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

This paper suggests the concept of shaping the pulsewidth-modulated (PWM) converter admittance. The analytic admittance matrix equation, which is derived from the small signal model of a PWM converter, is formulized. Based on the damping coefficient and the natural frequencies of phase-locked loop and dc voltage loop, the variations in admittance frequency responses are discussed. In addition, the frequency response can be adjusted through pole placement using the gain of the current controller. The controller gain for a stable operation is designed by detailed understanding of the frequency response shape. Furthermore, to stabilize the local electric power systems (EPSs) that invoke instability through the interaction between constant power load PWM converters, the admittance shaping loop is utilized for the distributed generator PWM converters. Through computer simulation and hardware experiments, the proposed shaping guidance can effectively stabilize local EPS.

Shaping of PWM Converter Admittance for Stabilizing Local Electric Power Systems

A switching loss of Modular Multilevel Converter(MMC) might increase drastically in HVDC system because the number of sub-module(SM) is proportional to the DC-link voltage. And, a special strategy for reducing switching frequency has been significant research issue in terms of overall operating efficiency of MMC for HVDC system. The voltage fluctuation of capacitor in SM, however, increases as the switching frequency decreases, and the capacitor with large capacitance which is the main portion of equipment cost for SM is required to mitigate the voltage fluctuation. In this paper, the switching frequency reduction strategy is proposed using the sorting method with a virtual capacitor voltage of individual SMs. In addition, this paper presents the 2nd order harmonic circulating current injection to suppress the voltage fluctuation. By numerical loss analysis, it is identified that the 2nd order harmonic current injection does not incur severe additional loss. Thanks to the harmonic current injection, the capacitance of SM capacitor could be reduced by 33% at the cost of only 0.05 % efficiency degradation in the given simulation condition. To evaluate the effectiveness of the proposed strategies, the computer simulation with 400 kV, 400MA, 221-level MMC has been performed and the results are discussed. Additionally, validity of the proposed strategies has been verified by 7-level down scaled prototype experimental setup.

A switching frequency reduction and a mitigation of voltage fluctuation of modular multilevel converter for HVDC

In this paper, it is described how the circulating current flows between inverters that have isolated dc link such as cascaded H-bridge (CHB) topology when they are operating in parallel. In this case, zero sequence circulating current, which normally flows through shared dc link, cannot flow between inverters in parallel. However, circulating current is provoked by asynchronous switching instant of devices, difference of dc-link voltages of parallel inverters, and unbalanced impedance. The circulating current should be suppressed by a sharing inductor that is inserted between inverters. Generally, all of the inverters should synthesize the same output voltage reference for load current control to minimize the size of this sharing reactor. However, this conventional method cannot guarantee diminishing circulating current in the transient state and even in the steady state. In this paper, in order to reduce sharing reactance, after deriving circulating current model from general case of n-parallel operation, circulating current control method is devised based on the model. This proposed algorithm is applied to active front end five-level-CHB inverter system for medium voltage drive. Simulation and experimental results are provided to verify the effectiveness of the proposed control scheme.

Parallel Operation of Inverters With Isolated DC Link for Minimizing Sharing Inductor

This paper presents a thrust loss suppression algorithm in an electric propulsion ship. From the viewpoint of the speed controller of the propulsion motor, the thrust loss due to cavitation can be regarded as a disturbance torque. The disturbance torque is estimated by means of a disturbance observer. Considering the estimated disturbance torque, the speed reference of the propeller is modified to suppress the thrust loss. The experimental results which performed in the cavitation tunnel with an electrically driven propulsion system are shown to verify the proposed algorithm.

Suppression of the Thrust Loss for the Maximum Thrust Operation in the Electric Propulsion Ship

For three-phase three-wire grid, a method to synchronize the converter to the grid voltage, which was distorted by low-order harmonics and unbalances, has been widely described. In addition to filtering them, the DC rejection can be achieved together with a frequency adaptive observer in order to eliminate DC bias in the voltage. The state equation for the observer is newly established, and the observer gains are examined in terms of their influences to the performance. The effectiveness of the proposed method is verified with simulations and experiments.

Seung-Ki Sul

Papers

Shaping of PWM Converter Admittance for Stabilizing Local Electric Power Systems

A switching frequency reduction and a mitigation of voltage fluctuation of modular multilevel converter for HVDC

Parallel Operation of Inverters With Isolated DC Link for Minimizing Sharing Inductor

Suppression of the Thrust Loss for the Maximum Thrust Operation in the Electric Propulsion Ship

Frequency adaptive observer to extract ac-coupled signals for grid synchronization