Research On Effect Of DAC Frequencies On Partial Discharge Characteristics Of Defects In XLPE Cables

Damped AC(DAC)detection technology is an important test method for cross-linked Polyethylene(XLPE)power cables commissioning test and preventive test.Due to the large capacitance of the cable tested,the DAC frequency applied in the field is generally low.However,the DAC frequency used in current laboratory research is higher.The effectiveness of the low-frequency DAC PD test and its equivalence with the power frequency PD test are not clear.Therefore,it is of great engineering significance to study the effect of low-frequency DAC from 20 Hz to 200 Hz on the partial discharge characteristics of common defects in XLPE cables.In this thesis,the DAC detection test platform for partial discharge characteristics of cables is built up.Three kinds of cable defect models,namely high voltage conducting spike,outer shield damage and suspension potential are designed and fabricated.Through simulation analysis,it is found that parallelling compensation capacitor will narrow the detection frequency band.Through experimental research,it is found that parallelling compensation capacitor will reduce the number of partial discharge pulses,but has little effect on the discharge quantity.Based on the experiment and simulation,an optimization method for the partial discharge test of the long-distance cable in the laboratory is proposed.Secondly,through the controlling inductance and capacitance in the circuit,the effect of the DAC frequency on the PD characteristics of the cable defects is studied,and the relationship between the DAC frequency and the PD characteristics is obtained.The test results show that within the DAC frequency range from 20 Hz to 200 Hz,in the same duration,the higher the DAC frequency,the larger the maximum apparent discharge quangtity,and the more the number of partial discharge pulses and the wider the phase distribution.The effect on the PD characteristics of doubling the DAC frequency is equivalent to increasing voltage amplitude of 1-2 kV.It is believed through theoretical analysis that the voltage frequency affects the accumulation of space charge in the extremely uneven electric field and affects the characteristics of discharge.Finally,the 50 Hz AC detection test platform for partial discharge characteristics of cables is built up,and the characteristics of cable defects under power frequency AC are studied.By comparing and analyzing,the equivalence between partial discharge test results of DAC and power frequency AC is evaluated.In the initial stage of the discharge,when the DAC frequency is higher than 120 Hz.the PDIV is consistent with that under the power frequency AC voltage.The PDIV under the DAC voltage of 28 Hz is slightly higher than that of the power frequency voltage,but both discharge quantities of PDIV are similar.In the stable stage of the discharge,the DAC in the frequency range of 80 Hz-150 Hz has the optimal equivalence of the partial discharge characteristics with the power frequency AC voltage.