BREAKDOWN OF ELECTRICAL MACHINE INSULATION UNDER THE SIMULTANEOUS ACTION OF ALTERNATING AND IMPULSE VOLTAGE

Yu. P. Pokhodkov, P. P. Besperstov, Yu. G. Gladkikh

(Presented by the scientific seminar of the Department of EIKT)

During operation, the insulation of electrical machines is subjected to commutation overvoltages of significant amplitude, which arise during transient processes. Such transient processes include: starting, stopping, switching from star to delta, reversing, a sudden change in the load on the motor shaft, etc. The magnitude of commutation overvoltages can reach 4–7 times the rated voltage, and in some cases even 15 times the rated value [1].

A comparison of the level of the maximum possible commutation overvoltages and the minimum values of the breakdown voltage of undamaged insulation makes it possible to conclude that, during machine operation, breakdown of the turn-to-turn and ground insulation of windings under the action of commutation overvoltages occurs only at a defective location. Defects in winding insulation consist of through punctures, cracks, and scratches, which may appear during the manufacture of the insulation, or during the manufacture of the winding, or under the influence of operating factors (elevated temperature, vibration, etc.).

Breakdown of insulation at a defective location under the action of commutation overvoltages does not always lead to failure of the motor winding, since the operating voltage is not capable of sustaining the discharge arc formed by the impulse of the commutation overvoltage. However, the discharge arc leads to the formation of a conducting track; consequently, with each subsequent breakdown of the insulation at the defective location, the probability of formation of a discharge arc increases, which leads to insulation failure.

In this work, the influence of the operating voltage, impulse amplitude, and operating current on the number of impulses until failure of the winding insulation of asynchronous motors was investigated. Failure of the winding insulation of the model in our case occurs when the rated operating voltage sustains the discharge arc at the defective location formed by the impulse voltage.

The study was carried out on stator models of an AO2-32-4 asynchronous motor. Artificial defects were introduced into the turn-to-turn and ground insulation of the winding. The defects consisted of a continuous annular cut in the turn-to-turn insulation and a through puncture in the ground insulation. A sinusoidal operating voltage of industrial frequency was applied to the model winding.

of industrial frequency was applied and, simultaneously, an impulse voltage was supplied. Figure 1 shows the basic circuit for determining the number of impulses until failure of the winding insulation.

The impulse amplitude was varied from 5 times the value of the operating voltage applied to the model winding to 12 times this value. The operating current was set within the limits \((0.9 \div 1.4) I_n\). An assessment of the discrepancy between the mean values of the number of impulses until failure of the winding insulation, at a 5-percent significance level for different currents and unchanged values of \(U_n\) and \(U_{\mathrm{imp}}\), according to Student’s agreement criterion, showed an insignificant discrepancy. Consequently, the magnitude of the operating current does not affect the number of impulses until failure of the winding insulation.

Fig. 1. Electrical circuit of the test installation

Based on the experimental results, an approximation was carried out of the distribution of the number of impulses until failure of the turn and ground insulation of the winding. The distribution of the number of impulses until failure is approximated by normal laws with parameters \(\overline{N}\) and \(\sigma\) \((N\) lies within the limits from 1 to \(\infty)\). Agreement between the practical and theoretical distributions, checked by Pearson’s goodness-of-fit criterion, indicates that their discrepancy is insignificant. The distribution parameters are given in Table 1 for turn insulation and in Table 2 for ground insulation. Approximation of the distributions of the number of impulses until failure of the winding insulation

Table 1

Table 2

makes it possible to calculate the probability of insulation failure under a single overvoltage impulse

\[ P_1 = 1 - F\left(\frac{1-\bar N}{\sigma}\right). \]

The dependence of the probability \(P_1\) on the operating voltage applied to the phase coil and on the impulse amplitude is given in Fig. 2 for turn insulation, and in Fig. 3 for ground insulation. On the basis of

Fig. 2. Dependence of the failure probability of defective turn insulation of the winding on \(U_n\) at various values of \(U_{\mathrm{imp}}\) after the first impulse breakdown.

Fig. 3. Dependence of the failure probability of defective ground insulation of the winding on \(U_n\) at various values of \(U_{\mathrm{imp}}\) after the first impulse breakdown.

From the experimental data, empirical formulas were derived for determining the number of impulses to failure of turn and ground insulation.

Number of impulses to failure of turn insulation

\[ \overline{N}= \frac{15.7[30\cdot(46.9-\ln U_{\text{imp}})-U_{\text{n}}]} {4.76-\ln U_{\text{n}}}. \]

Number of impulses to failure of ground insulation

\[ \overline{N}=(9.6-0.008U_{\text{n}}-\ln U_{\text{imp}})\cdot(5.35-\ln U_{\text{n}})\cdot0.705\cdot10^{4}. \]

where

\(U_{\text{n}}\) — operating voltage of the section, in [V];

\(U_{\text{imp}}\) — impulse amplitude, in [V];

\[ U_{\text{imp}}=K_{\text{o.op}}\cdot U_{\text{n}}; \]

\(K_{\text{o.op}}\) — coefficient of dangerous overvoltage.

Here the overvoltage coefficient is taken to mean the ratio of the impulse amplitude to the rated voltage. By the coefficient of dangerous overvoltage we mean a coefficient that creates, in a section of the motor phase, a voltage equal to or exceeding the breakdown voltage of an air gap with a thickness equal to twice the thickness of the turn insulation, or with a thickness equal to the sum of the thicknesses of the turn and ground insulation. In calculating the number of impulses to failure of the turn or ground insulation, we take into account only those impulses for which \(K_{\text{o.op}}\geq 1\). The theoretical curves calculated from the formulas obtained lie within the confidence intervals calculated from the experimental data.

Conclusions

1. Failures of the turn and ground insulation of the windings of AO2-series electric motors were investigated under the simultaneous action of impulse and alternating voltage.

2. It was established that, with an increase in the operating voltage \(U_{\text{n}}\) and the impulse amplitude \(U_{\text{imp}}\), the number of impulses to failure of the winding insulation decreases.

3. It was established that the number of impulses to failure of the winding is approximated by a normal distribution with parameters \(\overline{N}\) and \(\sigma\). The parameters of the distributions were determined.

4. A change in the magnitude of the operating current within the range \((1 \div 1.4)I_{\text{n}}\) does not affect the number of impulses to failure of the winding insulation.

5. Empirical formulas were derived for calculating the number of impulses to failure of defective turn and ground insulation of electrical-machine windings, as a function of the level of commutation overvoltages and the rated operating voltage.

REFERENCES

1. O. D. Goldberg, N. I. Suvorov et al. The influence of commutation overvoltages on the reliability of low-voltage asynchronous motors. Elektrotekhnika, 1968, No. 5.

2. Z. G. Kaganov. Internal overvoltages and other effects on the turn insulation of electric motors. In: Testing the Turn Insulation of Electrical Machines. Gosenergoizdat, 1959.

3. A. K. Mitropol’skii. Technique of Statistical Computations. Nauka, 1971.