For inverters, power quality is generally measured in terms of harmonic content and the magnitude of the ripple current. Current ripple for a conventional inverter is a function of inductor size, switching frequency and the number of discrete DC input levels (multi-level inverter). Zero average current error (ZACE) control provides low line frequency harmonics but requires high switching or a large inductor to provide low current ripple since only bi- level control is available. Modification of the ZACE concept is required to handle multi-level inverters. A new ZACE method is proposed that can handle the voltage transition in a single-phase multi-level inverter. Simulation results are presented for a grid-

connected single-phase five-level inverter to demonstrate the concept. The pulse width modulation (PWM) inverter functions around the concept of placing discrete voltages across a filtering inductor and load as shown in Fig. I. The inductor integrates the voltage waveform to produce a current. By varying the polarity and amplitude of the applied voltage, sinusoidal current waveforms can be created. The output current waveform, because it

is the integral of the applied voltage, contains ripple. It is desirable to limit the amplitude of the current ripple with respect to the amplitude of the current waveform. The magnitude of the ripple is a function of inductor size, switching frequency and the number of discrete input voltage levels.

## Friday, 9 March 2012

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