The interest in studying electrical conduction and polarization phenomena in polymers has increased since last year. Besides the classical problems of conduction, polarization and breakdown in polymeric insulators, interesting for the electric and electronic industry, new aspects are being considered by more and more research groups in the field. One of these is the application of the dielectric method, especially the depolarization technique, for studying molecular mobilities, i.e., multiple phase transitions in polymeric systems. Combined with mechanical relaxation, thermal analysis, magnetic resonance and with some additional techniques dielectric spectroscopy is becoming a powerful tool for studying the phase-structure of such complex polymer systems as blends, filled, and reinforced materials. This way the dielectric technique is helpful in quality control even of such systems which are not used in the electrical or electronic industry.

http://ieeexplore.ieee.org/iel7/7731603/7731604/07731995.pdf

Electrical properties of polymers

This paper describes the effects of frequency, voltage, temperature and mechanical stress on the times to breakdown of XLPE cable insulation subjected to highly divergent fields. Temperature and mechanical stress reduce the times to breakdown but tests with increasing voltage and high frequency produced unexpected results which could not be explained by the existing theory of electrical tree growth. Results of partial discharge measurements indicate that space charges are dominant in determining the shape of the tree and the time to breakdown of the insulation.

An Investigation into the Growth of Electrical Trees in XLPE Cable Insulation

We have investigated the partial discharge (PD) due to electrical treeing degradation in low-density polyethylene (LDPE), ethylene - vinyl acetate copolymer (EVA) and ethylene - acrylic acid copolymer (EAA) by a computer-aided partial discharge measurement system which allowed us to obtain phase-resolved PD pulse data. The experimental results revealed that the PD magnitude was strongly affected by the instantaneous applied voltage and that the occurrence of a PD was determined by the time derivative of the applied voltage (). The PD pulse-sequence analysis revealed the following: (i) a PD occurs in a discharge path which consists of a tree trunk and branches extending from the trunk; (ii) in each discharge path at most one PD occurs per half cycle. Based on these facts, a model of PDs due to electrical treeing was proposed. The influences of applied voltage and frequency were investigated by applying a triangular voltage. The number and average magnitude of PDs increased linearly with applied voltage whereas the PD charge per cycle increased quadratically. These results are in good agreement with the model.

https://iopscience.iop.org/article/10.1088/0022-3727/29/11/028/pdf

Partial discharges due to electrical treeing in polymers : phase-resolved and time-sequence observation and analysis

The study of electrical aging of polyethylene subjected to a 50 Hz ac voltage has shown that electroluminescence occured in the bulk of the polymer These light emissions (detected in the visible range of wavelength) were due to the buildup of space charge around the electrodes In a previous paper [J Appl Phys 54, 1532 (1983)] results concerning the role of dissolved gases in polyethylene were presented, where particular attention was paid to electroluminescence effects The present work aims to answer the two following questions: (1) what are the mechanisms causing the electroluminescence? (2) what is the nature of the luminescent centers? Study of the light emission with respect to the applied voltage form, the frequency, (when ac voltage is applied) and the voltage amplitude, shows that the light emission is due to radiative recombinations of carriers injected through the metal‐dielectric interface following the Fowler–Nordheim effect This process requires a double injection of carriers The influ

Mechanisms of electroluminescence during aging of polyethylene

The growth characteristics of electrical trees under DC and AC voltages are quite different. In order to investigate the influences of AC component in HVDC cable system on the electrical tree properties, the growth and discharge characteristics of electrical trees under AC-DC composite voltages were studied in this paper. The results showed that the AC component greatly accelerated the developing process of the electrical trees. The influences of the positive and negative DC bias voltages on the electrical tree growth properties were quite different. The growth rate under negative DC bias voltage was similar to that under pure AC voltage and pine-branch type electrical trees were more likely to form. In contrast, the growth rate of the electrical tree increased with the increase of the positive DC bias voltage and it was much faster than the negative DC biased one. More branch-like electrical trees would form under positive DC bias voltage. When the AC component decreased, the developing process of the electrical tree was fairly tough and it was easy to form bush-like electrical trees, which were the typical conducive electrical trees with fairly small discharges. Under the positive DC bias voltage, there was a fast re-growth process of electrical tree after the bush tree formed, and the positive DC bias voltage could promote the coming of the re-growth process. However, the detected partial discharge during this process was quite small. The test results indicated that it may pose a great threat to the safety of the cable insulation if there is a large AC component in the HVDC cable system.

Growth and partial discharge characteristics of electrical tree in XLPE under AC-DC composite voltage

Although the dc-tree-initiation voltage at a needle electrode in a plastic is very high, a tree can be formed by grounding the needle after charging it with a much lower dc voltage than the tree-initiation voltage. The tree formed by grounding will be called "a grounding tree". Since the formation of a tree under ac voltages is considered to be due to the repetition of charging and discharing, some relationship may exist between the ac tree and the grounding tree. The tree-initiation observed with a voltage composed of dc and ac shows that it is caused by a nearly constant value of the ac component, irrespective of the magnitude of the dc component. This also indicates that the mechanism of the intiation of an ac tree and that of a grounding tree are similar. An explanation is also given as to why the tree shape is branch-like or bush-like with ac voltages.

Y. Saito

Papers

On the Mechanism of Tree Initiation