*4.3. Harmonic Analysis*

Polluted and humidified insulators circulate a leakage current on their surface leading, under certain conditions, to a flashover. Odd order harmonics in general, and in particular the

3rd and 5th, have adverse effects on the electrical network [17]. Indeed, by analyzing the leakage current harmonics, through the ratio of the 3rd/5th harmonic, it possible to predict the functional conditions of and occurrences of flashover in ceramic insulators. Furthermore, by analyzing the values of the leakage current and the harmonic components of clean insulators under normal operating conditions, it is observed that in all insulators, the 5th harmonic value of the leakage current is greater than the 3rd. Therefore, this criterion can be used to detect abnormal operating conditions in polluted insulators. For medium and severe pollution with fog and humidity, the value of the 3rd harmonic becomes larger than that of the 5th harmonic. The increase in the 3rd harmonic is faster and greater than that of the 5th harmonic with the imminence of the flashover. As a result, the condition of the insulator becomes critical and the probability of flashover occurrence increases. In this case, the ratio of the 3rd to the 5th harmonic is greater than one [17,18]. The harmonics measured during the various tests carried out on the non-humidified and humidified insulators are presented in Figure 8. In order to assess the effect of humidification on the polluted insulators, the 1st, 3rd, and 5th harmonics were measured, and the results are shown in Figure 8. After humidification, the harmonic of the leakage current was also computed in order to establish the ratio of the 3rd and 5th harmonics presented in Figure 9.

**Figure 8.** Leakage current harmonics of non-humidified and humidified polluted insulators.

**Figure 9.** The 3rd/5th harmonic ratio of non-humidified and humidified polluted insulators.

From Figure 8, it can be seen that the 5th harmonic was greater than the 3rd one for all insulators. This allows deducing that these insulators, although in polluted conditions, are in normal operation. In real life conditions, adverse external conditions (such as rain, dew or fog) may wet the contaminated surface of insulators, hence increasing the levels of leakage current. The received samples must therefore be submitted to humidification to resume testing. This will make it possible to compare di fferent results of the leakage current recorded and to draw accurate conclusions. It can be seen from Figure 9 that the amplitude of the 5th harmonic was always higher than the 3rd one. This indicates the non-imminence of flashover, because the ratio of the 3rd to the 5th harmonic was below one in this case (Figure 10). However, the 1st, 3rd, and 5th harmonics are higher than the harmonics measured on the non-humidified polluted insulators. The 3rd harmonic is mainly related to the electric discharge activities (corona, creepage, and so on), and it can be used to detect sensitively initial discharge voltage and discharge intensity of the samples. For better comparison, the 3rd harmonic of the non-humidified and humidified polluted insulators are accommodated on the same graph (Figure 10).

**Non-humidified polluted insulatorsHumidified polluted insulators**

**Figure 10.** 3rd harmonic of non-humidified and humidified polluted insulators.

From this figure, it can be seen that the 3rd harmonic increased when insulators were humidified. These results are in agreemen<sup>t</sup> with the investigations reported in the literature [19].

#### *4.4. Pollution Level Assessment*

The salinity, ESDD, and NSDD results for the insulators tested are shown in Table 2. The low values of salinity indicate that the contamination contained low levels of sodium chloride, but in large part, other contaminants [20,21]. To determine the pollution site severity (SPS), di fferent limits are standardized by IEC [4], as shown in Table 3. Comparing the results in Table 2 with the IEC 60815 standard allows the determination of the pollution levels of the di fferent insulators that are presented in Table 4.

**Table 2.** Values of calculated salinity, equivalent salt deposit density (ESDD) and non-soluble deposit density (NSDD).



**Table 3.** IEC Pollution Severity [4].

**Table 4.** Pollution level of the tested insulators.


It can be seen that the insulators suffered from light to very high pollution levels. The results indicate that the area where these insulators are located has a high level of pollution, with an equivalent salt deposit of 0.2125 mg/cm2. This pollution should be attributed to the local environmental parameters dynamics, which provides different pollutant sources and amounts [22]. From the surrounding air analyses, it was found that the contaminants that may have been deposited on the surface of the insulators could have been: aluminum process bath mixes: artificial cryolite, trisodium hexafluoroaluminate, electrolytic bath with or without lithium, crushed electrolytic bath, electrolytic casting bath, enriched alumina (recycled) dry scrubbers, re-circulated alumina, fluorinated alumina, charged alumina, calcined alumina, dusts of anode butts (shot blasting machine), or solid tars-tailings of anode furnace ducts.

These results are in agreemen<sup>t</sup> with the recommendations described in the IEC standard, which mentions that [4]: "In the same environment, different types of insulators and even differently oriented accumulate pollution at different rates. Also, some insulators can be more efficient than others due to changes in the nature of the pollutant."
