Gustavo M. S. Azevedo

Modulation Techniques to Eliminate Leakage Currents in Transformerless Three-Phase Photovoltaic Systems

In some photovoltaic (PV) applications, it is possible to remove the transformer of a system in order to reduce losses, cost, and size. In transformerless systems, the PV module parasitic capacitance can introduce leakage currents in which the amplitude depends on the converter topology, on the pulsewidth modulation, and on the resonant circuit comprised by the system components. Based on the common-mode voltage model, modulation techniques are proposed to eliminate the leakage current in transformerless PV systems without requiring any modification on the converter and any additional hardware. The main drawback is that the proposed modulation technique for two-level inverters can only be used with 650-V dc link in the case of a 110-V (rms) grid phase voltage. Comparisons among the modulation techniques are discussed, and it is proven that the proposed modulation for two- and three-level inverters presents the best results. To validate the models used in the simulations, an experimental three-phase inverter is used.

Modeling of a variable speed wind turbine with a Permanent Magnet Synchronous Generator

The aim of this work is to analyze a typical configuration of a Wind Turbine Generator System (WTGS) equipped with a Variable Speed Generator. Nowadays, doubly-fed induction generators are being widely used on WTGS, although synchronous generators are being extensively utilized too. There are different types of synchronous generators, but the multi-pole Permanent Magnet Synchronous Generator (PMSG) is chosen in order to obtain its model. It offers better performance due to higher efficiency and less maintenance since it does not have rotor current and can be used without a gearbox, which also implies a reduction of the weight of the nacelle and a reduction of costs. Apart from the generator, the analyzed WTGS consists of another three parts: wind speed, wind turbine and drive train. These elements have been modeled and the equations that explain their behavior have been introduced. What is more, the whole WTGS has been implemented in MATLAB/Simulink interface. Moreover, the concept of the Maximum Power Point Tracking (MPPT) has been presented in terms of the adjustment of the generator rotor speed according to instantaneous wind speed.

Intelligent Connection Agent for Three-Phase Grid-Connected Microgrids

The high penetration of distributed generation power plants, based on renewable energy sources (RESs), is boosting the connection of power converters to the electrical network. This generation concept would permit to form local networks, microgrids, when the main grid falls due to any kind of contingency in the network. However, the connection and disconnection of these local networks may give rise to undesired transient overcurrents that should be avoided. In order to solve this drawback, this paper presents a method oriented to carry out a stable intentional disconnection/reconnection of local grids from the main electrical network under grid-fault conditions. This control method has been implemented in a grid-connected power converter that acts as an intelligent connection agent (ICA) and adapts its operation mode according to its connection state. The proposed control also manages the operation of a controlled switch, which is responsible of disconnecting/reconnecting the microgrid from the mains. In this paper, the behavior of the ICA under transient conditions will be discussed, and finally, its simulated and experimental performance will be shown.

A Method for Extracting the Fundamental-Frequency Positive-Sequence Voltage Vector Based on Simple Mathematical Transformations

In this paper, a novel scheme for obtaining the fundamental-frequency positive-sequence grid voltage is proposed. The method is based on four simple mathematical transformations; two of them are in the stationary reference frame, which are able to eliminate odd harmonics from the original signals. The other two transformations are implemented in a synchronously rotating reference frame in order to eliminate even harmonics. The output of the last transformation block is the input to a synchronous reference-frame phase-locked loop for detecting the frequency and position of the positive-sequence voltage vector. The proposed algorithm was verified through simulations and experiments by applying distorted and unbalanced signals, containing positive and negative-sequence components. The results are in agreement with those theoretically predicted and indicate that the proposed scheme has a great potential for use in grid-connected converter synchronization algorithms.

Evaluation of maximum power point tracking methods for grid connected photovoltaic systems

This paper presents a study of two maximum power point tracking methods for grid connected photovoltaic systems. The best operation conditions of the perturbation and observation and the incremental conductance are investigated in order to identify the performances of photovoltaic systems. Improvements of these methods can be obtained with the best adjustment of the sampling rate and the perturbation size. Practical considerations about the incremental conductance are discussed and some modifications to overcome its problems are done. A procedure to determine the parameters is explained and it helps to define which method is better to the grid connected photovoltaic system with only one conversion stage. The methods influence the quality of the currents injected in the grid and this influence becomes the main parameter for choosing the incremental conductance. The improvement with the choice of the best parameters is proved by means of simulation and experimental results.

Comparative Evaluation of Maximum Power Point Tracking Methods for Photovoltaic Systems

This paper presents a study of two maximum power point tracking methods for grid connected photovoltaic systems. The best operation conditions for the perturbation and observation and the incremental conductance methods are investigated in order to identify the efficiency performances of these most popular maximum power point tracking methods for photovoltaic systems. Improvements of these methods can be obtained with the best adjustment of the sampling rate and the perturbation size, both in accordance with the converter dynamics. Practical aspects about the incremental conductance method are discussed, and some modifications are proposed to overcome its problems. A procedure to determine the parameters is explained. This procedure helps to identify which method is better suited for grid connected photovoltaic systems with only one conversion stage. The methods’ influences on the quality of the currents injected in the grid are evaluated and compared. The performance improvement achieved with the choice of the best parameters is proved by means of simulation and experimental results performed on a low power test system. The simulation results have been obtained by modeling a photovoltaic system in MATLAB. A simplified model was used that employs only parameters of interest and therefore decreases simulation time. Experimental results corresponding to the operation of a grid connected photovoltaic converter controlled with a digital signal processor have been obtained. DOI: 10.1115/1.3142827

Photovoltaic inverters with fault ride-through Capability

This paper presents a new control strategy that allow the photovoltaic system operate under grid faults without overpass the rated current and assuring sinusoidal currents. In the classic control strategies used in photovoltaic systems the power delivered to the grid remains constant when a fault occurs, hence the current can reach dangerous values. Therefore the converter has a protection system to disconnect it avoiding its damage. Thus it must be connected manually when the electrical grid is re-established. The strategy presented here overcomes these drawbacks. It is compared with the classic strategy through simulations in PSCAD and the experimental results prove its effectiveness.

Losses and CMV evaluation in transformerless grid-connected PV topologies

Controlling the thermal losses in the semiconductors as well as the Common Mode Voltage (CMV), are important issues in the design of power electronics converters for photovoltaic applications. At present time there are several topologies that offer a good performance regarding losses and CMV. In this paper an evaluation of three of these converters topologies: H5, HERIC and NPC transformerless for single phase PV systems will be carried out by means of simulations performed with PSIM 7.05. This software permits to estimate accurately the switching and conduction losses, thanks to its Thermal Module. This analysis together with the CMV study for each case will permit to establish the pros and cons of each topology.

Efficiency Evaluation in Grid Connected Photovoltaic Energy Conversion Systems

This paper introduces a comparative study of efficiency for topologies in photovoltaic energy conversion systems. In special, a study of losses is presented and the methodology is used to compare different topologies for grid connected photovoltaic systems in such a way that can be chosen the option of best efficiency. The systems are also tested with photovoltaic generation as well as current harmonic and reactive power compensation simultaneously. The system that uses only inverters presents increased efficiency when compared to the conventional system. The synchronous reference frame method is used to control the three-phase inverter for all topologies. The proposed design is used to test efficiency for different pulse-width-modulation techniques and different loads in an electric system and simulation results demonstrate the good performance of the proposed configuration

Safe transient operation of microgrids based on master-slave configuration

Master-Slave configuration is a suitable alternative to droop control method used in microgrids. In this configuration, only one inverter is the master, while the others are slaves. The slave inverters are always current controlled whereas the master inverter should have two selectable operation modes: current controlled, when the microgrid is connected to the grid; and voltage controlled, when it is operating in island mode. In grid-connected mode, the master needs a synchronization system to perform the accurate control of its delivered power, and, in island mode, it needs a voltage reference oscillator that serves as a reference to the slave inverters. Based on the master-slave concept, this paper proposes a single system that perform both functions, i.e., it can act as a synchronization system or as a voltage reference oscillator depending on an input selector. Moreover, the system ensures a smoothly transition between the two operation modes, guaranteeing the safety operation of the microgrid. Experimental results are provided to confirm the effectiveness of the proposed system.