The influence of frequency of AC component on space charge behaviours in polyethylene under combined AC and DC electric fields
Combined ac and dc voltages may occur in high voltage power transmission system, which bring challenges to the dielectric materials. Generally speaking, space charge is well recognised under high voltage dc conditions but has received little attention under high voltage ac conditions. Under ac electric fields, the formation space charge in the material depends strongly on the frequency. The knowledge of space charge behaviour in dielectric materials under combined voltages is desirable in terms of both experimental and simulation. In this paper, a numerical model based on the bipolar charge injection/transport model is used to analyse characteristics of space charge in polyethylene under combined electric fields at room temperature. The DC component ratio and the overall root mean square (RMS) are kept the same, while the frequency of the AC component is altered from 0.05 Hz to 50 Hz. It has been found that the amount of the accumulated charges reduces with increasing frequency due to the more frequent polarity reversal. Moreover, the time to reach the steady states of charge amount increases with frequency as well.
Frequency dependence of interfacial space charge formed in laminated dielectrics under AC voltage application conditions
The frequency dependence of interfacial space charge was experimentally obtained for LDPE/EVA laminated dielectrics in the frequency range of dc to 50 Hz. It was found that there are high, low and intermediate transition frequency regions in this performance. Interfacial space charge might be governed in principle by the Maxwell-Wagner type polarization, but modified by a semi-blocking contact of the interface in the low frequency region, and by a limited number of carriers in the high frequency region. Increased voltage induces carriers in the interface to become less mobile or more trapped. Both phenomena occur in the transition region.
Trapping of excitons at chemical defects in polyethylene.
In a previous paper we studied an injected electron-hole pair in crystalline polyethylene (PE) and found that the exciton becomes weakly self-trapped in a narrow interchain pocket comprised between two gauche defects. Despite the large energy stored in the trapped excitation, there did not appear to be a direct nonradiative channel for electron-hole recombination. Actual polyethylene systems of practical use are, however, neither crystalline nor pure. To understand the fate of an electron-hole pair in the impure case, we studied by ab initio simulations the evolution of an exciton trapped on three common chemical defects found in polyethylene: a grafted carbonyl (C=O); an intrachain vinyl group (C=C); a grafted carboxyl (COOH). Ab initio simulations lead to predict three different outcomes: trapping, nonradiative recombination, and homolitic bond-breaking, respectively. This suggests that extrinsic self-trapping of electron-hole pairs over chemical defects inside the quasicrystalline fraction of PE could be relevant for electrical damage in high-voltage cables.