Search

Three-phase pulsewidth modulated converters are usually controlled to achieve sinusoidal currents in the grid side and tight voltage regulation in the dc-link capacitor. The implementation of linear controllers based on small-signal models is a well-established solution for these converters. However, the dynamic performance can be improved only to a limited extent and it deteriorates under different operating conditions which may result in the uncontrollability of the dc-bus voltage during transients. In this work, a state-plane model and control strategy are introduced to improve the dc-link dynamics for three-phase converters. The proposed method provides fast, reliable, and consistent transient responses under the entire operating range, including rectifier and inverter modes. First, a normalized model that describes the natural dynamic behavior of the converter is derived and represented in the state-plane. In this way, the dynamic evolution of the operating point is described in a graphical domain as circular trajectories with well-defined characteristics. The insight provided by these natural trajectories is used to develop a control strategy to improve the dc-link dynamics by selecting a unique combination of circular paths to solve the transients. As a result, the operating point follows well-defined trajectories to achieve fast and consistent dc-link voltage responses under a wide operating range. The effectiveness and feasibility of the proposed control strategy are validated by several simulation and experimental results. [ABSTRACT FROM AUTHOR]

The interest in distributed generators (DGs), especially those associated with renewable energies, has increased in recent years. The number of functionalities that their commonly associated voltage source inverters (VSIs) can deal with is rising, harmonic currents compensation being one of them. VSI control is in charge of accomplishing the different capabilities, and it must be done without exceeding the limited and variable dc voltage margin, if undesirable effects want to be avoided. If limitations are reached, capabilities will be reduced in order to remain within the limitations. At the same time, as the number of DGs is also increasing, when capabilities are not able to be coped with exclusively one VSI, the cooperation among VSIs appears as a solution. A key point is to establish a strategy in this kind of topology when one of the VSI is brought to its limits. In this paper, a new strategy is proposed, where even under limited situation VSIs always try to inject the maximum power extracted from the DG and achieve the harmonic compensation (multifrequency) by cooperation among the different parallel VSIs. Newest multifrequency saturation strategies are studied, modified, and applied to cooperative applications. Simulations, hardware in the loop (HiL), and experimental results are included and compared. [ABSTRACT FROM AUTHOR]

LLC resonant converters suffer from a startup inrush current that may push power switches beyond the safe operating area. The conventional method of limiting the startup inrush current is to adopt the frequency decrement method; however, doing so, results in the overdesign of magnetic components and the gate driver circuit, sluggish startup response, and circulating current in light and no-load conditions. To tackle these problems, this paper proposes a nonlinear controller called the average geometric controller (AGC), offering low-cost implementation requirements. The proposed controller uses the advantages of a new LLC average large-signal model, in order to analyze the large-signal nature of the converter. Studying the average nature of LLC converters enables the use of low-bandwidth sensors, and lower sampling rates, while improving the system performance. In addition to limiting the startup inrush current without employing a very high switching frequency, the proposed AGC provides an extreme dynamic startup response for LLC converters, with near zero voltage overshoot. In order to validate the theoretical analysis, experimental results of an LLC converter are presented using the proposed controller. Comparative experimental analysis is performed with a conventional method for limiting inrush current. Moreover, the accuracy of the average large-signal model is experimentally validated. [ABSTRACT FROM PUBLISHER]

Linear controllers and small-signal modeling techniques have been used for over 40 years to control power electronic converters around the equilibrium operating point. Outside the quiescent area, the shortcomings of linear controllers are well known and include uncertain large-signal transient behavior and sluggish dynamic/recovery response. In particular, resonant converters have inherent large-signal behavior, making the transient performance of small-signal linear controllers poor. This paper develops a geometric controller based on large-signal modeling to achieve enhanced dynamic response of the series resonant converter (SRC). The proposed modeling strategy provides detailed insight into the behavior of the SRC and creates an analytical framework to successfully perform geometric control. A new geometric control law is mathematically derived using average circular trajectories producing accurate and fast dynamics during startup, sudden load or reference changes. In order to validate the theoretical analysis, experimental and simulation results of an SRC are presented and compared to those of a linear and a nonlinear controller. The dynamic response is, on average, over 100 times faster than that of the designed linear controller, while overshoot is virtually eliminated. [ABSTRACT FROM AUTHOR]

The control of boost-derived converters presents a special challenge due to its non-minimum phase behavior, forcing a slow transient response when traditional linear techniques are employed. In order to improve the dynamic behavior, more complex controllers have been proposed (e.g., adaptive, nonlinear, boundary, etc.), but the improvements introduced by each one of them cannot be objectively assessed in the absence of a theoretical performance limit standard reference. This work introduces a benchmarking tool based on the evaluation of the converters’ performance under start-up, load current, and input voltage step-up/step-down transients. Six performance indices are defined using the theoretical performance limits as reference. In this way, the converters’ dynamic performance can be quantitatively assessed in six different aspects, enabling a thorough benchmarking procedure. The ideal parameters are found by exploring the performance limits of boost converters, providing valuable insight into the behavior of the topology. These reference parameters are found in a normalized domain and characterized by closed-form expressions, providing simple, intuitive, and general results valid for any combination of LC parameters and voltage levels. The characterization of the dynamic performance limits is validated by experimental results. The proposed benchmarking procedure is illustrated by evaluating the dynamic performance of two different dc–dc boost converters. [ABSTRACT FROM PUBLISHER]

Dual active bridge converters enable bidirectional power flow in buck and boost operating modes. This paper presents an advanced switching sequence and burst-mode strategy to balance conduction, switching, and magnetic losses under light, medium, and heavy loading conditions, leading to improved operating efficiency. The implementation of the switching sequence employs the natural state-plane trajectories of the converter and contributes to higher efficiency and the ability to perform burst mode. The proposed switching sequences improve the overall efficiency of the converter by enabling soft switching and adjusting the frequency to match the minimum RMS transformer current in the full operating range. Furthermore, it incorporates a fully controlled burst-mode switching sequence for light loading conditions to further extend the efficiency gains. As a result, maximum efficiency is obtained by taking advantage of all the possible switching structures of the converter. The analysis provides insight into the natural trajectories of the converter, which produce soft-switching transitions and enable the converter structures to achieve the target operating point directly. Simulation and experimental results are presented to validate the benefits of the switching sequence and illustrate the burst-mode operation. [ABSTRACT FROM PUBLISHER]

Traditional linear techniques are widely used for the control of basic dc–dc converters due to their simple implementation. However, due to the small-signal validity range of the models employed, the converters usually perform poorly under large transients, and the dynamic response can be improved only to a limited extent in order to ensure a stable behavior. On the other hand, faster dynamic response can be achieved with boundary controllers, which require faster sensors and more powerful processors. A novel control scheme that combines the advantages of fixed-frequency pulsewidth modulation with state-plane geometric analysis is introduced to obtain fast and reliable large-signal response. The natural evolution of the average state variables is described by a large-signal unified model, which provides the basis to develop a reliable nonlinear control scheme. The proposed technique is suitable for implementation in low-cost digital signal processors, using low-bandwidth sensing stages, and it features fast, sleek, and consistent dynamic response with constant switching frequency. Since the model developed accurately predicts the large-signal behavior, reliable and predictable responses can be obtained at any operating point. In this way, the transient response obtained shows reduced, consistent, and well-determined peaks in inductor current and capacitor voltage, avoiding magnetic saturation and system failures even during extremely large transients. Furthermore, the maximum rating specifications for the reactive components can be reduced, which, combined with the low requirements for sensors and processors, lowers the implementation cost and makes the controller a very appealing alternative for high-volume applications. The contributions made to the theoretical and applied field are valid for any combination of reactive components due to the normalized approach adopted. The theoretical concepts are supported by detailed mathematical procedures. The proposed theory and controller are validated by experimental results. [ABSTRACT FROM PUBLISHER]

For reliability reasons, the employment of small film capacitors instead of electrolytic ones is an interesting alternative for the dc-link in single-phase inverters for fuel cell applications. Due to the low capacitance that can be accomplished at an acceptable cost using this technology, there are large low-frequency voltage fluctuations (100/120 Hz and harmonics) in the dc-link caused by the double-frequency power transfer. By allowing these variations in the bus, the capacitor bank absorbs the current ripple from the inverter to avoid detrimental oscillations in the fuel cell. Traditional control strategies for inverters are usually designed to operate with nearly constant input voltage and are not able to effectively handle large (e.g., ) low-frequency input voltage fluctuations. This paper introduces the analysis of a swinging bus in the context of fuel cell standalone applications (i.e., voltage-source inverters) and proposes a nonlinear control approach to operate inverters with very large input voltage swing: the natural switching surface (NSS). Under the proposed scheme, the inverter presents excellent dynamic and steady-state characteristics, even at moderate switching frequency (e.g., 3.6 kHz). In order to illustrate the superior performance of the NSS, a comparison to a proportional-resonant (PR) controller is performed. Unlike the linear compensator, the NSS is able to reject the large bus voltage oscillations and achieve high-quality output voltage with low total harmonic distortion (THD). Simulation and experimental results are provided to illustrate the behavior of the swinging bus and to validate the NSS control scheme under the proposed demanding operating conditions. [ABSTRACT FROM AUTHOR]

This paper presents the use of a high-performance boundary controller for the full-bridge zero-voltage-switching topology. An enhanced dynamic response is obtained by employing the natural switching surface (SS), which is thoroughly derived in the normalized geometrical domain. The advantages of the normalization are the simple graphical representation, the generality for any combination of parameters, and the mathematical simplicity. Recently, nonisolated basic topologies have benefited from advancements in boundary control. The analysis and derivation in this work bring the benefit of outstanding dynamic performance to this isolated topology. As demonstrated in this work, the relationship between the leakage and output filter inductances makes the formulation of the natural trajectories for isolated converters possible. The resulting SS provides an excellent dynamic response during start-up, reference change, and sudden output loading conditions. Experimental results are presented to illustrate the characteristics and advantages of the control scheme and the converter operation with fixed switching frequency. [ABSTRACT FROM AUTHOR]

This study presents the application of the natural switching surface (NSS) to control single-phase power factor correctors (PFCs). The proposed boundary control scheme provides tight regulation and enhanced dynamic characteristics. The analysis is performed in the normalized geometrical domain to provide valuable graphical insight and generality. Low-frequency ripple effects are included as part of the converter reference to improve its behavior in steady state and load transient operation. In addition to the complete characterization of the system, a method to obtain fixed-frequency operation is presented by employing discrete-step references. The resulting switching surface (SS) provides fast output voltage regulation and input current dynamics, and low input harmonic distortion, achieving steady state in only a few switching actions under sudden load disturbances. The enhanced qualities of the natural SS are confirmed with simulation and experimental results of a 300 WPFC. [ABSTRACT FROM AUTHOR]