Proceedings of the Tomsk Order of the October Revolution
and Order of the Red Banner of Labor
Polytechnic Institute named after S. M. Kirov

Volume 282　　　　　　　　　　　　　　　　　　　　　　　　　1974

INVESTIGATION OF INTERTURN SHORT CIRCUITS IN THE WINDINGS OF HIGH-VOLTAGE TRANSFORMERS UNDER THE INFLUENCE OF IONIZATION IN GAS INCLUSIONS

V. I. Merkulov, Yu. N. Shumilov, V. V. Khamulyak

(Presented by the scientific seminar of the Department of Electrical Insulation and Cable Engineering)

In the manufacture of cast epoxy insulation, owing to a number of causes, gas inclusions, microcracks, and other kinds of defects are formed in it. Such causes may include: violation of the curing process, shrinkage of the compound, formation of microcracks as a result of thermal stresses, etc. [1].

The study of products and specimens made of epoxy compound shows that the distribution of gas inclusions within the volume of the insulation is random [2]. In this connection, the possibility cannot be excluded that such inclusions may appear on the surface of the winding or between its individual layers. It may be assumed that, when operating or test voltage is applied to the main insulation of a transformer, ionization arises in these gas inclusions; this will cause destruction of the turn insulation, a decrease in its resistance, and may lead to a short circuit between adjacent turns through the spark of a partial discharge and the reduced resistance of the turn insulation.

To verify this assumption, we carried out investigations to assess the influence of ionization in gas inclusions on the possibility of interturn short circuits. As specimens for the investigation, motorets simulating the winding of a high-voltage transformer were manufactured. The design of a motoret and the circuit for testing it are shown in Fig. 1.

Fig. 1. Electrical circuit for testing motorets for interturn short circuit: 1) air pores; 2) cast epoxy insulation 2.4 mm thick; 3) motoret winding; 4) high-voltage electrode

As the main insulation 2, an epoxy compound EZK-10 was used, the composition and manufacturing technology of which are described in [3]. The model winding was made by laying two conductors in a row, with no direct electrical contact between them, parallel to one another (one conductor in Fig. 1 is shaded). For the winding conductors, PEV-grade wire of diameter \(0.82\ \mathrm{mm}\) was used. To record an interturn short circuit between these conductors, a voltage of 4–5 volts was applied, simulating the induced voltage in the winding of a high-voltage transformer. The high voltage from \(TR_1\) was applied to the insulation gap between high-voltage electrode 4 and model winding 3. Thus, the design of the model represented a model of the main and turn insulation of a high-voltage transformer. When the turn insulation between adjacent conductors was destroyed, a short circuit occurred, which was recorded by the ignition of TN-02 neon lamps.

For conducting the studies, we manufactured two batches of specimens. The first batch, consisting of 17 specimens, had natural air inclusions formed during potting with EZK-10 compound. The second batch, consisting of 15 specimens, had artificial gas inclusions, specially applied to the surface of the model winding by baking a drop of BF-2 glue before potting with the compound. In most cases these inclusions had the form of flat air pores measuring \(2 \times 3\ \mathrm{mm}\) and \(0.5\ \mathrm{mm}\) thick.

The specimens were tested under alternating voltage at a frequency of \(400\ \mathrm{Hz}\), at a temperature of \(80^\circ\mathrm{C}\) and a voltage \(U = 24\ \mathrm{kV}\). Before the start of the test, the ionization inception voltage, \(U_{\mathrm{i.i.}}\), was measured for all specimens. The measurement results for \(U_{\mathrm{i.i.}}\) showed that, for the specimens of the second batch with artificial gas inclusions, the ionization inception voltage was \((2.2\text{--}5.1)\ \mathrm{kV}\), whereas \(U_{\mathrm{i.i.}}\) for the specimens of the first batch lay within \((13.0\text{--}15.6)\ \mathrm{kV}\). Upon opening the specimens of the first batch, we established that the natural air inclusions formed during potting were, as a rule, either spherical or ellipsoidal in shape, whereas the artificial pores were flat.

In this connection it may be assumed that the lower values of the ionization inception voltage for the second batch of specimens were due to the fact that the gas inclusions were flat. The test data for the specimens with respect to interturn short circuit are presented in the table.

Table

It follows from the table that, of the 17 tested specimens of the first batch, 14 specimens failed as a result of breakdown of the main insulation. In three specimens of this batch, an interturn short circuit was detected without destruction of the main insulation. Opening these specimens showed that there were air inclusions on the surfaces of the windings. In this case, products of insulation destruction were found in the pores; apparently, they served as a conducting bridge short-circuiting adjacent turns of the winding to one another. The resistance of this layer was of the order of a value

on the order of 30–100 ohms. In the remaining 14 specimens, air inclusions were found only within the volume of the insulation.

For the second batch of specimens, which had artificial gas pores on the surface of the windings, 10 of the 15 tested specimens failed as a result of an interturn short circuit. One specimen broke down simultaneously with the occurrence of an interturn short circuit, and only 4 specimens failed as a result of destruction of the main insulation. When these specimens were opened up, it was found that the artificial gas pores applied to the surface of the winding before casting had been destroyed and filled with compound. This shows that the probability of an interturn short circuit as a result of a sharp decrease in the resistance of the interturn insulation under the action of ionization depends to a significant extent on the location of air inclusions in the volume of the insulation.

Considering that in actual designs, for which a random distribution of air inclusions is characteristic, the probabilities of these inclusions being located in the volume of the insulation and on the surface of the windings are equivalent, it may be said that an interturn short circuit represents a serious danger.

Thus, the tests conducted to investigate the possibility of interturn short circuits in the windings of high-voltage transformers show:

1. Ionization processes in gas cavities at transformer windings are one of the causes of interturn short circuits.

2. The occurrence of an interturn short circuit caused by ionization is determined by the probability of the presence of air inclusions on the winding surface.

REFERENCES

1. K. I. Chernyak. Epoxy Compounds and Their Application. Sudpromgiz, 1967.

2. M. Z. Gelman, B. M. Ryabov. Ionization characteristics of polymerized insulation. Proceedings of LPI, No. 258, 1965.

3. K. I. Chernyak. Nonmetallic Materials in Shipboard Electrical and Radio-Engineering Apparatus. “Sudostroenie,” 1970.