Carlo Cecati

An Improved Circulating Current Injection Method for Modular Multilevel Converters in Variable-Speed Drives

Modular multilevel converters (MMC) represent an interesting and emerging topology in medium-voltage motor drive applications. The main challenge of using such a topology in variable-speed drives is the large voltage ripple of submodule capacitors at low speed with constant torque. In this paper, an improved circulating current injection method is proposed, which does not completely eliminate the capacitor voltage ripple, but maintains it bounded within reasonable values. As a result, magnitude of injected circulating current is reduced, leading to converter efficiency improvement and reduction of semiconductor current ratings. Dimensioning of submodule capacitance is also discussed, which is an important consideration when designing the MMCs in variable-speed drives. The proposed method has been successfully validated by simulation and experimental results.

A Single-Stage LED Driver Based on SEPIC and LLC Circuits

A single-stage light-emitting diode (LED) driver has been developed integrating a single-ended primary-inductor converter with a half-bridge LLC resonant converter. The proposed topology reduces systems cost, enhancing reliability. Because the LLC resonant part maintains soft-switching characteristics, switching losses are relatively low. Due to careful parameters selection, the system bus voltage can be kept low as needed in high-power LED drive designs. Some experiments using a 100-W prototype were performed to validate theoretical analysis. The obtained power factor was as high as 0.99, and due to soft-switching operations efficiency was up to 92% at full load.

Optimal Low Switching Frequency Pulsewidth Modulation of Current-Fed Three-Level Converter for Solar Power Integration

Large-scale photovoltaic energy conversion systems have been installed at many places across the world and they handle higher power levels. In high power conversion, low device switching frequency operation is preferred in order to satisfy the thermal constraints of semiconductor devices and also to improve efficiency. However, low device switching frequency operation leads to higher total harmonic distortion (THD) of the converter output currents. Synchronous optimal pulsewidth modulation (SOP) is an emerging low device switching frequency modulation technique, which has been successfully implemented for voltage source inverters without compromising on THD. The goal of our research is to propose, analyze, and implement a modified SOP technique for current source inverter topologies. The additional switching constraints for these inverter topologies have been included in the modified SOP technique. The experimental results obtained from a low-power prototype demonstrated its effectiveness.

Dynamic modeling and optimal control for hybrid electric vehicle drivetrain

In this paper, the series-parallel drivetrain architecture present in Hybrid Electric Vehicles is studied and modeled in detail using Matlab/Simulink. The system is composed of two permanent magnet synchronous machines in addition to the internal combustion engine all mechanically combined using a power split device. The novelty of this model is to use exclusively basic Simulink blocks resulting in a less execution time especially advantageous during implementation. Furthermore, the new modeling approach is focused on optimal control of both engine and electric machines in wide speed region. The obtained simulation results are proving the accuracy of the simpler dynamic model and the efficiency of its control.

A Proposal for a Multi-Agent based Synchronization Method for Distributed Generators in Micro-Grid Systems

A synchronization technique based on the Multi-Agent Systems approach, is proposed for a group of Distributed Generators belonging to a Micro-Grid. The Average Time Synchronization consensus algorithm is used. A detailed description of system’s hardware architecture is given and several simulations of the dynamic are performed. Since the synchronization take place on a dedicated layer, different from the power grid one, the proposed technique does not require voltage and current measurements. This gives scalable, flexible and resilient characteristics to the system by construction.

Graphical THD minimization procedure for single phase five-level converters

This paper proposes two graphycal procedure to mitigate h harmonics and to minimize the total harmonic distortion (THD), respectively. The paper considers a five-level inverter and fundamental frequency modulation: it computes the two switching angles able to reduce low order harmonics and able to minimize the THD. Simulation results are obtained and, in order to validate the proposed procedure, an experimental prototype is built and experimental results are carried out. The comparison between simulation and experimental results confirms the accuracy of the proposed graphycal procedure. It is shown that the switching angles to mitigate the third-fifth and the third-fifth-seventh harmonics exist only for few values of modulation index. The angles obtained by the THD minimization procedure and the corrensponding values of the amplitudes of the harmonic components are shown to verify the respect of grid code.

Mixed harmonic elimination and reduction technique for single phase nine level converters

This paper presents a procedure that works at fundamental frequency to compute switching angles in single phase 9-level converters to eliminate the third and odd triple harmonics and to reduce other low order harmonics, obtaining a low total harmonic distortion (THD). The DC voltages feeding the converter, that are equal, but variable, allow to obtain switching angles independent on modulation index. The proposed procedure consists in a minimization of THD by means of a suitable graphical procedure. Its mathematically demonstrated that the third and all odd multiple of the third harmonic are deleted. Simulation results, obtained by using Matlab/Simulink, are shown.

Grid connected converter impedance calculation under virtual synchronous machine control

This paper presents a converter impedance calculation method under virtual synchronous machine (VSM) control. Firstly, the conventional virtual synchronous machine control strategy is presented, in which the active and reactive power control is realized. Secondly, the small signal model with VSM control is deduced, thus a calculation method for impedance in d-q frame is proposed. The effect produced by voltage and frequency adjustment in VSM control to d-q frame voltage is fully analyzed. This method also extends to capacitor voltage close loop control for static var compensation field. At last, the calculated impedance is compared with the measured impedance using Matlab/Simulink. Results validate the correctness of the proposed impedance calculation method.

Asymmetric control with hybrid/full bridge modular multilevel converter for low frequency low voltage drive application

This paper proposed one novel control method for modular multilevel converter(MMC) to tackle its low frequency operation problems as the module capacitor voltage ripples. Comparing with conventional control, this paper proposed one asymmetric control by alternating work sates of upper and lower arms, to achieve excellent capacitor voltage ripples suppression effect. To avoid the limited modulation range and enhance performance, hybrid or full bridge MMC structure is employed. The detailed control is analytical discussed, with full simulation results to verify the proposed methods.

On flatness-based controller for shunt-connected VSC with LCL-filter for voltage dip mitigation in a weak grid

In this paper, a grid connected shunt converter for mitigation of voltage sags in weak grids is described. The VSC keeps the grid voltage magnitude at the point of connection constant by injecting reactive power to compensate for the voltage dip. A novel flatness-based control (FBC) is proposed for this purpose that gives higher performance and robustness with an inductor/capacitor/inductor (LCL)-filter in between the VSC and the grid. Simulation model includes the practical limitation of voltage source converter output voltage. Then, the simulation results for the FBC under balanced voltage dips are presented and compared with traditional proportional integral controller. Finally, the sensitivity analysis of the FBC for grid impedance and to system parameter variations is presented.