Partial discharge and breakdown of protrusion on conductor in GIS under oscillating lightning impulses

Gas insulated switchgear (GIS) is widely used in power system. With the increase of voltage level and substation scale, filed impulse voltage withstand test is increasingly important to ensure GIS safety as AC voltage withstand test becomes non-effective for the exposure of all defects. Standard lightning impulse (SLI) is normally adopted in factory, whereas oscillating lightning impulse (OLI) is more suitable for field use for its high generation efficiency. Thus discharge characteristics of typical defect in GIS under OLI need to be studied. Sharp protrusions on conductor are typical defects in GIS and are critical at lightning impulse voltages. This paper therefore aims to study discharge characteristics of protrusion on conductor under OLI. An impulse voltage generator was first established to generate four oscillating lightning impulses. A 12 mm long protrusion defect was then set up on conductor in a 330 kV GIS chamber. There was an ultra high frequency (UHF) sensor in the neighboring chamber to measure partial discharge (PD) and a voltage divider was used to measure applied voltage waveform. PD characteristics, 50% probability breakdown voltage (BDV50) and Voltage-time characteristics (V-t characteristics) were experimentally studied. The results show that breakdowns always occur in the vicinity of each wave peak under OLI. Obvious polarity effect exists that BDV50 is much lower for positive polarity. Wavefront has strong influence on the discharge characteristics. For positive polarity, short wavefront indicates that first streamer discharge will directly lead to breakdown. Pre-breakdown discharge is followed by breakdown when wavefront is longer, in which corona stabilization effect is observed. For negative polarity, corona stabilization effect is more obvious. Breakdowns first occur in the third oscillating period with 1.2 μs wavefront and there exists PDs in the first and second period. With 13 μs wavefront, breakdowns only occur in the first period. The results of the study are expected to provide important clues for the choosing of voltage waveform and analysis of test results in field test for GIS.