Switching converters are used in electric drive applications to produce variable voltage, variable frequency supply which generates harmful large dv/dt and high-frequency common mode voltages (CMV). Multilevel inverters generate lower CMV as compared to conventional two-level inverters. This paper presents simple carrier-based technique to control the common mode voltages in multilevel inverters using different structures of sine-triangle comparison method such as phase disposition (PD), phase opposition disposition (POD) by adding common mode voltage offset signal to actual reference voltage signal. This paper also presented the method to optimize the magnitude of this offset signal to reduce CMV and total harmonic distortion in inverter output voltage. The presented techniques give comparable performance as obtained in complex space vector-based control strategy, in terms of number of commutations, magnitude, and rate of change of CMV and harmonic profile of inverter output voltage. Simulation and experimental results presented confirm the effectiveness of the proposed techniques to control the common mode voltages. 1. Introduction In the medium-voltage, high-power adjustable speed drive (ASD) system, AC supply is first converted into DC (known as DC buffer stage) and then converting back this DC into variable voltage variable frequency AC supply using inverter (voltage source or current source type). Figure 1 shows the general block diagram of AC-DC-AC induction motor drive system. The front-end converter may be uncontrolled or controlled voltage source or current source rectifier while the motor side converter can be conventional two-level or multilevel VSI, CSI. In VSI fed drive, the DC link capacitor ( and ) is sufficiently large and DC link inductor is not required whereas in CSI fed drive, is sufficiently large and is not needed [1, 2]. Figure 1: General block diagram of AC-DC-AC VSI or CSI fed ASD (only in CSI and only , in VSI). The DC buffer stage consists of only large capacitor in the case of voltage source inverter and only large inductor in the case of current source inverter. This process involves switching power semiconductors to manipulate the magnitude and frequency of the output waveforms. The switching action of rectifiers and inverters results in common mode voltages which are essentially zero-sequence voltages superimposed with switching noise which will appear at rectifier, inverter, and motor terminals [1, 2]. If not mitigated, they appear on the neutral of the stator winding with respect to ground, which should be zero when the
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