Research On The Effects Of Voltage Stabilizer On The AC And DC Insulation Properties Of XLPE And Their Mechanisms

At present,the voltage grade and working stability of crosslinked polyethylene(XLPE)insulated HVAC and HVDC power cable still cannot meet the growing social need.Voltage stabilizer can improve the electrical strength and long-term stability of XLPE insulation material,and has played an important role in improving the voltage level of power cable.In recent years,the amount of patents and reports on voltage stabilizer from internationally renowned manufacturers for power cable and insulation material has gradually increased.The use of voltage stabilizer shows a good potential in further improving the working voltage grade of power cable.Therefore,further research on voltage stabilizer is helpful to promote the development and domestic manufacture of insulation material for power cable working at higher voltage.In order to select excellent voltage stabilizer and systematically investigate its effect on the AC and DC insulation performance of XLPE,a screening scheme combining theoretical chemical calculation and DC breakdown strength test was adopted to research 17 kinds of aromatic ketones.It was found that the DC breakdown strength increment caused by voltage stabilizer is significantly positively correlated with its electron affinity and is significantly negatively correlated with its HOMO-LUMO energy gap but is hardly corelated to its ionization potential.Three aromatic ketones which contain graftable vinyl group and can significantly increase the DC breakdown strength of LDPE were selected as graftable voltage stabilizers for further study.The influences of the three voltage stabilizers on the AC insulation performance of XLPE were systematically investigated.ALRB was found to possess the best effect among the three voltage stabilizers.At the addition content of 0.8phr,ALRB can significantly inhibit the initiation and growth of AC electrical tree and improve the AC breakdown strength of XLPE at different temperatures.Besides,its negative influnces on the relative dielectric constant and dielectric loss of XLPE are insignificant.After long-term AC electrical aging,the effect of ALRB improving the AC breakdown strength of XLPE is still continuable.Subsequently,the graftability of ALRB was characterized by infrared spectroscopy,the influnces of ALRB grafting on the basic physical and chemical properties of XLPE insulation,namely crystallization,crosslinking,mechanical property and thermal oxygen aging property were investigated.It was found that ALRB can be grafted onto XLPE molecules by addition reaction on its vinyl group in the thermal cross-linking process,and the graftable voltage stabilizer ALRB has good compliance to the existing power cable production process and little negative effect on the basic physical and chemical properties of XLPE.To graft voltage stabilizer ALRB onto XLPE is a modification method of high practical value.The graftable voltage stabilizer ALRB was experimentally proved to have excellent resistance to emigration,its effect of increasing the DC breakdown strength of XLPE is efficiently maintained after a long time.The influences of ALRB grafting on the DC breakdown strength,conductivity and space charge characteristic of XLPE at different temperatures were systematically investigated.Meanwhile,the DC insulation performance of commercial ±500kV HVDC cable material(±500kV XLPE)was also systematically tested as a reference.The results indicate that ALRB grafting can significantly improve the DC breakdown strength of XLPE at different temperatures and the DC breakdown property of ALRB grafted XLPE is significantly superior to ±500kV XLPE.The mechanism is that ALRB possesses both a high electron affinity and a narrow energy gap,which endow ALRB molecule the ability to capture high-energy electron and dissipate its energy through excitation.ALRB grafting slightly increases the temperature dependence of XLPE conductivity,but its negative influence on the electric field distribution of DC cable insulation layer is insignificant.ALRB grafting can affect the trap distribution of XLPE by reducing the deep trap density and increasing the shallow trap density and then result in a more uniform space charge distribution,less charge accumulation and faster dissipation.With ±500kV XLPE as a reference,the effects of the additive content,voltage and temperature on the DC grounded tree initiation and growth of XLPE before and after ALRB grafting modification were studied.The results indicate that the increase of voltage and temperature can both accelerate the initiation and growth of DC grounded tree,the inhibition effect of ±500kV XLPE on the DC grounded tree initiation and growth is better than that of ordinary XLPE.0.4~0.8phr ALRB grafting can significantly inhibit the initiation and growth of DC grounded tree in XLPE,and the effect increases with additive content.At the content of 0.4phr,the inhibition effect of ALRB on DC grounded tree is basically equal to that of ±500kV XLPE.Although the charge injection and migration are not inhibited by the grafting of ALRB voltage stabilizer,the initiation and growth of DC grounded tree can still be suppressed by ALRB through the weakening of the damage caused by space charge release process.