Andres Mora

Control Scheme for an Induction Motor Fed by a Cascade Multicell Converter Under Internal Fault

Cascade multicell (CM) converters are characterized by their high modularity, allowing their power to be easily increased. This also allows any faulty module (cell) to be isolated, so the load can be fed by the remaining operative cells. This leads to operation with an unbalanced number of cells. The use of modified voltage references to avoid the unbalanced operation in steady state has been proposed in the literature (denoted as fundamental phase-shifted compensation). This paper presents the transient operation from normal to faulty operation when such voltages are used and an induction motor is driven by the converter. In the first place, a simple way to include fundamental phase-shifted compensation (FPSC) in closed-loop operation, specifically with a field-oriented control (FOC) scheme, is presented. Then, the use of two supervisor controllers is proposed to modify the FOC speed and flux references. These new references avoid hard torque pulsation while automatically determining the maximum power available from the converter and applying it to the load. Experimental results obtained with the proposed technique in a low-power CM converter are shown for two different cases, constant torque load and fan curve load, showing a significant improvement on the signal behavior.

Model Predictive Torque Control for Torque Ripple Compensation in Variable-Speed PMSMs

This paper presents a new and simple finite-control set model predictive control strategy to reduce the torque ripple in permanent-magnet synchronous machines (PMSMs). The method is based on minimizing a cost function that considers the flux linkage torque harmonics obtained from a discrete-time model of the machine. The power converter switching state that minimizes this cost function is selected and applied during a whole sampling period. Additionally, it is proposed to mitigate the other source of torque ripple, known as cogging-torque, using a feed-forward signal applied to the torque control loop. A hybrid method that uses the output information from an observer and look-up table is presented to obtain a good cogging-torque estimation and thus an accurate mitigation of this disturbance torque at low rotational speed. Experimental results demonstrate the good performance of the torque ripple compensation methods presented in this paper.

Model Predictive Control of Modular Multilevel Matrix Converter

This paper presents a new and simple Finite Control Set Model Predictive Control (FCS-MPC) strategy of the Modular Multilevel Matrix Converter (M3C). This converter is one of the direct AC/AC power converters suitable for medium-voltage high-power machine drives with regenerative capacity. One of the main feature of this converter is that does not need external dc-voltage supplies and thus, all capacitor voltages have to be regulated to the desired value. Therefore, this paper provides a cost function that considers error terms related to the output current and capacitor voltages. Moreover, a compensation input current is presented in order to improve the dynamics response of the capacitor voltage average value. Simulation results illustrate that the proposed algorithm is capable to achieving good performance, even in critical operation point of the M3C.

A novel LVRT control strategy for Modular Multilevel Matrix Converter based high-power Wind Energy Conversion Systems

The trend of multimegawatt wind turbines has positioning multilevel converters as a promising solution for high-power Wind Energy Conversion Systems (WECSs). Furthermore, due to the high penetration of wind energy into the electrical network, some rather strict grid regulations have been development in case of fault into the grid power. Mainly, grid codes set Low Voltage Ride Through (LVRT) requirements for grid connected WECS. In this scenario, this paper presents a novel modelation and control strategy to fulfil Low Voltage Ride Through requirements using a Modular Multilevel Matrix Converter for interfacing a high power wind turbine.

Balancing energy and low frequency operation of the Modular Multilevel Converter in Back to Back configuration

The modular multilevel topologies are the next generation of power converters for applications where high power-voltage is necessary, such as traction systems, marine propulsion and Wind Energy Conversion Systems. In this paper, a new model and its respective control scheme for the Modular Multilevel Converter are proposed. The model is able to represent separately the dynamic of each port and to show the degrees of freedom in the converter, which are two circulating currents and the common mode voltage. These three degrees of freedom were used to perform energy balancing and to mitigate the voltage fluctuations at low AC frequency operation. Additionally, the proposed control strategy is extended to control two Modular Multilevel Converters in Back to Back configuration. Extensive theoretical analysis and computer simulation validate the effectiveness and viability of the presented control algorithm.

Control of Wind Energy Conversion Systems Based on the Modular Multilevel Matrix Converter

The nominal power of single wind energy conversion systems (WECS) has been steadily increasing, now reaching power ratings close to 10 MW. In the power conversion stage, medium-voltage power converters are replacing the conventional low-voltage back-to-back topology. Modular multilevel converters have appeared as a promising solution for multi-MW WECSs, due to their modularity and the capability to reach high nominal voltages. This paper discusses the application of the modular multilevel matrix converter to drive multi-MW WECSs. The modeling and control systems required for this application are extensively analyzed and discussed in this paper. The proposed control strategies enable decoupled operation of the converter, provide maximum power point tracking capability at the generator side, grid code compliance at the grid side (including low-voltage ride-through control) and good steady state and dynamic performance for balancing the capacitor voltages in all the clusters. Finally, the effectiveness of the proposed control strategy is validated using simulation and through experimental results obtained with a 27-power-cell prototype.

Dead-Time and Semiconductor Voltage Drop Compensation for Cascaded H-Bridge Converters

Compensation of nonlinear effects generated by dead-time and semiconductors voltage drop has been widely studied in the literature about two-level converters. This paper takes a closer look at those analyses for multilevel converters, specifically for a cascaded H-bridge (CHB) converter modulated with phase-shifted pulse width modulation. The interaction between the cells is analyzed for a generic n-cell converter, from which a general expression for the distortion generated by the nonlinearities has been obtained. Based on this expression, a compensation signal for the overall converter is calculated on each sample time, which is a fraction of the triangular carrier used to modulate each cell, thus significantly improving the quality of the output voltage signals. Experimental validation of the proposed compensation method is presented using a three-phase four-cell CHB converter prototype of 5.5 kW.

Improved control strategy of the modular multilevel converter for high power drive applications in low frequency operation

Modular Multilevel Converters (M2C) are considered an attractive solution for high power drives. However, its operation at low rotational speeds can produce undesired voltage fluctuations in the M2C capacitors. In this paper, two methodologies to improve the converter performance in this speed range are analysed and tested. The first strategy proposes the control of the inner converter currents combining a synchronous dq rotating frame and resonant controllers to improve the current tracking and to reduce the voltage fluctuations. The second strategy achieves the reduction of the voltage fluctuations by adjusting the DC Port voltage as a function of the machine frequency. Both methods are validated by simulation and experimental work, where a 18 cell M2C prototype is applied to drive an induction machine.

Finite-Set Model-Predictive Control Strategies for a 3L-NPC Inverter Operating With Fixed Switching Frequency

In this paper, finite-set model-predictive control (FS-MPC) methodologies for a grid-connected three-level neutral-point-clamped converter are investigated. The proposed control strategies produce fixed switching frequency, maintaining all the advantages of predictive control such as fast dynamic response, inclusion of nonlinearities and restrictions, and multivariable control using a single control loop. The first of the proposed FS-MPC strategies is based on a multiobjective cost function, designed to regulate both the inverter currents and the balancing of the dc-link capacitor voltages. The second FS-MPC strategy is derived from the first one, and it is based on a cost function that regulates only the grid current, with the balancing of the capacitor voltages being realized by controlling the duty cycles of the redundant vectors. The proposed control systems are experimentally validated using a 5-kW prototype. The experimental results show a good performance for both strategies, in steady-state and transient response, with a total harmonic distortion lower than

Resonant control system for a 7-leg back-to-back converter for interfacing variable speed generators to 4-wire loads

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