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Measured Partial Discharge Inception Voltage for a Cavity at Different Applied Frequencies Cecilia Forssén and Hans Edin KTH Electrical Engineering Abstract The present paper studies experimentally how the Partial discharges (PD) can be measured at variable frequency of the applied voltage influences the PDIV. frequency of the applied voltage. The interpretation of The PDIV is measured with applied frequency in the such measurements requires a physical understanding of range 0.1 to 100 Hz for disc-shaped cavities insulated in the observed PD frequency dependence. This paper polycarbonate. Measurements are done with different studies the influence of the applied frequency on the pre-excitation of the specimen to study how this affects partial discharge inception voltage (PDIV). The PDIV the PDIV. It is also briefly investigated how the cavity is measured for insulated disc-shaped cavities at applied ages due to PD, with the purpose of avoiding aging frequency in the range 0.1 to 100 Hz. It is found that the effects in the PDIV measurements. PDIV increases with increasing applied frequency, probably as an effect of the statistical time lag. In 2. Experimental methods addition, the measured PDIV is lower than that predicted by Paschens’s law. The effect of pre- 2.1. Test object excitation on PDIV is also studied. A period without The specimen used for this study is a disc-shaped cavity voltage supply prior to each PDIV measurement is seen embedded in polycarbonate (relative permittivity 3), see to cause a faster increase in the PDIV with frequency. Figure 1. It is made by pressing together three 1 mm thick polycarbonate plates with a drilled hole centered in the middle plate. The cavity has a diameter of 10 mm 1. Introduction and a thickness of 1 mm. The polycarbonate plates come from the supplier with a protection plastic that is Partial discharges (PD) are usually measured at a single removed just before the measurements. The plates are frequency of the applied voltage, in general 50/60 Hz. inspected for irregularities in the drilled hole. The plates There are however reasons for measuring PD also at are not cleaned. other applied frequencies. A lower applied frequency (commonly used is 0.1 Hz) reduces the power and size needed for the voltage supply equipment. Another option is to measure PD at variable applied frequency (i.e. at more than one frequency) [1]. This method provides more information than measurements at a single frequency since the variation in applied frequency changes the local state at defects in the insulation. These changes can be used to better characterize the defects, provided that the frequency dependence of PD is interpreted physically. Fig. 1 – Schematic picture of specimen (rotational symmetry, One important property of PD is the partial discharge measures given in millimeter). inception voltage (PDIV). This is the lowest applied voltage where discharges appear. Another property is 2.2. Measurement system the statistical time lag. This is the time from that the The phase resolved PD measurement system is electric field is above the critical level for discharge described in [1] and is based on an ICM (Insulation until the discharge occurs. The statistical time lag is Condition Monitoring) system [3]. caused by a lack of free electrons to start a discharge. If For the PDIV measurements a function generator is the applied voltage changes significantly over a period used to generate a sinusoidal signal with amplitude that comparable to the statistical time lag, this influences increases linearly from zero at ramp rate 0.1 kV/s. The measurements of the PDIV. Therefore these first discharge is detected with an oscilloscope (LeCroy measurements are sensitive to the rate of change of the 9310AM, 400MHz, 100MS/s). applied voltage and to changes in the statistical time lag. Experiments [2] show that light irradiation affects PDIV measurements in cavities, presumably since the electron 2.3. Aging tests generation rate in the cavity changes with the irradiation The purpose of the aging tests is to find a time interval conditions. where the PD activity in the cavity only changes slightly due to aging phenomena. It is then assumed that PDIV measurements within this time interval are not so much affected by aging. Aging tests are done on two equal specimens. A sinusoidal voltage with amplitude 10 kV and frequency 50 Hz is applied for 7 h with interruptions (about 40 s long) for phase resolved PD measurements at 50 Hz. 2.4. PDIV measurements Partial discharge inception voltage measurements are done on two equal specimens. Prior to the PDIV measurements the specimens are conditioned (aged) for 1.5 h at 10 kV and 50 Hz. The PDIV is measured with an applied sinusoidal voltage with frequency in the range 0.1 to 100 Hz and with amplitude linearly increasing from zero at a ramp rate of 0.1 kV/s. The PDIV is defined as the amplitude of the applied voltage when the first discharge is detected. It is calculated as the product of the ramp rate and the time to the first discharge. In addition to the influence of the frequency on the PDIV, also the effect of pre-excitation of the specimen is investigated. With “pre-excitation” is meant voltage applied to the specimen prior to the PDIV measurement. Three cases are studied: 1. Pre-excitation at variable frequency The PDIV is measured ten times at each frequency. Prior to the first measurement at each new frequency, a voltage with that new frequency and with amplitude 10 kV is applied for 30 min. 2. No pre-excitation The specimen is left without voltage supply for 14 h. Then PDIV is measured ten times at each frequency. Prior to the first measurement at each new frequency, the specimen is left without voltage supply for 30 min. 3. No pre-excitation (10 min pause) Same as 2 but the specimen is left without voltage supply for 10 min between each measurement and only five measurements are done at each frequency. The effect of ramp rate on the PDIV measurements is also investigated by varying the ramp rate in the range 0.1 to 0.4 kV/s. Ten PDIV measurements at 100 Hz are done at each ramp rate. Since this is the highest frequency used in this study, the statistical time lag should have the largest impact on the PDIV measurements here. Prior to the first measurement at each new ramp rate, a voltage with frequency 50 Hz and amplitude 10 kV is applied for 30 min. 3. Results and discussion 3.1. Aging The aging tests show similar results for the two equal Fig. 2 – PD pattern before aging (a) and after 1.5 h (b), 4 h (c) specimens. Figure 2 shows the PD pattern before aging and 7 h (d) aging. Mind the differences in y-axis scaling! and after 1.5, 4 and 7 h aging, respectively. The PD pattern changes greatly during the first 5 – 30 min of aging; the maximum apparent charge roughly halves. After 1.5 h and up to 7 h aging there are no distinct changes in the PD pattern and the maximum apparent charge is about constant. The change in PD pattern with aging is probably due to decreasing statistical time lag for discharges and increasing conductivity of the cavity surface, as discussed in [4]. This causes lower apparent charge since discharges occur at weaker field in the cavity. In the present study, an aged specimen that is left without voltage supply for some time (30 min or longer) is seen to resume its PD pattern from the early stages of aging (1 to 5 min aging). If voltage is applied for another 30 min (after the interruption), the PD pattern gets the same shape as after the first 90 min of aging. This is somewhat similar to observations in [5] where Fig. 3 – Average PDIV at 100 Hz measured with different an aging test (on a spherical cavity in epoxy) is seen to ramp rates. Error bars show 90 % confidence intervals. give anomalous PD patterns during the first minutes of voltage supply after a period with voltage removed. Figure 4 shows PDIV measured at different applied As conclusion from the aging tests, measurements done frequencies, and with different pre-excitation (as after 1.5 h (and up to 7 h) are not significantly affected described in Section 2.4). The measured PDIV is clearly by aging. Therefore all specimens are conditioned for frequency dependent in the cases of pre-excitation at 1.5 h prior to the PDIV measurements. In addition, variable frequency (a), and no pre-excitation (b). The leaving a specimen without voltage supply changes the PDIV increases as the frequency goes up. The PD activity, at least temporarily. As far as possible this explanation is probably a larger influence of the should be avoided, if not intended. statistical time lag at higher frequencies, i.e. at shorter period times. As the applied voltage amplitude has 3.2. PDIV exceeded the critical level for discharge, the time spent The PDIV measurements show similar results for two above this level during each voltage cycle is shorter at equal specimens. There is a considerable scatter in some high frequency than at low frequency. Therefore PDs of the PDIV measurement results. Certainly part of this may come later at high frequency resulting in a higher scatter is due to measurement errors but the most is PDIV. probably caused by the statistical nature of PD. The case of no pre-excitation and 10 min pause between Therefore, throughout this paper, the results of PDIV each measurement (c) is somewhat different from the measurements are shown as average values with error other two cases. The PDIV at the lowest frequency (0.1 bars indicating 90 % confidence intervals (normal Hz) is similar to the other cases but the increase in distribution is assumed). PDIV with frequency is faster. The explanation may be Figure 3 shows the effect of the ramp rate on the that the statistical time lag is longer in case (c) than in measured PDIV. No drastic influence of the ramp rate is (a) and (b), since there has been a pause of 10 min with seen for the ramp rates tested here. There is a tendency no applied voltage prior to each measurement. A longer to a decrease in PDIV at the lowest ramp rate (0.1 kV/s) statistical time lag makes the effect of delay of although it is not pronounced. The data is also less discharges visible at lower frequencies. scattered at lower ramp rate. Both these observations In [7] PDIV for an insulated disc-shaped cavity in are expected since at lower ramp rate there is less polyethylene (placed in a dark box) is measured at influence from the statistical time lag. They are also in different applied frequencies. If compared to this study, accordance with results from [2] where a systematic the most similar case is (c). The results in [7] however decrease in measured PDIV was seen as the ramp rate show no clear frequency behavior of the PDIV. was reduced. It was not possible to lower the ramp rate The scatter in data is smallest in case (a), larger in (b) below 0.1 kV/s in this study, otherwise that would have and largest in (c). This probably comes since the been instructive. statistical time lag is shorter in the case of pre-excitation If Paschen’s law (for gaseous breakdown between plane (a) than in the other two cases, and since measurements metal electrodes) is used to estimate the PDIV of the were only repeated five times in (c). Moreover there is a current specimen, a value of 7.7 kV is obtained. As seen large scatter in the 100 Hz measurements for both (b) from Figure 3, the measured PDIV is lower than this for and (c). This is caused by a longer statistical time lag all ramp rates. This is in contradiction to [6] where the here since these are the first measurements done after measured PDIV of disc-shaped cavities was seen to the 14 h without voltage supply. obey Paschen’s law. The disagreement may be due to Also Figure 4 shows that the measured PDIV is lower differences in the measurement methods but no detailed (up to 40 %) than that predicted by Paschens’law. The description of this is given in [6]. difference is largest for the cases with smallest 4. Conclusions influence of the statistical time lag. Partial discharge inception voltage (PDIV) is measured in disc-shaped cavities at different frequencies of the applied voltage. The measured PDIV is lower than that predicted by Paschen’s law. The PDIV is seen to increase with increasing frequency. This may be explained by a larger influence from the statistical time lag at higher frequencies, causing discharges to be delayed and occur at higher applied voltages. The effect of pre-excitation (voltage applied prior to PDIV measurements) on the PDIV is also investigated. If the specimen is left without voltage supply for 10 min prior to each PDIV measurement, the PDIV is seen to increase faster with frequency than if the PDIV measurements are done without pauses. A probable explanation is that leaving the specimen without voltage supply increases the statistical time lag. This causes the effect of delay of discharges to appear at lower frequencies. 5. Acknowledgements This study was sponsored by the Centre of Competence in Electric Power Engineering (EKC2), Sweden. 6. References [1] H. Edin, Partial Discharges Studied with Variable Frequency of the Applied Voltage, Ph. D. Thesis, KTH, Stockholm, Sweden, 2001. [2] A. Cavallini, F. Ciani, G. Mazzanti, and G.C. Montanari, “First Electron Availability and Partial Discharge Generation in Insulation Cavities: Effect of Light Irradiation”, IEEE Trans. on Dielectrics and Electrical Insulation, 2005, pp. 387-394. [3] Power Diagnostix Systems GmbH, Bruesseler Ring 95a, 52074, Aachen, Germany, (www.power-diagnostix.com, last visited March 2007). [4] P.H.F. Morshuis, Partial Discharge Mechanisms, Ph. D. Thesis, Delft University of Technology, Fig. 4 – Average PDIV measured with pre-excitation at Delft, the Netherlands, 1993. variable frequency (a), no pre-excitation (b) and no pre- [5] P. Morshuis, A. Cavallini, G.C. Montanari, F. excitation and 10 min pause between each measurement (c). Puletti, and A. Contin, “The Behavior of Physical Error bars show 90 % confidence intervals. and Stochastic Parameters from Partial Discharges in Spherical Voids”, in Proc. IEEE Conf. on Properties and Applications of Dielectric Materials, Xian, China, 2000, pp. 304-309. [6] H.C. Hall and R.M. Russek, “Discharge inception and extinction in dielectric voids”, IEE Proc. Power Engineering, 1954, vol. 101, pp. 47-55. [7] W. Hauschild, A. Cavallini, and G.C. Montanari, “Effect of Supply Voltage Frequency on Testing of Insulation System”, IEEE Trans. on Dielectrics and Electrical Insulation, 2006, vol. 13, pp. 1189- 1191.