscow, “Nauka,” 1969.
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Z. G. Kaganov, N. V. Krutov. Problems of preventive maintenance of turn insulation of electric motors. In: Insulation of Electrical Machines. Moscow, GEI, 1958.
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N. V. Smirnov. Inspection and Testing of Windings of Electrical Machines and Apparatus. Moscow, Gosenergoizdat, 1959.
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V. N. Korolev. Tests of insulation of high-voltage machines. Elektrotekhnika, 1964, No. 12.