Electro-thermal breakdown of hvdc cables and effect of long-term ageing on space charge in cable dielectrics

Abstract

With the advent of High Voltage Direct Current (HVDC) technology, underground power cable transmission, with polymeric insulation, owing to its several inherent benefits, is eclipsing over the conventional mass impregnated and oil-impregnated paper insulation. Polymeric HVDC cables have excellent electrical and thermal properties, especially in design stress, which is comparatively high, compared to mass and oil impregnated insulation. In general, cable performance has assumed paramount importance in power transmission. The performance of power cables is linked to the electric and thermal profiles of the dielectric. Standard recommendations, such as CIGRÉ TB-496 and IEC-62067, provide guidelines for manufacturers as well as utilities for the testing of cables. However, for high voltages (more than ~500 kV) extruded cable technology is still limited mostly to laboratory or developmental experience. Therefore, it is important to have viable simulation tools, not only for design and development but also for the condition assessment of the cables. This work presents the simultaneous simulation of interdependent electric and thermal fields inside a dc cable, under loaded conditions using analogous distributed circuit models for thermal and electrical phenomena. The complex nonlinearities are integrated with such circuit models, leading to the simulation of electro-thermal runaway, for the first time, in a dc cable. Using the suggested, nonlinear circuit models, complete dynamics of the temperature profile and electric field inside the cable dielectric can also be obtained under various test and operating conditions until electro-thermal runaway limits in the time domain. The results are useful for the design and development of dc cables with safety factors, apart from understanding the electro-thermal limits. The models are validated with rigorous and careful experiments. In general, when the polymeric cable operates under continuous load and sometimes under overloaded conditions, it is subjected to high thermal stress, which causes polymer degradation. Ageing is a slow process, causing the weakening of insulation. In HVDC cables, space charge accumulation inside the bulk of cable insulation is a major issue which has been deemed critical to their breakdown and thus is a topic of great interest in the HVDC cable industry. However, space charge is not that severe issue in the case of High Voltage Alternative Current (HVAC) cables due to periodic polarity changes. Space charge is known to accumulate even in a fresh specimen, however, particularly in an aged polymeric material, due to the degradation of the material, space charge is believed to be extremely significant. The presence of space charge in the insulation bulk affects the electric field, making its distribution non-uniform across the insulation, which may cause a premature breakdown in the insulation. So, in this work, the author intends to investigate the effect of long-term thermal ageing on the accumulation of space charge and electric field dynamics in a polymeric material (LDPE- Low-Density Polyethene), which is a widely used base material for polymeric cable insulation). To achieve thermal ageing, the prepared LDPE sheet samples were placed in a hot air oven for long durations. Thermal ageing has been carried out under different ageing temperatures and durations. The space charge was measured after certain ageing periods using the PEA (Pulsed Electro-Acoustic) method. The experiments were done for low and high electric fields. All experiments were done at ambient temperature. The corresponding electric field enhancement has been investigated. The space charge density and electric field enhancement factors have been estimated. Also, in highly aged samples (256 days), the space charge is measured in the vicinity of breakdown. A clear difference in the trend of packet-like charge movement is observed in the highly aged samples when compared to the packet charge of fresh samples reported until now. Interesting insights on the injection and movement of charges through the bulk, mean accumulated charge density, field enhancement factor, as well as the shift of space charge regimes with ageing, are presented, which were apparently not given adequate attention in the literature so far. The results are believed to be useful for HVDC insulation designers and utilities.