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

Conventional capacitors contribute to a considerable scale of system level size, cost and failure, and suffer from one or more issues on energy density, cost, and reliability. A two-terminal active capacitor, which has the same level of convenience as passive capacitors, is proposed recently to overcome the above issues. In this paper, the modeling of the active capacitor is investigated and a voltage feed-forward compensation scheme is proposed for overshoot reduction. Therefore, the impedance, equivalent capacitance, ESR, and ESL, of the active capacitor can be specified. A case study based on a single-phase full-bridge rectifier with an active capacitor demonstrates the theoretical analyses.

Impedance characteristics modeling of a two-terminal active capacitor

The ac resistance and parasitic capacitance of the inductor are the primary considerations in the winding design for the dual-active bridge converter (DAB). They are dependent of up to four independent structure variables. The interactive restrictions between those variables makes the design difficult. In this paper, the core-related capacitances between the central limb, side limb, yoke and winding are derived, and a local optimization for it is proposed. Moreover, a total winding capacitance design method is proposed by mapping the four dimensional problem into two dimensions. The analysis and design are verified by finite element method simulations and experimental results on a 100 kHz prototype are performed.

Winding design of series AC inductor for dual active bridge converters

This paper proposes a capacitor voltage oscillation reduction method by using third harmonic zero-sequence current for Modular Multilevel Cascade Converter (MMCC) with Single Delta Bridge Cells (SDBC). A practical case study on an 80 MVar/ 33 kV MMCC-SDBC based STATCOM is used to demonstrate the method. The impact of the third harmonic zero-sequence current level of the capacitor oscillation reduction and the electro-thermal stresses on IGBT modules is investigated. An optimal parameter of the current level is obtained by compromising the above two performance factors. The capacitor bank volume is reduced by 23 % by applying the proposed method.

A DC-link Capacitor Voltage Oscillation Reduction Method for a Modular Multilevel Cascade Converter with Single Delta Bridge Cells (MMCC-SDBC)

As a key component in modular multilevel converter (MMC), the reliability of submodule (SM) capacitor is a major concern. To get the health state of SM capacitor and take proactive action, this paper develops a condition monitoring method without any extra sensors or complicated algorithms. In the proposed method, one SM in each arm is bypassed symmetrically at a time, and the energy stored in each capacitor dissipates through the internal bleeding resistor. The discharging time proves to be an effective indicator of capacitor degradation. The information of all SMs are acquired with a proper rotating sequence. This condition monitoring process could be enabled regularly and demonstrate the degradation trend accurately. Besides, it does not affect the normal operation of MMC, and works well in different control or modulation strategies. Experimental results verify the feasibility of the proposed method.

Condition Monitoring Method for Submodule Capacitor in Modular Multilevel Converter

Static Synchronous Compensators (STATCOMs) have been applied for offshore wind power plants to satisfy the grid codes under both normal operation and grid fault conditions. This paper investigates the performances of two types of Modular Multilevel Cascade Converters (MMCC) for STATCOM applications, with the special focus on asymmetrical Low-Voltage-Ride-Through (LVRT) operation under grid faults. The design of an 80 MVar / 33 kV STATCOM is first presented. The electro-thermal stress of the power modules used in each type of MMCC is analyzed with voltage balancing control under grid faults. Finally, the reactive power compensation capability of the MMCC solutions is compared under different scenarios of grid faults, with considerations of the device temperature limits.

Huai Wang

Papers

Impedance characteristics modeling of a two-terminal active capacitor

Winding design of series AC inductor for dual active bridge converters

A DC-link Capacitor Voltage Oscillation Reduction Method for a Modular Multilevel Cascade Converter with Single Delta Bridge Cells (MMCC-SDBC)

Condition Monitoring Method for Submodule Capacitor in Modular Multilevel Converter

Reactive power compensation capability of a STATCOM based on two types of Modular Multilevel Cascade Converters for offshore wind application