1,720,971 research outputs found
Analysis of multi-sampled pulsewidth modulation and its application in grid-tied converters
No full textPower electronic systems are essential parts of most modern technologies that depend on converting electrical energy from one form to another. One of such applications is the electric power grid, where more and more grid-tied converters are present, bringing a true paradigm shift in the generation, transmission, distribution, and consumption of electricity.
To enable their highly-sophisticated usage as energy-processors, power electronic converters are actively controlled.
For a set hardware design, it is the task of the control system to enable highest dynamic performance, which determines the overall system capabilities. The actuation of any power electronic converter is determined by the modulation strategy, which dynamically adjusts its switching states in order to achieve the desired system behavior.
Nowadays, power electronic control systems are typically implemented within digital control platforms, as they offer much higher levels of flexibility and unmatched regulation capacities with respect to analog control systems.
However, significant demerits of digital control are the associated delays, which are not present when using analog controllers.
The focus of this dissertation is the multi-sampled pulsewidth modulation (MS-PWM), which relies on executing the control action more than twice per modulation period with a target of achieving analog-like dynamic performance while retaining all the benefits of the digital control systems. This dissertation brings the analysis of some fundamental aspects of this modulation strategy, as well as an investigation of its applicability in grid-tied power electronic converters.Power electronic systems are essential parts of most modern technologies that depend on converting electrical energy from one form to another. One of such applications is the electric power grid, where more and more grid-tied converters are present, bringing a true paradigm shift in the generation, transmission, distribution, and consumption of electricity.
To enable their highly-sophisticated usage as energy-processors, power electronic converters are actively controlled.
For a set hardware design, it is the task of the control system to enable highest dynamic performance, which determines the overall system capabilities. The actuation of any power electronic converter is determined by the modulation strategy, which dynamically adjusts its switching states in order to achieve the desired system behavior.
Nowadays, power electronic control systems are typically implemented within digital control platforms, as they offer much higher levels of flexibility and unmatched regulation capacities with respect to analog control systems.
However, significant demerits of digital control are the associated delays, which are not present when using analog controllers.
The focus of this dissertation is the multi-sampled pulsewidth modulation (MS-PWM), which relies on executing the control action more than twice per modulation period with a target of achieving analog-like dynamic performance while retaining all the benefits of the digital control systems. This dissertation brings the analysis of some fundamental aspects of this modulation strategy, as well as an investigation of its applicability in grid-tied power electronic converters
Nonlinearities of Multi-Sampled Phase-Shifted PWM in Unbalanced Multi-Cell Converters
Full text linkThis article reveals, analyzes and proposes the method to mitigate nonlinear effects of multi-sampled multi-update (MSMU) digital phase-shifted pulsewidth modulators (PS-DPWM) that appear in unbalanced multi-cell voltage-source converters (MC-VSCs). For balanced MC-VSCs, the harmonic cancellation of PS-DPWM allows for an increase in the sampling frequency, ensuring that the average current is acquired at the peaks, valleys, and intersections of all triangular carriers. For unbalanced operation, which is typically encountered in practice, e.g., due to cell voltage mismatch in multilevel MC-VSCs and inductance mismatch in interleaved MC-VSCs, harmonic cancellation of PS-DPWM is compromised and, thus, the increased sampling frequency brings switching ripple in the feedback signal. Since in MSMU control the modulating signal is also updated at peaks, valleys, and intersections of all carriers, this may cause vertical intersections between the modulating signal and the carriers, resulting in specific nonlinear effects. The nonlinearities are shown to introduce limit-cycle oscillations and output waveform distortion. A method to prevent such detrimental impact of MSMU-PS-PWM is also proposed. A simple analytical procedure is proposed to quantify the analyzed non-linear effects, revealing that they are more emphasized for higher levels of imbalance and control bandwidth. Moreover, the modulator nonlinearity is shown to decrease as the number of cells increases. The analyses are verified in simulations and experiments, using laboratory prototypes of three- and four-level MC-VSCs
All-Port MIMO Admittance Passivity for Robust Stability of DC–DC Interlinking Converters
Full text linkThis article explores the passivity properties of a dc–dc converter's unterminated all-port MIMO admittance matrix, for analyzing and preventing the converter's destabilizing impact in grid-connecting (interlinking) scenarios with an arbitrary, even meshed, termination (grid impedance). In addition to the passivity properties of the converter's unterminated input and output self admittances, the coupling passivity property is examined, which accounts for the possible destabilizing impact of port-coupling. The dependence of these properties on the control loop parameters is exemplified using a current-controlled buck converter. Examples of two techniques for enhancing a converter's all-port MIMO admittance passivity, by active damping impedance emulation and multisampled pulse width modulation, are examined and shown to be effective. The proposed methodology is validated both in frequency and time domain, using control hardware-in-the-loop simulations, as well as experimentally, using a laboratory prototype
High Performance Multi-Sampled Control for Power Electronics Converters
No full textThis article presents a review of multi-sampled pulse-width modulation (MS-PWM), where feedback sampling, control execution, and modulating signal update are performed more than twice per modulation period. Consequently, this modulation strategy enables very fast dynamic performance, robust stability, and noise suppression in two-level, multi-level, and interleaved power electronics converters. First, small-signal models are presented to show the positive impact of MS-PWM on delay reduction. Reduced modulation delay allows for an increase of the control loop bandwidth and higher disturbance rejection, which is demonstrated with several application examples including impedance-based stability of grid-connected converters. Next, nonlinear effects that may arise with MS-PWM due to the switching ripple content in the feedback signal are analyzed and some measures for their suppression are presented. Finally, the capabilities of MS-PWM to strongly attenuate white and switching noise, while preserving fast dynamic performance, are analyzed. Thereby, various linear and non-linear feedback filtering methods are discussed. Numerous experimental validations, performed on different setups with two-level and multi-level converters, are provided to support the analyses
Median Filters for Switching Noise Mitigation in Oversampled Power Electronics Control Systems
No full textThis article addresses switching noise propagation and its suppression in multi-rate control systems, where the inductor current is oversampled, digitally filtered and the control execution is decimated to single(double)-sampled pulse-width modulation (PWM). The operating points where the sampling instants occur near the switching ones are critical for switching noise sensitivity. It is shown that, when the number of the corrupted samples is high with respect to the number of samples used for filtering, typically considered moving average filters (MAFs) bring a detrimental impact. As an alternative, median filter (MED) is proposed. To decouple the ranking within MED from the switching ripple, repetitive ripple removal is added beforehand. Multi-rate PWM control with the proposed feedback filtering outperforms state-of-the-art double-sampled PWM by eliminating the well-known switching noise sensitivity for small and large duty cycles. In addition, noise-sensitive operating point regions that appear with MAF are successfully suppressed
On the Applicability of SISO and MIMO Impedance-Based Stability Assessment of DC–DC Interlinking Converters
Full text linkThis article presents a formal mathematical correlation between the standardly used port-level (terminated) single-input single-output (SISO), and the recently acknowledged device-level (unterminated) multiple-input multiple-output (MIMO) impedance-based method for the stability assessment of dc–dc interlinking converters. Based on this, the conditions that must be met to ensure the correct stability assessment by the SISO method applied to a single port-pair are derived. It is shown that without prior knowledge on whether these conditions are met, the SISO method must be applied to every port-pair to account for possible port-level hidden dynamics . Alternatively, the MIMO method can be used, which is revealed to inherently account for any port-level hidden dynamics . It is further analyzed which method is advantageous in terms of computational complexity, intuitiveness, and simplicity for applications featuring meshed grids or multiport interlinking converters, as well as in terms of interpreting the resulting stability margins. Finally, suitability of the MIMO method for termination-independent stability-oriented controller design and stability assessment based on measurements is highlighted. The presented methodology is illustrated for a simplified dc system with a current-controlled buck converter. Analytical stability predictions are validated using hardware-in-the-loop simulations and also experimentally, using a laboratory hardware prototype
Switching Noise Propagation and Suppression in Multi-Sampled Power Electronics Control Systems
Full text linkThis article addresses switching noise propagation and its suppression in current-controlled systems with multisampled pulsewidth modulation (MS-PWM). MS-PWM enables very high control bandwidths by reducing digital delays. However, when the sampling instants occur near the commutation ones, system performance is prone to being impaired by switching noise. It is analyzed how using the typically considered moving average filters (MAFs) in feedback may have an adverse effect, especially when the number of the noise-corrupted samples is high compared with the number of averaged samples. It is also shown that, without any filters, MS-PWM on its own may mask the negative impact of noise, due to modulator-related nonlinear effects. However, these nonlinearities can lead to an undesirable response to transients and output waveform distortion. Hence, this article proposes MS-PWM with median-based feedback filtering. To avoid ranking within median filter (MED) being affected by the switching ripple, repetitive ripple removal (RRR) is added before MED. The effectiveness of the proposed strategy in suppressing the switching noise is verified in simulations and experiments, during dc and ac operations. RRR+MED successfully suppresses noise-sensitive operating point regions that appear with MAF. Finally, it is shown that, even with added RRR+MED, MS-PWM still retains dynamic improvements over the standard DS-PWM without any filters, offering better reference tracking and disturbance rejection
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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