26-09-2013, 12:33 PM
EXPERIMENTAL INVESTIGATION OF FAULT-TOLERANT CONTROL FOR QUAD-INVERTER CONVERTERS
ABSTRACT
Fault-tolerant control strategies for quad-inverter based multiphase-multilevel converters are proposed and experimentally verified in this paper. Explicitly, the conversion scheme consists of four standard 2-level three-phase voltage source inverters (VSIs), able to supply a dual three-phase induction motor in open-end stator winding configuration, quadrupling the utility power of a single VSI within given voltage and current ratings. The developed modulation scheme has the capability to generate multilevel output voltage waveforms in healthy conditions, equivalent to the one of a 3-level VSI, and to share the total motor power among the four dc sources in each switching cycle. This sharing potentiality is investigated under post-fault operating conditions, when one VSI completely insulated due to a severe failure on it. In such circumstances, the quad-inverter system can perform with reduced power rating by a proper modulation of the remaining three healthy VSIs. The proposed contribution is devoted to investigate inherent active redundancy of the used quad-inverter, under one, two or three failed inverters, to ensure system operability in the degraded modes. Once the fault is diagnosed, the post-fault operating condition under one severe failed inverter is investigated by experimental tests, to confirm the simulation results. The resulting two degrees of freedom are exploited, if required, either to balance power sharing between the two three-phase motor windings, or to balance the power sharing among the three remaining dc sources. It has been verified that during post-fault conditions, with one failed inverter, the total power rating is reduced to the half, and one degree of freedom in the power sharing is lost. Significantly, the remaining two degrees of freedom can be exploited to equally share the total power between the two three-phase open-winding loads, leading to optimum inverter efficiency , or among the three remaining dc sources, leading to a balanced charge status in case of battery power supplies.