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

Power electronics are widely used in renewable energy systems to achieve lower cost of energy, higher efficiency and high power density. At the same time, the high reliability of the power electronics products is demanded, in order to reduce the failure rates and ensure cost-effective operation of the renewable energy systems. This paper thus describes the basic concepts used in reliability engineering, and presents the status and future trends of Design for Reliability (DfR) in power electronics, which is currently undergoing a paradigm shift to a physics-of-failure approach. Two case studies of a 2 MW wind turbine system and a 6 kW photovoltaic system are presented to demonstrate the use of the DfR approach as a design tool to predict lifetime of components (e.g., power devices) as well as the overall systems (e.g., power converters) based on the required operating conditions (i.e., mission profile).

Design for reliability in renewable energy systems

Due to the small volume and high efficiency, transformerless inverters have gained much popularity in grid-connected PV applications, where minimizing leakage current injection is mandatory. This can be achieved by either modifying the modulation schemes or adding extra power switching devices, resulting in an uneven distribution of the power losses on the switching devices. Consequently, the device thermal loading is redistributed and thus may alter the entire inverter reliability performance, especially under a long-term operation. In this consideration, this paper assesses the device reliability of three transformerless inverters under a yearly mission profile (i.e., solar irradiance and ambient temperature). The mission profile is translated to device thermal loading, which is used for lifetime prediction. Comparison results reveal the lifetime mismatches among the power switching devices operating under the same condition, which offers new thoughts for a robust design and a reliable operation of grid-connected transformerless PV inverters with high efficiency.

/pdf/reliability-assessment-of-transformerless-pv-inverters-1jmdoutdz9.pdf

Reliability Assessment of Transformerless PV Inverters Considering Mission Profiles

Reliability assessment of power conditioner considering maintenance in a PEM fuel cell system

To evaluate the reliability of power electronic systems, it is necessary to estimate the junction temperatures of power devices under periodic power loss profiles due to the fundamental-frequency current. However, a large number of periodic power loss profiles under long-term reliability evaluation (e.g., 1 year) are challenging to compute quickly. On the other hand, in order to improve computational efficiency, unvalidated simplified thermal analysis methods may impair the confidence of the reliability outcome. Therefore, this paper proposes an analytical model for power semiconductors with a compromise of the thermal estimation accuracy and computational efficiency. With the proposed model, it is indicated that minimum computation efforts can be obtained for thermal estimation under a maximum allowable error. The proposed method, thus, enables computation-efficient thermal stress analysis for power semiconductor devices with a preset modeling error. The effectiveness of the proposed method has been verified by simulations and experiments on a power electronic system with 1200-V/50-A insulated gate bipolar transistor (IGBT) modules.

A Simplification Method for Power Device Thermal Modeling With Quantitative Error Analysis

With the increasing penetration of renewable power, its reliability and cost-effective production are becoming more important. A filter is inserted between the grid-connected inverter and the power grid to reduce the PWM switching harmonics, which may become a fragile part seen from the power electronics converter perspective. As the grid-connected inverter is typically designed with additional reactive power capability, this paper tries to investigate the additional stresses of the filter capacitor introduced by the reactive power injection. According to an electro-thermal stress evaluation, the time-to-failure distribution of a single LCL filter capacitor is investigated in detail. Moreover, the increasing equivalent series resistance along with the operational period is taken into account. Aiming towards a system-level reliability analysis, a Weibull distribution based reliability of an individual capacitor can be linked to the reliability of a capacitor bank by using a reliability block diagram. A case study of a 2 MW wind power converter shows that the lifetime is significantly reduced from the individual capacitor to the capacitor bank. Besides, over-excited reactive power injection further reduces the lifetime of the LCL filter capacitors.

/pdf/reactive-power-impacts-on-lcl-filter-capacitor-lifetime-in-4vkzgjb0bb.pdf

Huai Wang

Papers

Design for reliability in renewable energy systems

Reliability Assessment of Transformerless PV Inverters Considering Mission Profiles

Reliability assessment of power conditioner considering maintenance in a PEM fuel cell system

A Simplification Method for Power Device Thermal Modeling With Quantitative Error Analysis

Reactive Power Impacts on LCL Filter Capacitor Lifetime in Grid-Connected Inverter