Showing papers by "Marco Liserre published in 2005"

Synchronization methods for three phase distributed power generation systems - An overview and evaluation

Abstract: Nowadays, it is a general trend to increase the electricity production using distributed power generation systems (DPGS) based on renewable energy resources such as wind, sun or hydrogen. Jf these systems are not properly controlled, their connection to the utility network can generate problems on the grid side. Therefore, considerations about power generation, safe running and grid synchronization must be done before connecting these systems to the utility network. This paper is mainly dealing with the grid synchronization issues of distributed systems. An overview of the synchronization methods as well as their major characteristics is given. New solutions to optimize the synchronization methods when running on distorted grid conditions are discussed. Simulation and experimental results are used to evaluate the behavior of the synchronization methods under different kind of grid disturbances such as voltage dips, harmonics and notches.

Grid Impedance Estimation via Excitation of $LCL$ -Filter Resonance

Abstract: Inverters adopted in distributed power generation, active filter, and uninterruptible power supply are often connected to the grid through an inductance-capacitor-inductance (LCL) filter. The impedance of the LCL filter has a typical frequency characteristic with a resonance peak. Hence, the LCL filter has to be damped in order to avoid instability. However, the resonance of the LCL filter can be also excited in a controlled way in order to individuate the resonance frequency in the spectrum (using for example the fast Fourier transform). This paper proposes to use a controlled excitation in measuring the grid impedance, since this one influences also the resonance frequency. This paper will address some possible limits, some solutions, and some implementation issues (e.g., how to obtain a controlled resonance in the filter without damaging the system) in order to use the resonant peak for grid impedance detection. The analysis is validated both by simulations and experimental results.

An energy-based control for an n-H-bridges multilevel active rectifier

Abstract: This paper deals with the control of a multilevel n-H-bridges front-end rectifier. This topology allows n distinct dc buses to be fed by the same ac source offering a high loading flexibility suitable for traction applications as well as for industrial automation plants. However, this flexibility can lead the system to instability if the dc buses operate at different voltage levels and with unbalanced loads. Thus, linear controllers, designed on the basis of the small-signal linearization, are not effective any longer and stability can not be ensured as large-signal disturbances occur. The use of a passivity-based control (PBC) designed via energy considerations and without small-signal linearization properly fits stability problems related to this type of converter. The system has been split into n subsystems via energy considerations in order to achieve the separate control of each dc bus and its stability in case of load changes or disturbances generated by other buses. Then, a set of n passivity-based controllers (one for each subsystem) is adopted: the controllers are linked using dynamical parameters computed through energy balance equations. Hence, the system dc buses are independent and stable as experimental results demonstrate.

Implementation issues of a fuzzy-logic-based three-phase active rectifier employing only Voltage sensors

Abstract: Switch-mode rectifiers are becoming more and more interesting in those applications where bidirectional power flow, power-factor correction, and electromagnetic interference reduction are required. Typical active rectifiers include current or voltage sensors, however, in order to obtain low-cost systems, their number should be reduced. In this paper, a current-sensorless active rectifier with feedforward fuzzy logic control is presented and discussed. The mathematical description of the system and the design of the fuzzy logic controller are introduced and analyzed; moreover the implementation on a SH7047 microcontroller is presented with emphasis on the most significant implementation issues. Experimental results are reported and discussed.

Low-cost digital implementation of proportional-resonant current controllers for PV inverter applications using delta operator

Abstract: The performances of the P+Resonant controller in case of current control for a single phase grid connected inverter have been proved to be superior to the PI controller, since it is able to remove the phase error of the control at the fundamental frequency of the grid. It offers also the possibility of selective harmonic compensation. However, in case of digital implementation on a low-cost fixed-point DSP, the limited computational power and the limited numerical representation precision can restrict the utilization of it. The present paper proposes a different way of digital implementation of the P+Resonant controller with selective harmonic compensation on a low-cost fixed-point DSP. The resonant part of the P+R has been implemented as a second order filter based on delta operator. The current controller, together with harmonic compensation for the 3/sup rd/, 5/sup th/, and 7/sup th/ harmonics has been successfully tested in practice with a 1.5 KW PV inverter, achieving a 1.2% total harmonic distortion (THD) of the grid current.

A generalised hybrid multilevel modulation technique developed in case of non-integer ratio among the dc-link voltages

Abstract: This paper proposes a new multilevel modulation technique designed to work with different dc-link voltage levels for each of the inverter employed in a cascade structure. This modulation technique belongs to the family of hybrid modulation techniques but it does not require that the dc-link voltage levels are in an integer ratio among them. Hence it is suited for both distributed generated multilevel converters fed by different renewable sources and UPFC, employing multilevel converters, where the regulation of the dc voltage levels can be used to meet different design requirements. Moreover the proposed generalized hybrid multilevel technique obtains optimal harmonic cancellation in respect to the carrier shifting technique that fails to eliminate some side-band harmonics due to the different dc-voltage levels. In this paper the new modulation technique is described, being compared with the traditional hybrid modulation technique, that in case of non-integer relation among the dc-links voltage levels fails in obtaining a proper first harmonic value, and with the carrier shifting technique that results in an higher harmonic content.

Islanding detection method for single-phase distributed generation systems based on inverters

Abstract: The detection of the islanding condition of a distributed generation system is crucial for safety reason, as discussed in IEEE Standards, and specifically required by some national codes Several anti-islanding methods have been investigated and classified as passive (measurement of the natural effects of islanding) and active (based on the measurement of the effects due to transients or harmonics deliberately introduced in the system) In case the power drained by the load matches the power generated by the DG inverter the effect of islanding are small and the passive detection methods fail However the active methods that have been developed to overcome these limits create disturbances that can interact with those generated by other DG systems In this paper a new anti-islanding method is proposed It exploits the natural sensitivity to disturbances of a grid voltage sensorless control to highlight islanding condition The adopted grid voltage sensorless control is adapted to a single-phase system with the use of resonant controllers on the basis of the internal model control law: a resonant controller-based observer results Then a Kalman-filter-based algorithm is used to detect the islanding condition on the basis of the energy mismatch between the estimated 3/sup rd/ and 5/sup th/ harmonics and the real ones Simulation and experimental results support the analysis

Active front end adjustable speed drives under grid voltage sags: effects and dynamical performance evaluation

Abstract: In this paper an evaluation of voltage sags effects on active front end adjustable speed drives (ASD's) performance has been carried out The operation of such devices is affected by several disturbances that can occur on the electrical grid According to a number of surveys on power quality, voltage sags represent the main cause of disturbances which effects are even more evident than the ones related to harmonics Therefore, the behavior and the dynamical performances of field-orientated and direct torque control drives under grid symmetrical voltage sags are investigated and simulation as well as experimental results are presented This kind of analysis provides useful advices for future work on how drive control technique can be modified and implemented in order to fulfill some important requirements above all on speed regulation and grid currents