# Study of Partial Discharge Inception Voltage in Inverter Fed Electric Motor Insulation Systems
## 2 Literature Review
### Variables Affecting PDIV
#### 2.1.1 Environmental Conditions  
Two principal factors affecting PDIV are the surrounding air properties and enamel properties of the sample. Both of them might be influenced by environmental factors. In the following sections, the effect of pressure, relative humidity (RH), and temperature on PDIV/RPDIV (repetitive partial discharge inception voltage) are gathered. The RPDIV is the minimum peak-to-peak impulse voltage at which more than five PD pulses occur on ten voltage impulses of the same polarity according to IEC 60034-18-41 [10]. Pressure  
The pressure principally affects the PDIV due to its influence on the air breakdown voltage. As described in [PERSON]'s law (explained in Section 2.2), the breakdown voltage for a given gas (air) and electrode material can be defined as a function of the product of the gas pressure and the gap distance [25]. The PDIV is the minimum voltage at which the voltage across the air gap is equal to the breakdown voltage of the air. Therefore, the surrounding air pressure has a significant effect on the PDIV. Several investigations showed that the evolution of the PDIV has a \(U\) shape with an increase in pressure. [PERSON] et al. [26] measured the PDIV of twisted pairs with a 50 Hz sinusoidal waveform. Starting at 5 mbar, the authors detected that the PDIV decreased until the minimum was reached at 50-70 mbar. Then, it increased with the increase in pressure, creating the aforementioned \(U\) shape. In addition, [PERSON] et al. [27] note that the increase in pressure in from 100 to 1013 mbar increases the PDIV for twisted pairs. [PERSON] et al. [28] obtained the same influence of the pressure in a range of 200-1000 mbar on RPDIV. The results reported in the mentioned studies [26; 27; 28] are compared in Figure 1. The PDIV results are normalised to 1000/1013 mbar and are depicted in a pressure range from 5 to 1013 mbar. In this case, the characteristic \(U\) shape is only seen in the results from [26], as the minimum PDIV value is at around 50-70 mbar and [27; 28] measured above this pressure. To sum up, starting at a low pressure, the PDIV is inversely affected by the pressure until it reaches a minimum, followed by an increasing proportional tendency, leading to a _U_-shaped evolution. Figure 1: Comparison of the dependence of PDIV on pressure ([PERSON] et al. [26], [PERSON] et al. [28], and [PERSON] et al. [27]). Humidity  
Regarding the humidity, both air and enamel properties are affected. According to [PERSON] and [PERSON] [29] (at 25 and 50 \({}^{\circ}\)C, RH 10-90% and PDIV values above 5000 V), the PDIV increases with increasing humidity until a peak value is reached. Afterward, the PDIV decreases with an increase in the humidity. Therefore, they suggest that the PDIV is affected by two mechanisms, the aforementioned air properties and enamel properties. The peak value of PDIV relates to the point at which the effect of the first mechanism is neutralised by the second mechanism (critical RH [20]). Similarly, [PERSON] et al. [20] noticed the same effect on voltage levels above 5300 V at 23 \({}^{\circ}\)C in a range of RH 35-80%. According to the first mechanism, before the peak PDIV value, a proportional relationship between the humidity and PDIV is seen, due to the increase in air breakdown voltage. When the moisture content is increased (an increase in RH), the electric breakdown strength of the air is increased. This observation is in good agreement with [30]. The author explains that the breakdown voltage of the humid air is increased with the increase in RH from 0% (dry air) to 40%. The water vapour has a higher breakdown strength than air, therefore, the mixture of water vapour and air (higher RH) has a higher breakdown voltage. Additionally, the authors of [20] observed that, in this range, when decreasing the RH, the area of PD activity is concentrated in a small area, deteriorating the enamel more. Even though no PDIV is studied, reducing the area will result in a lower PD activity probability and the PDIV will be defined by a higher voltage level [31].