In this paper a voltage modulation method based on a triangular carrier wave for the three-phase four-leg voltage source converter is described. The four-leg converter can produce three output voltages independently with one additional leg. The proposed modulation method for the four-leg converter can be implemented with a single carrier by a simple but useful "offset voltage" concept. The method is equivalent to the so called three-dimensional space vector PWM method, but its implementation is much easier. The maximum magnitude of the balanced three-phase voltage and the maximum magnitude of zero sequence voltage, which can be synthesized simultaneously, are derived. The feasibility of the proposed modulation technique is verified by computer simulation and experimental results. These results show that a proposed carrier-based pulsewidth modulation (PWM) technique can be easily implemented without conventional computational burden.Index Terms-Carrier-based PWM, offset voltage, three-dimensional space vector PWM, three-phase four-leg converter, zero sequence voltage.
In a three-phase four-wire utility system, three-phase unbalanced loads and many single-phase nonlinear loads result in a neutral line current that is zero-sequence current. To deal with unbalanced and nonlinear loads in a high power system, this paper suggests a multilevel four-leg pulsewidth modulation (PWM) voltage-source inverter (VSI) as a topology for the high power applications where a function is required to control the zero-sequence component as well as dq components. This paper proposes a carrier-based PWM method for a multilevel four-leg PWM VSI along with introducing a novel offset voltage. The proposed offset voltage makes it possible for the switching sequence of all the legs to be optimized for minimizing the harmonic distortion of the output voltage irrespective of the number of inverter levels.
The proposed PWM method based on digital signal processors is implemented and tested by using a prototype three-level four-leg VSI. The feasibility of the proposed PWM method is verified by the spectral analysis, simulation, and experimental results.Index Terms-Multilevel four-leg, offset voltage, optimal switching sequence, pulsewidth modulation, three-phase fourwire, voltage-source inverter, zero-sequence current.
NOMENCLATUREfn Pole voltage references. V * as , V * bs , V * cs Phase output voltage references. V * af , V * bf , V * cf Output voltage reference in three-phase four-leg VSI. V 1 Fundamental component voltage. V n nth harmonic component voltage. f s Carrier frequency. f o Output fundamental frequency. THD Total harmonic distortion,= V 2 /V 2 1 −1.
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