Space Charge Behavior in Epoxy-Paper-Epoxy Three Layers Composites

Generally, UHV converter transformer bushing cores use epoxy resin and insulating paper composite materials as the main insulating dielectric. It is necessary to study the space charge behavior of epoxy resin/insulating paper composites at high temperature and field strength. According to the insulation structure of dry-type bushing core, two epoxy/paper/epoxy three layers Composite samples with different thickness were made(sample1#: epoxy0.24mm, paper0.09mm, epoxy0.24mm; sample 2#: epoxy0.24mm, paper0.13mm, epoxy0.10mm). Both voltage-on and voltage-off space charge tests were performed for 1h by Pulsed Electro-Acoustic (PEA) method under different electric field (10kV/mm, 20kV/mm and 40kV/mm). The test results and analysis show: 1. Because of the acoustic reflection signals superposition, sample1# with same thickness of two epoxy layers is difficult to obtain the real multilayer space charge behavior by PEA. Sample2# with different thickness of two epoxy layers can avoid the acoustic reflection signals superposition effectively. 2. At 60 degrees C, under different electric field, the space charge tests show there is another charge peak Kc in impregnated paper. It is due to the incompletion of paper impregnation. At this time, the charge distributing information is reliable. Under 10kV/mm similar to 40kV/mm, negative charge injection cannot be observed. And the injected depth around anode is small. The sample with different thickness of two epoxy layers can avoid the acoustic reflected signals superposition to PEA method measuring interference effectively. At 60 degrees C, the quantity of charge at the interface is due to the difference of dielectric constant and conductivity between two layers. After voltage-on test, interfacial polarization not reached steady state. Interfacial charge revealed a trend of continued increase. This research will provides the basis for multilayer dielectric space charge tests and analysis.