Capacitor Voltage Deviation Research Articles

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 also keeping the capacitor voltages balanced. 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.

P 230. Third generation controllable pulse parameter transcranial magnetic stimulation device

Commercially available transcranial magnetic stimulation (TMS) devices induce electric field (E-field) pulses with damped cosine shape and provide very limited control over the pulse parameters. Furthermore, conventional TMS devices consisting of a single power stage cannot produce rapid bursts (<10 ms inter pulse interval) with equal or increasing strength since the energy on the capacitor diminishes from pulse to pulse due to circuit losses, and the time between the pulses is too short for the capacitor charger to supply the lost charge. Consequently, conventional TMS topologies require combining the output of multiple power stages to generate paradigms such as paired-pulse and quadripulse stimulation. Addressing these limitations we have developed a third generation controllable pulse parameter TMS device (cTMS3) that allows extensive adjustment of the pulse parameters such as the pulse width, number and shape of phases, and directionality (ratio of positive to negative phase amplitude), as well as enables the generation of powerful pulse bursts. To summarize the cTMS3 device capabilities and compare it to conventional TMS and other cTMS devices. The coil current and electric field were measured with a Rogowski current sensor and a search coil, respectively. cTMS3 uses a circuit topology with two energy storage capacitors and four controllable switches. It can generate rapid-rate (⩽50 Hz) trains of mono/bi/polyphasic near-rectangular electric field pulses with adjustable amplitude (maximum capacitor voltages of 1 kV and 2.6 kV), pulse width (phase duration of 10 μs to over 200 μs), and directionality (positive/negative phase amplitude ratio range of 1–52). In contrast to earlier cTMS devices, in cTMS3 the pulse phases can assume 4 possible electric field levels, proportional to either of the two capacitor voltages, the capacitor voltage difference, or 0. Thus, cTMS3 can produce waveforms with a staircase shape that can approximate conventional sinusoidal current pulses ( Fig. 1 ). To counteract the loss of capacitor charge in rapid pulse bursts, the pulse width of the second and subsequent pulses can be increased, allowing their stimulation strength to be equal or even higher than the first pulse ( Fig. 2 ). In Fig. 2 , the voltage on the two energy storage capacitors is reduced by 2% and 17%, respectively, after the first pulse, but the second pulse is 67% longer than the first pulse. Consequently, the strength of the second pulse, as measured by modeled neural membrane depolarization (membrane time constant = 196 μs) is 45% higher than the first pulse. Thus, paired-pulse paradigms, where the second pulse can have higher strength than the first pulse, can be implemented. Similarly, increasing the pulse width in a rapid burst of pulses can enable quadripulse stimulation. cTMS3 offers flexibility of pulse shape adjustment unparalleled in other available TMS devices. The combination of high energy efficiency of the near rectangular pulses, pulse energy recovery, and pulse width control allow the generation of rapid bursts of pulses with equal or even increasing strength that can implement paired-pulse and quadripulse stimulation. These features could extend the capabilities of TMS as a research and diagnostic tool, and could enable optimization of the stimulus in therapeutic interventions.

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

SUMMARY 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 that 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 of time 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 estimates 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. Copyright © 2013 John Wiley & Sons, Ltd.

Finite-Set Model-Based Predictive Control for Flying-Capacitor Converters: Cost Function Design and Efficient FPGA Implementation

Recently, there has been an increase in the use of finite-set model-based predictive control (FS-MBPC) for power-electronic converters. However, the computational burden for this control scheme is very high and often restrictive for a good implementation. This means that a suitable technology and design approach should be used. In this paper, the implementation of FS-MBPC for flying-capacitor converters in field-programmable gate arrays (FPGAs) is discussed. The control is fully implemented in programmable digital logic by using a high-level design tool. This allows us to obtain very good performances (both in control quality, speed, and hardware utilization) and have a flexible, modular control configuration. The good performance is obtained by exploiting the FPGA's strong points: parallelism and pipelining. Furthermore, an improved cost function for the voltage control of the flying capacitors is proposed in this paper. Typical cost functions result in tracking control for the flying-capacitor voltages, although this does not correspond with the desired system behavior. The improved cost function offers a capacitor voltage control that corresponds more closely with the desired behavior and adds a limitation on the capacitor voltage deviation. Furthermore, the selection of the weight factor in the cost function becomes less critical.

Voltage Balancing in Five-Level Diode-Clamped Power Converters

Abstract This paper addresses the voltage imbalance problem of the dc-link capacitors in multilevel power converters. Considering the five-level diode-clamped converter, a mathematical analysis of the capacitor voltage difference dynamics is carried out. It leads to a new problem statement that relates the voltage balancing objective to a problem of ensuring the practical stability of a nonlinear system in the presence of disturbances. Then, exploiting the properties and knowledge of the disturbance patterns, a novel and simple controller is presented. Simulation results are included to validate the performance of the proposed controller.

DSP-based implementation of capacitor voltage balancing strategy for a three-phase three-level bidirectional rectifier

Digital signal processing (DSP)-based implementation of a modified space-vector modulation strategy for balancing the DC-bus capacitor voltages of a three-phase neutral-point clamped bidirectional converter is presented. The proposed modulation strategy is simple in nature. It first identifies the switching states responsible for the unbalancing of capacitor voltages and then, based on the knowledge of polarity of capacitor voltage difference and direction of power flow, uses the appropriate redundant switching vectors to achieve the balancing of capacitor voltages. The analysis and control of the converter is described. The proposed control algorithm is validated through experimental results obtained on a laboratory prototype of the converter. The performance of the converter is found excellent in terms of the line-side and load-side power quality. A perfect balancing of the DC-bus capacitor voltages is achieved under balanced as well as unbalanced load conditions. The performance of converter is also investigated with distorted mains voltages and found satisfactory. DSP of dSPACE has been used for real-time simulation and implementation of the proposed control algorithm.

DC Link Capacitor Voltage Balancing in a Three-Phase Diode Clamped Inverter Controlled by a Direct Space Vector of Line-to-Line Voltages

In this paper, the direct modulation strategy of a three-level inverter with self stabilization of the dc link voltage is extended to a five-level inverter. Therefore, a new modeling and control strategy of a five-level three-phase diode-clamped inverter (DCI) is presented. The obtained modeling shows that modulated multilevel voltages are obtained by combination of eight different three-level functions, which are called modulation functions. Therefore, a space-vector scheme without using a Park transform is explained. Based on this algorithm, the location of the reference voltage vector can be easily determined. Then, the voltage vectors are selected to generate corresponding levels and simultaneously their durations are calculated. More over, the redundancies of different switch configurations for the generation of intermediate voltages are used to limit the deviation of capacitor voltages. Experimental results are given to illustrate the proposed control strategy of the three-phase three-level diode clamped inverter. Then, obtained results for a five-level three-phase DCI with the extended version of the control strategy are presented to show the good performances of the proposed balancing modulation.