**3. Experimental Arrangement**

The insulators shown in Figure 1 were sampled from different locations around the Rio Tinto's factory in Saguenay (Canada). Three methods were used to assess the pollution level of the insulators: ESDD, NSDD, and the leakage current.

**Figure 1.** Samples of 13.2 kV ceramic insulators investigated in the study.

#### *3.1. ESDD and NSDD Assessment*

The assessment of the pollution of a site is possible by measuring both ESDD and NSDD on insulators generally in service. The measurements were made under standard IEC TS 60815-1 [4]. The probe of the YOKOGAWA Model SC 72 was used to measure the conductivities. These conductivities at different temperatures were corrected at 20 ◦C. The ESDD and NSDD were calculated according to IEC TS 60815-1 [4].

#### *3.2. Leakage Current Measurement*

The pollution of insulators can be assessed by measuring the leakage current to avoid any possible breakdowns [15]. Figures 2 and 3 show the measuring circuit and the leakage current test device, respectively. The high voltage (HV) AC source was connected to the insulator via a capacitive voltage divider. The circuit consisted of a 380 V/100 kV–10 kVA testing transformer, whose primary winding was connected to the autotransformer integrated in a semi-automatic or manual automatic control system. This later adjusted the voltage at the desired value. The system included a data acquisition that collected the main electrical characteristics. *R* was a power resistor (500 Ω, 500 W), connected between the insulator and the ground that measured the leakage current. The voltage was measured via a capacitive divider.

**Figure 2.** Schematic configuration of the leakage current-measuring circuit.

**Figure 3.** An overview of the experimental setup.

In order to evaluate the impact of humidification, the insulator samples were stored in a chamber where the relative air humidity was controlled by a water and glycerin mixture. The humidification concept can be found in the literature [16].

#### **4. Results and Discussion**

#### *4.1. Leakage Current Assessment*

Figure 4 shows the comparison of the measured leakage current on the humidified and non-humidified polluted insulators. The applied voltage was 7.62 ± 6% kV. From this figure, it can be observed that the leakage current increased with the relative humidity. This confirms the important role played by moisture on the leakage current flowing at the surface of polluted insulators.

**Figure 4.** Leakage current on the humidified and the non-humidified polluted insulators.

In order to better explain the effect of humidification on the polluted insulators and the high values of the leakage current measured after humidification, the surface resistance of the non-humidified and humidified insulators was computed (Figure 5). This figure expresses the result differently for the sake of readers more familiar with viewing resistance. The results show that the non-humidified insulators have a very high surface resistance.

**Figure 5.** Resistance of non-humidified and humidified polluted insulators.

The leakage current density of the non-humidified and humidified polluted insulators is shown in Figure 6. It can be seen that the leakage current density of these insulators after humidification was much higher than that of non-humidified insulators. This increase can be explained by the fact that, due to contamination on the surface, the insulator can retain more moisture, thus increasing the leakage current. Moisture is a naturally occurring phenomenon and on a macro level will vary with the location and weather. High relative air humidity is very stressful to the insulator.

**Figure 6.** Leakage current density of non-humidified and humidified polluted insulators.

#### *4.2. Phase Di*ff*erence between Leakage Current and Applied Voltage*

It is a well-known fact that the electric field distribution is greatly influenced by the pollution layer's characteristics (conductivity and thickness) [3]. With increasing pollution layer conductivity, the electric field intensity increases with a concomitant increase in the leakage current. In order to highlight the capacitive nature of these insulators, the calculation of the phase shifts between the measured leakage current and the applied voltage was performed for the humidified and the non-humidified polluted insulators (Figure 7). From the analysis of these results, it appears that the waveform of the leakage current had a phase shift of about 90◦ with respect to the applied voltage. This shows the dominance of the capacitive e ffect of these di fferent non-humidified polluted insulators. The results clearly show that the phase shift between the applied voltage and the measured leakage current had decreased substantially for all humidified polluted insulators but did not tend toward zero (this is based on subtracting complex current in dry conditions from wet conditions and seeing the resulting phase, which lies between 35◦ and 64◦, not near 0◦).

**Non-Humidified polluted insulatorsHumidified polluted insulators**

**Figure 7.** Comparison of phase shifts between non-humidified and humidified polluted insulators.
