| In the recent years the use of variable speed ac/dc motor drives has been significantly increased. Variable speed drives in medium voltage range (2300V and 4160V) are widely employed in the industry for applications such as in water treatment plants, oil and natural gas industry, foodstuff industry, marine engineering, cement industry, paper industry, power utilities, cold as well as hot rolling mills etc. Multilevel inverters have drawn tremendous interest recently in variable speed drive applications due to their advantages regarding harmonics, power factor, torque pulsations, and common mode voltage. However, the present variable speed drives with multilevel inverters have not matured enough and suffer from the disadvantages of relatively high system cost, lower reliability and not being fault tolerant to system faults.; In response to disadvantages of the existing multilevel inverters, this research proposes new multilevel inverter topologies and modular adjustable speed drive systems. The first approach in this research utilizes three three-phase inverters to realize a multilevel inverter with three-level output voltage. The second approach employs three three-phase inverters together with a 0.33 pu output transformer and generates four-level output voltage across the motor terminals. Both approaches are modular, utilize minimum number of switching devices and result in eliminated input current harmonics below harmonic order of 17. In the third approach, the motor windings are grouped into several delta-connected three-phase winding groups and each delta winding grouped is powered by a separate three-phase inverter module. The adjustable speed drives with this approach are realized with modular inverters as well as modular motor, which increases system reliability and reduces the voltage insulation requirement of the motor as well as the inverters. This translates into significant cost savings. Lastly, the fourth approach proposes several modular generator/converter topologies suitable for utility interface of wind/microturbine and flywheel type electromechanical energy conversion systems. The proposed modular generator/converter topologies are flexible to meet different voltage and power levels, modular, fault tolerant to generator/converter faults.; Analysis, simulation and design equations for the proposed approaches are presented. Finally, experimental results on laboratory prototypes are provided for the proposed approaches. |
