Five-level GTO inverters for large induction motor drives

The development of large induction motor drives with low torque ripple and fast dynamic response for new or retrofit applications has been limited by the device ratings and problems of series connections. This paper investigates the use of a five-level GTO voltage-sourced inverter for large induction motor drives. The advantages of such a drive are that single GTO thyristors may be used at each level, thereby avoiding the need for series connection of the thyristors. The thyristors are well protected from overvoltages by the clamping action of the DC supply capacitors. The disadvantages are that each DC level requires a separate supply, four in the case of the five-level inverter, and that the devices are not equally loaded. This paper reviews the basic operation of the five-level inverter and possible PWM voltage/frequency control techniques for the specific application of induction motor drives. The simulation results clearly show the unequal loading of the devices and the need for independent voltage supplies for the five levels. It is shown that a combination of several PWM techniques offers the best solution for the drives application. The conclusions indicate that large induction motors with ratings up to 22 MVA, 7.46 kV may be supplied by the five-level inverter using available 4.5 kV, 3.0 kA GTO thyristors. The recommended supply for such an inverter with full regenerative operation over the complete speed range is four, four-quadrant converters in a quasi-24-pulse configuration