René Vargas

Model Predictive Control—A Simple and Powerful Method to Control Power Converters

This paper presents a detailed description of finite control set model predictive control (FCS-MPC) applied to power converters. Several key aspects related to this methodology are, in depth, presented and compared with traditional power converter control techniques, such as linear controllers with pulsewidth-modulation-based methods. The basic concepts, operating principles, control diagrams, and results are used to provide a comparison between the different control strategies. The analysis is performed on a traditional three-phase voltage source inverter, used as a simple and comprehensive reference frame. However, additional topologies and power systems are addressed to highlight differences, potentialities, and challenges of FCS-MPC. Among the conclusions are the feasibility and great potential of FCS-MPC due to present-day signal-processing capabilities, particularly for power systems with a reduced number of switching states and more complex operating principles, such as matrix converters. In addition, the possibility to address different or additional control objectives easily in a single cost function enables a simple, flexible, and improved performance controller for power-conversion systems.

Predictive Control of a Three-Phase Neutral-Point-Clamped Inverter

A new predictive strategy for current control of a three-phase neutral-point-clamped inverter is presented. The algorithm is based on a model of the system. From that model, the behavior of the system is predicted for each possible switching state of the inverter. The state that minimizes a given quality function is selected to be applied during the next sampling interval. Several compositions of are proposed, including terms dedicated to achieve reference tracking, balance in the dc link, and reduction of the switching frequency. In comparison to an established control method, the strategy presents a remarkable performance. The proposed method achieves comparable reference tracking with lower switching frequency per semiconductor and similar transient behavior. The main advantage of the method is that it does not require any kind of linear controller or modulation technique, achieving a different approach to control a power converter.

Guidelines for weighting factors design in Model Predictive Control of power converters and drives

Model Predictive Control with a finite control set has emerged as a promising control tool for power converters and drives. One of the major advantages is the possibility to control several system variables with a single control law, by including them with appropriate weighting factors. However, at the present state of the art, these coefficients are determined empirically. There is no analytical or numerical method proposed yet to obtain an optimal solution. In addition, the empirical method is not always straightforward, and no procedures have been reported. This paper presents a first approach to a set of guidelines that reduce the uncertainty of this process. First a classification of different types of cost functions and weighting factors is presented. Then the different steps of the empirical process are explained. Finally, results for several power converters and drives applications are analyzed, which show the effectiveness of the proposed guidelines to reach appropriate weighting factors and control performance.

Predictive Current Control of an Induction Machine Fed by a Matrix Converter With Reactive Power Control

A different approach to perform the control of an induction machine fed by a matrix converter (MC) is presented in this paper. The proposed technique is based on predictive control and effectively controls input and output variables to the power converter, as expected from an MC. The method allows the use of all valid switching states, including rotating vectors that are not considered in most control techniques, as space vector modulation or direct torque control for induction machines fed by MCs. Experimental results show the excellent performance of the proposed approach, with low-distortion input currents, adjustable power factor, sinusoidal output currents with smooth frequency transitions, and good speed control in motoring and regeneration conditions, even working under an unbalanced power supply. The implementation and comprehension of the method should be considered simple compared to other control strategies with similar features. The high computational effort required should not be a problem considering recent progresses in digital signal processors-and even less in years to come.

Predictive Torque Control of an Induction Machine Fed by a Matrix Converter With Reactive Input Power Control

This paper presents a new control method for a matrix-converter-based induction machine drive. A discrete model of the converter, motor, and input filter is used to predict the behavior of torque, flux, and input power to the drive. The switching state that optimizes the value of a quality function, used as the evaluation criterion, is selected and applied during the next discrete-time interval. Experimental results confirm that the proposed strategy gives high-quality control of the torque, flux, and power factor with a fast dynamic control response. The key implementation issues are analyzed in depth to give an overview of the realization aspects of the proposed algorithm.

Predictive Approach to Increase Efficiency and Reduce Switching Losses on Matrix Converters

The matrix converter stands as an alternative in power conversion. It has no energy storage devices, performing the energy conversion by directly connecting input with output phases through bidirectional switches based on power semiconductors, allowing high-frequency operation. For this reason, it is known as the all-silicon power converter, featuring reduced size and weight. Forced commutations of the high number of semiconductors cause switching losses that reduce the efficiency of the system and imply the use of large heat sinks. This paper presents a novel method to reduce switching losses based on predictive control. The idea is to predict switching losses for every valid switching state of the converter, if applied during the next sampling time, and then, select the optimum state based on an evaluation criterion. The proposed strategy was experimentally tested on an 18-kVA matrix converter driving an 11-kW induction machine, reducing energy losses and increasing efficiency up to 3% compared to the basic strategy. As a consequence, the converter misuses less energy and requires smaller heat sinks.

Predictive Strategy to Control Common-Mode Voltage in Loads Fed by Matrix Converters

Common-mode voltages (CMVs) cause overvoltage stress to the winding insulation and bearings deterioration, reducing the lifetime of electric machines. This paper presents a predictive strategy that effectively mitigates CMVs from matrix converters (MCs), without affecting its functionality and allowing the use of rotating vectors. The method was experimentally tested on an MC feeding an induction machine, mitigating CMVs in 70% and reducing abrupt changes. The reduction is achieved with no tradeoff on the performance of the drive until reaching 40%, point where further reduction comes with an increase on the total harmonic distortion of line side currents. The designer can adjust the method in order to protect the AC machine, extending its lifetime and reducing negative effects of CMVs, and still comply with the standard for connection to the grid due to the flexibility allowed by the proposed strategy.

Resonances and overvoltages in a medium voltage fan motor drive with long cables in an underground mine

Underground mining is characterized by space constraints forcing the use of long cables to feed large machines, where the application of variable speed drives can generate overvoltages at the machine terminals. This paper presents the analysis and mitigation of harmful overvoltages detected in inverter-fed 1400 HP 13.8 KV motors used for air ventilation in an underground mine. The possible causes of the overvoltages, namely wave reflection and resonance are described theoretically and simulated. On-site direct voltage measurements at the 13.8 kV motor terminals confirmed that resonance was the dominant phenomenon. Based on this study a suitable low-pass filter was designed, constructed and installed. Finally, the adequate operation of such filter is demonstrated by further on-site measurements.

Predictive Control of an Induction Machine Fed by a Matrix Converter With Increased Efficiency and Reduced Common-Mode Voltage

Predictive control represents an optimization-oriented alternative for the control of power converters and drives. Predictive torque control of induction machines has been shown to achieve excellent initial results. The objective of this paper is to help develop this promising control approach by introducing new elements to improve its performance. The resulting algorithm improves the efficiency of the converter from 91.1% to 92.6% and achieves a common-mode voltage mitigation of 50%, compared to the basic control method. A tradeoff is observed in the power quality. The algorithm gives the designer the ability to select the best operating point for each particular application and to optimize the converters performance. Experimental results are presented to validate the proposals.

Predictive torque control with input PF correction applied to an induction machine fed by a matrix converter

A new modulation and control strategy for a matrix converter induction motor drive is presented in this paper. A discrete model of the converter, motor and input filter is used to predict the behaviour of torque, flux and input power of the system. The switching state that optimizes the value of a quality function is selected. The predictive control method, referred to as predictive torque control for matrix converters is introduced in this paper, including a strategy to control the input power factor. One of the fundamental differences that the proposed approach presents is that it considers all valid voltage vectors generated by a matrix converter, including rotating vectors that are neglected by most control strategies.