# Study of Partial Discharge Inception Voltage in Inverter Fed Electric Motor Insulation Systems
## 5 Discussion
### Variables Affecting PDIV
The measurement of an accurate PDIV value is not possible, as the error is due to the overshoot, whether DC or peak voltage is considered as the PDIV. Nonetheless, it is noteworthy that, in [54], there is no effect at the peak PDIV for 56% overshoot (Figure 5). This difference from other studies could be related to the availability of the starting electron. The author of [46] explains that the higher PDIV resulting from high overshoots is due to the low probability of having a starting electron due to the short time above the inception level in each overshoot. Hence, the peak PDIV value is increased, as in [26; 45]. Conversely, the author of [53] notes that, with high ringing (several oscillations/overshoots before achieving DC voltage), the increase in the average voltage (even maintaining peak voltage) increases the probability of starting the electron availability, as the inception level is surpassed several times. In this way, the PD provability increases and the PDIV decreases. As a consequence, the results [54] suggest that there is a high ringing in the used waveform. As the inception voltage is surpassed with a high starting electron probability, the peak PDIV would be defined by a similar value of inception voltage and the DC PDIV will be defined by a lower voltage level. It could be said that, for high ringing, a high overshoot affects the DC PDIV but not peak PDIV, as shown in [54]. However, this hypothesis cannot be verified as there is no information about waveform ringing in [54]. In cases where measurements are developed without overvoltage and avoiding charge accumulation, there is no consistent answer in the literature. Some conclude that there is no effect of rise time on the PDIV, whereas others observed a decrease in the PDIV when decreasing the rise time. Considering that the rise time does affect the PDIV, with a lower rise time, the initial electron generation starts faster, reaching the critical electron number earlier, so the PDIV can be defined by a lower applied voltage, as long as the inception voltage is overcome. Further study is still necessary. To end with electrical variables, the influence of the pulse width on the PDIV is not clear. For high rise times, the effect of the pulse width appears to be negligible, as the critical initial electron number is reached during rising. For lower rise times, the authors of [43] stated that the pulse width decreased PDIV, whereas, in [57], it increased or did not change (depending on the voltage nonconducting duration). The main difference between [43] and the rest of the mentioned studies [52; 54; 57] is that the pulse width is achieved from different conventions of switching frequency and duty rate. Consequently, the nonconducting duration could vary from the maximum to the minimum pulse width, affecting the PDIV results, as explained in [57]. For example, if a 100 kHz switching frequency and 0.2 duty ratio and 1 kHz switching frequency and 0.8 duty ratio are used to acquire 2 us and 800 us pulse widths, respectively, the nonconducting duration could vary from 8 to 200 us. Hence, it could be rather difficult to distinguish whether the pulse width or the charge accumulation is influencing the PDIV. Nonetheless, the influence of the nonconducting duration cannot be distinguished, as there are no data [43]. Overall, it could be concluded that the effect of the pulse width cannot be studied independently of the rise time and duty. As far as we know, the effect of the pulse width considering the nonconduction duration has not been studied for a short pulse width with a short rise time. Further investigation is necessary to achieve more consistent conclusions in this area. A multifactorial study could be used to analyse the real effect of the waveform characteristics.