Effects of transients on breakdown of XLPE cable insulation
The trend to underground considerable parts of the high voltage grid connections reinforces the need to study the effect of transients in the network on the cable insulation system. Even though the high voltage cables and accessories are designed to withstand certain types of transients, they may break down when subjected to transient situations, especially the superimposed transients which are produced by lightning and switching impulses in the combined line-cable systems. Therefore, it is essential to investigate the effects of transients on the breakdown of cable insulation. In this work, cross linked polyethylene (XLPE) was investigated, which is widely used for power cable insulation. XLPE samples were produced by hot pressing. Transients measured in the field were assessed and characterized by a set of parameters. Based on the transient parameters, DC and AC voltage waveforms with different frequencies, superimposed waveforms with different harmonic frequencies and amplitudes were designed as the testing voltage waveforms. Breakdown voltage of XLPE samples were measured under the designed testing voltage waveforms. The result shows that, under AC sinusoidal waveforms, the higher the frequency, the lower the breakdown strength of XLPE samples. Under superimposed waveforms, the effect of harmonic frequency seems insignificant. On the other hand, harmonics of superimposed waveform influences the breakdown strength. High harmonic amplitude leads to lower breakdown strength. These phenomena might be explained by the dielectric power losses in the insulation material. Below the Debye frequency, the loss tangent tand of insulation material increases with frequency increasing, since more friction losses of dipoles occur with polarity changing. This leads to higher dielectric power losses under the same applied voltage. In other words, the higher the frequency, the faster the dielectric power losses increase, the lower the breakdown voltage is. However, for superimposed waveforms, the frequency of harmonic doesn’t have significant effects on the breakdown of XLPE insulation by itself. The harmonic frequency and amplitude influence the breakdown in combination, due to which at larger harmonic amplitude, XLPE insulation also breaks down at lower voltage.
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