Modeling and Full Decoupling Control of a Grid-Connected Five-Level Diode-Clamped Converter

This paper presents a novel approach to deal with the regulation of the dc-link capacitor voltages and ac-side currents in a grid-connected five-level diode-clamped converter. Due to the controllability problems of this topology, guaranteeing a solid current control and, mainly, a correct dc-link voltage sharing, represents a complex technical challenge. With the purpose of coping with it, an averaged model that describes the system dynamics at both sides of the converter is presented, assuming that a modulation strategy is integrated in the system to generate the switching sequence. In order to derive the proposed model, no restriction concerning the use of only the three nearest vectors to the desired voltage reference is taking into account. Then, several changes of variables are carried out in the model equations to obtain control input decoupling for control purposes, while reducing the complexity of the model as well. Finally, the voltage and current controllers are designed separately using different control inputs in a straightforward way. Neither auxiliary hardware nor complicated mathematical calculations are required to achieve the control objectives. The effectiveness and good performance of the system under the proposed control approach is validated by simulation results, suggesting that the five-level diode-clamped converter can be a solid solution as an interfacing system connected to the utility grid for, e.g., industrial drives or renewable energy applications.Ministerio de Economía y Competitividad y Unión Europea : DPI2016-75294-C2-1-RMinisterio de Economía y Competitividad y Unión Europea : DPI2013-41891-

Integrated Control of Five-Level Diode-Clamped Rectifiers

This paper presents an approach for dealing with the control of five-level, diode-clamped rectifiers. The control of such converters is challenging since the voltage balancing among capacitors is not a trivial task. Some of the existing approaches that cope with this problem use specific modifications of one of the traditional modulation techniques such as using redundant vectors in SVM. The main feature of the proposed technique is that part of the modulation is considered in the system equations and, in this way, the voltage balance can be solved designing a specific controller for this problem. As a result, several levels are used within a switching period. Furthermore, it is shown that the proposed approach hardly affects the control of active and reactive powers and total dc-link voltage in such a way that the well-known direct power control (DPC) can be applied. As a consequence, the resultant modulation stage is simpler than other techniques based on, e.g., space vector modulation (SVM). The effectiveness and good performance of the system under the proposed control approach are validated by both simulation and experimental results.Ministerio de Economía y Competitividad y Unión Europea : DPI2016-75294-C2-1-

Asymptotic rejection of sinusoidal disturbances based voltage balance control in back-to-back power converters

This paper addresses the imbalance problem of the dc-link capacitor voltages in the three-level diode-clamped back-to-back power converter. In order to cope with it, a mathematical analysis of the capacitor voltage difference dynamics, based on a continuous model of the converter, is first carried out. It leads to an approximated model which contains explicitly several sinusoidal functions of time. In view of this result, the voltage imbalance phenomenon can be addressed as an output regulation problem, considering the sinusoidal functions as exogenous disturbances. Thus, a novel approach to deal with the mentioned problem in the back- to-back converter is presented. Then, the particular features of the disturbances are used to design several controllers. They all follow an asymptotic disturbance rejection approach. In this way, the estimations of the disturbances are used to apply a control law that cancels them while regulating the capacitor voltage balance as well. Finally, the performance of the proposed control laws is evaluated, presenting the simulation results obtained when the different controllers are implemented.MICINN-FEDER DPI2009-0966

Voltage balancing in three-level neutral-point-clamped converters via Luenberger observer

This paper addresses the problems associated with the dc-link capacitor voltages of the three-level neutral-point-clamped power converter: the imbalance of the capacitor voltages as well as the presence of an ac-voltage low-frequency oscillation in the dc link of the converter. In order to cope with them, a mathematical analysis of the capacitor voltage difference dynamics, based on a direct average continuous model, is carried out, considering a singular perturbation approach. The analysis leads to a final expression where a sinusoidal disturbance appears explicitly. Consequently, the two problems can be handled together using the ordinary formulation of a problem of regulating the output of a system subject to sinusoidal disturbances, applying classical control theory to design the controller. In this way, the controller is designed including the disturbance estimate provided by a Luenberger observer to asymptotically cancel the disturbance, while keeping also balanced the capacitor voltages. Experiments for a synchronous three-level neutral-point-clamped converter prototype are carried out to evaluate the performance and usefulness of the converter working as a grid-connected inverter under the proposed control law.MICINN-FEDER DPI2009-09661Junta de Andalucía P07-TIC-0299

A controller for practical stability of capacitor voltages in a five-level diode-clamped converter

The balancing of the dc-link capacitor voltages represents the main technical challenge in multilevel diode-clamped converters. In particular, when the number of capacitors is greater than two, these power conversion systems must be actively controlled to achieve a correct voltage sharing. Focusing on the five-level diode-clamped converter topology, this paper deals with the voltage imbalance phenomenon from a control point of view. For that purpose, the voltage balancing objective is addressed as a problem of ensuring the practical stability of a nonlinear system under the presence of external disturbances. Considering this approach, a novel control method is proposed to regulate the outputs of the system. It is based on several steps, which should be carried out during the controller design stage, that lead to define a discrete-time controller with sampling frequency corresponding to the triple of the ac-source frequency. Finally, the performance of the proposed controller is analyzed under varying operating conditions to demonstrate the capability and viability of the controller to attain the control goals.Unión EuropeaMinisterio de Economía y CompetitividadJunta de Andalucí