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Analytical solutions of pulsewidth-modulation (PWM) strategies for multilevel inverters are used to identify that alternative phase opposition disposition PWM for diode-clamped inverters produces the same harmonic performance as phase-shifted carrier PWM for cascaded inverters, and hybrid pwm for hybrid inverters, when the carrier frequencies are set to achieve the same number of inverter switch transitions over each fundamental cycle. Using this understanding, a PWM method is then developed for cascaded and hybrid inverters to achieve the same harmonic gains as phase disposition PWM achieves for diode-clamped inverters. Theoretical and experimental results are presented in the paper.Index Terms-Analytic solutions, bipolar discontinuous pulsewidth modulation, cascaded, diode-clamped and hybrid multilevel inverters.

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Stationary frame proportional plus integral (PI) current-regulated voltage-source inverters are conventionally regarded as unsatisfactory for ac systems because they cannot eliminate steady-state errors. This paper presents a technique for interpreting the stationary/rotating frame transformations as modulation processes in the Laplace domain, which move the control function from one part of the frequency spectrum to another. The technique is used to compare stationary and synchronous frame PI regulators on a common basis to better understand the advantages of a synchronous frame regulator. A new form of stationary frame resonant regulator is then derived which achieves zero steady-state error without requiring the complex transformations of a synchronous frame regulator. The performance of this new regulator is evaluated and found to be similar to that of the synchronous frame PI regulator.Index Terms--transformations, frequency domain, linear current regulators, three-phase power converters.

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