# Study of Partial Discharge Inception Voltage in Inverter Fed Electric Motor Insulation Systems
## 5 Discussion  
### Variables Affecting PDIV  
Starting with environmental conditions, there is a consensus that PDIV's dependency on pressure has a \(U\) shape, where the PDIV decreases and then increases with an increase in the pressure. In spite of the decreasing PDIV tendency with pressure being clear, the decrease perceived by [PERSON] et al. [26] and [PERSON] et al. [28] is slightly higher than that found by [PERSON] et al. [27] (Figure 1). This difference may result from the use of different insulation materials, which are not specified in the articles. Moving on to relative humidity, at high voltage (>5300 V) there are two main mechanisms related to an increase in the humidity. On the one hand, there is the increase in the air breakdown strength and decrease in the PD activity surface. On the other hand, there is the increase in insulation bulk charge accumulation and relative and imaginary part of permittivity. At low RH values, the first mechanism is predominant and the PDIV increases until a peak value with the increase in RH. Then, the PDIV decreases due to the predominance of the second mechanism. At voltages lower than 3450 V, however, the second mechanism seems to be predominant in the whole range of RH. Still, some authors describe an inverse effect on the PDIV or even no effect of the RH on cleaned samples. Thus, further investigation is required. Figure 18: Comparison of PDIV models and experimental measurements for different temperatures (sample group C): (**a**) PDIV; (**b**) error. For environmental conditions, all the authors in the literature review agree that an increase in temperature decreases the PDIV due to the air electric breakdown strength decrease and the change in the relative permittivity of the enamel. Air breakdown strength always decreases with temperature, decreasing the PDIV. The relative permittivity, conversely, can increase/decrease with temperature depending on the material and its effect on the PDIV can change. If the relative permittivity decreases, air and relative permittivity may affect the PDIV in opposite ways. In this article, we studied whether air properties or relative permittivity prevail in the PDIV. We concluded that changes in the air properties are the dominating factor affecting the PDIV. The relative permittivity should decrease by around 25% of the current value at 180 \({}^{\circ}\)C to neutralise the decreasing effect of the air breakdown strength on the PDIV. Hence, in spite of both air properties and relative permittivity being considered to evaluate how the PDIV changes with the temperature, the temperature always has an inverse effect on the PDIV. For the electrical factors, the PDIV is not affected by the waveform (sinusoidal, square bipolar, or unipolar) and polarity but the peak-to-peak applied voltage value is the main factor to define the PDIV. Nevertheless, it is important to consider both the waveform and polarity in the case of consequent PDIV measurements, due to the charge accumulation (the memory effect). The authors agree that the frequency does not affect the PDIV as long as the permittivity of the insulation material does not depend on the tested frequency range. The negligible difference observed by [PERSON] et al. [47] (was that the PDIV changed by 6% at 1-10 kHz), which could be because the pulse width changes while the duty is maintained for different frequencies. According to [43], the pulse width could affect the PDIV. Frequency affects the RPDIV by the enhancement of free electron availability due to charge accumulation at frequencies below 10 kHz, decreasing the RPDIV while increasing the switching frequency. Additionally, the increase in overshoot average amplitude has an analogous effect on PD provability and, therefore, decreases the RPDIV at frequencies up to 75-100 kHZ. Over 100 kHz, the RPDIV tends to stabilise with the increase in frequency. Regarding the rise time, when the PDIV is measured with high overshoot, the overshoot level and ringing are the major factors affecting the PDIV.