Study of Polyethylene Deformation in an Electric Field
Abstract Generated abstract
This study tests the thermoelectromechanical theory of polymer breakdown, which attributes the temperature-dependent decrease in polyethylene electric strength to specimen thinning under electrostatic compressive forces. The authors measured deformation in 0.05 and 0.1 mm polyethylene specimens under a linearly increasing voltage using an optical balance arrangement sensitive to submicron changes, and recorded breakdown fields at temperatures from 20 to 80 degrees C. In all cases deformation before breakdown was less than 1 micron, far below the 20 to 40 microns predicted by the theory, leading the authors to conclude that this breakdown mechanism is not supported by their experiments.
Full Text
Study of Polyethylene Deformation in an Electric Field
V. S. Dmitrevsky, V. I. Merkulov
(Presented by the scientific seminar of the Department of Electrical-Insulation and Cable Engineering)
The authors [1, 2, 3] proposed a theory of electromechanical breakdown of polymers, which explains the decrease in their electric strength with increasing temperature as a result of a reduction in the specimen thickness by electrostatic compressive forces. Calculations carried out by them showed that the thickness of polymer specimens before breakdown is \(0.6 d_0\), where \(d_0\) is the initial thickness of the specimen. In order to verify this hypothesis, we measured the deformation of polyethylene before breakdown.
The measurements were carried out under a linearly increasing voltage at a rate of \(0.5\ \mathrm{kV/sec}\), using a device whose basic schematic diagram is shown in Fig. 1. It can be seen from Fig. 1 that the deformation of the specimen
Fig. 1. Device for measuring the deformation of a specimen before breakdown:
1 — specimen, 2 — high-voltage electrode, 3 — measuring electrode,
4 — balance arm with a mirror, 5 — directed-light source, 6 — instrument scale
\((\Delta)\) can be expressed through the deflection on the instrument scale \((n)\) by the following equation:
\[ \tan \alpha=\frac{\Delta}{x}=\frac{n}{2l},\ \text{i.e.}\ \frac{\Delta}{x}=\frac{n}{2l},\ \text{then}\ n=\frac{2\Delta l}{x}, \]
where \(\Delta\) is the deformation of the specimen before breakdown by electrostatic compressive forces;
\(n\) is the magnitude of the deflection on the instrument scale;
\(x\) is the short arm of balance beam 4;
\(l\) is the distance from the fixed axis of balance beam (4) to the instrument scale.
For \(\Delta = 1\,\mu\), \(x = 5\ \mathrm{mm}\), \(l = 5\ \mathrm{m}\), \(n = 2\ \mathrm{mm}\), i.e., for an arm ratio \(x/l = 0.001\), the sensitivity of the device is \(0.5\,\mu\) per \(1\ \mathrm{mm}\) of the instrument scale.
Table 1 presents the experimental results obtained on polyethylene specimens with thicknesses of 0.05 and 0.1 mm.
Table 1
| \(T^\circ\mathrm{C}\) | 20 | 40 | 60 | 80 |
|---|---|---|---|---|
| \(E_{\mathrm{br,av}}\), kV/mm, at \(d_0 = 0.05\ \mathrm{mm}\) | 420 | 390 | 275 | 250 |
| \(E_{\mathrm{br,av}}\), kV/mm, at \(d_0 = 0.1\ \mathrm{mm}\) | 420 | 390 | 270 | 250 |
| \(\Delta\mu\), at \(d_0 = 0.05\ \mathrm{mm}\) | \<1 | \<1 | \<1 | \<1 |
| \(\Delta\mu\), at \(d_0 = 0.1\ \mathrm{mm}\) | \<1 | \<1 | \<1 | \<1 |
\(E_{\mathrm{br,av}}\), kV/mm—the average breakdown field strength.
As can be seen from Table 1, the deformation of the specimens before breakdown was less than \(1\,\mu\), whereas according to the thermoelectromechanical theory it should have been equal to 20 and 40 microns, respectively, for specimens with thicknesses of 0.05 and 0.1 mm.
Thus, the experiments carried out show that, apparently, the theory of thermoelectromechanical breakdown of polymers is untenable. It is characteristic that, when creating good conditions for heat removal from the specimen, Lawson [4] did not obtain a dependence of \(E_{\mathrm{br}}\) of polyethylene on temperature. This also indicates the untenability of the thermoelectromechanical theory of breakdown.
References
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W. G. Lawson. “Temperature dependence of the intrinsic electric strength of polythene.” Nature, 1965, 206, No. 499, 1248–1249.