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

In order to minimize the effect of the grid harmonic voltages, harmonic compensation is usually adopted for a grid-tied inverter. However, a large variation of the grid inductance challenges the system stability in case a high-order passive filter is used to connect an inverter to the grid. Although in theory, an adaptive controller can solve this problem, but in such a case the grid inductance may need to be detected online, which will complicate the control system. This paper investigates the relationship between the maximum gain of the controller that still keeps the system stable and the Q-factor for a grid-tied inverter with an RL series or an RC parallel damped high-order power filter. Then, a robust passive damping method for LLCL-filter-based grid-tied inverters is proposed, which effectively can suppress the possible resonances even if the grid inductance varies in a wide range. Simulation and experimental results are in good agreement with the theoretical analysis.

/pdf/a-robust-passive-damping-method-for-llcl-filter-based-grid-337m128y5u.pdf

A Robust Passive Damping Method for LLCL-Filter-Based Grid-Tied Inverters to Minimize the Effect of Grid Harmonic Voltages

This paper introduces a reliability-oriented design tool for a new generation of grid-connected photovoltaic (PV) inverters. The proposed design tool consists of a real field mission profile (RFMP) model (for two operating regions: USA and Denmark), a PV panel model, a grid-connected PV inverter model, an electrothermal model, and the lifetime model of the power semiconductor devices. An accurate long-term simulation model able to consider the one-year RFMP (solar irradiance and ambient temperature) is developed. Thus, the one-year estimation of the converter device thermal loading distribution is achieved and is further used as an input to the lifetime model. The proposed reliability-oriented design tool is used to study the impact of mission profile (MP) variation and device degradation (aging) in the PV inverter lifetime. The obtained results indicate that the MP of the field where the PV inverter is operating has an important impact (up to 70%) on the converter lifetime expectation, and it should be considered in the design stage to better optimize the converter design margin. In order to have correct lifetime estimation, it is crucial to consider also the device degradation feedback (in the simulation model), which has an impact of 20-30% on the precision of the lifetime estimation for the studied case.

Reliability Oriented Design Tool For the New Generation of Grid Connected PV-Inverters

Reliability analysis is an important tool for assisting the design phase of a power electronic converter to fulfill its life-cycle specifications. Existing converter-level reliability analysis methods have two major limitations: 1) being based on constant failure rate models; and 2) lack of consideration of long-term operation conditions (i.e., mission profile). Although various studies have been presented on power electronic component-level lifetime prediction based on wear-out failure mechanisms and mission profile, it is still a challenge to apply the same method to the reliability analysis of converters with multiple components. Component lifetime prediction based on associated models provides only a $B_{X}$ lifetime information (i.e., the time when X % items fail), but the time-dependent reliability curve is still not available. In this paper, a converter-level reliability analysis approach is proposed based on time-dependent failure rate models and long-term mission profiles. Two different methods to obtain the component-level time-to-failure are illustrated by a case study of dc/dc converters for a 5 kW fuel cell-based backup power system. The reliability analysis of the converters with and without redundancy is also performed to assist the decision making in the design phase of the fuel cell power conditioning stage.

/pdf/mission-profile-based-system-level-reliability-analysis-of-2v9flefgh3.pdf

Mission Profile Based System-Level Reliability Analysis of DC/DC Converters for a Backup Power Application

Reliability of Power Electronic Converter Systems

Electrolytic capacitor with a dc-side inductor is a typical dc-link filtering configuration in grid-connected diode rectified adjustable speed drives (ASDs). The criteria to size the dc-link filter are mainly from the aspects of stability and power quality. Nevertheless, the reliability of the dc-link filter is also an essential performance factor to be considered, which depends on both the component inherent capability and the operational conditions (e.g., electrothermal stresses) in the field operation. Nowadays, unbalanced voltage has the most frequent occurrence in many distribution networks. It brings more electrical-thermal stress to the component, affecting the reliability of the capacitors. In order to study the reliability performance of the LC filter in an ASD system quantitatively, this paper proposes a mission profile based reliability evaluation method for capacitors. Different from the conventional lifetime estimation, a nonlinear accumulated damage model is proposed for the long-term estimation, considering the nonlinear process of equivalent series resistor growth and capacitance reduction during the degradation. Based on the proposed lifetime estimation procedure, four case studies are investigated: first, lifetime benchmarking of capacitors in LC filtering and slim capacitor filtering configurations; second, scalability analysis for the lifetime of capacitors in terms of system power rating and grid-unbalanced levels; third, lifetime estimation of capacitors in the dc-link filter with long-term mission profile; and fourth, the impact of the capacitor sizing on the lifetime of the dc-link capacitor under grid-balanced and grid-unbalanced conditions. The results serve as a guideline for proper selection of dc-link configurations and parameters to fulfill a specification in ASDs.

/pdf/lifetime-estimation-of-dc-link-capacitors-in-adjustable-1643bie7mf.pdf

Huai Wang

Papers

A Robust Passive Damping Method for LLCL-Filter-Based Grid-Tied Inverters to Minimize the Effect of Grid Harmonic Voltages

Reliability Oriented Design Tool For the New Generation of Grid Connected PV-Inverters

Mission Profile Based System-Level Reliability Analysis of DC/DC Converters for a Backup Power Application

Reliability of Power Electronic Converter Systems

Lifetime Estimation of DC-Link Capacitors in Adjustable Speed Drives Under Grid Voltage Unbalances