Candidate: Ibrahim Marwan Jarrar
Title: A Study on Turn-to-Turn Insulation of Medium Voltage Motors Fed by Variable Frequency Drives
Date: November 30, 2022
Time: 10:00 AM
Place: EIT 3145
Supervisor(s): Jayaram, Sheshakamal - Cherney, Ed (Adjunct)
Abstract:
The use of inverter drives in machine applications is expanding rapidly due to their advantages in terms of energy savings and speed control. Advances in the drive industry have made it possible for inverter drives to be used in medium and high voltage applications. The pulse width modulation (PWM) approach, which is often employed in motor inverter drives, is one of the most popular techniques used in these drives to generate the output waveform. Unlike AC-fed machines, the voltage waveform generated through PWM is comprised of multi-width square shape impulses of short rise time and high repetition rate to generate the final AC voltage at the targeted fundamental frequency. The downside is that these drives cause extra stresses on machine insulation that AC-fed machines do not experience. These inherited stresses that accompanied the PWM waveform accelerate the ageing of machine insulation and have led to premature failures. As a result, recognizing these stresses is critical for evaluating the endurance of turn insulation.
The International Electrotechnical Commission (IEC) 60034-18-42 standard, issued specifically for the qualification of Type II insulation systems of inverter-fed machines, recognizes the impact of features of the voltage source converter supply on the ageing of different components of machine insulation. In terms of interturn stresses, the standard identifies the impulse rise time, jump voltage, and switching frequency, to be the main factors affecting the life of turn insulation. Nonetheless, under the assumption of a comparable ageing rate between sinusoidal and repetitive impulse stresses, it is permissible to utilize a sinusoidal voltage waveform rather than a repetitive impulse waveform to evaluate the endurance of turn insulation. Thus, the direct effect of the waveform rise time on ageing is neglected and the influence of the rise time is confined to defining the voltage amplitude stressing the turn insulation. The effect of switching frequency is also limited to determining the number of cycles to failure given that the peak-to-peak voltage is the same for both sinusoidal and impulse waveforms applied. This assumes a frequency-independent voltage endurance coefficient and allows for a linear adjustment of the effect of switching frequency based on the testing frequency regardless of the shape or type of the test waveform.
Considerable research has been done to address the adverse effects of different waveform factors on turn-to-turn insulation. Whether in support or challenging the standard, many of the studies done to address these effects have focused on analyzing their impacts on the occurrence of partial discharges (PDs) within turn insulation while only a few researchers have extended their investigations to creating and comparing the resulting life curves under repetitive impulse energization. Furthermore, the standard identifies the jump voltage as a main component impacting the insulation life but does not specify or signify the unique influence of the overshoot factor. As might be widely established, a shorter waveform rise time may result in a larger terminal overshoot depending on the length of the cable and the mismatch impedance with the load, creating extra stresses on the machine insulation. Unfortunately, there seems to be limited work done to separate the influence of the slew rate from the overshoot. Research done to characterize the impact of features of the voltage source converter supply usually focuses on maintaining similar peak voltages regardless of the overshoot ratios, and thus, neglecting a factor that might have a significant impact on ageing.
In this research, therefore, the impact of the waveform switching frequency, overshoot and rise time on the time-to-failure and the endurance of Type II machine insulation is investigated. Reference life curves at 1 kHz and 4 kHz are derived utilizing unipolar repetitive square-impulses of 15% overshoot and 300 ns rise time. The effects of the impulse overshoot and rise time in the ranges of 0% to 30% and 400 ns to 1000 ns are also evaluated and reflected on the established reference life plots. Moreover, an antenna-based PD measurement system is utilized to record and investigate the significant factors affecting the life performance of the insulation systems. Two types of samples are used in this work; a back-to-back turn insulation sample and a single cavity layered insulation. The influence of varying the switching frequency, the rise time, and the overshoot components on the characteristics of the PDs during ageing are investigated to identify the effect of each factor while relating their influence on the life of turn insulation. The interaction between these factors is highlighted and examined utilizing the design of experiment principles. Finite element method simulation using COMSOL® Multiphysics is also utilized to explain and support the experimental findings and to suggest and describe the ageing and failure mechanism associated with the test samples.
Based on the endurance test results, the analysis of PD measurements, and the reported time-to- failure data, it is evident that turn insulation is subjected to additional stress factors when energized by an inverter supply as opposed to the stresses exhibited by sinusoidal waves. This indicates an incomparable ageing rate between the two types of waveforms, even at similar peak voltages applied, resulting in inadequate use of sinusoidal supply to qualify inverter-fed machines. The overshoot component of the waveform is found to have a substantial impact on the endurance of turn insulation apart from the jump voltage impact, and therefore, is identified as a significant factor influencing the life of turn insulation independent from other factors. Moreover, the reported results of the time-to- failure analysis and the PD measurements advocate a non-linear relationship between the switching frequency and the life of turn insulation; whereas, the waveform rise time is found to have a negligible but consistent impact within the considered range.