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.
Highlights
To avoid temporary partial discharge (PD) events and to ensure stable void conditions during diagnostic PD-testing, IEC 60034-27 [1] and IEEE std 1434 [2] suggest a procedure for preconditioning of stator bar insulation
The cutoff frequency based on material parameters for new samples, given in Table 1, and equation (2) is less than 1 mHz
The large observed spread cannot be explained by a time lag and it makes no meaning to use the PD repetition rate n to describe the PD activity
Summary
To avoid temporary partial discharge (PD) events and to ensure stable void conditions during diagnostic PD-testing, IEC 60034-27 [1] and IEEE std 1434 [2] suggest a procedure for preconditioning of stator bar insulation. It is recommended to use at least 1-15 minutes of pre-conditioning at maximum test voltage. Recommended duration is, flexible, as it is stated that pre-conditioning up to 1 hour may be preferable. For example it is reported that in case of polycarbonate insulated samples stable PD conditions were reached after 1.5 hours of pre-conditioning [3]. The main purpose of this work is to experimentally investigate how PD parameters change with varying frequency AC test voltage after different durations of short-circuiting time after the one-hour pre-conditioning period. The observed phenomena are discussed with respect to a classical theory of interpretation of PD measurements
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