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

/pdf/overview-of-control-and-grid-synchronization-for-distributed-2nx31ovpe9.pdf

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.

/pdf/a-review-of-single-phase-improved-power-quality-ac-dc-2pm6h4zjxq.pdf

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

This paper presents a review of recently used direct torque and flux control (DTC) techniques for voltage inverter-fed induction and permanent-magnet synchronous motors. A variety of techniques, different in concept, are described as follows: switching-table-based hysteresis DTC, direct self control, constant-switching-frequency DTC with space-vector modulation (DTC-SVM). Also, trends in the DTC-SVM techniques based on neuro-fuzzy logic controllers are presented. Some oscillograms that illustrate properties of the presented techniques are shown.

Direct torque control of PWM inverter-fed AC motors - a survey

This paper presents a comprehensive study on the state-of-the-art and emerging wind energy technologies from the electrical engineering perspective. In an attempt to decrease cost of energy, increase the wind energy conversion efficiency, reliability, power density, and comply with the stringent grid codes, the electric generators and power electronic converters have emerged in a rigorous manner. From the market based survey, the most successful generator-converter configurations are addressed along with few promising topologies available in the literature. The back-to-back connected converters, passive generator-side converters, converters for multiphase generators, and converters without intermediate dc-link are investigated for high-power wind energy conversion systems (WECS), and presented in low and medium voltage category. The onshore and offshore wind farm configurations are analyzed with respect to the series/parallel connection of wind turbine ac/dc output terminals, and high voltage ac/dc transmission. The fault-ride through compliance methods used in the induction and synchronous generator based WECS are also discussed. The past, present and future trends in megawatt WECS are reviewed in terms of mechanical and electrical technologies, integration to power systems, and control theory. The important survey results, and technical merits and demerits of various WECS electrical systems are summarized by tables. The list of current and future wind turbines are also provided along with technical details.

High-Power Wind Energy Conversion Systems: State-of-the-Art and Emerging Technologies

This paper endeavors to present a comprehensive summary of the causes and effects of voltage unbalance and to discuss related standards, definitions, and mitigation techniques. Several causes of voltage unbalance on the power system and in industrial facilities are presented as well as the resulting adverse effects on the system and on equipment such as induction motors and power electronic converters and drives. Standards addressing voltage unbalance are discussed and clarified, and several mitigation techniques are suggested to correct voltage unbalance problems. This paper makes apparent the importance of identifying potential unbalance problems for the benefit of both the utility and customer.

Assessment of Voltage Unbalance

In this paper, a direct torque control (DTC) method of an induction machine is proposed which enables the minimum torque ripple control, while maintaining constant switching frequency. Some drawbacks of the conventional DTC are the relatively large torque ripple in a low speed range and the variation of switching frequency according to the amplitude of hysteresis bands and the motor operating speed. In the proposed strategy, an RMS torque-ripple equation is derived using instantaneous torque equations and, at each switching cycle, an optimal switching instant which satisfies the minimum torque-ripple condition is determined based on the ripple equation. The proposed strategy improves the performance of the DTC by combining a low-torque-ripple characteristic in steady state with the fast torque dynamics. Experimental results prove the feasibility of the proposed strategy as compared with the conventional method.

New direct torque control of induction motor for minimum torque ripple and constant switching frequency

In this paper, a new phase detecting method is proposed for PWM rectifiers and active filters considering distorted utility conditions. The accurate phase angle information of utility voltage is essential to the active power conversion systems tied to the utility line. The angle information can be easily corrupted by the distorted utility voltage such as voltage unbalance and harmonics in the waveform. Thus, large harmonics may be caused in the current or voltage reference signal whose phase angle is based on the detected voltage phase angle. First, focusing on the distorted utility condition, effects of the several phase angle detecting methods on the operation of PWM rectifiers and active filters are investigated. Then a new voltage phase angle detector is proposed using voltage sequence detector and PI controller. The proposed method does not cause harmonics in the reference signal by adequately eliminating unbalance components and high-order harmonic terms in the utility voltage. The basic principle of the proposed method is described in detail. Experimental results prove the feasibility of the proposed strategy compared with the conventional method.

A new phase detecting method for power conversion systems considering distorted conditions in power system

SiC devices are gaining acceptance in the motor drive industry. This paper compares the power loss and efficiency between two options that can be used with SiC-based variablefrequency drives (VFDs). In the first option, the SiC VFD is equipped with an output sine-wave filter with carrier frequency at 50 kHz. A $dv/dt$ filter is used for the second option with the carrier frequency reduced to 8 kHz. Both options are compared with a standard Si insulated-gate bipolar transistor (IGBT) VFD operating at a carrier frequency of 8 kHz with no output filter. The focus of this paper is to present different filtering options for SiC VFDs. The $dv/dt$ filter is designed to meet the same specification as that of the standard Si IGBT VFD with no output filter, so as to present a fair comparison between a standard Si IGBT VFD and the next-generation SiC VFD. Results using a 460-V 11-kW system show that the SiC VFD with an output sine-wave filter has a lower efficiency compared with SiC VFD with a $dv/dt$ filter. Influence of the various filtering options on leakage current in the motor drive system has also been studied, and the results are presented in this paper.

Power Loss, System Efficiency, and Leakage Current Comparison Between Si IGBT VFD and SiC FET VFD With Various Filtering Options

An MC is a direct AC-AC converter that has benefits of high reliability, small size, and high efficiency over a conventional PWM converter-inverter system. This article introduced the Yaskawa MC product specifications, applications, and techniques that helped commercialization of a low-voltage and medium-voltage MC. The 9-114 kVA (200 V and 400 V classes) for a low voltage MC and 200-6,000 kVA (3.3 kV, 6.6 kV) for a medium-voltage MC have thus far been commercialized. Three typical applications of an MC were also introduced along with the related technology that helped to achieve desired results for each application. The application of an MC is still expanding to other fields in industry. Authors are continuing their efforts in improving performance and expanding the product lineup.

Development of MCs and its Applications in Industry [Industry Forum]

In this paper, effects of the hysteresis bands on the direct torque control (DTC) of an induction machine are investigated, and a method to control the switching frequency of inverter by a variable amplitude of the hysteresis band is proposed for DTC of an induction machine. A major drawback of the conventional DTC is an unpredictable inverter switching frequency which varies due to operating speed, load condition and parameters of the induction machine. Therefore the amplitude of the hysteresis band should be large enough to avoid excessive inverter switching at any operating region, which inevitably causes relatively large torque ripple especially in the low speed region. This paper proposes an effective switching frequency regulation method which consists of hysteresis band controller and switching frequency command generator. The effectiveness of the proposed strategy is analyzed and compared with the conventional method.

Jun-Koo Kang

Papers

New direct torque control of induction motor for minimum torque ripple and constant switching frequency

A new phase detecting method for power conversion systems considering distorted conditions in power system

Power Loss, System Efficiency, and Leakage Current Comparison Between Si IGBT VFD and SiC FET VFD With Various Filtering Options

Development of MCs and its Applications in Industry [Industry Forum]

Direct torque control of induction machine with variable amplitude control of flux and torque hysteresis bands