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

Abstract

Detection of partial discharges (PDs) is widely used as a condition assessment tool for high voltage equipment. Application of low frequency test voltage is often preferred in the case of test objects with a large capacitance. The question addressed here is how results from PD-measurements performed at low frequencies correspond to that occurring at 50 Hz power frequency. Different theoretical models for void voltage were examined and compared to experiments performed on laboratory samples of mica/epoxy, including embedded cylindrical voids. All test objects were preconditioned at 10 kV and 50 Hz for 5 min before partial discharge inception voltage (PDIV) testing by stepwise increasing the test voltage from 0 to 10 kV. The PDIV test was first completed at 300 Hz before being repeated at decreasing frequencies down to 0.1 Hz. The temperature was varied in the range of 20° to 155°C. The results at high frequencies showed that a pure capacitive model fits well to the measurements. Measured dielectric response in mica/epoxy explained the decreasing PDIV at low frequencies and high temperatures. A high PDIV was measured at a combination of low temperatures and low frequencies. This was correlated with a reduced void resistance of the electrically stressed void sidewalls caused by the PD activity during the preconditioning period. This indicates that the effect of PD by-products decays faster at higher temperatures. Values of PDIV are, therefore, expected to be dependent on both temperature and frequency.

Effects of Frequency and Temperature on Partial Discharge Characterization of Stator Windings

“…These observations accord well with the reported trend in PD magnitudes over time for stator windings in [14] that PD magnitude was lower in used bars compared to new windings. PDs produce acidic by-products when in contact with the surrounding gas and dielectric surface, resulting in a significantly increased surface conductivity [15], [16]. In time, aging caused by persistent PD activity is likely to erode the surface of the void and can result in further increased surface conductivity [15].…”

PD-Activity in Generator Stator Bar insulation versus Voltage Frequency and Temperature

“…Probably indicating initial effects caused by the preconditioning and test procedure itself. This was in [3] deduced to be correlated to the effect of preconditioning and PD by-products acting as charge carriers, hence increasing the void conductivity. The maximum test voltage was, however, too low to get a good comparison between measured values and estimated values from the model.…”

“…How the voltage across the void v varies with the applied voltage a and frequency is determined by the electrical model used to describe the test object. The relation between the applied voltage a and the void voltage c is then given by the following equations [3]: where the subscripts b and c represent the series insulation and void respectively, is the thickness, is the relative permittivity, is the conductivity, is the angular frequency, and * is the complex permittivity.…”

“…This is based on a standard test circuit, also found in for example IEC 60034-27. The test setup is the same as is presented in our previous publication [3].…”

Investigation of Factors Affecting Partial Discharges on Epoxy Resin: Simulation, Experiments, and Reference on Electrical Machines

“…The MATLAB/Simulink model (IEC 60270 Standard), which was created in order to simulate the PD, is shown in Figure 8. The circuit consists of [27][28][29][30][31]:…”