Analysis of common mode voltage-“neutral shift” inmedium voltage PWM adjustable speed drive (MV-ASD) systems

In this paper, an analysis of common mode voltage or “neutral lift” in the new emerging voltage source inverter type medium voltage adjustable speed drive (MV-ASD) systems is presented. Both cascaded multilevel (CML) inverter and 3-level NPC inverter topologies are analyzed. An equivalent circuit model to determine the common mode voltage stress is presented. Analysis and simulation results are discussed and worst case common mode voltage excursion is computed for an example 800 hp, 4160 V MV-ASD. It is shown that certain system components are excessively stressed and in the MV-ASD system these data are particularly useful in specifying system components and for proper design of the system. Possible effects of common mode voltage and its dv/dt on medium voltage motor bearings are discussed. A new multilevel PWM strategy is introduced which results in zero common-mode voltage. Simulation results are shown to illustrate the effectiveness of these schemes. Finally, experimental results from a 800 hp, 4160 V, MV-ASD system are presented     

Multilevel inverter by cascading industrial VSI

In this paper, the modularity concept applied to medium-voltage adjustable speed drives is addressed. First, the single-phase cascaded voltage-source inverter that uses series connection of insulated gate bipolar transistor (IGBT) H-bridge modules with isolated DC buses is presented. Next, a novel three-phase cascaded voltage-source inverter that uses three IGBT triphase inverter modules along with an output transformer to obtain a 3-p.u. multilevel output voltage is introduced. The system yields in high-quality multistep voltage with up to four levels and low dv/dt, balanced operation of the inverter modules, each supplying a third of the motor rated kVA. The concept of using cascaded inverters is further extended to a new modular motor-modular inverter system where the motor winding connections are reconnected into several three-phase groups, either six-lead or 12-lead connection according to the voltage level, each powered by a standard triphase IGBT inverter module. Thus, a high fault tolerance is being achieved and the output transformer requirement is eliminated. A staggered space-vector modulation technique applicable to three-phase cascaded voltage-source inverter topologies is also demonstrated. Both computer simulations and experimental tests demonstrate the feasibility of the systems.     

A new medium-voltage PWM inverter topology for adjustable-speeddrives

In this paper, a new pulsewidth modulation (PWM) inverter topology suitable for medium-voltage (2300/4160 V) adjustable-speed drive systems is proposed. The modular inverter topology is derived by combining three standard three-phase inverter modules and a 0.33-pu output transformer. The output voltage is high-quality multistep PWM with low dv/dt. Further, the approach also guarantees balanced operation and 100% utilization of each three-phase inverter module over the entire speed range. These features enable the proposed topology to be suitable for powering constant-torque, as well as variable-torque type loads. Clean power utility interface of the proposed inverter system can be achieved via an 18-pulse input transformer. Analysis, simulation and experimental results are shown to validate the concepts     

A new modular motor-modular inverter concept for medium-voltageadjustable-speed-drive systems

In this paper, a new modular motor-modular inverter concept for medium-voltage adjustable-speed drive (MV-ASD) systems is introduced. It is shown that standard MV motor winding connections can be reconnected into several three-phase groups, each powered by a separate three-phase pulsewidth modulation inverter, resulting in a high-performance MV-ASD system. The proposed approach is fault tolerant and can continue to operate at reduced power levels, under inverter and/or motor faults. An example 250 hp four-pole 60 Hz 2300 V/4160 V motor winding connection diagram along with the associated inverter configurations is analyzed. Results from analysis are presented on the aspects of faults and motor operation under partial winding excitation. Experimental results on a low-voltage (460 V) 10 hp ASD are demonstrated