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

Due to the parameter mismatch, the unbalanced power loss distribution among SMs in the modular multilevel converter (MMC) can be introduced and further deteriorated by the low-frequency asynchronous switching transients related to no-carrier modulation techniques. The unbalanced thermal stress can reduce the reliability of the MMC and increase the complexity of cooling system design. Nevertheless, an internal balance mechanism exists in the MMC thanks to the capacitor voltage balancing. It contributes to an even conduction loss dissipation among SMs, which is studied and revealed in this paper. Moreover, a computationally light conduction loss estimation method is proposed correspondingly relying on the characteristics of semiconductors and the arm current only. Simulations and experiments are conducted to verify the effectiveness the proposed method.

/pdf/balanced-conduction-loss-distribution-among-sms-in-modular-a17krk11eq.pdf

Balanced Conduction Loss Distribution among SMs in Modular Multilevel Converters

This article discloses part of an invention on plug-and-play converter-level on -state voltage measurement methods for power semiconductor devices. To exclude the external power supply required in on -state voltage measurement circuits, a self-power solution is proposed to provide the required bidirectional low-voltage power sources. The application of the measurement circuit with the proposed self-power solution is demonstrated for a single-switch, a single-phase inverter, and a three-phase inverter.

A Self-Power Method for a Converter-Level on -State Voltage Measurement Concept

This study explores the integration issues of next-generation high-penetration photovoltaic (PV) systems, where the grid is becoming more decentralised and vulnerable. In that case, the PV systems are expected to be more controllable with higher efficiency and reliability. Provision of ancillary and intelligent services, such as fault ride-through and reactive power compensation, is the key to attain higher utilisation of solar PV energy. Such functionalities for the future PV inverters can contribute to reduced cost of energy, and thus enable more cost-effective PV installations. To implement the advanced features, a flexible power controller is developed in this study, which can be configured in the PV inverter and flexibly change from one to another mode during operation. Based on the single-phase PQ theory, the control strategy offers the possibilities to generate appropriate references for the inner current control loop. The references depend on system conditions and also specific demands from both system operators and prosumers. Besides, this power control strategy can be implemented in commercial PV inverters as a standardised function, and also the operation modes can be achieved online in predesigned PV inverters. Case studies have verified the effectiveness and flexibilities of the proposal to realise the advanced features.

/pdf/power-control-flexibilities-for-grid-connected-multi-1mtkr9t1mj.pdf

Power control flexibilities for grid-connected multi-functional photovoltaic inverters

Many efforts have been made to improve the performance of power electronic systems with active capacitive DC-link module in terms of power density as well as reliability. One of the attractive solution is an active capacitive DC-link with the series-connected circuit because of handling small-rated power. However, in the existing control method of this circuit, the DC-link current of the backward-stage or forward-stage need to be sensed for extracting the ripple components, which limits the flexibility of the active DC-link module. Thus, in this paper, a voltage control method of an active capacitive DC-link module is proposed. Current sensor at the DC-link will be cancel from the circuit. The controller of the series-connected circuit requires internal voltage signals of the DC-link module only, making it possible to be fully independent without any additional connection to the main circuit. Impedance based analysis and modeling are presented in this paper. Simulation and experiment verify the accuracy of proposed independent control strategy in 500 W single-phase application.

A voltage control method for an active capacitive DC-link module with series-connected circuit

Power electronics is the enabling technology for maximizing the power captured from renewable electrical generation, e.g., the wind and solar technology, and also for an efficient integration into the grid. Therefore, it is important that the power electronics are reliable and do not have too many failures during operation which otherwise will increase cost for operation, maintenance and reputation. Typically, power electronics in renewable electrical generation has to be designed for 20–30 years of operation, and in order to do that, it is crucial to know about the mission profile of the power electronics technology as well as to know how the power electronics technology is loaded in terms of temperature and other stressors relevant, to reliability. Hence, this chapter will show the basics of power electronics technology for renewable energy systems, describe the mission profile of the technology and demonstrate how the power electronics is loaded under different stressors. Further, some systematic methods to design the power electronics technology for reliability will be given and demonstrated with two cases—one is a wind power and the other is photovoltaic application.

Huai Wang

Papers

Balanced Conduction Loss Distribution among SMs in Modular Multilevel Converters

A Self-Power Method for a Converter-Level on -State Voltage Measurement Concept

Power control flexibilities for grid-connected multi-functional photovoltaic inverters

A voltage control method for an active capacitive DC-link module with series-connected circuit

Design for Reliability of Power Electronics in Renewable Energy Systems