IEEE Transactions on Industrial Electronics

This paper presents a simple control method for balancing the DC-link voltage of three-level neutral-point-clamped inverters, while providing enhanced ride-through and common-mode voltage (CMV) elimination. The method uses DC-DC power converter technology on the DC link for balancing and ride-through enhancement, and a modified pulsewidth-modulation switching algorithm for CMV elimination. Simulation and experimental results are supplied to confirm the validity of the proposed method, which includes full digital signal processor control.

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.

Stochastic logic is based on digital processing of a random pulse stream, where the information is codified as the probability of a high level in a finite sequence. This binary pulse sequence can be digitally processed exploiting the similarity between Boolean algebra and statistical algebra. Given a random pulse sequence, any Boolean operation among individual pulses will correspond to an algebraic expression among the variables represented by their respective average pulse rates. Subsequently, this pulse stream can be digitally processed to perform analog operations. In this paper, we propose a stochastic approach to the digital implementation of complex controllers using programmable devices as an alternative to traditional digital signal processors. As an example, a practical realization of nonlinear dissipative controllers for a series resonant converter is presented.

This paper presents a method for designing the control loop for DC-to-DC power converters when uncertainties exist in the AC characteristics of the converter's load. In the proposed method, a converter is initially considered as a stand-alone module feeding a current sink load and the control loop of the converter is then designed in a way that maximizes the robustness of the converter's closed-loop performance against the unknown AC dynamics of a potential load. As a result, the proposed control design method can provide the predictable closed-loop performance for a converter when it is loaded with an actual load whose AC characteristics are unknown in advance.

In this paper, the requirements imposed by a direct torque control (DTC) strategy on multilevel inverters are analyzed. A control strategy is proposed in order to fulfill those requirements when a flying-capacitor multilevel inverter is used. Simulation and practical results will confirm the performance of the proposed strategy when using the multilevel inverter to control an induction motor by the DTC principle. Also, the advantages of using a multilevel inverter with a DTC strategy are shown by simulation results.

A highly dynamic control scheme with very low torque ripple-direct self control (DSC) with torque hysteresis control-for very high-power medium-voltage induction motor drives fed by a double three-level inverter (D3LI) is presented. In this arrangement, two three-level inverters that are connected in parallel at their DC sides are feeding the open motor windings. The DSC, well known from two- and three-level inverters, is adapted to the D3LI and optimized for a minimum torque ripple. An 18-corner trajectory is chosen for the stator flux of the induction machine since it is approaching the ideal circle much better than the hexagon known from DSC for two-level inverters, without any detriment to the torque ripple. The machine and inverter control are explained and the proposed torque quality and dynamics are verified by measurements on a 180-kW laboratory drive.

This paper presents a novel proposal of developing converter output voltage waveforms and novel converter topologies. The main idea is based on the assumption that the total converter output space vector is composed of two orthogonal space vectors. Two basic proposals are discussed. The first one is related to a converter built of two standard inverters: a main inverter (MI) and an auxiliary one. The converter output voltage space vector is composed of two orthogonal vectors generated by the respective inverters. The total power of the auxiliary inverter does not exceed 20% of the MI power. Thanks to the presented control method, the harmonic content of the output voltage is significantly reduced. The second proposal is related to a novel converter topology denoted as OVT-IHC. The converter is built of one two-level inverter and three isolated H-bridge circuit units. The structure and its performance are also discussed in the paper. The converter in question is able to generate 133 different output space vectors and permits achievement of a stepped adjustment of the RMS output voltage. Both topologies presented in the paper indicate some characteristics and advantages of multilevel inverters. The converters acting on the basis of the orthogonal vectors idea possess promising properties and are suitable to applications in medium-power converters. The paper presents main features and contribution to the theory.

Modulation strategies for multilevel inverters have typically focused on synthesizing a desired set of three phase sinusoidal voltage waveforms using a fixed number of DC voltage levels. This results in the average current injection and hence the net power drawn from the multiple DC bus terminals to be unmatched and time varying. Subsequently, the DC-bus voltages are unregulated, requiring corrective control action to incorporated. In this paper, the principle of reciprocity transposition in introduced as a means for modeling the DC-bus current injection simultaneously as the modulation strategy is formulated. Furthermore, a new sinusoidal pulsewidth-modulation strategy that features constant and controllable current injection at the DC-bus terminals while maintaining output voltage waveform quality is introduced. The proposed strategy is general enough to be applied to converters with an even number of levels and an odd number of levels. Analytical results comparing the performance of the proposed modulator with a conventional multiple carrier modulator are presented for example multilevel converters with four and five levels. Computer simulation results verifying the analytical results are presented for a four-level converter.