Pablo Lezana

Predictive Current Control of a Voltage Source Inverter

This paper presents a predictive current control method and its application to a voltage source inverter. The method uses a discrete-time model of the system to predict the future value of the load current for all possible voltage vectors generated by the inverter. The voltage vector which minimizes a quality function is selected. The quality function used in this work evaluates the current error at the next sampling time. The performance of the proposed predictive control method is compared with hysteresis and pulsewidth modulation control. The results show that the predictive method controls very effectively the load current and performs very well compared with the classical solutions

PWM regenerative rectifiers: state of the art

New regulations impose more stringent limits on current harmonics injected by power converters that are achieved with pulsewidth-modulated (PWM) rectifiers. In addition, several applications demand the capability of power regeneration to the power supply. This work presents the state of the art in the field of regenerative rectifiers with reduced input harmonics and improved power factor. Regenerative rectifiers are able to deliver energy back from the dc side to the ac power supply. Topologies for single- and three-phase power supplies are considered with their corresponding control strategies. Special attention is given to the application of voltage- and current-source PWM rectifiers in different processes with a power range from a few kilowatts up to several megawatts. This paper shows that PWM regenerative rectifiers are a highly developed and mature technology with a wide industrial acceptance.

Survey on Fault Operation on Multilevel Inverters

This paper is related to faults that can appear in multilevel (ML) inverters, which have a high number of components. This is a subject of increasing importance in high-power inverters. First, methods to identify a fault are classified and briefly described for each topology. In addition, a number of strategies and hardware modifications that allow for operation in faulty conditions are also presented. As a result of the analyzed works, it can be concluded that ML inverters can significantly increase their availability and are able to operate even with some faulty components.

Operation of a medium-voltage drive under faulty conditions

This paper presents a method for operating cascaded multilevel inverters when one or more power cells are damaged. The method is based on the use of additional switches in the power circuit to bypass the faulty cell. To control the cells, the angle of phase shifting in the carrier signals is modified according to the number of operating cells, to minimize the load voltage distortion, when the inverter operates in failure mode. The reference signals of the pulsewidth-modulation modulators are also modified to increase the output voltage. Simulation and experimental results show the effectiveness of this method, which significantly increases the reliability of the drive.

Model Predictive Control of an Asymmetric Flying Capacitor Converter

Multilevel converters and, in particular, flying capacitor (FC) converters are an attractive alternative for medium-voltage applications. FC converters do not need complex transformers to obtain the DC-link voltage and also present good robustness properties, when operating under internal fault conditions. Unfortunately, with standard modulation strategies, to increase the number of output voltage levels of FC converters, it is necessary to increase the number of cells and, hence, the number of capacitors and switches. In this paper, we develop a finite-state model predictive control strategy for FC converters. Our method controls output currents and voltages and also the FC voltage ratios. This allows one to increase the number of output voltage levels, even at high power factor load conditions and without having to increase the number of capacitors and switches. Experimental results illustrate that the proposed algorithm is capable of achieving good performance, despite possible parameter mismatch.

Cascaded Multilevel Inverter With Regeneration Capability and Reduced Number of Switches

Multilevel converters are a very interesting alternative for medium and high power drives. One of the more flexible topologies of this type is the cascaded multicell converter. This paper proposes the use of a single-phase reduced cell suitable for cascaded multilevel converters. This cell uses a reduced single-phase active rectifier at the input and an H-bridge inverter at the output side. This topology presents a very good performance, effectively controlling the waveform of the input current and of the output voltage and allowing operation in the motoring and regenerative mode. The results presented in this paper confirm that this medium voltage inverter effectively eliminates low frequency input current harmonics at the primary side of the transformer and operates without problems in regenerative mode.

High-voltage multilevel converter with regeneration capability

This paper presents a multilevel converter with regeneration capability. The converter uses several power cells connected in series, each working with reduced voltage and with an active front end at the line side. This paper presents the following: (1) the control method of each cell; (2) the use of phase-shifting techniques to reduce the current and voltage distortion; and (3) criteria to select the connection of the cells. The converter generates almost sinusoidal currents at the load and at the input and works with very high power factor.

Large current rectifiers: State of the art and future trends

This paper presents the different technologies used in the generation of large controlled currents, in the kiloamperes range. After a brief review of processes requiring large currents, the paper discusses the working principles of thyristor phase-controlled rectifiers commonly used in these applications. Chopper-rectifiers using high-current insulated gate bipolar transistors are introduced as an alternative being considered in recent projects. The pulsewidth-modulated current-source rectifier, currently used in medium-voltage motor drives, is also analyzed as a future alternative for rectification in industrial processes. In addition, this paper presents the most important requirements and specifications to be considered in the applications of these high-power units. A system comparison is developed between thyristor and chopper-rectifiers in terms of quality of control, harmonics, power factor, losses, and efficiency.

Multicarrier PWM With DC-Link Ripple Feedforward Compensation for Multilevel Inverters

Like most power converter topologies, multilevel inverters are controlled with modulation techniques that are conceptually based on nonlinear waveform synthesis assuming constant dc-link voltages. However, real applications have load and supply dependent dc-links that usually present important low frequency ripple, which is also modulated and transmitted to the load, generating undesirable low frequency voltage and current distortion. This paper introduces a simple but effective dc-link ripple feedforward strategy into traditional carrier-based modulation techniques. The dc-link ripples are measured and used to modify the carriers or the reference directly in the modulation stage. Simulation and experimental results show the accuracy of the proposed method, eliminating low order harmonics in the load current.

Extended Operation of Cascade Multicell Converters Under Fault Condition

Multilevel converters are an interesting alternative for high power drives, due to their good quality output signals. Despite their advantages, the large number of components required increases the fault probability. Among the multilevel topologies, the cascade multicell converter presents advantages when operating under internal fault conditions, due to its high modularity. Previous works proposed to compensate the unbalanced operation due to a fault by changing the canonical fundamental output phase shift to precalculated angles, depending on the fault condition. This solution assumes that, if the maximum output phase voltage on each leg is used, the maximum line-to-line voltage will be at a maximum as well. This paper shows how this assumption is not always valid and presents the optimum angles and modulation indexes that must be used in order to obtain the maximum balanced load voltages.