• Journal of Power Electronics
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• 제14권6호
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• pp.1166-1177
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• 2014

A new type of topology with medium-frequency-transformer (MFT) isolation for medium voltage wind power generation systems is proposed in this paper. This type of converter is a high density power conversion system, with high performance features suitable for next generation wind power systems in either on-shore or off-shore applications. The proposed topology employs single-phase cascaded multi-level AC-AC converters on the grid side and three phase matrix converters on the generator side, which are interfaced by medium frequency transformers. This avoids DC-Link electrolytic capacitors and/or resonant L-C components in the power flow path thereby improving the power density and system reliability. Several configurations are given to fit different applications. The modulation and control strategy has been detailed. As two important part of the whole system, a novel single phase AC-AC converter topology with its reliable six-step switching technique and a novel symmetrical 11-segment modulation strategy for two stage matrix converter (TSMC) is proposed at the special situation of medium frequency chopping. The validity of the proposed concept has been verified by simulation results and experiment waveforms from a scaled down laboratory prototype.

• The Journal of Korean Physical Therapy
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• 제13권3호
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• pp.719-726
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• 2001

Transdermal drug delivery offers various advantages over conventional drug delivery systems, such as avoidance gastrointestinal degradation and hepatic first-pass effect. encourages patient compliance. and possible sustained release of drugs. However, transdermal transport of drugs is low permeability of the stratum corneum, the superficial layer of the skin. Many physicochemical and biological factors influencing transdermal transport is described together with the corresponding experimental and clinical results. Phonophoresis is medical treatment with drugs introduced into the skin by ultrasound energy. Enhanced drug penetration is through to result from the biophysical alterations of skin structure by ultrasound waves. The frequency used for phonophoresis is usually from 20 kHz to 15MHz. Phonophoresis can be categorized in to three ranges: low-frequency range(below 1 MHz). therapeutic frequency range(1 to 3MHz), and high-frequency range(above 3 MHz). The depth of penetration of ultrasound into skin is inversely proportional to the frequency. Cavitation may cause mechanical stress. temperature elevation, or enhanced chemical reactivity causing drug transport. One theory is that ultrasound affects the permeation of the stratum corneum lipid structure as the limiting step in permeating through the skin. The range of indications for phonophoresis is wide. Aspecific classification of the range of indications is obtained by classification of pathological conditions. The continuous research is needed for many interesting issucs of phonophoretic transdermal delivory in new future.

• JSTS:Journal of Semiconductor Technology and Science
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• 제7권4호
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• pp.267-273
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• 2007

A fractional-N frequency synthesizer supports quadruple bands and multiple standards for mobile broadcasting systems. A novel linearized coarse tuned VCO adopting a pseudo-exponential capacitor bank structure is proposed to cover the wide bandwidth of 65%. The proposed technique successfully reduces the variations of KVCO and per-code frequency step by 3.2 and 2.7 times, respectively. For the divider and prescaler circuits, TSPC (true single-phase clock) logic is extensively utilized for high speed operation, low power consumption, and small silicon area. Implemented in $0.18-{\mu}m$ CMOS, the PLL covers $154{\sim}303$ MHz (VHF-III), $462{\sim}911$ MHz (UHF), and $1441{\sim}1887$ MHz (L1, L2) with two VCO's while dissipating 23 mA from 1.8 V supply. The integrated phase noise is 0.598 and 0.812 degree for the integer-N and fractional-N modes, respectively, at 750 MHz output frequency. The in-band noise at 10 kHz offset is -96 dBc/Hz for the integer-N mode and degraded only by 3 dB for the fractional-N mode.

• International Journal of Naval Architecture and Ocean Engineering
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• 제3권1호
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• pp.20-26
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• 2011

A weakly nonlinear seakeeping methodology for predicting motions and loads is presented in this paper. This methodology assumes linear radiation and diffraction forces, calculated in the frequency domain, and fully nonlinear Froude-Krylov and hydrostatic forces, evaluated in the time domain. The particular approach employed here allows to overcome numerical problems connected to the determination of the impulse response functions. The procedure is divided into three consecutive steps: evaluation of dynamic sinkage and trim in calm water that can significantly influence the final results, a linear seakeeping analysis in the frequency domain and a weakly nonlinear simulation. The first two steps are performed employing a three-dimensional Rankine panel method. Nonlinear Froude-Krylov and hydrostatic forces are computed in the time domain by pressure integration on the actual wetted surface at each time step. Although nonlinear forces are evaluated into the time domain, the equations of motion are solved in the frequency domain iteratively passing from the frequency to the time domain until convergence. The containership S175 is employed as a test case for evaluating the capability of this methodology to correctly predict the nonlinear behavior related to wave induced motions and loads in head seas; numerical results are compared with experimental data provided in literature.

• Nuclear Engineering and Technology
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• 제42권3호
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• pp.319-328
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• 2010

Bonded tendons have been used in reactor buildings at some operating nuclear power plants in Korea. Assessing prestress force on these bonded tendons has become an important pending problem in efforts to assure continued operation beyond their design life. The System Identification (SI) technique was thus developed to improve upon the existing indirect assessment technique for bonded tendons. As a first step, this study analyzed the sensitivity of the key parameters to prestress force, and then determined the optimal parameters for the SI technique. A total of six scaled post-tensioned concrete beams with bonded tendons were manufactured. In order to investigate the correlation of the natural frequency and the displacement to prestress force, an impact test, a Single Input Multiple Output (SIMO) sine sweep test, and a bending test using an optical fiber sensor and compact displacement transducer were carried out. These tests found that both the natural frequency and the displacement show a good correlation with prestress force and that both parameters are available for the SI technique to predict prestress force. However, displacements by the optical fiber sensor and compact displacement transducer were shown to be more sensitive than the natural frequency to prestress force. Such displacements are more useful than the natural frequency as an input parameter for the SI technique.

• KEPCO Journal on Electric Power and Energy
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• 제8권1호
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• pp.31-36
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• 2022

This paper presents the measuring process of dynamic properties of offshore wind power foundation and provides consideration of each step. This Guideline enables to maintain consistent measuring procedure and therefore increase the reliability of test results. Small scaled suction bucket foundation was fabricated to represent the commercial support structure installation mechanism and two cases(free-free, free-fixed) of dynamic tests were performed at workshop. From the tests, the importance of dynamic properties of connection part between suction bucket and tower was figured out. More over, types and configuration of measuring devices are recommended which can help find the natural frequency of wind turbine foundation correctly. In field test, it was found that the natural frequency of suction bucket foundation was increased linearly with the penetration depth due to the confining effect of ambient soil. Meanwhile, it was not easy to get an enough excitation force with normal impact hammer because the N.F of suction bucket model was in the lower range of 0 Hz ~ 5 Hz. Therefore, new excitation method which has enough force and can excite lower frequency range was devised. This study will help develop safety check procedure of suction bucket foundation in field at each installation stage using the N.F measurement.

• 한국통신학회논문지
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• 제34권5C호
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• pp.548-554
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• 2009

직교 주파수 분할 다중화 (orthogonal frequency division multiplexing: OFDM) 시스템은 주파수 옵셋에 매우 민감하다. 따라서 주파수 옵셋 추정은 OFDM 시스템에서 매우 중요하다. 최근 Ren에 의해 제안된 주기도표 기반의 주파수 옵셋 추정 기법은 세 개의 단계로 구성되어 있으며, 정밀하게 주파수 옵셋을 추정할 수 있다. 하지만 두 번째 단계와 세 번째 단계에서의 주파수 옵셋 추정 가능 범위가 좁다는 단점이 있다. 이는 전체 신호의 대역폭파 비교하였을 때 추정 가능 범위가 좁으며, 첫 번째 단계에서의 시스템 복잡도를 증가시킨다. 따라서 본 논문에서는 OFDM 시스템에서 넓은 주파수 옵셋 추정 범위를 제공하며 낮은 복잡도를 지니는 새로운 기법을 제안한다. 모의실험을 통하여 제안된 기법의 주파수 옵셋 추정 범위가 Ren의 기법보다 넓으며, 시스템 복잡도는 낮음을 알 수 있다.

• 한국정보전자통신기술학회논문지
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• 제13권6호
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• pp.483-488
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• 2020

최근에는 수요자의 다양한 요구를 충족하는 서비스를 제공하기 위해서 여러 개의 무선통신 시스템을 하나로 통합한 무선기기에 대한 수요가 증가하고 있다. 다양한 무선통신 시스템으로 구성된 무선기기에는 많은 주파수 고정형 필터가 필요하다. 주파수 가변 필터는 다수의 주파수 고정형 필터를 대체할 수 있으며, 주파수 가변 필터를 사용하면 시스템을 보다 효율적으로 구성할 수 있다. 본 논문에서는 800 ~ 2400MHz 대역에서 동작하는 3-폴 주파수 가변 대역통과필터(BPF)를 설계하였다. 주파수 가변범위를 넓히기 위해서 튜닝 스크류는 스텝을 갖도록 설계하였고, 튜닝 스크류의 조절을 용이하게 하기 위해서 리니어 모터를 사용하였다. 구현한 주파수 가변 BPF는 설계한 주파수 범위에서 동작하며, 삽입손실은 최대 2.82dB, 중심주파수에서 ± 50MHz 이격된 지점에서 최소 18.7dB의 감쇠 성능을 갖는다.

• 한국항행학회논문지
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• 제16권6호
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• pp.935-943
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• 2012

본 논문에서는 위성항법기반 교통인프라 기술개발의 네트워크 수신국 사이트 선정을 위하여 사전 조사, 현장 조사, 현장 환경 분석의 3단계 선정 절차를 제시하였으며 실제 테스트베드인 오창 지역을 기준으로 4개의 후보지를 도출하였다. IGS, NGS 등과 같은 국제상시관측기관이 제안하는 사이트 선정 권고안을 정리하여 위성항법기반 교통인프라 기술개발에 적합한 요구사항을 도출하고 이를 만족하는 1차 후보군을 선정하는 작업이 사전조사이며, 1차 후보군에 대한 육안 가시검사 및 환경, 기타 조사를 통하여 2차 후보군을 선정하는 작업이 현장조사이다. 또한 선정된 2차 후보군에 대해 상공장애도 분석과 전파환경 분석을 통해 최종적으로 수신국 사이트 후보지를 선정하는 것이 현장 환경 분석이다. 위와 같은 일련의 선정 절차를 통해 1차 16개 후보군, 2차 7개 후보군, 3차 최종 4개의 수신국 사이트를 효과적으로 선정할 수 있었다.

• 한국전자파학회논문지
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• 제14권1호
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• pp.8-13
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• 2003

본 논문에서는 이동통신 품질 보호를 위한 주파수 1 GHz CISPR 허용기준 설정의 적정성을 조사하였으며, 주파수 1 GHz 이상의 장해파 측정에 있어 최대값 추출의 정확성을 높이기 위해 사전 시험에 이용될 수 있는 피시험체의 최적 회전 스텝 분해능에 대해 조사하였다. 마이크로 프로세스의 클럭속도 500 MHz 및 1.7 GHz인 퍼스널 컴퓨터를 피시험체로 측정한 결과, 클럭속도 500 MHz의 경우, 미국 표준과학연구원 Koepke 방식에 의해 추출된 최적 스텝 분해능은 40 개이었으며, 클럭속도 1.7 GHz 퍼스널 컴퓨터의 경우 스텝 분해능은 36개로 평가되었다. 완전 스캔(fully scan) 방식의 측정이 요구되는 기가헤르쯔 측정에서 회전 스텝 분해능에 대한 예측은 측정의 정확성뿐만 아니라 측정 시간을 상당히 단축시켜 줄 것이다.