# Study of Partial Discharge Inception Voltage in Inverter Fed Electric Motor Insulation Systems
###### Abstract  
This article performs a thorough evaluation of different environmental and electrical waveform characteristics affecting PDIV on electric motor winding insulation. Temperature received special attention, as it clearly affects PDIV. Additionally, the PDIV models found in the literature were reviewed. Considering that the winding of the electric motor can reach 180 \({}^{\circ}\)C during operation, the experiments were performed at high temperatures in order to evaluate the performance of PDIV models. Several models were studied regarding their accuracy at high temperatures and the advantages and disadvantages of each one were identified. Based on the analysis, a simple analytical model to estimate the PDIV of twisted pairs depending on the temperature was proposed. All in all, the proposed model was the best compromise between computational requirements and PDIV estimation accuracy at high temperatures. Finally, future lines were identified. Further studies are necessary that consider the humidity, rise time, and pulse width effect on PDIV. Moreover, new models regarding both, environmental and waveform characteristics are necessary to accurately estimate PDIV. Extended volume-time theory seems to be a good basis for that. electric motors; electric motor insulation; insulation degradation; Partial Discharge (PD); Partial Discharge Inception Voltage (PDIV); electrical stress; environmental stress; PDIV model +
Footnote †: journal: Applied Sciences  
for winding insulation and can cause partial discharge (PD) activity, being one of the most common fault factors in winding insulation [12]. In particular, turn-to-turn winding insulation (considered the weakest point of the winding insulation system) is most affected by PD deterioration [7; 13; 14]. In order to protect the insulation from the issues caused by WBG converters, different solutions are proposed in the literature. A possible solution is the use of filters to reduce the \(\mathrm{d}\
u/\mathrm{d}t\)[15]. Additionally, the use of multi-level inverters was also shown to lengthen the lifetime of the insulation due to the reduction in PD amplitude [16]. Even using these solutions, in the case of the insulation being exposed to higher electric stress than expected and reaching partial discharge inception voltage (PDIV), a PD event will probably occur and the deterioration of the insulation system will be accelerated [14; 17; 18; 19]. In addition to electrical factors, environmental factors may enhance PD activity and, therefore, insulation deterioration. For the same application and waveform characteristics, depending on the surrounding conditions, PD events can appear or not, due to their influence on PDIV [20]. It is known that during service, PDIV decreases because of the mentioned electrical and environmental factors [21; 22; 23; 24]. When the PDIV reaches the operation voltage, there might be constant PD activity. This will result in even faster deterioration and a probable insulation breakdown. Thus, the aim of the insulation design should be to ensure that PD-free service lasts as long as possible. For that, it is important to have a margin between the initial PDIV and operating voltage. The higher this margin is, the longer the time required for the degradation to have a PDIV equal to the operating voltage. Therefore, the PDIV of the turn-to-turn insulation is a key factor in the design of the insulation system. Accurate models for PDIV estimation regarding different operation factors are necessary. Although mechanical factors can also be stress factors, they are not easy to consider. These factors, such as vibrations, may cause defects on the insulation surface, which are difficult to examine and replicate. The present article has two main objectives. First, to summarize from the existing literature how WBG-derived electrical factors (such as waveform and polarity, frequency, rise time and pulse width), and environmental factors (pressure, temperature, and relative humidity) affect PDIV. Mechanical factors are not considered in this study.