Citation: Mirza, A.Y.; Bazzi, A.; Nguyen, H.H.; Cao, Y. Motor Stator Insulation Stress Due to Multilevel Inverter Voltage Output Levels and Power Quality. Energies 2022, 15, 4091. https://doi.org/10.3390/ en15114091 Academic Editor: José Gabriel Oliveira Pinto Received: 14 March 2022 Accepted: 30 May 2022 Published: 2 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). energies Article Motor Stator Insulation Stress Due to Multilevel Inverter Voltage Output Levels and Power Quality † Arshiah Yusuf Mirza * , Ali Bazzi, Hiep Hoang Nguyen and Yang Cao Electrical and Computer Engineering, University of Connecticut, Storrs, CT 06269, USA; bazzi@uconn.edu (A.B.); hiep.nguyen@uconn.edu (H.H.N.); yang.cao@uconn.edu (Y.C.) * Correspondence: arshiah.mirza@uconn.edu † This article is an extended version of our conference paper published in 2021 IEEE Energy Conversion Congress and Exposition (ECCE), Vancouver, Canada, 10–14 October 2021; pp. 5569–5572. Abstract: Multilevel Inverters (MLIs) are widely sought after in medium-voltage applications like electric ships, electric aircraft, and renewable energy integration due to excellent advantages like lower device stress, better power quality, and modularity. However, non-sinusoidal excitations from MLIs pose a serious problem to motor-insulation and lead to their premature breakdown. This paper investigates stress in medium-voltage motor insulation when the stator winding is excited by 3-, 5-, and 7-level multilevel inverter output waveforms. The effect of firing angle on insulation stress is also studied for each of the multilevel inverters. Results show that in addition to the number of output voltage levels, PWM wave shape is a critical factor which affects the insulation stress. Both these factors work together to impact the insulation health. A strong correlation is shown between the increase in the voltage root mean square (RMS) value and increase in dielectric stress when ignoring the dv/dt impact for a fixed DC input voltage and operating frequency of the inverter. Similarly, the dielectric stress in the stator insulation increased with an increase in firing angle for each of the MLIs. This paper shows a potential that both the RMSs can be optimized to reduce the insulation stress and improve the power quality of MLIs in medium voltage drives. Keywords: multilevel inverters; 3-level; 5-level; 7-level; voltage; dv/dt; frequency; stress; motor stator ground-wall insulation; cable insulation; insulation stress; dielectric stress; leakage current; RMS; THD 1. Introduction Multilevel inverters (MLIs) have revolutionized the power electronic drives indus- try [1]. With their many topologies and control schemes catering to a large variety of appli- cations, MLIs have been replacing conventional 2-level inverters, especially in medium- and high-voltage applications. One of the major advantages of MLIs is the higher power quality when compared to the conventional two-level inverters. It is possible to produce output voltages with very low Total Harmonic Distortion (THD) resembling sine waves, reducing the effort for filtering [2]. Despite the numerous advantages, that have been widely explored, their effect on insulation stress is relatively unknown. The operating voltages, switching frequencies, and the wave shapes of inverter outputs tremendously affect the insulation systems in loads that are fed by MLIs. Conventional insulation systems in motor drives were designed for sinusoidal excitations at power frequency. High frequency staircases, such as waveform outputs by multi-level inverters, stress the insulation differently and their effect on insula- tion breakdown is not fully understood. Additionally, wide-bandgap devices like Silicon Carbide (SiC) and Gallium Nitride (GaN) have a large dielectric breakdown strength and enable high switching frequencies, which consequently increase the dv/dt in motor drives. Owing to the large dv/dt, the insulation breaks down prematurely before its lifetime. Energies 2022, 15, 4091. https://doi.org/10.3390/en15114091 https://www.mdpi.com/journal/energies