01_Mary_ELF_spalcechg_interface

01_Mary_ELF_spalcechg_interface - I EEE Transactions on...

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IEEE Transactions on Dielectrics and Electrical Insulation Vol. 8 No. 5, October 2001 771 Electroluminescence Measurements to Detect Accumulated Charge at the Electrode-Insulator Interface D. Mary and D. Malec Paul Sabatier University Toulouse, France ABSTRACT Electroluminescence (EL) spectrum analysis is proposed to detect \charge accumulation in in- sulators. The objective is to find the nature of accumulated space charges at the electrode- insulator interface by using only EL measurements recorded under a moderate ac 50 Hz field. The differences between spectra obtained with virgin samples and treated samples are ana- lyzed. Some experience obtained with 500 pm thick low-density polyethylene (LDPE) Sam- ples are described to illustrate the expected modification of the spectrum vs. interfacial space charges. Direct space charge measurements, using the laser induced pressure propaga- tion (LIPP) method are used to verify our assumptions. Good agreement is found between analysis and experimental data space charges. 1 INTRODUCTION UMEROUS investigations have been performed in order to study N space charge build-up under dc and ac polarization in polymer insulators, and more recently in ceramic insulator. These charges can originate from the dissociation of bulk charges and/or from injection processes. Since the development of new techniques for direct space charge measurement, some studies have related the space charge di- rectly to the external discharge current [I, 21, the conduction current [3, 41 and also the dielectric breakdown [5-81 of polymer insulators. 2 THEORETICAL Under a moderate ac applied field (< 30 kV/mm), we consider that in virgin samples the majority of charges injected under one polarity of the voltage is lost by recombination with carriers injected under the reverse polarity, as also proposed in [U] and verified in [9, 101. It is the reason why no high charge gradients can develop if the time of polarization is short (4 week). Because the applied field is relatively moderate, it is clear that the detected electroluminescent activity is only due to recombination processes (no ionization in the bulk, no air dis- CONSIDERATIONS Electroluminescence (EL) measurements already have been used charges in the electrode area). When no charge can develop and move to detect space charge accumulation in polymer insulators. Indeed, in the bulk during the 50 Hz polarization (frozen because the stress is both recombination and impact-ionization processes generate radiative an ac field), the interfaces are the only areas of the sample where re- emission. Our preliminary studies have demonstrated the recombina- I combination processes can occur. Consequently, measurements may tion process of accumulated charges at electrode-insulator interfaces, be used naturally to detect any charge buildup in the interface areas.
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This note was uploaded on 06/11/2011 for the course ELECTRICAL 124 taught by Professor Ghjk during the Spring '11 term at Institute of Technology.

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01_Mary_ELF_spalcechg_interface - I EEE Transactions on...

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