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

The science and technology of ultracapacitors are reviewed for a number of electrode materials, including carbon, mixed metal oxides, and conducting polymers. More work has been done using microporous carbons than with the other materials and most of the commercially available devices use carbon electrodes and an organic electrolytes. The energy density of these devices is 3¯5 Wh/kg with a power density of 300¯500 W/kg for high efficiency (90¯95%) charge/discharges. Projections of future developments using carbon indicate that energy densities of 10 Wh/kg or higher are likely with power densities of 1¯2 kW/kg. A key problem in the fabrication of these advanced devices is the bonding of the thin electrodes to a current collector such the contact resistance is less than 0.1 cm2. Special attention is given in the paper to comparing the power density characteristics of ultracapacitors and batteries. The comparisons should be made at the same charge/discharge efficiency.

Ultracapacitors: Why, How, and Where is the Technology

This paper reviews the current status and implementation of battery chargers, charging power levels, and infrastructure for plug-in electric vehicles and hybrids. Charger systems are categorized into off-board and on-board types with unidirectional or bidirectional power flow. Unidirectional charging limits hardware requirements and simplifies interconnection issues. Bidirectional charging supports battery energy injection back to the grid. Typical on-board chargers restrict power because of weight, space, and cost constraints. They can be integrated with the electric drive to avoid these problems. The availability of charging infrastructure reduces on-board energy storage requirements and costs. On-board charger systems can be conductive or inductive. An off-board charger can be designed for high charging rates and is less constrained by size and weight. Level 1 (convenience), Level 2 (primary), and Level 3 (fast) power levels are discussed. Future aspects such as roadbed charging are presented. Various power level chargers and infrastructure configurations are presented, compared, and evaluated based on amount of power, charging time and location, cost, equipment, and other factors.

/pdf/review-of-battery-charger-topologies-charging-power-levels-85yy2igx2w.pdf

Review of Battery Charger Topologies, Charging Power Levels, and Infrastructure for Plug-In Electric and Hybrid Vehicles

http://ieeexplore.ieee.org/iel5/5610941/5624686/05624745.pdf

Power electronics

The aim of this paper is to present a review of current control techniques for three-phase voltage-source pulsewidth modulated converters. Various techniques, different in concept, have been described in two main groups: linear and nonlinear. The first includes proportional integral (stationary and synchronous) and state feedback controllers, and predictive techniques with constant switching frequency. The second comprises bang-bang (hysteresis, delta modulation) controllers and predictive controllers with on-line optimization. New trends in current control-neural networks and fuzzy-logic-based controllers-are discussed, as well. Selected oscillograms accompany the presentation in order to illustrate properties of the described controller groups.

Current control techniques for three-phase voltage-source PWM converters: a survey

Dynamic voltage restorer (DVR)s are known as effective and economic means to compensate for voltage sags and swells. This paper proposes a novel method to generate reference waves synchronized with the grid whether or not the line voltages are distorted by a fault. The proposed reference wave generator (RWG) looks for the normal voltage waveforms in the grid from the corrupted line voltages and synthesizes the expected positive sequence reference waves for DVRs. There is no transient problem on the generated reference waves when the line voltages are distorted by faults. Experimental results verified the proposed theory very well.

Reference wave generation in dynamic voltage restorers by use of PQR power theory

This paper describes the two control techniques to perform the sensorless vector control of interior permanent magnet synchronous motors (IPMSM) by injecting the high frequency voltage to the stator terminal. The first technique is the estimation algorithm of the rotor initial position including the identification of the rotor magnet N-S polarity at standstill. IPMSM possess the saliency which produces the ellipse of the stator current when the high frequency voltage is injected to the motor terminal. The major axis angle of the current ellipse gives rotor position information at standstill. The center of the current ellipse shifts because of the magnetic saturation effect from the origin, which makes it possible to identify the rotor magnet N-S polarity. The averaging method of the multi-cycles stator current is utilized to identify the rotor magnet N-S polarity. This averaging method increases the accuracy of the position estimation and solves the current ripple problem. The second control technique is a sensorless control algorithm that injects the high frequency current to the stator terminal in order to estimate the rotor position and speed. The rotor position and speed for sensorless vector control is achieved by appropriate signal processing to extract the position information from the stator current even in the low speed range. Experimental results are presented to verify the feasibility of the proposed control schemes.

Vector control of interior permanent magnet synchronous motor without a shaft sensor

For stable and efficient operation of IPMSM, the stator flux should be observed across a wide speed range including low-speed. However, not only a de offset but also low-order harmonics are contained in the estimated flux due to various reasons. In this paper, a distortion-minimizing flux observer is proposed to suppress the dc offset and harmonic flux errors by adopting frequency-adaptive observers. A frequency-adaptive observer is appropriate to extract the fundamental component while keeping a simple structure even if a rotating speed of IPMSM varies. The fundamental flux could be estimated quickly by the proposed current- and voltage- model based flux observer, which is implemented with not only the frequency-adaptive observer but also harmonic extractors. Robustness and effectiveness of the proposed methods are verified with simulation and experimental results. Through the proposed algorithms, the fundamental stator flux can be estimated while maintaining high dynamic performance over a wide speed range.

Distortion-Minimizing Flux Observer for IPMSM Based on Frequency-Adaptive Observers

This paper describes how to reconstruct the three phase current using the only single shunt in a three-level Neutral-Point-Clamped (NPC) PWM inverter. The area where phase currents are measurable is analyzed and the reconstruction method is proposed in three-level PWM inverter. According to the characteristics of the immeasurable areas in voltage plane, the voltage injection method, which relocates the original voltage reference in the immeasurable area to that in a measurable area in the voltage plane, has been devised. For this relocation, the compensating voltage is added and subtracted to the voltage reference in a switching period. And a control algorithm to inject the compensating voltage is also proposed. The proposed reconstruction of the phase currents in the single shunt sensing inverter has been tested by the computer simulation at various reference voltages, being in or passing in the immeasurable area. The results show the satisfactory measurement of the phase currents in overall operating conditions.

Single shunt current sensing technique in three-level PWM inverter

If a small DC-link capacitor is used in the DC-link fed by single phase AC source, the DC-link voltage fluctuates severely with twice of the source frequency. To handle this fluctuation, the concept of ‘average voltage constraint’ is proposed in this study. On the basis of the concept, a flux weakening scheme generating d-axis current reference (i ds r*) for Interior Permanent Magnet Synchronous Machine (IPMSM) is devised. And, q-axis current reference (i qs r*) is modified for the unity power factor operation to AC source. The proposed scheme has been applied to inverter driven 1 kW compressor of an air-conditioner. From the experimental results, it has been verified that the compressor operates well at required operating condition regardless of the severe fluctuation of DC-link voltage due to the reduced DC-link capacitor. Also the frequency spectrum of the AC source current reveals that the harmonics of the source current meet IEC 61000-3-2 Class A and that the overall power factor is above 97% without any additional circuit such as an input filter and the power factor correction circuit.

Seung-Ki Sul

Papers

Reference wave generation in dynamic voltage restorers by use of PQR power theory

Vector control of interior permanent magnet synchronous motor without a shaft sensor

Distortion-Minimizing Flux Observer for IPMSM Based on Frequency-Adaptive Observers

Single shunt current sensing technique in three-level PWM inverter

Control of three phase inverter for AC motor drive with small DC-link capacitor fed by single phase AC source