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 proposes a new two-phase PWM scheme that can reduce the ground to stator neutral voltage in three-phase diode rectifier/inverter system with corner grounded delta-connected input source. According to the sign of voltage difference between ground and DC-bus midpoint voltage, the proposed PWM use only one zero space vector in one control period. Thus, the peak-to-peak value of ground to stator neutral voltage can be reduced to 2/3 compared with conventional three-phase SVPWM. Without any degradation of control performance of inverter, the proposed PWM scheme can be easily implemented in software and requires no extra hardware except a simple neutral voltage measurement circuit.

A new space vector pulsewidth modulation strategy for reducing ground to stator-neutral voltage in inverter-fed AC motor drives

This paper describes a dc-link voltage control method of a single-phase back-to-back cascaded H-bridge inverter (SBCI) for a medium-voltage motor drive system. The main advantage of the SBCI topology over the conventional regenerative cascaded H-bridge topology with a three-phase active front-end (AFE) is a simple system structure, which is composed of an input transformer, a power cell, a current sensor, etc. However, the challenging points of the SBCI are larger voltage ripple in the dc-link capacitor and imbalance of dc-link voltages of each phase. The asymmetric dc-link voltage of each power cells could cause unstable operation such as over-modulation due to the lack of the dc-link voltage of a particular phase and result in over-voltage or under-voltage faults. In this paper, the control strategy of the dc-link voltage for the SBCI that uses the negative-sequence voltage of the converter is described. The proposed control method is verified with a computer simulation whose target is a 6.6-kV–1.25-MW medium-voltage drive system. Also, through the experimental setup with the prototype SBCI whose power rating is 16.2 kVA, the dc-link voltage of each AFE has been controlled within a 0.5% error of its reference value at the full load.

Control Strategy of DC-Link Voltage for Single-Phase Back-to-Back Cascaded H-Bridge Inverter for MV Drive With Interfacing Transformer Having Tertiary Winding

This paper describe a design method of an LCL filter for the three-phase two-level voltage-source PWM converters. The grid current harmonics regulation is the primary condition for the operation of the grid-connected converters to satisfy. If the grid voltage is assumed to be ideally sinusoidal, PWM switching is the only factor to produce harmonics on the grid current, especially around the bands of the switching frequency. However, every single harmonic cannot be evaluated for the filter design. In this paper, it is found that there exists the most significant harmonic frequency at which the harmonic current is dominant. Theoretical description of the PWM waveform and the transfer function of the filter's impedance are needed for the derivation of the most significant frequency. Once the most significant frequency is recognized, the total inductance to satisfy the grid current harmonic regulations is determined by selecting proper resonant frequency. Along with the information of the designed total inductance and preset resonant frequency, additional conditions such as the filter's total energy, converter current ripple and converter capacity are utilized to specify the values of each component of the filter. The effectiveness of the proposed method is verified with simulation and experimental results.

Non-iterative LCL filter design for three-phase two-level voltage-source PWM converters

This paper presents the optimal load sharing control method of grid connected gas engine generation system. This micro-grid system consists of engine, generator, load, grid and power converters. These components can be connected in various ways to make a micro-grid system. Several kinds of system structures are suggested and their characteristics and annual running costs are compared to decide the optimal system structure. The annual running cost can be saved by sharing the load power with gas engine generator when the electricity rate is higher than the generation cost. The system control method is proposed to achieve this objective, to minimize the annual running cost. The validity and performance of the proposed system structure and its control algorithm are verified by experimental results. These experimental results reveal excellent load sharing capability and running cost reduction.

Control of Utility Connected Gas Engine Generation System

This paper presents the theoretical analysis on the errors in voltage synthesis and position estimation due to the voltage distortion in position-sensorless operation. Especially, the pulsating square wave signal injection method into the estimated d-axis would be focused on, revealing the nature that the position estimation performance depends on the frequency of the injected signal. From the analysis, it would be shown that, in no load condition, the voltage distortion of one phase mainly has a 2nd order harmonic component in negative sequence by the signal of the frequency to the same switching frequency of the inverter and it leads to position estimation error with 3rd order harmonic component. In the case of a half frequency of the switching frequency or in case of heavy load conditions, the effect of inverter nonlinearity does not affect the position estimation error. Therefore, only the effect of cross-coupling inductances is reflected in these cases, and the position estimation error appears in the form of 6th order harmonics. Simulation and experimental results verify the validity of the analysis.

Seung-Ki Sul

Papers

A new space vector pulsewidth modulation strategy for reducing ground to stator-neutral voltage in inverter-fed AC motor drives

Control Strategy of DC-Link Voltage for Single-Phase Back-to-Back Cascaded H-Bridge Inverter for MV Drive With Interfacing Transformer Having Tertiary Winding

Non-iterative LCL filter design for three-phase two-level voltage-source PWM converters

Control of Utility Connected Gas Engine Generation System

Analysis on the position estimation error in position-sensorless operation using pulsating square wave signal injection