There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

다중혈관 관상동맥 환자에서 y-문합을 이용하여 양쪽 내흉동맥만을 사용한 우회술의 조기 성적

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

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

Power electronics

Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

/pdf/light-emitting-diodes-ubzde6g9qc.pdf

Light-emitting diodes

Advances in power electronics enable efficient and flexible processing of electric power in the application of renewable energy sources, electric vehicles, adjustable-speed drives, etc. More and more efforts are devoted to better power electronic systems in terms of reliability to ensure high availability, long lifetime, sufficient robustness, low maintenance cost and low cost of energy. However, the reliability predictions are still dominantly according to outdated models and terms, such as MIL-HDBK-217H handbook models, Mean-Time-To-Failure (MTTF), and Mean-Time-Between-Failures (MTBF). A collection of methodologies based on Physics-of-Failure (PoF) approach and mission profile analysis are presented in this paper to perform reliability-oriented design of power electronic systems. The corresponding design procedures and reliability prediction models are provided. Further on, a case study on a 2.3 MW wind power converter is discussed with emphasis on the reliability critical components IGBTs. Different aspects of improving the reliability of the power converter are mapped. Finally, the challenges and opportunities to achieve more reliable power electronic systems are addressed.

Design for reliability of power electronic systems

Current grid standards largely require that low-power (e.g., several kilowatts) single-phase photovoltaic (PV) systems operate at unity power factor (PF) with maximum power point tracking (MPPT), and disconnect from the grid under grid faults by means of islanding detection. However, in the case of wide-scale penetration of single-phase PV systems in the distributed grid, disconnection under grid faults can contribute to 1) voltage flickers, 2) power outages, and 3) system instability. This article explores grid code modifications for a wide-scale adoption of PV systems in the distribution grid. In addition, based on the fact that Italy and Japan have recently undertaken a major review of standards for PV power conversion systems connected to low-voltage networks, the importance of low voltage ride-through (LVRT) for single-phase PV power systems under grid faults is considered, along with three reactive power injection strategies. Simulations are presented for a PV power system with a LVRT capability and ancillary services. An example of a full-bridge single-phase grid connected system is tested experimentally to demonstrate the potential benefits. Additionally, grid codes for advanced PV systems with the discussed features are summarized.

/pdf/wide-scale-adoption-of-photovoltaic-energy-grid-code-zp4qehaqcc.pdf

Wide-Scale Adoption of Photovoltaic Energy: Grid Code Modifications Are Explored in the Distribution Grid

A technique for reduction of the dc-link capacitance in a capacitor-supported system is presented. The concept is based on connecting a voltage source in series with the dc bus line to compensate the ripple voltage on the dc-link capacitor, so as to make the output have a near zero ripple voltage. Since the voltage compensator processes small ripple voltage on the dc link and reactive power only, it can be implemented with low-voltage devices. The overall required energy storage of the dc-link, formed by a reduced value of dc-link capacitor and the voltage compensator, is reduced, allowing the replacement of popularly used electrolytic capacitors with alternatives of longer lifetime, like power film capacitors, or extending the system lifetime even if there is a significant reduction in the capacitance of electrolytic capacitors due to the aging effect. Comprehensive analysis on the static and dynamic characteristics of the system, and hold-up time requirement will be discussed. The proposed technique is exemplified on an ac-dc-dc power conversion system. Theoretical predictions are favorably verified by experimental results.

/pdf/use-of-a-series-voltage-compensator-for-reduction-of-the-dc-2zt9a9aazh.pdf

Use of a Series Voltage Compensator for Reduction of the DC-Link Capacitance in a Capacitor-Supported System

As the development and installation of photovoltaic (PV) systems are still growing at an exceptionally rapid pace, relevant grid integration policies are going to change consequently in order to accept more PV systems in the grid. The next-generation PV systems will play an even more active role like what the conventional power plants do today in the grid regulation participation. Requirements of ancillary services like low-voltage ride-through (LVRT) associated with reactive current injection and voltage support through reactive power control have been in effectiveness in some countries, e.g., Germany and Italy. Those advanced features can be provided by next-generation PV systems and will be enhanced in the future to ensure an even efficient and reliable utilization of PV systems. In light of this, reactive power injection (RPI) strategies for single-phase PV systems are explored in this paper. The RPI possibilities are as follows: 1) constant average active power control; 2) constant active current control; 3) constant peak current control; and 4) thermal optimized control strategy. All those strategies comply with the currently active grid codes but are with different objectives. The proposed RPI strategies are demonstrated first by simulations and also tested experimentally on a 1-kW singe-phase grid-connected system in LVRT operation mode. Those results show the effectiveness and feasibilities of the proposed strategies with reactive power control during LVRT operation. The design and implementation considerations for the characterized RPI strategies are also discussed.

/pdf/reactive-power-injection-strategies-for-single-phase-4zppxtpvyo.pdf

Reactive Power Injection Strategies for Single-Phase Photovoltaic Systems Considering Grid Requirements

Reliability is one of the key issues for the application of Insulated Gate Bipolar Transistors (IGBTs) in power electronic converters. Many efforts have been devoted to the reduction of IGBT wear out failure induced by accumulated degradation and catastrophic failure triggered by single-event overstress. The wear out failure under field operation could be mitigated by scheduled maintenances based on lifetime prediction and condition monitoring. However, the catastrophic failure is difficult to be predicted and thus may lead to serious consequence of power electronic converters. To obtain a better understanding of catastrophic failure of IGBTs, the state-of-the-art research on their failure behaviors and failure mechanisms is presented in this paper. Moreover, various fault-tolerant design methods, to prevent converter level malfunctions in the event of IGBT failure, are also reviewed.

/pdf/catastrophic-failure-and-fault-tolerant-design-of-igbt-power-1b3e14j5s1.pdf

Huai Wang

Papers

Design for reliability of power electronic systems

Wide-Scale Adoption of Photovoltaic Energy: Grid Code Modifications Are Explored in the Distribution Grid

Use of a Series Voltage Compensator for Reduction of the DC-Link Capacitance in a Capacitor-Supported System

Reactive Power Injection Strategies for Single-Phase Photovoltaic Systems Considering Grid Requirements

Catastrophic failure and fault-tolerant design of IGBT power electronic converters - an overview