A. Dell'Aquila

Genetic algorithm-based design of the active damping for an LCL-filter three-phase active rectifier

The use of a LCL-filter mitigates the switching ripple injected in the grid by a three-phase active rectifier. However stability problems could arise in the current control loop. In order to overcome them a damping resistor can be inserted, at the price of a reduction of the efficiency. On the contrary the use of the active damping seems really attractive but it is often limited by the use of more sensors respect to the standard control and by the complex tuning procedure. This paper introduces a new active damping method that does not need the use of more sensors and that can be tuned using genetic algorithms. It consists of adding a filter on the reference voltage for the converters modulator. The tuning process of this filter is easily done, for a wide range of sampling frequencies, with the use of genetic algorithms. This method is used only for the optimum choice of the parameters of the filter and an on-line implementation is not needed. Thus the resulting active damping solution does not need new sensors or complex calculations. Moreover, in the paper particular attention is devoted to the dynamics of the system due to the introduction of the active damping.

Control Issues in Single-Stage Photovoltaic Systems: MPPT, Current and Voltage Control

Photovoltaic Systems (PVS) can be easily integrated in residential buildings hence they will be the main responsible of making low-voltage grid power flow bidirectional. Control issues on both the PV side and on the grid side have received much attention from manufacturers, competing for efficiency and low distortion and academia proposing new ideas soon become state-of-the-art. This paper aims at reviewing part of these topics (MPPT, current and voltage control) leaving to a future paper to complete the scenario. Implementation issues on Digital Signal Processor (DSP), the mandatory choice in this market segment, are discussed.

Stability improvements of an LCL-filter based three-phase active rectifier

Three-phase active rectifiers guarantee sinusoidal input currents and controllable dc voltage at the price of a high switching frequency ripple that can disturb and reduce efficiency of other EMI sensitive equipment connected to the grid. This problem could be solved choosing a high value for the ac inductors making them expensive and bulky. Moreover the dynamic of the system becomes poor with so high value of inductance. On the contrary to adopt an LCL-filter configuration allows to use reduced values of the inductances (preserving dynamic) and to reduce the switching frequency pollution emitted in the grid. However the stability of the system should be rigorously studied. A poor analysis made on qualitative considerations could lead to excessive damping (unnecessary increase of the losses) or insufficient damping (the system seems to be stable but it is not). In this paper the damping, both passive (based on the use of resistors) and active (based on the modification of the control algorithm), is studied using the z-plane root locus approach and looking to dynamic performances and losses. In fact it is necessary to verify the dynamic effects by the introduction of damping resistors or by the modification of the control algorithm to perform active damping. The analysis is validated both with simulation and experiments.

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

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.

Overview of Anti-Islanding Algorithms for PV Systems. Part I: Passive Methods

This paper offers an overview of the passive methods used for islanding detection. A monitoring PLL has been adopted for the estimation of the voltage amplitude and frequency. Passive inverter resident methods discussed are the over/under voltage (OUV), over/under frequency (OUF), the voltage harmonic monitoring (with a study on the effects of the grid impedance value and of the inverter DC voltage ripple) and the phase monitoring (different from the classical phase jump method). A harmonic synchronization PLL is used to monitor the 3 rd, 5th and 7th harmonics. All the proposed algorithms are validated by simulations and experimental results obtained in accordance with the IEEE Std. 929-2000 procedure

Wavelet-Based Islanding Detection in Grid-Connected PV Systems

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.

A stable three-phase LCL-filter based active rectifier without damping

Industrial-LCL-based grid converters using an active rectifier need to be designed in view of robustness, stability and high efficiency. In this paper the design of an active rectifier that does not need damping whether passive or active is described. This allows obtaining stability without the decrease of the efficiency typical for the passive damping methods or the increase of cost due to more sensors or more complex control algorithms that is typical for the active damping methods. This has been achieved with a careful choice of sensor position and of the passive elements in the LCL-filter. In fact if the current sensors are on the grid side, rather than on the converter side, the current loop is much more near to the stability. Moreover, if the grid side inductance is a fraction of the converter side one, the current loop is again much more near to the stability. Thus, with a proper design the system can be made stable at some switching frequencies even without any damping.

A passivity-based multilevel active rectifier with adaptive compensation for traction applications

The control of a single-phase multilevel H-bridge rectifier suitable for traction applications is considered. Such a converter often presents instability problems making difficult its design and uncertain its behavior. In this paper the use of a passivity-based controller is experimentally investigated. Such a controller achieves stability, unity power factor, good balancing between the two DC links, and satisfactory behavior even in the presence of load changes. A right choice of the damping parameters allows the fulfilling of system requirements in terms of DC-link voltage error and grid current distortion.

An Anti-Islanding Method for Single-Phase Inverters Based on a Grid Voltage Sensorless Control

The detection of the islanding condition of a distributed generation (DG) system is crucial for safety reasons, as discussed in the IEEE standards and specifically required by some national codes. Several anti-islanding methods that are resident in the inverter have been investigated and classified as passive (measurement of the natural effects of islanding) or 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 is small, and the passive methods fail. However, the active methods, which 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 of a grid-voltage sensorless control to disturbances to highlight the islanding condition. The adopted grid-voltage sensorless control is adapted to a single-phase system with the use of resonant controllers based on the internal model control law: resonant-controller-based observer results. Then, a Kalman-filter-based algorithm is used to detect the islanding condition based on the energy mismatch between the estimated third and fifth harmonics and the real ones. Experimental results support the analysis

A Comparative Analysis of Real-Time Algorithms for Power Signal Decomposition in Multiple Synchronous Reference Frames

The monitoring and rejection of voltage and current harmonics in power electronics applications such as power quality conditioners or distributed generation systems require correct estimation algorithms especially if the harmonic amplitudes are time varying. Power signal decomposition in multiple synchronous rotating reference frames (MSRFs) is considered one of the best solutions. The most commonly employed implementations of this signal transformation are based on phase-locked loops (PLLs), recursive discrete Fourier transforms (RDFT), or discrete Kalman filtering (DKF). In this paper, a rigorous analysis of the performance of these implementations has been carried out. Complete tests have been performed to evaluate the computational burden, the frequency domain response, and the tolerance to low frequency amplitude variations. The results make it possible to select the proper method depending on the requirements of each application.