Influence of Metal oxide and Metalloid Nano Particles on the Dielectric Response of HVDC Cable Nano Dielectric

Influence of nano fillers like MgO, SiO₂ and clay on the dielectric response of HVDC cable dielectric is investigated. Surface treated metal oxide MgO, metalloid SiO₂nanoparticles and nano clay of different concentrations in low density polyethylene was considered for the study. LDPE-nano dielectric was prepared by using Brabender twin screw extruder and thin samples were obtained using hot plate vulcaniser. Electrical & dielectric properties were determined. The study indicated that, inclusion of nano fillers in the LDPE matrix resulted in an increase in contact angle and lower wetting energy of the composite. Surface potential decay was observed to be slow with composites indicating increased resistance to surface charge mobility. The relative permittivity and dielectric loss factor of nano dielectric increase with filler concentration and with decreased frequency. The polarization currents decrease exponentially with time and stabilize much lower than LDPE. The results are presented and discussed.

I. Introduction

Polyethylene nano dielectrics have assumed greater importance as electrical insulation in HVDC cables due to its superior electrical, thermal & mechanical properties [1]–[5]. A small quantity of surface modified inorganic nanometer scale fillers like, MgO, SiO₂, alumina, silica, layered silicate, ZnO etc. are added to conventional polymers such as polyethylene (LLDPE, LDPE, MDPE, and HDPE) to improve the properties of nano dielectric [4]. Surface modification of fillers is usually done by treating the nano fillers with functional silanes to have good distribution and dispersion of fillers in the polymer matrix and to prevent agglomeration [2], [3]. Evaluation of polymer nano dielectrics for high voltage application is usually made by determining the properties like permittivity, dielectric loss, electrical conductivity, space charge, dielectric breakdown, partial discharge resistance, treeing characteristics, etc. [4]. Further, measurement of contact angle helps one to understand the hydrophobicity of the material. It can quantitatively help one to determine the wettability of solid surface by liquid drop. Surface chemical heterogeneity and roughness are the key parameters that affect the wettability of a solid dielectric [6]. The surface potential decay process study is vital to understand the possible changes in trap distribution and the charge transport process brought about by filler inclusion in the polyethylene matrix [7]. The study of dielectric response of LDPE-nano composite in the frequency domain and polarization and depolarization current measurements in time domain is scanty which is very much necessary to understand its long-term performance and its reliability in HVDC extruded power cables. Preliminary studies on LDPE nano composites at low voltage (140 Vrms) carried out by the authors [8] indicated the influence of surface treatment on dielectric response. However, it is very necessary to understand the dielectric response at higher voltages (1400 Vrms) in order to ascertain its suitability for high voltage applications in addition to electrical properties like surface decay potential, contact angle measurement. Systematic laboratory investigations were carried to understand the influence of nano fillers on the dielectric response of LDPE nanocomposites. Dielectric response studies were carried out both in frequency and time domain. The results are presented and discussed.