Ricardo Quadros Machado

A Line-Interactive Single-Phase to Three-Phase Converter System

This paper describes a line-interactive single-phase to three-phase converter. A typical application is in rural areas supplied by a single-wire with earth return system. The traditional objective of feeding a three-phase induction motor is not anymore the main concern for such conversion. Due to the evolution of the farm technology, some of the local loads (as electronic power converters, computers, communication equipments, etc) require high power quality that is intended as sinusoidal, symmetrical, and balanced three-phase voltage. Additionally, to maximize the power from the feeder, the system provides a unity power factor to the grid. A three-phase voltage source inverter-pulsewidth modulation converter is used for this purpose. The power converter processes a fraction of the load power and the energy necessary to regulate the dc link voltage. As it does not need to supply active power, it is not necessary to have a source at the dc side. However, if island mode operation is needed, a dc source must be available at the dc link to supply the load. The control strategy, design criteria, and experimental results are presented

Three-phase to single-phase direct connection rural cogeneration systems

This paper proposes a solution for the direct connection of a three-phase induction generator to a single-phase feeder. This high quality power system is intended to be used in micro-hydro power plants, without control of the turbine. The generated power that is not consumed by the local load is driven to the single-phase feeder. The power flow control is provided by a three-phase PWM inverter, which guarantees the local power quality and controls the power flow through the single-phase feeder. This converter allows balancing the induction generator currents, voltages and frequency. The paper describes the inverter control strategy, presents design criteria of the controllers and shows experimental results.

Selective disturbance compensation and comparisons of active filtering strategies

This paper presents an alternative method to evaluate selectively disturbance compensation. Instantaneous decompositions to be applied directly on voltages and current signals are used to identify the distorting and unbalancing components, and can be used to compensate either nonactive power, power oscillations, harmonics and load imbalances in a different way as provided by the traditional pq theory. Three control strategies of active filters were tested and compared for solving typical power quality problems. Simulation results are presented to show the main differences among the control strategies facing nonlinear, unsymmetrical and variable loads under generic voltage conditions. According to these results, it is shown that some shunt active filtering strategies can even improve the PCC voltages under resonance conditions.

Wavelet and PCA to Power Quality Disturbance Classification Applying a RBF Network

The quality of electric power became an important issue for the electric utility companies and their customers. It is often synonymous with voltage quality since electrical equipments are designed to operate within a certain range of supply specifications. For instance, current microelectronic devices are very sensitive to subtle changes in power quality, which can be represented as a disturbance-induced variation of voltage amplitude, frequency and phase (Dugan et al., 2003). Poor power quality (PQ) is usually caused by power line disturbances such as transients, notches, voltage sags and swells, flicker, interruptions, and harmonic distortions (IEEE Std. 1159, 2009). In order to improve electric power quality, the sources and causes of such disturbances must be known. Therefore, the monitoring equipment needs to firstly and accurately detect and identify the disturbance types (Santoso et al., 1996). Thus, the use of new and powerful tools of signal analysis have enabled the development of additional methods to accurately characterize and identify several kinds of power quality disturbances (Karimi et al., 2000; Mokhtary et al., 2002). Santoso et al. proposed a recognition scheme that is carried out in the wavelet domain using a set of multiple neural networks. The network outcomes are then integrated by using decision-making schemes such as a simple voting scheme or the Dempster-Shafer theory. The proposed classifier is capable of providing a degree of belief for the identified disturbance waveform (Santoso et al., 2000a, 2000b). A novel classification method using a rule-based method and wavelet packet-based hidden Markov models (HMM) was proposed bay Chung et al. The rule-based method is used to classify the time-characterized-feature disturbance and the wavelet packet-based on HMM is used for frequency-characterizedfeature power disturbances (Chung et al., 2002). Gaing presented a prototype of waveletbased network classifier for recognizing power quality disturbances. The multiresolutionanalysis technique of discrete wavelet transforms (DWT) and Parseval’s theorem are used to extract the energy distribution features of distorted signals at different resolution levels. Then, the probabilistic neural network classifies these extracted features of disturbance type identification according to the transient duration and energy features (Gaing, 2004). Zhu et

Decentralized control for renewable DC Microgrid with composite energy storage system and UC voltage restoration connected to the grid

In this paper we propose a new decentralized control strategy applied to a DC Microgrid in order to manage the power delivery of storage devices into a common DC-link, avoiding high-bandwidth communication (HBC) between the storage devices (SD) and alternative sources. Batteries and Ultracapacitors (UC) are used as SD and the common DC-link is fed by alternative sources such as photovoltaic panels, wind turbines and fuel cells as well. The batteries are used to supply/absorb extra power in steady-state regime while the UC absorbs the power transients caused by variations on the power production or load connections. The proposed strategy uses as input for the batteries control only the DC-link voltage and state of charge (SOC), while for the UC only the DC-link voltage and UC terminal voltage are used to achieve the power sharing among the storage devices, equalization of the batteries and voltage restoration of the UC without HBC. The DC-link voltage is not restored in order to work as the sharing signal between storage devices. Additionally, the DC Microgrid is connected to the AC grid in order to deliver the extra power to the distributed system; however, the voltage variation on the DC-link does not affect the power quality of the produced energy. Simulated and experimental results are presented to demonstrate the feasibility of the proposed approach.

Analysis of the maximum available time to switch the operation control mode of a distributed generation during an islanding occurrence

The operation of distributed generation systems (DG) is subjected to events such as unintentional islanding occurrences. To prevent the activation of the DGs protections and power supply interruption, when an islanding occurs, the DG must change from grid-connected operation mode to isolated operation mode and keep the voltage and the frequency within acceptable operational levels. The available time to switch the operation control mode should be investigated to avoid false islanding detections and to minimize the negative impact on the quality of the generated energy. This paper analyzes the maximum time for switching the operation control mode of the DG, after an unintentional islanding occurrence. Simulations were carried out on the PSCAD/EMTDC. The simulated DG system is based on a diesel generator and distinct loads. Using a classical control strategy (CCS) and a fuzzy control strategy (FCS) for voltage, frequency, active and reactive powers, the maximum available switching time is analyzed. The FCS provided longer available times than a strategy using classical controllers.

A grid-connected multilevel converter for interfacing PV arrays and energy storage devices

This paper presents a design procedure and a control strategy for a grid-connected single-phase multilevel converter. The proposed system uses two series connected H-bridge modules, one fed by photovoltaic panels, the other by energy storage devices. The former switches at line frequency, while the latter operates in PWM. The system is designed to minimize the voltage stress on the switches, while the control strategy is such that a constant active power is delivered to the grid with high power factor, even if the energy produced by the photovoltaic panels is variable. The paper illustrates the power converter design procedure, the hybrid PWM method and the small signal modeling used to design the controllers of each inverter. In addition, the performance of the multilevel converter is verified by means of simulation and experimental results, which show the system ability to operate as expected.

Análise de conversores CC-CA conectados em redes de distribuição e utilizados em sistemas de geração distribuída

This paper presents a controlled power converter for three-phase grid connected systems. When the power converter operates connected to the grid the current/power is used as control variable. However, if the power converter is operating in stand alone mode (on contingency or islanding mode) the control variable must be changed to voltage. Finally, the paper analyzes the effects of different loads on the islanding detection mode.