Renewable energy sources like wind, sun, and hydro are seen as a reliable alternative to the traditional energy sources such as oil, natural gas, or coal. Distributed power generation systems (DPGSs) based on renewable energy sources experience a large development worldwide, with Germany, Denmark, Japan, and USA as leaders in the development in this field. Due to the increasing number of DPGSs connected to the utility network, new and stricter standards in respect to power quality, safe running, and islanding protection are issued. As a consequence, the control of distributed generation systems should be improved to meet the requirements for grid interconnection. This paper gives an overview of the structures for the DPGS based on fuel cell, photovoltaic, and wind turbines. In addition, control structures of the grid-side converter are presented, and the possibility of compensation for low-order harmonics is also discussed. Moreover, control strategies when running on grid faults are treated. This paper ends up with an overview of synchronization methods and a discussion about their importance in the control

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Overview of Control and Grid Synchronization for Distributed Power Generation Systems

Solid-state switch-mode rectification converters have reached a matured level for improving power quality in terms of power-factor correction (PFC), reduced total harmonic distortion at input AC mains and precisely regulated DC output in buck, boost, buck-boost and multilevel modes with unidirectional and bidirectional power flow. This paper deals with a comprehensive review of improved power quality converters (IPQCs) configurations, control approaches, design features, selection of components, other related considerations, and their suitability and selection for specific applications. It is targeted to provide a wide spectrum on the status of IPQC technology to researchers, designers and application engineers working on switched-mode AC-DC converters. A classified list of more than 450 research publications on the state of art of IPQC is also given for a quick reference.

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A review of single-phase improved power quality AC-DC converters

This paper deals with a crucial aspect in the control of grid-connected power converters, i.e., the detection of the fundamental-frequency positive-sequence component of the utility voltage under unbalanced and distorted conditions. Specifically, it proposes a positive-sequence detector based on a new decoupled double synchronous reference frame phase-locked loop (DDSRF-PLL), which completely eliminates the detection errors of conventional synchronous reference frame PLL's (SRF-PLL). This is achieved by transforming both positive- and negative-sequence components of the utility voltage into the double SRF, from which a decoupling network is developed in order to cleanly extract and separate the positive- and negative-sequence components. The resultant DDSRF-PLL conducts then to a fast, precise, and robust positive-sequence voltage detection even under unbalanced and distorted grid conditions. The paper presents a detailed description and derivation of the proposed detection method, together with an extensive evaluation using simulation and experimental results from a digital signal processor-based laboratory prototype in order to verify and validate the excellent performance achieved by the DDSRF-PLL

Decoupled Double Synchronous Reference Frame PLL for Power Converters Control

Phase, amplitude and frequency of the utility voltage are critical information for the operation of the grid-connected inverter systems. In such applications, an accurate and fast detection of the phase angle, amplotude and frequency of the utility voltage is essential to assure the correct generation of the reference signals and to cope with the new upcoming standards. This paper presents a new phase-locked-loop (PLL) method for single-phase systems. The novelty consists in generating the orthogonal voltage system using a structure based on second order generalized integrator (SOGI). The proposed structure has the following advantages: — it has a simple implementation; — the generated orthogonal system is filtered without delay by the same structure due to its resonance at the fundamental frequency, — the proposed structure is not affected by the frequency changes. The solutions for the discrete implementation of the new proposed structure are also presented. Experimental results validate the effectiveness of the proposed method.

A New Single-Phase PLL Structure Based on Second Order Generalized Integrator

This paper deals with a fundamental aspect in the control of grid-connected power converters, i.e., the detection of the positive-sequence component at fundamental frequency of the utility voltage under unbalanced and distorted conditions. Accurate and fast detection of this voltage component under grid faults is essential to keep the control over the power exchange with the grid avoiding to trip the converter protections and allowing the ride-through of the transient fault. In this paper, the systematic use of well known techniques conducts to a new positive-sequence voltage detection system which exhibits a fast, precise, and frequency-adaptive response under faulty grid conditions. Three fundamental functional blocks make up the proposed detector, these are: i) the quadrature-signals generator (QSG), ii) the positive-sequence calculator (PSC), and iii) the phase-locked loop (PLL). A key innovation of the proposed system is the use of a dual second order generalized integrator (DSOGI) to implement the QSG. For this reason, the proposed positive-sequence detector is called DSOGI-PLL. A detailed study of the DSOGI-PLL and verification by simulation are performed in this paper. From the obtained results, it can be concluded that the DSOGI-PLL is a very suitable technique for characterizing the positive-sequence voltage under grid faults.

New positive-sequence voltage detector for grid synchronization of power converters under faulty grid conditions

This paper presents a detailed analysis of PLL structures for utility connected systems. The three-phase PLL (phase-locked loop) structure capable of fast tracking the utility voltage vector is studied. A single-phase PLL structure, derived from the three-phase PLL topology, is discussed and its behavior under distorted utility conditions is analyzed. Additionally, the conventional zero-crossing PLL is studied and an alternative implementation for this PLL is proposed in order to improve its dynamic behavior. Simulation and experimental results from a DSP-based system are included to support the theoretical analysis.

PLL structures for utility connected systems

Phase angle, frequency and amplitude of the utility voltage vector are basic information for an increasing number of grid-connected power conditioning equipments, such as PWM rectifiers, uninterruptible power systems (UPS), voltage sag compensators and the emerging distributed generation systems. For these applications, accurate tracking of the utility voltage vector is essential to ensure correct operation of the control system. This paper presents a comparative study of synchronous reference frame PLL algorithms for single-phase systems. Simulation and experimental results, including operation of the PLL structures under distorted utility conditions are presented, to allow a performance evaluation of the PLL algorithms.

Performance evaluation of PLL algorithms for single-phase grid-connected systems

The aim of this paper is to present a computer-based tool to aid in the process of detecting cavitation in centrifugal pumps. The method proposed in this paper, namely, the Load Torque Signature Analysis, employs electrical signals from the motor to estimate the torque developed by the pump and determines the occurrence and intensity of the phenomenon through its frequency spectrum information.

Incipient Detection of Cavitation Phenomenon in Centrifugal Pumps

The need for detection of rotor faults at an earlier stage, so that maintenance can be planned ahead, has pushed the development of monitoring methods with increasing sensitivity and noise immunity An important issue in such effort is the modeling of the induction machine including rotor bar and end-ring faults, with a minimum of computational complexity In this paper, a new and simpler approach for the modeling of induction machines with rotor asymmetries is employed in the simulation and analysis of an induction motor with rotor asymmetries Simulation and finite element analysis results are presented to support the proposed approach

Simulation and analysis of induction machines with rotor asymmetries

The application of cascaded multilevel converters in large-scale photovoltaic plants connected to the utility grid has many advantages, such as the absence of sinusoidal filters and line-frequency transformers. Improved overall performance under partial shading conditions is possible due to the distribution of arrays among a large number of maximum power point trackers. In this situation, although the cells operate at different power levels, the three-phase currents injected into the point of common coupling must be balanced. A suitable control technique is required to achieve this goal. This paper presents a complete algorithm to handle three-phase power balance and single-phase power harvesting simultaneously. Simulation results assuming a medium-voltage system connected to a typical utility grid illustrate the dynamic and steady-state performance capability of the proposed strategy. Experimental results of a low-voltage, solid-state transformer-based, three-cell prototype are also included.

Braz de Jesus Cardoso Filho

Papers

PLL structures for utility connected systems

Performance evaluation of PLL algorithms for single-phase grid-connected systems

Incipient Detection of Cavitation Phenomenon in Centrifugal Pumps

Simulation and analysis of induction machines with rotor asymmetries

Operation of a Grid-Tied Cascaded Multilevel Converter Based on a Forward Solid-State Transformer Under Unbalanced PV Power Generation