Suppression of Second-Order Harmonic Current for Droop-Controlled Distributed Energy Resource Converters in DC Microgrids

Droop-controlled distributed energy resource converters in dc microgrids usually show low output impedances. When coupled with ac systems, second-order harmonics typically appear on the dc-bus voltage, causing significant harmonic currents at the converters resource side. This paper shows how to reduce such undesired currents by means of notch filters and resonant regulators included in the converters control loops. The main characteristics of these techniques in terms of harmonic attenuation and stability are systematically investigated. In particular, it is shown that the voltage control-loop bandwidth is limited to be below twice the line frequency to avoid instability. Then, a modified notch filter and a modified resonant regulator are proposed, allowing to remove the constraint on the voltage loop bandwidth. The resulting methods (i.e., the notch filter, the resonant regulator, and their corresponding modified versions) are evaluated in terms of output impedance and stability. Experimental results from a dc microgrid prototype composed of three dc-dc converters and one dc-ac converter, all with a rated power of 5kW, are reported

Plug and Play DC-DC Converters for Smart DC Nanogrids with Advanced Control Ancillary Services

This paper gives a general view of the control possibilities for dc-dc converters in dc nanogrids. A widely adopted control method is the droop control, which is able to achieve proportional load sharing among multiple sources and to stabilize the voltage of the dc distribution bus. Based on the droop control, several advanced control functions can be implemented. For example, power-based droop controllers allow dc-dc converters to operate with power flow control or droop control, whether the hosting nanogrid is operating connected to a strong upstream grid or it is operating autonomously (i.e., islanded). Converters can also be equipped with various supporting functions. Functions that are expected to play a crucial role in nanogrids that fully embrace the plug-and-play paradigm are those aiming at the monitoring and tuning of the key performance indices of the control loops. On-line stability monitoring tools respond to this need, by continuously providing estimates of the stability margins of the loops of interest; self- tuning can be eventually achieved on the basis of the obtained estimates. These control solutions can significantly enhance the operation and the plug-and-play feature of dc nanogrids, even with a variable number of hosted converters. Experimental results are reported to show the performance of the control approaches

Analysis of an On-Line Stability Monitoring Approach for DC Microgrid Power Converters

An online approach to evaluate and monitor the stability margins of dc microgrid power converters is presented in this paper. The discussed online stability monitoring technique is based on the Middlebrook's loop-gain measurement technique, adapted to the digitally controlled power converters. In this approach, a perturbation is injected into a specific digital control loop of the converter and after measuring the loop gain, its crossover frequency and phase margin are continuously evaluated and monitored. The complete analytical derivation of the model, as well as detailed design aspects, are reported. In addition, the presence of multiple power converters connected to the same dc bus, all having the stability monitoring unit, is also investigated. An experimental microgrid prototype is implemented and considered to validate the theoretical analysis and simulation results, and to evaluate the effectiveness of the digital implementation of the technique for different control loops. The obtained results confirm the expected performance of the stability monitoring tool in steady-state and transient operating conditions. The proposed method can be extended to generic control loops in power converters operating in dc microgrids

Vanadium redox flow batteries: Potentials and challenges of an emerging storage technology

open4noIn this paper an overview of Vanadium Redox Flow Battery technologies, architectures, applications and power electronic interfaces is given. These systems show promising features for energy storage in smart grid applications, where the intermittent power produced by renewable sources must meet strict load requests and economical opportunities. This paper reviews the vanadium-based technology for redox flow batteries and highlights its strengths and weaknesses, outlining the research lines that aim at taking it to full commercial success.openSpagnuolo, Giovanni, Guarnieri, Massimo; Mattavelli, Paolo; Petrone, Giovanni;Guarnieri, Massimo; Mattavelli, Paolo; Petrone, Giovanni; Spagnuolo, Giovann

Personalized Medicine in Skull Base and Sinonasal Tumors

: Skull base and sinonasal tumors (SBSNTs) represent a considerable challenge for clinicians in view of their rarity, anatomical complexity of the site of origin, and great histological variety [...]

Synchronous-Frame Harmonic Control for High-Performance AC Power Supplies

In order to achieve the reduction of voltage distortion in ac power supplies (ACPSs), this paper describes an implementation of synchronous-frame control for selected frequencies in the output voltage. The regulation of the fundamental output voltage, as well as that of some low-order harmonics, is achieved using a synchronous-frame controller for each selected frequency in addition to a conventional control. The conventional part conserves good dynamic performance under load changes, while rotating-frame controllers allow a slow, but very precise compensation of the residual errors within the assumption that the harmonics produced by distorting load are slowly varying. Moreover, motivated by a fixed-point implementation, a set of refinements and modifications of the original scheme is proposed, allowing a reduction of signal processing requirements and a new control algorithm structure less sensitive to quantization and rounding errors. This solution is particularly effective for high-power fully digitally controlled ACPSs, where the voltage loop bandwidth is usually not large enough to provide regulation at harmonic frequencies. The proposed control scheme has been implemented using a fixed-point single-chip digital signal processor (ADMC401 by Analog Devices). Experimental results on a 3-kVA three-phase converter prototype show the effectiveness of the proposed approach

An improved modulation strategy for the three-phase Z-source inverters (ZSIs)

Z-source inverters (ZSIs), compared to the conventional two-stage architecture, embrace some interesting features, like the reduced size and complexity of the entire conversion system. Many research activities have been established to improve the performance of the so-called ZSI since it has been proposed in 2003, and several modifications have been introduced since then. These modifications include the structure of the ZSI, i.e. modifying the topology itself, and its modulation scheme as well. From the modulation perspective, the existing modulation strategies suffer from some demerits, such as the increased number of switch commutations at high current during the entire fundamental period and the utilization of extra reference signals. In this paper, an improved modulation strategy is proposed in order to enhance the performance of the three-phase ZSIs and the equivalent topologies. The proposed modulation strategy, which is called simple-boost modified space vector (SBMSV) modulation, reduces the number of switch commutations for shorter period during the fundamental cycle, simplifies the generation of the gate signals by utilizing only three reference signals, and achieves a single switch commutation at a time. This modulation strategy is analyzed and compared to the conventional equivalent modulation strategy, where a reduced-scale 1 kVA three-phase ZSI is designed and simulated using MATLAB/PLECS models. Finally, the designed 1 kVA three-phase ZSI is implemented experimentally in order to verify the proposed modulation strategy, the reported analysis, and the simulation results