88 / 2018-08-24 13:20:43

Effect of Thermal Ageing on Space Charge Behavior of HVDC XLPE Materials

Thermal Ageing;,Space Charge,XLPE,HVDC transmission,Electrical conductivity

HVDC cross-linked polyethylene (XLPE) cables have become more and more attractive for a bulk power transmission over long distance and global energy connections. However, as the voltage level becomes higher, the space charge problem has been a main challenge for the HVDC XLPE cables in operation, especially for the cables at thermal ageing. The aim of the paper is to elucidate the influence of thermal ageing on the space charge characteristics, physical and chemical properties of DC XLPE materials.A commercial compound LE-4235 for 320 kV HVDC XLPE cable was used for sample preparations. ,. XLPE films were made by a heating press method in the laboratory. The compound pellets were heated in a special designed model for thermal press at 180 oC with pressing force of 200 kN. After the press process, the samples were placed in an oven and thermally aged at 130 oC. Space charges were measured by a pulse electro-acoustic (PEA) method setup developed in our high voltage laboratory, the polatization time was 30 minutes, wherea the depolarization time was 10 minutes. Fourier Transform Infrared (FT-IR) was used to measure the chemical properties before and after the ageing. Electrical conductivity of the materials was measured by a setup developed in the laboratory. Trap energy distributions were estimated to show the variations of shallow traps and deep traps during the ageing process.The results showed that the reference samples had a few hetero charge accumulations in the samples. This is probably due to the remained crosslinking byproducts causing the hetero charge to accumulate near the electrode. After ageing for several days, the distribution of space charge in the samples was controlled, and the electric field in the sample became relatively stable. With an increase of the ageing time, there were homo-charges accumulated near the electrode. Space charge decay properties were investigated to depict the space charge mobility when depolarization through the thermal ageing process. The results showed that the space charge decay became more and more difficult with the thermal ageing. The trap energy distributions were presented by the conductivity measurements, which indicated the variations of shallow traps and deep traps. The density of deep traps was enlarged, whereas the depth became deeper which made it easy to capture ions and caused the space charge accumulation. The results of the FT-IR showed variations of different functional groups during the thermal ageing process. It was found that the space charge characteristics of HVDC XLPE cable materials were relatively stable through the thermal ageing process. There were a few hetero space charges accumulating near the electrode due to byproducts remained in the materials. Homo-charges were accumulated in the samples after the thermal ageing. During the thermal ageing process, a turning point was found that from heterogeneous accumulation to homogeneous accumulation. It became more convenient for space charge injections after ageing. The deep traps in the samples expanded and became deeper. The trap energy enlarged to restrict the transport of space charge as the chemical bonds rupture during the thermal ageing process.