*2.2. Experimental Part*

There are two major categories, according to which the dielectric measurements regarding any High Voltage (HV) equipment are classified: the destructive and non-destructive tests. In the context of the destructive tests, AC, positive and negative LI BDV are measured for the two NF samples along with a base oil sample. In regard to the non-destructive ones, PDIV, as well as the variation of the apparent charge for various voltage levels, have been measured.

AC BDV is the fundamental parameter for the characterization of any insulation system, as long the transformer nominally operates under AC voltage. For the determination of this property, each one of the samples under investigation is stressed under AC voltage/60 Hz using a Baur DTA 100 C generator, as illustrated in Figure 2a, according to the conditions of IEC 60,156 standard. The generator can produce up to 100 kV increasing the voltage at a rate of 2 kV/s until breakdown occurs between two Rogowski electrodes, which ensure a uniform electric field distribution, with the gap distance set at 2.5 mm. At the beginning of the experimental procedure, the electrodes are polished and cleaned. 10 sets of 6 breakdown events (60 in total) are performed, with the applied interval being 2 min between two successive breakdowns and 5 min between two sets. The results are fit to Weibull and normal distributions in order the AC BDV in low probability levels to be estimated [7,23,34].

**Figure 2.** AC BDV and PD apparatuses: (**a**) BAUR DTA test cell with Rogowski electrodes. (**b**) Schematic diagram of the PD setup.

Regarding the measurement of LI BDV, this is another property of major importance as it is an indication for the strength of the insulation in emergency conditions, such as lightning strikes. A two-stage impulse generator is used, producing up to 400 kV, with positive and negative polarity, according to IEC 60,897 standard. A point–sphere geometry is used, ensuring a highly divergent field geometry, with the gap distance set at 25 mm. The radius of curvature is 50 μm, while the diameter of the sphere is 12.7 mm. Five voltage applications are conducted for each voltage level with each sample being replaced with a new one after every BD event, ensuring a good reliability for the processing of the results. The applied lightning impulse voltage 1.2/50 μs, is monitored on oscilloscope Tektronix DPO4104; 1 GHz/5 GS s-1.

In terms of the conduction of PD measurements, a HV test transformer (HIGH VOLT GmbH Transformer PEOI 40/100,100 kV) is used with a TETTEX Instruments PD Detector DDX-9101 to measure the apparent charge to a nominal capacitor (HIPOTRONICS capacitor PSF 100-1). The schematic diagram of the configuration is displayed in Figure 2b. The test cell consists of two plate electrodes with the gap between them set at 0.75 mm. Nomex® Dupont™ insulating paper with 0.75 mm thickness impregnated to the NFs and base oil samples, is put in the gap. The AC voltage value for which the measured apparent charge exceeds the value of 10 pC is considered as the PDIV for each sample under investigation. The insulating papers impregnated to iNF, sNF, and base oil samples are stressed under AC voltage while the transformer is increasing the voltage at a rate of 0.1 kV/s until the measured apparent charge exceeds the threshold value. Each experiment is repeated 4 times for ensuring the reliability of the obtained results.
