Spectrum Estimation of Input Current Ripple on a Wide Class of Multilevel Grid-Tied Converters

Multilevel (ML) converters are frequently used to implement grid-tied ac-dc conversion systems. Their design may benefit from multiobjective optimization techniques, which typically involves time-consuming circuit simulations in order to obtain input current estimations suitable for input inductor and electromagnetic interference filter design. Herein, a closed-form expression of the input current ripple is derived to ease harmonic content estimations. The proposed approach separates the fundamental grid-current component from its ripple and models the latter like an amplitude modulation, where the modulating signal is its envelope and the carrier is the triangular current waveform. First, a general waveform analysis of ML converters is performed to derive the voltage across the grid-side inductor, then the associated current ripple is modeled. Experimental results on an ML converter prototype are reported to validate the analytical results

Black-Box Large-Signal Average Modeling of DC-DC Converters Using NARX-ANNs

This paper investigates the use of non-linear autoregressive exogenous (NARX) artificial neural networks (ANNs) to achieve black-box average dynamic models of dc-dc converters capable of capturing the main converter non-linearities. Non-linearities may include, for example, dynamic behavior variations due to changes of operating point or operating mode (e.g., discontinuous conduction mode, continuous conduction mode). This paper presents design guidelines for determining the NARX-ANN architecture and the dataset to be used in the training process. Dataset definition includes the choice of the perturbations for stimulating the aimed system behaviors and optimizations for dataset size reduction. The proposed approach is first derived for a dc-dc boost converter. To verify the generality of the proposed method, the same methodology is also applied to a Ćuk converter. In both cases, the proposed NARX-ANN modeling provided accurate results, with only limited deviations observed in the time-domain responses to step variations of duty-cycle and output current. The proposed model provided accurate small-signal behavior under different operating conditions. The validity of the approach is evaluated experimentally by considering a boost converter prototype

Analysis and Design of a Partial-Power Post-Regulator Based DC/DC Converter for Automotive Applications

This paper proposes and analyzes a partial-power (PP) converter for electric vehicle charging applications, where high efficiency over a wide range of battery voltages is required. The proposed converter employs a first dual-output isolation stage and a second dual-input buck post-regulator. In such a post-regulator, only a fraction of the transferred power is processed, whereas the other part of the output power flows directly to the load without any conversion losses. This allows high efficiency even with output voltages that may vary over a wide range. The isolation stage is always operated at resonance, it behaves as a dual-output dc-transformer (DCX), ensuring very high efficiency. The conversion structure, analysis, and design considerations are shown considering a prototype rated 10 kW and interfacing a 800 V dc-link with an output bus in the voltage range 250 V - 500 V, which is common in electric vehicle battery-charging applications

Online Loss Reduction of Isolated Bidirectional DC-DC Quad-Active Bridge Converters

Isolated multi-port converters can interconnect several sources and loads operating at different voltage levels by their ports, with the advantage of galvanic isolation and shared magnetics. However, challenges exist for such converters, due to coupling between the ports, the high number of modulation variables, and, in general, their modeling complexity. This paper discusses the operation of the quad-active bridge (QAB) and presents its improved operation by a model-free four-dimensional ripple correlation control (4D-RCC). First, fundamental component analysis of the converter is carried out for the QAB, driving a decoupling matrix that ensures good closed-loop control performance for each port individually. Then, the 4D-RCC is presented in the case of the QAB to perform an online optimization of the overall converter efficiency. The presented optimization exploits orthogonal signals for the simultaneous adjustment of the duty-cycles of the four ports of the QAB. The validation of the analytical results and the proposed optimization approach are reported by Matlab/Simulink simulations and experimental results considering a converter prototype rated 5kW

Optimization Approaches for RMS Current Reduction of Triple Active Bridge Converters

Isolated multi-port converters can interconnect different loads and energy sources at their ports, while utilizing a limited number of switching devices and magnetic components, which offers potential advantages in terms of power density. However, being the multiple ports coupled among each other, the number of modulation variables and operating modes increases, which poses challenging optimization issues. This paper exploits three different optimization approaches used to optimize the performance of a triple active bridge converter (TAB) by minimizing the ports total true rms current. The three approaches shown herein are based on an offline gradient descent search, online multidimensional ripple correlation search, and artificial neural network. All the approaches are validated through simulation and experimental results considering a TAB converter prototype rated 5 kW

Multidimensional Ripple Correlation Technique for Optimal Operation of Triple Active-Bridge Converters

The paper presents a multidimensional ripple correlation search technique of optimal operating points of triple active bridge (TAB) converters. Such converters present multiple modulation parameters that should be exploited to achieve high operation efficiency. On the other hand, the several degrees of freedom available make the identification of optimal parameters a challenging task, not easily tackled analytically or in closed form. A model-free on-line search method based on the ripple correlation technique is then proposed in this paper. The proposed method finds the optimum modulation parameters of TAB converters utilizing a three-dimensional ripple correlation control. The key property of the proposed solution is the adoption of orthogonal perturbation signals, where the orthogonality is simply obtained using different injection frequencies. The multidimensional correlation technique originally shown herein can be applied to other generic optimization problems. The proposed search is verified through a hardware-in-the-loop validation setup and an experimental prototype rated $5kW$

High Frequency Passivity Properties of Grid-Connected Admittance with Double-Sampling Asymmetric Dual-Edge Modulator

Small-signal stability and dynamic interactions among power electronic converters (PECs) and electrical grids are widely analyzed using the impedance-based approach. To reduce such interactions, a desirable feature of PECs is that their admittance exhibits dissipative behavior. Due to control delays, the PEC admittance usually exhibits non-dissipative zones around and above the crossover frequency of the inner control loop, possibly reducing the stability margins of the contemporary electrical grids. This paper proposes the double-sampling asymmetric dual-edge digital pulse-width modulator (ADE-DPWM) as an effective way to improve the passivity properties of the PEC admittance. Even in the digital implementation, the ADE-DPWM features zero delay and almost unity magnitude up to half of the switching frequency. Moreover, this paper examines the influence of ADE-DPWM on the PEC admittance even at higher frequencies, where destabilization of poorly damped grid resonances may be influenced by sampling and pulse-width modulation sidebands. Due to an operating point dependent ADE-DPWM small-signal model, the high-frequency passivity properties highly depend on the steady-state operating point. The analytical predictions are shown to be in excellent agreement with the experimental admittance measurements up to twice the switching frequency for all tested steady-state operating points

Per-Phase Power Controller for Smooth Islanded Transitions in Three-Phase Three-Wire Systems

This manuscript describes the operation of a droop-based controller for three-phase converters in the case of the absence of a neutral connection to the grid. The controller is capable of output power tracking and smooth transitions into the islanded operation. While independent per-phase control of the converter output power is possible if a neutral connection is present, its absence implies additional constraints to be considered. Focusing on this latter case, the controller described herein allows the independent control of the active power at the output of each phase of the converter and a smooth transition to the islanded operation. These features are paramount in future smart power systems, such as smart microgrids, for implementing demand–response, power-flow management, and uninterrupted power operation