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

DC power systems connecting to single-phase DC/AC inverters with nonlinear loads will have their DC sources being injected with AC ripple currents containing a low-frequency component at twice the output voltage frequency of the inverter and also other current harmonics. Such a current may create instability in the DC power system, lower its efficiency, and shorten the lifetime of the DC source. This paper presents a general waveform control method that can mitigate the injection of the low-frequency ripple current by the single-phase DC/AC inverter into the DC source. It also discusses the inhibiting ability of the waveform control method on other coexisting harmonics, while the DC source delivers AC power to a nonlinear load. With the application of the waveform control, the average DC output power is supplied by the DC source, while the other harmonics pulsation power can be confined to the AC side (between the capacitors and the AC load). Theoretical analyses and experiment results are provided to explain the operation and the inhibiting ability, and to showcase the performance of the approach in tackling the harmonics. Results validate that the proposed solution can achieve significant mitigation of harmonic component at DC side with nonlinear load as well as a high quality output voltage without the need for extra large capacitive energy storage element implementation.

Waveform control method for mitigating harmonics of inverter systems with nonlinear load

This paper proposes a new bidirectional resonant dc-dc converter suitable for wide voltage gain range applications (e.g., energy storage systems). The proposed converter overcomes the narrow voltage gain range of conventional resonant DC-DC converters, and meanwhile achieves high efficiency. It is realized by configuring a full-bridge mode and a half-bridge mode operation during each switching cycle. The fixed-frequency phase-shift control schemes are applied and the normalized voltage gain can be always from 0.5 to 1, regardless of the load. The transformer root-mean-square (RMS) currents in both the forward and the reverse modes have a negligible variation with respect to the voltage gain, which is beneficial to low conduction losses throughout a wide voltage range. Moreover, the applied MOSFETs are soft-switched, allowing for minimum switching losses. Both the operation principles and characteristics of the proposed converter are analyzed. Finally, a 1-kW converter prototype is built and the experimental results verify the theoretical analyses.

A fixed-frequency bidirectional resonant DC-DC converter suitable for wide voltage range

The dc-link capacitor is considered as a weak component in Photovoltaic (PV) inverter system and its reliability needs to be evaluated and tested during the product development. Conventional reliability testing methods do not consider the real operating conditions (e.g., mission profile) of the dc-link capacitor during the test. Therefore, the validation of the reliability performance of the dc-link capacitor under its mission profile is still a challenge. To address this issue, a new reliability testing concept for the dc-link capacitor in PV inverters is proposed in this paper. In contrast to the conventional method, the proposed reliability testing method realizes the test profile through the modification of the original mission profile (e.g., solar irradiance and ambient temperature) in order to maintain the test condition as close to the real application as possible. A certain acceleration factor is applied to the solar irradiance amplitude and the ambient temperature level during the mission profile modification in order to increase the thermal stress of the dc-link capacitor during test, and thereby effectively reduce the testing time. The results show that the testing time can be reduced to 2.5 % of the real field operation lifetime, if the solar irradiance amplitude is increased by 20 % and the ambient temperature is elevated to 75 °C.

/pdf/mission-profile-based-accelerated-testing-of-dc-link-2eh6c9wkmf.pdf

Mission Profile-based Accelerated Testing of DC-link Capacitors in Photovoltaic Inverters

The inevitable electrothermal coupling among capacitors in a bank will lead to nonnegligible errors to the temperature as well as the lifetime prediction of the individual capacitor. In this article, the analytical thermal models with the corresponding design for capacitor banks are proposed considering the thermal coupling effect. First, a measurement-based lump thermal model and a physical-model-based analytical thermal model are proposed. Then, with the help of the proposed thermal models, the design method for the thermal matching of capacitors in a bank is proposed by optimizing the parameters of individual capacitors, such as the rated parameters and actual loading. To validate the accuracy of the proposed design, case studies based on a capacitor bank with nine capacitors are presented.

Analytical Modeling and Design of Capacitor Bank Considering Thermal Coupling Effect

This paper discusses reliability issues of three different types of capacitors (ceramic, electrolytic and film) applied in the input side of an impedance-source photovoltaic (PV) microconverter having an ultra-wide dc gain range. The study considers a wear-out failure analysis based on a daily PV mission profile. The general operation and control principle are first described. Then the selection criteria of the capacitors are analyzed. Experimental measurements of capacitors temperature are translated into accumulated damage using their corresponding lifetime models. Finally, conclusions are drawn regarding the applicability of different capacitor technologies in impedance-source PV microconverters, where the film capacitor has the lowest damage.

Huai Wang

Papers

Waveform control method for mitigating harmonics of inverter systems with nonlinear load

A fixed-frequency bidirectional resonant DC-DC converter suitable for wide voltage range

Mission Profile-based Accelerated Testing of DC-link Capacitors in Photovoltaic Inverters

Analytical Modeling and Design of Capacitor Bank Considering Thermal Coupling Effect

Reliability Study of Input Side Capacitors in Impedance-Source PV Microconverters