| The semiconductive layer is an important part of the high voltage cable.It is located between the metal core and the insulating layer,which can make them closely connect,eliminate the air gap,and prevent the breakdown of the insulating layer caused by air gap discharge.At the same time,it can uniform the electric field,suppress the electric field distortion at the interface,and ensure the safe operation of the cable.The improvement on the performance of the semiconductive layer plays an important role in the power transmission of a higher voltage.In this article,the electrical properties of the semiconductive layer are improved.The first is to suppress the positive temperature coefficient(PTC)effect,which will affect the performance of semiconductive layer at high temperature.The second is to improve the ability of the semiconductive layer to inhibit the injection of space charge into the insulating layer,which helps to reduce the accumulation of space charge.The accumulation of space charge is one of the factors leading to the failure of insulating layer.In order to achieve the above two purposes,the following research was conducted:(1)Two kinds of carbon black with similar resistivities but different particle sizes were selected to study the influence of the particle size on the resistivity threshold and the PTC effect of the semiconductive layer.The original idea of this study was to make a semiconductive layer with a lower resistivity threshold.After reaching the resistivity threshold,the carbon black was continuously added,so that the PTC effect of the semiconductive layer can be weakened without increasing the original content of carbon black.However,it was discovered that the semiconductive layer made by the carbon black with a small particle size not only has a lower resistivity threshold,but also a stronger conductive network.Therefore,the PTC effect of the semiconductive layer containing 20% carbon black with small particle size was weaker than that of the semiconductive layer containing 30% carbon black with large particle size.(2)Adding conductive polyaniline to improve the performance of the semiconductive layer.On the one hand,the conductive polyaniline with long-range conductivity can enhance the conductive network of semiconductive layer and reduce the PTC effect when the semiconductive layer expands at high temperature.On the other hand,polyaniline with a unique conductive mechanism and high dielectric constant can change the electron migration path in the semiconductive layer,and weaken the effect of the external electric field on the interface barrier,thereby improving the ability of the semiconductive layer to inhibit the injection of space charge into the insulating layer.In order to maximize the role of conductive polyaniline,the static freezing method was firstly used to make carbon black and polyaniline into a strip-shaped conductive composite,and then the formula for semiconductive layer was improved.By doing these,the semiconductive layers with excellent performance were obtained.Resistivity test results show that when the content of polyaniline is 5%,the PTC effect of the semiconductive layer is the weakest.Compared with the semiconductive layer without polyaniline,its highest resistivity is reduced by 57%,and the PTC strength is reduced by 23%.The space charge test results show that when the amount of polyaniline is 3%,the semiconductive layer has the best ability to inhibit space charge.Under the electric field of-10 k V/mm and-30 k V/mm,the amount of space charge in the insulating layer decreases by 39% and 42% respectively,and the migration of space charge to the inside of the insulating layer can be partially suppressed.In this study,the conductive polymer with unique physical and chemical properties and conductive mechanisms was introduced for the first time,which provides a new way for the improvement of semiconductive layer.(3)In the thermally stimulated depolarization current(TSDC)test,the current peak reversal phenomenon appeared,which seriously affected the accuracy of the TSDC.In this paper,the changes of the electron injection and trapping process under low and high electric field are analyzed,and the impact of these changes on the detrapping and migration of electrons during the measurement process is explained.The reason for the inversion peak was found.By coating the surface of the insulating layer with silicone oil,the direction of electron migration in the TSDC measurement process is changed,the inversion peak is eliminated.The accuracy of TSDC is improved,which helps to evaluate the ability of the semiconductive layer to suppress space charge.In addition,the study of electron injection and detrapping process in this part also has an important guiding role in improving the ability of the semiconducting layer in the suppression of space charge. |
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