Showing papers by "Marco Liserre published in 2009"

Evaluation of Current Controllers for Distributed Power Generation Systems

Abstract: This paper discusses the evaluation of different current controllers employed for grid-connected distributed power generation systems having variable input power, such as wind turbines and photovoltaic systems. The focus is mainly set on linear controllers such as proportional-integral, proportional-resonant, and deadbeat (DB) controllers. Additionally, an improved DB controller robust against grid impedance variation is also presented. Since the paper discusses the implementation of these controllers for grid-connected applications, their evaluation is made in three operating conditions. First, in steady-state conditions, the contribution of controllers to the total harmonic distortion of the grid current is pursued. Further on, the behavior of controllers in the case of transient conditions like input power variations and grid voltage faults is also examined. Experimental results in each case are presented in order to evaluate the performance of the controllers.

Voltage Support Provided by a Droop-Controlled Multifunctional Inverter

Abstract: This paper presents a single-phase multifunctional inverter for photovoltaic (PV) systems application. The converter provides active power to local loads and injects reactive power into the grid providing voltage support at fundamental frequency. The proposed topology is controlled by means of the droop-control technique. Hence, it allows the obtaining of voltage-sag-compensation capability, endowing voltage ride-through to the system. A model and analysis of the whole system is given to properly choose the control parameters. Simulation and experimental results validate the proposed control using a 5-kVA PV converter.

A Single-Phase Voltage-Controlled Grid-Connected Photovoltaic System With Power Quality Conditioner Functionality

Abstract: Future ancillary services provided by photovoltaic (PV) systems could facilitate their penetration in power systems. In addition, low-power PV systems can be designed to improve the power quality. This paper presents a single-phase PV system that provides grid voltage support and compensation of harmonic distortion at the point of common coupling thanks to a repetitive controller. The power provided by the PV panels is controlled by a Maximum Power Point Tracking algorithm based on the incremental conductance method specifically modified to control the phase of the PV inverter voltage. Simulation and experimental results validate the presented solution.

Wavelet-Based Islanding Detection in Grid-Connected PV Systems

Abstract: Distributed power generation systems (DPGSs) based on inverters require reliable islanding detection algorithms (passive or active) in order to determine the electrical grid status and operate the grid-connected inverter properly. These methods are based on the analysis of the DPGS voltage, current, and power in time or frequency domain. This paper proposes a time-frequency detection algorithm based on monitoring the DPGS output power considering the influence of the pulsewidth modulation, the output LCL filter, and the employed current controller. Wavelet analysis is applied to obtain time localization of the islanding condition. Simulation and experimental results show the performance of the proposed detection algorithm in comparison with existing methods.

Reactive Power Control for Improving Wind Turbine System Behavior Under Grid Faults

Abstract: This letter aims to present a generalized vector-based formulation for calculating the grid-side current reference to control reactive power delivered to the grid. Strategies for current reference generation were implemented on the abc stationary reference frame, and their effectiveness was demonstrated experimentally, perhaps validating the theoretical analysis even under grid fault conditions.

The high efficiency transformer-less PV inverter topologies derived from NPC topology

Abstract: The grid-connected photovoltaic (PV) systems are an important part of renewable energy sources and their integration is getting more and more widespread. In order to improve the efficiency, practicality and reliability of the PV systems, many kinds of new inverter topologies have been proposed to avoid using a grid isolation transformer. The NPC topology and two other derived topologies are presented and analyzed in this paper. Validated by experiments, it has been proven that they are very suitable for using in transformer-less PV applications due to high efficiency, low leakage current and EMI. Simulations using Simulink and the PLECS toolbox have been done for evaluating efficiency of different NPC topologies and some experimental results are presented in this paper to validate the operation of the different topologies.

The PWM strategies of grid-connected distributed generation active NPC inverters

Abstract: The Neutral Point Clamped topology due to high efficiency, low leakage current and EMI, its integration is widely used in the distributed generation (DG) systems. However the main disadvantage of the NPC inverter is given by an unequal distribution of the losses in the semiconductor devices, which leads to an unequal distribution of temperature. By using the Active NPC topology, the power losses distribution problem is alleviated. The modulation strategy is a key issue for losses distribution in this topology. In this paper two known strategies are discussed and a new proposed PWM strategy, namely the Adjustable Losses Distribution (ALD) PWM strategy is proposed for better losses distribution in the Active NPC (ANPC) topology. Simulations using Simulink and the PLECS toolbox have been done for evaluating efficiency of different NPC topologies and some experimental results are presented in this paper to validate the operation of the different strategies.

Synchronization techniques for grid connected wind turbines

Abstract: Phase locked loop (PLL) algorithms are very important for grid synchronization in most of the grid-connected power converters applications. Particularly new grid codes for wind parks pose serious challenges to PLL. The performance of the PLL should not be affected under distorted grid conditions such as: transients, short interruptions, sags, swells, voltage unbalance, harmonics, flickers, frequency and phase variations. This paper presents a performance comparison of different phase locked loop techniques: Synchronous Reference Frame-Phase Locked Loop (SRF-PLL), Enhanced Phase-Locked-Loop (EPLL) and Quadrature-Phase Locked Loop (QPLL).

An overview of anti-islanding detection algorithms in photovoltaic systems in case of multiple current-controlled inverters

Abstract: Grid-connected Distributed Generation Power Systems (DGPS) based on inverters require the employed controller to include an islanding detection algorithm in order to determine the grid status and operate properly. In certain cases, such as low-power low-cost current-controlled inverters used in residential photovoltaic (PV) systems, the inverter must be stopped once the islanding condition is detected according to the standard and grid-code limits. Passive, active and monitoring-based algorithms for islanding detection have been analyzed in literature considering one DGPS connected to the local Electrical Power System (EPS). Only in case of active methods, such as Active Frequency Drift (AFD), the effect of two DGPS in the same local EPS on the islanding capabilities has been analyzed. This paper analyzes the performance of diverse passive and active islanding detection methods (IDM) in multiple inverters connected at the same local EPS interact.

Speed sensorless control of a PMSG for small wind turbine systems

Abstract: Wind turbine systems (WTS) based on Permanent Magnet Synchronous Generators (PMSG) have been gaining more and more importance over the last years. In fact, these generators are self-excited, allowing operation at high power factor and high efficiency. Additionally, the elimination of the gearbox allows a reduction in costs and maintenance. To further reduce the system complexity and the maintenance process, sensors normally used to sense the speed of these generators could be eliminated. This aspect is particularly important for wind turbine installation in harsh environment characterized by extreme weather conditions. To this purpose this paper investigates how a Phase Locked Loop (PLL) can be profitably used to estimate the rotor speed. A systematic analysis is presented as well as simulation and experimental results proving the feasibility of such solution.

Characterization of differential-mode filter for grid-side converters

Abstract: Pulse Width Modulated Voltage Source Converters (PWM-VSC) could be the universal interface for distributed power producers, loads, storage, power conditioning and transmission systems to the future smart grid. The differential mode filter used to connect the PWM-VSC to the electric grid has a crucial role in the proper operation of the converter and in filtering PWM switching harmonics. The paper aims to offer a complete characterization of differential mode filters with attention paid to the model of the inductors. Experimental results validate the theoretical study.

Implementation on DSP TMS320F2812 of the control of the grid converter of a small wind turbine system

Abstract: The paper discusses implementation issues related to the control of a three-phase grid-inverter for a wind-turbine system (WTS). The grid-side converter is controlled in order to maintain constant the DC voltage also in presence of grid voltage variations and the sinusoidal currents in phase with the corresponding voltages. PI and P+Resonat controllers are considered. For the P+Resonant controller different digital implementations are evaluated: Shift operator based and Delta operator based. A performance comparison, in terms of the C-code execution time is presented. The control algorithm based on PI controller has been experimentally verified.

Management of power excess in wind turbine system

Abstract: Variable speed wind turbine generators provide the opportunity to capture more power than fixed speed turbines. However also the limitation of the produced power is an important issue. The paper is focused on the management of the power in excess in a variable-speed wind turbine equipped with a PMSG (Permanent Magnet Synchronous Generator) connected to the grid through a back-to-back converter. It is investigated the role of the braking chopper to dissipate the power in excess that can not be injected in the grid and the blade pitch control to limit the aerodynamic power in above-rated wind speeds in order to keep the turbine within its design limits. The interaction between the blade pitch control and the braking chopper is discussed. Finally, some simulation results in the case of a ramping wind speed and under real wind conditions are reported.

Dr. Bimal K. Bose: A reference for generations [Editor's Column]

Abstract: This special issue honors Dr. Bimal K. Bose and this editorial includes an interview with Dr. Bose.

Discrete wavelet transforms applied to the evaluation of the averaged load conductance in shunt active power filters

Abstract: Shunt Active Power Filters (SAPF) allow power system disturbances associated to the current consumption of non-linear and/or unbalanced loads to be compensated Diverse control algorithms have been proposed in literature in order to obtain accurate and fast compensation results under steady-state and transient conditions The FBD method considers the averaged load conductance in order to obtain an accurate reference signal for compensation purposes, as a drawback, a whole period at the fundamental grid frequency is required in the most general case for evaluation of the averaged load conductance This paper proposes the application of wavelet transforms in order to obtain an accurate and fast estimation of the averaged load conductance