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

Reliability of dc-link capacitors in modern design of power electronic converters is an important aspect that needs to be considered The requirement of applying condition monitoring for health status estimation in many reliability-critical applications have been a focused demand The existing capacitor condition monitoring methodologies are suffering from shortcomings such as, low estimation accuracy, extra hardware, and increased cost, and thereby, they are rarely adopted by industry Therefore, development of new methods that are based on advanced software algorithms and data processing techniques requiring no extra hardware will be more attractive to industry In this paper, a condition monitoring methodology is proposed and applied on the dc-link capacitor in a three phase Front-End diode bridge motor drive The proposed condition monitoring methodology estimates the capacitance value of the dc-link capacitor based on Artificial Neural Network (ANN) algorithm Two ANNs (ANN1 and ANN2) are proposed, trained and evaluated based on time-domain and frequency-domain parameters Experiments are conducted to validate the proposed methodology and the effectiveness of the proposed method is examined through an error analysis

Capacitance estimation algorithm based on DC-link voltage harmonics using artificial neural network in three-phase motor drive systems

Capacitor is one of the reliability critical components in power electronic systems. In the last two decades, many efforts in the academic research have been devoted to the condition monitoring of capacitors to estimate their health status. Industry applications demand more reliable power electronics products with preventive maintenances. Nevertheless, most of the developed capacitor condition monitoring technologies are rarely adopted by industry due to the complexity, increased cost and other relevant issues. An overview of the prior-art research in this area is therefore needed to justify the required resources and the corresponding performance of each of the key method. It serves to provide a guideline for industry to evaluate the available solutions by technology benchmarking, as well as to advance the academic research by discussing the history development and the future opportunities. Therefore, this paper firstly classifies the capacitor condition monitoring methods into three categories, then the respective technology evolution from 1993 to 2015 is summarized. Remarks on the state-of-the-art research and the future opportunities targeting for practical industry applications are given.

A review of the condition monitoring of capacitors in power electronic converters

Capacitors have been witnessed as one of the weak points in grid-connected PhotoVoltaic (PV) applications, and thus efforts have been devoted to the design of reliable DC-link capacitors in PV applications. Since the hot-spot temperature of the capacitor is one of the failure inducers, instantaneous thermal modeling approaches considering mission profiles for the DC-link capacitor in single-phase PV systems are explored in this paper. These thermal modelling approaches are based on: a) fast Fourier transform, b) look-up tables, and c) ripple current reconstruction. Moreover, the thermal modelling approaches for the DC-link capacitors take into account the instantaneous thermal characteristics, which are more challenging to the capacitor reliability during operation. Such instantaneous thermal modeling approaches enable a translation of instantaneous capacitor power losses to capacitor thermal loading from the operating conditions. As a consequence, it offers new insights into the temperature monitoring and reliability-oriented design of the DC-link capacitors, and thus a more reliable operation of single-phase grid-connected PV systems can be enhanced. Study results on a 3-kW single-phase grid-connected PV system have been adopted to demonstrate a look-up table based modelling approach, where real-field daily ambient conditions are considered.

/pdf/instantaneous-thermal-modeling-of-the-dc-link-capacitor-in-1jo584kpp9.pdf

Instantaneous thermal modeling of the DC-link capacitor in PhotoVoltaic systems

Transformerless photovoltaic (PV) inverters are going to be more widely adopted in order to achieve high efficiency, as the penetration level of PV systems is continuously booming. However, problems may arise in highly PV-integrated distribution systems. For example, a sudden stoppage of all PV systems due to anti-islanding protection may trigger grid disturbances. Thus, standards featuring with ancillary services for the next generation PV systems are under a revision in some countries. The future PV systems have to provide a full range of services as what the conventional power plants do, e.g. Low Voltage Ride-Through (LVRT) under grid faults and grid support service. In order to map future challenges, the LVRT capability of three mainstream single-phase transformerless PV inverters under grid faults are explored in this paper. Control strategies with reactive power injections are also discussed. The selected inverters are the full-bridge inverter with bipolar modulation, full-bridge inverter with DC bypass and the Highly Efficient and Reliable Inverter Concept (HERIC). A 1 kW single-phase grid-connected PV system is analyzed to verify the discussions. The tests confirmed that, although the HERIC inverter is the best candidate in terms of efficiency, it is not very feasible in case of a voltage sag. The other topologies are capable of providing reactive current during LVRT. A benchmarking of those inverters is also provided, which offers the possibility to select appropriate devices and to further optimize the system.

/pdf/low-voltage-ride-through-of-single-phase-transformerless-1chnlicfxb.pdf

Low voltage ride-through of single-phase transformerless photovoltaic inverters

The modular multilevel converter (MMC) is attractive for medium- or high-power applications because of the advantages of its high modularity, availability, and high power quality. Reliability is one of the most important challenges for the MMC consisting of a large number of power electronic devices. The diode open-circuit fault in the submodule (SM) is an important issue for the MMC, which would affect the performance of the MMC and disrupt the operation of the MMC. This paper analyzes the impact of diode open-circuit failures in the SMs on the performance of the MMC and proposes a protection scheme for the MMC under diode open-circuit faults. The proposed protection scheme not only can effectively eliminate the possible caused high voltage due to the diode open-circuit fault but also can quickly detect the faulty SMs, which effectively avoids the destruction and protects the MMC. The proposed protection scheme is verified with a downscale MMC prototype in the laboratory. The results confirm the effectiveness of the proposed protection scheme for the MMC under diode open-circuit faults.

/pdf/protection-scheme-for-modular-multilevel-converters-under-2pw4j8clz7.pdf

Huai Wang

Papers

Capacitance estimation algorithm based on DC-link voltage harmonics using artificial neural network in three-phase motor drive systems

A review of the condition monitoring of capacitors in power electronic converters

Instantaneous thermal modeling of the DC-link capacitor in PhotoVoltaic systems

Low voltage ride-through of single-phase transformerless photovoltaic inverters

Protection Scheme for Modular Multilevel Converters Under Diode Open-Circuit Faults