Electrical Tree Initiation And Growth Properties In HVDC Cables

As HVDC transmission techonology is being developed,cross-linked polyethylene(XLPE)DC cable has been used widely,owing to its' superior electrical and mechanical properties.However,some adverse operation conditions may decrease the initiation voltage and accelerate the growth of DC electrical trees in the insulation,such as terminal flashover fault,high frequency(HF)harmonic interference,transient overvoltage shock and so on.In this paper,we investigated the initiation properties of DC grounded electrical tree,DC superimposed with harmonic and impulse voltage.Meanwhile,we also studied the growth features of DC electrical tree thoroughly.The main research results are as follows.We discovered the rules of initial properties with DC pre-stressed time and amplitude.The results indicated that the length of electrical tree increased and reached saturation gradually with DC pre-stressed time enlarging,especially the above phenomenon was more notable under positive voltage condition.Moreover,the length increased distinctly with DC pre-stressed amplitude rising.The initiation characters with different DC pre-stressed time and amplitude were closely related to the injection depth and range of space charges.In order to study the initiation properties of DC superimposed with harmonic voltage(50Hz,1kHz,2kHz)and DC-impulse(lightning impulse(LI),switching impulse(SI))composite voltages,the testing system of electrical tree was established.It was found that harmonic voltage component could reduce initial voltage of DC electrical tree,thus being decreased with the harmonic frequency increasing.There was a polarity effect on initiation characters under DC-impulse composite voltages.It should be pointed out that with DC voltage increasing,the initiation impulse voltage decreased for samples of the same polarity;however,for those samples of the opposite polarity,there were no obvious changes when the amplitude of DC pre-stress was smaller than impulse voltage,and the initiation impulse voltage increased if DC voltage was larger.Besides,the electrical tree initiation voltages under positive impulse voltage were smaller than those under negative impulse voltage.The growth characters and partial discharge properties of electrical tree under positive and negative DC voltage were elaborated.The experimental results indicated that the electrical tree would grow rapidly during boosting process and a short time when DC voltage just reached a constant value;however,the growth length was barely changed during the middle and later periods of constant DC pre-stressed stage.The growth speed under positive voltage was higher obviously and with the DC pre-stressed time increasing,a stagnation phenomenon of electrical tree might occur under negative DC voltage condition,even electrical tree channel would disappear.Based on the growth characteristics and electrical tree channel discharge properties,stream theory was proposed to explain the growth mechanism of DC electrical tree.