Measurement of capacitance and dielectric dissipation factor (power factor) is a proven diagnostic to assess the condition of high voltage bushings. In the past, dielectric response measurements in the frequency domain with frequency domain spectroscopy (FDS) – also called dielectric frequency response (DFR) or in the time domain with polarization depolarization current (PDC) – were applied to transformer insulation to determine water content in the cellulose. These methods have also been successfully applied to high voltage bushings.
A common method for monitoring dielectric losses in bushing insulation is addition of the currents of the bushings of the three phases. If the currents are equal, the sum of the three phase currents is zero. Small deviations, however, cannot be recognized because the voltages of the three phases are not always symmetric. This is the reason that imbalance in the three currents can sometimes be misinterpreted. A newer approach uses voltage transformers or other bushings of the same phase as reference. This way, more precise capacitance and tangent delta measurements become possible. Capacitance of the bushings can also be used to measure or monitor any partial discharges generated in the transformer or in the bushings. Normally, on-site PD measurement is difficult due to heavy interference in the substation. But simultaneously measuring PD at different receiver frequencies, allows pulse shapes to be analysed. This can prove helpful to differentiate between the PD pulses inside the winding or those close to the instrument, i.e. within the bushing. This INMR article from 2015, based on a contribution by Michael Krüger of Omicron in Austria and Wojciech Koltunowicz of Omicron Energy Solutions in Germany, proposed additional methods of insulation testing and monitoring and also describes their advantages. Measurement results of oil impregnated paper (OIP), resin impregnated paper (RIP) and resin bonded paper (RBP) bushings are presented for new and aged bushings and limits for assessment are discussed.
The methods described are integral. For singular insulation faults such as partial discharges, PD measurement is much more sensitive. For RBP bushings in particular, there is often more than one PD source in the insulation, e.g. in cracks and voids. With new methods such as impulse form analysis in the time domain or frequency spectrum analysis, different PD sources can be separated from one another and also from interference. This enables more detailed diagnosis and also, by better interference suppression, more sensitive on-site PD measurements. All such methods are illustrated with practical case studies, which illustrate their importance and efficiency.
High voltage bushings are essential parts of power transformers, circuit breakers and of other power system apparatus. Indeed, more than 17% of all transformer failures are caused by defective bushings and such failures can destroy a transformer. For example, a recent survey by CIGRE Working Group A2.37 found out that 30% of all bushing faults resulted in transformer burn and 10% in burst or explosion. Therefore, regular diagnostic measurement is essential for safe operation of transformers.
Measurement of Dielectric Losses
Measurement of capacitance and dissipation, or power factor, has been common now for decades and is normally performed at line frequency. Table 1 shows the 50/60Hz limits for DF/PF and partial discharges (PD) according to IEC 60137 and IEEE C57.19.01
Measurement of Dielectric Response on New RIP, RBP and OIP Bushings
Fig. 1 shows the tan δ curves of new RIP, RBP and OIP high voltage bushings. The frequency range is 15 to 400 Hz and test voltage is 2 kV. The curves are rather flat and the minimum is below the lowest test frequency of 15 Hz. The values at 50 Hz fulfill the limits as per Table 1.
In Fig. 2, one can see an RIP bushing that was stored outside without protection of the oil side. The non-protected side became humid over time and the change in tan delta can be clearly seen. Moisture increases the tan d, particularly at low frequencies, with the minimal of the tan δ curves shifting to higher frequencies with increasing moisture content.
Limits for Dissipation Factor at Different Frequencies
Table 1 outlines limits and typical values for the dissipation factor for 50/60 Hz. Measurement of the dissipation factor at other frequencies should become a standard. Low frequency results (e.g. 15 Hz) allow for sensitive moisture assessment; measurements at high frequencies (e.g. 400 Hz) allow sensitive detection of contact problems at the measuring tap or at the innermost layer connection or of high resistive partial breakdowns between grading layers. Table 2 shows indicative limits for new and aged bushings at different frequencies, with all tests done at a test voltage of 2 kV