Effects of LDPE/nanofilled LDPE interface on space charge formation

Space charge characteristics of double-layer dielectric, which consists of low density polyethylene (LDPE) and nano-titania filled LDPE (named TiO2-LDPE), have been tested by pulsed electro acoustic (PEA) method under different high voltage direct current (HVDC) electric fields. The TiO2-LDPE material is fabricated by doping nano-TiO2 into free additive LDPE with different weight proportions of 0.5 parts per hundred parts of resin (phr), 2 phr and 5 phr, respectively. Furthermore, the effects of LDPE/TiO2-LDPE interface on space charge behavior are discussed when the LDPE layer is both on the side of anode and cathode by reversing the double-layer dielectric. The results indicate that positive carriers accumulate at the interface of samples whose LDPE layer is located on the side of anode when electric field increases to certain value. However, negative carrier accumulation happens to the interface of samples whose LDPE layer is on the side of cathode. The higher the electric field, the larger the amount of interface charges accumulated. But there is no obvious difference of charge amount among systems with different nano-TiO2 concentrations. The formation of the interface space charge is considered as the result of conductivity difference between LDPE and TiO2-LDPE as well as influence of surface morphology. Moreover, it can be seen that the charge peak at cathode broadens when the LDPE layer is on the side of anode. At the same time, a small positive charge peak appears near cathode and the charge peak at anode broadens when the LDPE layer is on the side of cathode. The case is the most obvious under 5 phr. It is believed to be caused by induced charges of the interface charges. The induced charges have opposite polarity and distribute near the electrodes. The results provide an effective space charge controlling method, which makes it possible to locate space charge in the position required. It will be helpful for further optimization of HVDC cable.