*3.3. Positive and Negative Lightning Impulse Breakdown Voltage*

The up-and-down method was implemented in order to estimate the mean LI BDV under positive and negative polarity of base oil and the two NF samples after conducting measurements in various voltage levels according to the demands of IEC 60,897 standard. The mean LI BDVs as well as the average times to breakdown are exhibited in Table 6. Figures 8 and 9 depict the oscillograms of the LI BDVs at 50% breakdown probability under positive and negative polarity, respectively.


**Table 6.** Mean positive and negative LI BDVs and time to breakdown.

**Figure 8.** Oscillogram of the positive LI BDVs of the under-study samples at 50% breakdown probability level.

**Figure 9.** Oscillogram of the negative LI BDVs of the under-study samples at 50% breakdown probability level.

The LI strength of iNF under positive polarity is enhanced by 28.5% with respect to the base oil with the corresponding improvement of sNF being limited at 6%. The mean BD times of the samples after the application of positive LIV are almost steady, unlike with the corresponding ones under negative polarity. Under the application of negative LIV, the time to BD of iNF is restricted by 29.5% with respect to the one of the base oil, while sNF sample demonstrates almost the same BD time. In terms of their behavior under negative LIV, both NF samples show limited increase, as displayed in Table 6 and Figure 8. It is also evident that the addition of alumina NPs narrows the gap between positive and negative LI BDV.
