Five level GTO inverters for large induction motor drives

The use of a five-level GTO (gate turn-off thyristor) voltage sourced inverter for large induction motor drives is investigated. 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 the devices are not equally loaded. The authors review the basic operation of the five level inverter and possible PWM voltage/frequency control techniques for the specific application of induction motor drives. 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. It is found that large induction motors with ratings up to 22 MVA, 7.46 kV may be supplied by the five-level inverter using presently available 4.5 kV, 3.0 kA GTO thyristors