# Study of Partial Discharge Inception Voltage in Inverter Fed Electric Motor Insulation Systems
## 2 Literature Review
### Variables Affecting PDIV
#### 2.1.1 Environmental Conditions
The second mechanism, however, creates an inverse relationship between the PDIV and humidity. When RH is increased, the condensation of water on the insulation can enhance the electric field. [PERSON] et al. [32] measured an increased space charge in the bulk of polymers with nanocomposite fillers. The same author observed in [33] that the space charge accumulated in the insulation bulk often has often the same polarity as the injecting electrode (e.g., a positive charge accumulation near the positive electrode). This increases the overall electric field in the air gap. Consequently, the PDIV is defined by a lower voltage value. Similarly, [PERSON] et al. [20] mentioned that PD activity might also be affected by the RH. Other investigations suggested that the relative and imaginary part of permittivity increase with humidity, which decreases the PDIV as well [34; 35]. [PERSON] et al. [35] analysed the electric field strength in the air gap (\(E_{\mathrm{gap}}\)), taking into account the relative permittivity (\(\epsilon_{\mathrm{r}}\)) using a plane capacitor model (Equation (1), with \(d_{\mathrm{air}}\) being the gap length and \(t_{\mathrm{ins}}\) the insulation layer thickness). The electric field intensity increases with the relative permittivity and, therefore, the RH. However, based on optical emission spectroscopy (OES) measurements, the author concluded that not only the increase of \(\epsilon_{\mathrm{r}}\) of the enamel wire increases the \(E_{\mathrm{gap}}\) but also other mechanisms (not specified). \[E_{\mathrm{gap}}=\frac{V}{d_{\mathrm{air}}+2t_{\mathrm{ins}}/\epsilon_{ \mathrm{r}}} \tag{1}\]  
For lower voltage values (<3450 V), [PERSON] et al. [20] defined the critical RH value below the analysed RH range (35-80%). This suggests that the second mechanism is apparently dominant in the whole range of humidity for low voltage. This agrees with the observation of several authors at lower voltage levels at room temperature (25 \({}^{\circ}\)C) [35; 36; 37; 38], 60 \({}^{\circ}\)C [35], 80 \({}^{\circ}\)C [35], and 90 \({}^{\circ}\)C [39]. [PERSON] et al. [39], on the contrary, noted at 30 \({}^{\circ}\)C an increase in the PDIV with an increase in RH. This is explained by the surface discharge predominating, due to the semiconductive layer generated in the insulation with the interaction between moisture and byproducts formed at PD. Therefore, the influence of the RH depends on the exposure temperature. Moreover, [PERSON] et al. [38] concluded that, after washing the samples with pure water, there was no effect of humidity on PDIV in a range of 50-95%. In this case, the decreasing effect of the PDIV (in samples before washing) was due to a change in surface conductivity because of contaminant (Na,K, Cl, etc.) adhesion with touching by bare hands, which indicates that the other impact factors considered previously are negligible. To summarise, generally speaking, two main mechanisms result from an increase in the relative humidity. At low RH values, the PDIV increases with the increase in the RH due to the increase in the air breakdown strength and decrease in the PD activity surface, until a peak PDIV value is reached. Then, the PDIV is inversely affected by the RH. The increase in the space charge accumulation in insulation bulk and relative and imaginary part of permittivity, resulting from the increase in RH, decreases the PDIV. At high voltage levels (>5300 V), both mechanisms could be developed, whereas, at low voltage levels (<3450 V), the second mechanism is predominant. Even so, some authors found an inverse effect of conductivity on PDIV, or no effect of RH, if samples cleaned with pure water are measured. Further investigation is necessary, considering the voltage level, temperature, and cleanness of the samples when the RH effect on PDIV is studied.