Publication: Multilevel inverter output characteristics analysis at different switching frequencies based on off grid system
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Date
2023-07
Authors
Fatin Amirah binti Mohamed Azahar
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Abstract
Multilevel Inverter has become a growing demand for higher-power applications with low harmonics for industrial technology. According to the applied
switching frequency, modulation techniques for multilevel inverters can be classified into three parts, fundamental switching frequency, mixed switching frequency, and high switching frequency. This study is focussed on multilevel inverter Output characteristics analysis at different switching frequencies ranging from fundamental switching frequency to high switching frequency. The purpose is to analyse the output waveforms produced by cascaded H-Bridge multilevel inverters at different levels based on off-grid system. An efficient technique to reduce total harmonic distortion in cascaded H-bridge is investigated. The Output waveform and the output characteristics of the Multilevel Inverter were fully evaluated and compared by using MATLAB Simulink simulation software. The results produced by both multilevel modulations were tested in terms of THD for voltage and current by using different values of variable frequency. The THD values for the output voltage and current waveforms are compared. The value for blocking voltage is also been tested and compared. The output power quality is calculated while the average current of THD is reduced after increasing the level of voltage. For high switching frequency, Multicarrier PWM topology which
is Phase Opposition Disposition Level Shifted PWM (PODPWM) is implemented. As for low switching frequency modulation, a calculation of switching angles technique will be used. Both Multilevel modulation techniques are proposed for five, seven, and nine-level cascaded H-bridge. The evaluation and study performance of the Multilevel Inverter through simulation studies is achieved at the end of the studies. The average voltage and current THD under purely resistive (R) loads of the LSF of 5-L, 7-L, and 9-L CHBMLI are found to be 17.60%,12.20% and 9.29%. Under resistive-inductive (RL) load, the average current THD are reduced based on the inductor values. In this research, the challenging part is to perform high switching frequency modulation when the number of levels is high because high switching frequency uses a lot of carrier signals and it led to high number of switches. Reducing THD of output voltages requires an increase in the number of levels of the multilevel inverter. However, a high number of levels increases the control complexity and introduces voltage imbalance
problems.