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

Grid-tied inverters have been widely used to inject the renewable energies into the distributed power generation systems. However, a large variation of the grid impedance challenges the stability of the high-order power filter based grid-tied inverter. Many passive and active damping methods have been proposed to overcome this issue. Recently, a composite passive damping method for a high-order power filter based grid-tied inverter with an RC parallel damper and an RL series damper was presented to eliminate this problem, but at the cost of more material and power losses. In this paper, a hybrid damping method with a digital filter and an RC parallel damper is proposed. The design of the digital filter is developed using a normalized method. The validity is verified through the simulations and the experiments on a 500 W, 110 V/50 Hz prototype, while the grid inductance varies from 0.15 mH to 5 mH.

/pdf/a-hybrid-damping-method-for-llcl-filter-based-grid-tied-4qfu3h44aa.pdf

A hybrid damping method for LLCL-filter based grid-tied inverter with a digital filter and an RC parallel passive damper

Fuel cells are becoming a promising energy source for various applications due to its relatively high efficiency and reliability, and low pollution. They have been applied in backup power systems in telecom applications, where reliability and availability are high priority performance factors. This paper describes a lifetime prediction method for the power semiconductors used in the power conditioning of a fuel cell based backup system, considering both the long-term standby mode and active operation mode. The annual ambient temperature profile is taken into account to estimate its impact on the degradation of MOSFETs during the standby mode. At the presence of power outages, the backup system is activated into the operation mode and the MOSFETs withstand additional thermal stresses due to power losses. A study case of a 1 kW backup system is presented with two annual mission profiles in Denmark and India, respectively. The ambient temperature, occurrence frequency of power outages, active operation time and power levels are considered for the lifetime prediction of the applied MOSFETs. Comparisons of the accumulated lifetime consumptions are performed between standby mode and operation mode, and between Denmark and India, respectively.

/pdf/real-mission-profile-based-lifetime-estimation-of-fuel-cell-2ah9m5qbol.pdf

Real mission profile based lifetime estimation of fuel-cell power converter

Thermal loading of wind power converters is critical to their reliability performance. Especially for IGBT modules applied in a converter, both of the mean value and variation of the junction temperature have significant impact on the lifetime. Besides other strategies to reduce the thermal loading of the IGBT modules, the power losses and thereby the thermal stresses can be controlled by varying the PWM switching frequency according to power loading conditions. This paper investigates the feasibility to apply this flexible modulation strategy in a 3 MW wind power converter application to reduce the temperature fluctuations due to wind speed variations. The trade-off between the reduced amplitude of temperature fluctuations and the additional power losses that may be introduced is quantitatively studied.

The feasibility study on thermal loading control of wind power converters with a flexible switching frequency

DC capacitors in power electronic converters are a major constraint on improvement of power density as well as reliability. In this paper, according to the degradation data of electrolytic capacitors through the accelerated test, the time-to-failure of the capacitor cell is acquired and it can be further extended to lower stress levels. Then, in a case study of a fuel cell backup power application, the mission profile based lifetime expectancy of the individual capacitor and the capacitor bank is estimated in terms of the standby mode and operation mode. The lifetime prediction of the capacitor banks can be obtained and compared with and without the degradation effect.

/pdf/degradation-effect-on-reliability-evaluation-of-aluminum-53utyifhau.pdf

Degradation effect on reliability evaluation of aluminum electrolytic capacitor in backup power converter

A multiobjective Pareto optimization routine is pro-posed to systematically assess the performance factors concerning annual degradation, volume, and cost. As an essential performance criterion of PV microinverters, the reliability is included in the multiobjective optimization, which has not been seen in the previous literature. Thus, a practical trade-off among annual degradation, volume and cost can be made for a two-stage PV microinverter. Firstly, the operation principle of the inverter stage in the boundary conduction mode (BCM) is analyzed for electrical aspect modeling. Then, a system-level electro-thermal model is built for the PV microinverter. After that, critical performance metrics, i.e., reliability, cost, and volume, are modeled before a multiobjective Pareto optimization strategy is proposed and executed. The proposed optimization method leads to a remarkable reliability improvement while maintaining low cost and volume.

Huai Wang

Papers

A hybrid damping method for LLCL-filter based grid-tied inverter with a digital filter and an RC parallel passive damper

Real mission profile based lifetime estimation of fuel-cell power converter

The feasibility study on thermal loading control of wind power converters with a flexible switching frequency

Degradation effect on reliability evaluation of aluminum electrolytic capacitor in backup power converter

Cost-Volume-Reliability Pareto Optimization of a Photovoltaic Microinverter