110 / 2018-08-24 19:22:04

Space Charge and Breakdown Strength Behavior of PP/POE/MgO Nanocomposites

Polypropylene, Magnesium Oxide, Space Charge, Breakdown Strength, Tensile Strength

Abstract- With the development of long-distance transmission technology, insulation material properties for cables have attracted much attention. In this study, Polypropylene (PP) is considered as a potential insulation cable material to replace conventional cross-linked polyethylene (XLPE), as it withstands higher operating temperatures and has comparable electrical properties, as well as meeting the requirement as an environment-friendly material, since PP does not require crosslinking.

In order to improve the relatively poor mechanical properties of PP in comparison with conventional cable insulation material like XLPE, polypropylene and polyolefin elastomer (PP/POE) blends were prepared by melt blending. 10 wt% and 20 wt% POE were introduced into PP respectively, to investigate the mechanical property using a tensile test. The POE used is poly(propylene-co-ethylene) (PE02) which shows a higher stiffness and brittleness than pure PP at room temperature. In HV DC applications, space charge accumulation is the main issue that restricts the development of polymer as the cable insulation material. Any significant accumulation of such charge may accelerate aging and shorten the service life of the insulating materials, especially during polarity reversal. From recent research, nano-doping has been shown to be a potential way to suppress the accumulation of space charge, as well as to enhance the breakdown strength. In this work, nano-MgO composites blended with PP/POE blends were prepared to investigate the space charge and AC/DC breakdown strength behavior. Two mass concentrations of nano-MgO composites, 5 wt% and 10 wt%, were prepared with and without surface treatment. In this study, Triethoxy (octyl) silane (C8-E) was chosen as the silane coupling agent for better interaction with functional groups on the nano-MgO particle surface with the polymer. Either functionalized or untreated nano-MgO were dispersed in isopropyl alcohol (IPA) and then added into PP/POE solution, vacuum dried for 3 days and pressed 5 times.

PP/POE blends show a significant improvement of the tensile strength compare to pure PP when analyzing the stress-strain curves and Young’s modulus. The mechanical strength greatly increases from blending with 10 wt% PE02 to 20 wt% PE02. As the content of nano-MgO increases from 5 wt% to 10 wt%, a negative impact on the mechanical properties of PP/POE blends can be observed, which is partly recovered by the blending with POE.

Inclusion of PE02 and nano-MgO either with surface treatment and untreated slightly decrease the AC breakdown strength compared to the pure PP reference. The minimum AC breakdown strength is PP with 20 wt% PE02 and 10 wt% untreated nano-MgO and AC breakdown strength is 159.9 kV/mm which is 14.1% lower than pure PP of 181.6 kV/mm. However, the systems are more reliable for fillgrades of nano-MgO of 5%, as evident from the stronger breakdown strength exhibited in lower Weibull failure probabilities. Compared with the pure PP, DC breakdown strength drops considerably after inducing PE02 and nano-MgO, except for samples with 10wt% PE02 and 5wt% treated-MgO, which increase the DC breakdown strength from 361.4 kV/mm to 384.4 kV/mm. A narrowing of the Weibull distribution in both AC and DC breakdown strength could be observed after adding nano-MgO. This could be the result of an improved interface region in terms of compatibility between the polymer and ceramic nano-MgO filler, which is also evident from the uniform dispersion of nanoparticles. Both AC and DC breakdown strength decrease in PP blends with PE02 from 0wt% to 20wt%. The increased breakdown strength is attributed to the surface treatment of nano-MgO. It is suggested to be a result of fewer agglomerations of nanoparticles, i.e. a better dispersion of particles.

Compared with the pure PP reference, the space charge accumulation near the cathode is efficiently suppressed by the addition of a low amount of nano-MgO when applying a 20 kV/mm electric field. One possible reason for that is when adding a polar nano-MgO filler into non-polar base material PP, an increasing number of shallow traps could help dissipate charge more efficiently. In addition, the introduction of MgO nanoparticles promotes the generation of more charge carriers in PP. The space charge suppression gets worse when increase the nano concentration from 5% to 10 %, a significant accumulation of space charge can be observed around the cathode. That is caused by the high concentration of nanoparticles and subsequent abundance of additional trapping sites, which may lead to the observed deterioration of electrical properties. As the increasing concentration of POE shows, POE excess can also have a negative impact on the space charge suppression, which is likely due to the different dielectric properties of PP and PE02 and a resulting mismatch. The nanoparticles with surface treatment show an improvement of the inhibition on space charge accumulation for PP/POE blends compared to specimen with nano-MgO in PP.

It has been shown in experiments that the PP with 10 wt% PE02 and 5 wt% of surface treated-MgO has higher breakdown strength, while the introduction of POE and nano-MgO can significantly improve the mechanical flexibility. In combination with the observed suppression of space charge accumulation, this shows that PP/POE/MgO nanocomposites are a viable option for HVDC cable insulation material in the future.