Study On The Space Charge Injection In Ldpe By Surface Modification Of Electrode

Development of electric power industry,high voltage direct current(HVDC)has attracted much more attention recently.For long-distance transmission,HVDC cables have large transmission capacity and suffer lower electrical losses.Under an electrical field,space charge is easy to occur within the polymer insulating layer.This existing space charge can locally distort the electrical field and is an important factor in determining electrical ageing and breakdown.It becomes a research focus to reduce the space charge in HVDC cable.There are mainly two ways to suppress the space charge.Firstly,space charges with the same polarity can be reduced by modifying the surface of electrode.Secondly,the nature of applied materials can be altered to suppress the space charges with different polarities.Space charge can be depressed if the semi-conductive shielding layer and insulating layer were modified.Surface modification of the electrode is also a method of suppressing space charge.This method is scarcely used and found in literatures.Different types and nature of electrode materials can cause difference in the injection of space charge and charge polarity.In this paper,surface modified copper used as electrode were applied to emit electrons.Space charge injection in low density polyethylene(LDPE)was measured by Pulsed Electro-Acoustic(PEA).A magnetron sputtering method was utilized to coat Mo and Cr on a piece of copper.Surface topography of the molybdenum coating on the copper was analyzed by scanning electron microscopy(SEM).The results show that Cr and Mo particles has deposited on the copper.And the Mo film is denser than Cr film.The coated electrode,semi-conductive layer and LDPE layer were combined to form a composite structure.Distribution of space charge in LDPE was measured through PEA.Because electronegativity value of Cr is smaller than Cu,there is no significant increase in work function.Cr-coated electrode has little effect on the inhibition of space charge.However,electronegativity value of Mo is larger than Cu,the Mo coated electrode can contribute a 0.3-0.4 eV increment to the work function of the copper electrodes.It becomes more difficult for electrons to cross the potential barrier and inject into the LDPE layer.That is,the space charge injection is suppressed.Electrodes,made of copper were sputtered at different parameters(voltage and time)and the charge distributions in LDPE sample were tested by PEA.Results show that,with the increase of the deposition voltage,the amount of injected space charge decreased,when the coating voltage is 440 V,few space charge injected into LDPE sample.Besides,with the increase of coating time can also inhibit the injection of space charge in LDPE.When the PEA test was completed,electrons and holes still exist in the deep trap within LDPE sample.The amount of trapped charge and peak current were analyzed by Thermally Stimulated Current(TSC).Results show that with the increment of the coating voltage,the amount of trapped charge and peak current decreases dramatically.Continued coating Mo for a longer time,the injection of space charge would has a drastic reduction.