Time Development of Voltage Frequency Dependence of Partial Discharge Activity in Voids

Abstract

Condition assessment of high voltage equipment based on partial discharge measurements is often performed after a voltage pre-conditioning period. The aim of this paper is to present results from experimental examinations of time variance of partial discharge activity and to propose physical explanations of the phenomena observed. Experiments were performed on laboratory made 3 mm thick discs of generator bar insulation, consisting of mica and glass fiber reinforced epoxy with a 0.5 mm thick cylindrical void surfaces of 10 mm in diameter. The effect of conducting and insulating void surfaces was examined using copper tape as upper and lower electrodes of the voids. All objects were tested by 12 one-minute long AC voltage frequency sweeps at frequencies from 50 Hz to 0.1 Hz distributed in time from start of the experiment, after initial one-hour constant 50 Hz voltage application and during object short-circuiting for 20 h. The main result shows that in case of insulating voids the apparent charges vanished after the one-hour constant 50 Hz voltage application. After a grounding period of 5 minutes, the charge magnitudes slowly increased with time until reaching steady state after about 4-8 hours. Test objects with conductive void surfaces showed such reduction in case of PD testing at 0.1 Hz only. At voltage frequencies above 10 Hz the measured PD magnitudes were found to be nearly constant, close to the expected high theoretical value. This indicate that PD by-products strongly affected the void resistivity and thereby affect the PD activity of insulating voids. These by-products are temporary and disappear with time.

Partial discharge inception voltage of voids enclosed in epoxy/mica versus voltage frequency and temperature

“…The total apparent charge during the 5 s conditioning between the different frequencies generally increased with time. This indicated that the preconditioning effect faded with time, similarly to what was seen in [16]. This means that the effect of preconditioning was weaker at lower frequencies than at higher frequencies due to the frequency test order.…”

“…The effect of such preconditioning disappears with time [16]. Generally, at elevated temperatures, the PD byproducts diffuse or are neutralised faster than they are produced.…”

“…The conditioning period at 50 Hz between different frequencies was reduced to 5 s to not influence the void condition but, rather, act as a reference if PDs at 50 Hz changed during the frequency sweep. Both the maximum and total apparent charge during this 5 s period were recorded and used as they can be related to the void voltage [16].…”