Role Of Applied Voltage Waveforms On Partial Discharge Patterns Of Electrical Treeing In Low Density Polyethylene

摘要

High voltage apparatuses play an important role in transmitting a huge amount of electrical energy. In order to keep the apparatuses in normal condition, the insulation system should be maintained. Solid as well liquid insulating materials are widely used in high voltage apparatuses. During operation of the apparatuses, the insulation may experience ageing. Appearance of electrical discharges in high voltage apparatuses with reasonable magnitude and frequency can be interpreted as abnormal condition. In order to know the origin of the discharges it is necessary to investigate the characteristics of the discharge. This study reports the experimental results on the characteristics of Partial Discharges (PD) in Low Density Polyethylene (LDPE) under sinusoidal, triangular and rectangular voltages. Electrode used in the experiment was steel needle-plane arrangement with tip radius of curvature of 3 μm. This sharp needle allow to produce a very high electric field of several MV m-1 at the needle tip. The PD in LDPE was generated around the tip of needle electrode when an electrical tree appeared. The PD pulses were measured using a personal computer-based measurement system. Discharges occurred in the sample were detected with an RC detector with a lower cut off frequency of 250 kHz. The output of the detector was measured by PD analyzer and digitized. Using the measuring system the discharge magnitude, frequency as well as the phase of discharge occurrence in the applied voltage cycles is measured and the digital data are stored in the PC. The analysis was focused on the Φ-q-n pattern and the time sequence between consecutive pulses. Here, Φ is the phase angle of PD occurrence, q is the PD magnitude and n is the number of PD pulses. The experimental results showed that the applied voltage greatly affected the PD in the LDPE. Under sinusoidal applied voltage, the PD pulses due to electrical tree in LDPE distributed around the zero cross. The occurrence is strongly related to the time derivative of the applied voltage. This was clarified under triangular applied voltage where PD pulses distributed almost uniformly as can bee seen in PD pulse sequences. Φ-q-n pattern under rectangular applied voltage made more clearly where PD pulses only took place around the zero cross where the value of dv/dv was high. The PD magnitude of the electrical tree reflected the instantaneous applied voltage. This was clarified under triangular applied voltage where triangular patterns were obtained. The experimental results under rectangular voltage strongly supported the findings under sinusoidal and triangular applied voltage. Based on the experimental results equivalent circuits for electrical treeing PD in LDPE is proposed.