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Alternative numerical method for identification of flutter on free vibration
Chun, Nakhyun,Moon, Jiho,Lee, Hak-Eun Techno-Press 2017 Wind and Structures, An International Journal (WAS Vol.24 No.4
The minimization method is widely used to predict the dynamic characteristics of a system. Generally, data recorded by experiment (for example displacement) tends to contain noise, and the error in the properties of the system is proportional to the noise level (NL). In addition, the accuracy of the results depends on various factors such as the signal character, filtering method or cut off frequency. In particular, coupled terms in multimode systems show larger differences compared to the true value when measured in an environment with a high NL. The iterative least square (ILS) method was proposed to reduce these errors that occur under a high NL, and has been verified in previous research. However, the ILS method might be sensitive to the signal processing, including the determination of cutoff frequency. This paper focused on improving the accuracy of the ILS method, and proposed the modified ILS (MILS) method, which differs from the ILS method by the addition of a new calculation process based on correlation coefficients for each degree of freedom. Comparing the results of these systems with those of a numerical simulation revealed that both ILS and the proposed MILS method provided good prediction of the dynamic properties of the system under investigation (in this case, the damping ratio and damped frequency). Moreover, the proposed MILS method provided even better prediction results for the coupling terms of stiffness and damping coefficient matrix.
Seismic response evaluation of 154 kV transformer porcelain bushing by shaking table tests
Nakhyun Chun,Bubgyu Jeon,Sungwan Kim,Sungjin Chang,Suwon Son 국제구조공학회 2022 Structural Engineering and Mechanics, An Int'l Jou Vol.84 No.2
The use of electricity and communication between electronic devices is increasing daily, which makes the stability of electrical power supply vital. Since the 1990s, large earthquakes have occurred frequently causing considerable direct damage to electrical power facilities as well as secondary damage, such as difficulty in restoring functions due to the interruption of electric power supply. Therefore, it is very important to establish measures to protect electrical power facilities, such as transformers and switchboards, from earthquakes. In this study, a 154 kV transformer whose service life had expired was installed on the base fabricated by simulating the field conditions and conducting the shaking table tests. The dynamic characteristics and seismic behavior of the 154 kV transformer were analyzed through the resonance frequency search test and seismic simulation test that considers the front, rear, left, and right directions. Since the purpose of this study is to analyze the acceleration amplification in the bushing due to the acceleration amplification, the experimental results were analyzed focusing on the acceleration response and the converted acceleration amplification ratio rather than the failure due to the displacement response of the transformer. The seismic force amplification at the transformer bushing was evaluated by simulating the characteristics of electrical power facilities in South Korea, and compared with the IEC TS 61463 acceleration amplification factor. Finally, the amplification factor at zero period acceleration (ZPA) modified for each return period was summarized. The results of this study can be used as data to define the amplification factor at ZPA of the transformer bushing, simulating the characteristics of electrical power facilities in Korea.
전낙현(Nakhyun Chun),김정훈(Jeonghun Kim),황경민(Kyeong-Min Hwang) 대한전기학회 2021 대한전기학회 학술대회 논문집 Vol.2021 No.10
경주 · 포항 지진 피해 이후로 행정안전부에서 내진설계기준 공통적용사항 발표를 통해 설계지진하중 상향을 요구하였다. 한전에서는 1,000년, 2,400년, 4,800년 재현주기에 맞춰 송배전설비 내진설계기준 (DS-0050)을 개정하였다. 기능수행수준, 즉시복구수준, 붕괴방지수준의 3가지 내진성능수준을 정의하였고 전력 설비의 특성에 맞춰 구조요소와 기능요소로 분류하여 각각의 요구수준을 정의하였다. 4,800년에 해당하는 송변전 설비 중 기반암(S1 지반) 노두에 해당하는 최대지반가 속도(PGA)값은 0.286g로 분류하였다. 또한, 송배전설비 자체의 증폭이나 건축 구조물 안에 설치되어 있는 설비의 경우 이를 동시에 고려한 증폭계수를 도출할 수 있도록 하였다. 이를 위해 한전 보유 설비들을 실증실험을 통해 기기 증폭비 그리고 변전소 구조물 7가지 타입을 수치해석을 통해 구조물내부증폭비를 결정하였다.
전낙현 ( Chun Nakhyun ),장정범 ( Jang Jung-bum ),황경민 ( Hwang Kyeong-min ) 한국구조물진단유지관리공학회 2021 한국구조물진단유지관리공학회 학술발표대회 논문집 Vol.25 No.2
경주, 포항 지진 이후로 기반시설의 내진성능수준 상향요구로 인해 송·변·배전 설비로 구성되어 있는 전력설비의 내진설계 기준(2021)은 S1기준 PGA가 최대 0.286g로 상향 되었다. 기존 전력설비는 등가정적해석법을 기반으로 설계 및 검증 되어 온 반면 개정기준은 변압기 실험의 의무화로 개정되었고 일부 핵심설비 또한 실험이 권장되고 있다. 일부 설비가 지진과 설비의 동적 상호작용으로 인해 불안정한 동적응답이 확인됨에 따라 강화된 사항이다. 이로 인해 동적 특성에 주요한 영향을 미치는 감쇠비에 대한 평가가 중요하게 되고 이를 정밀하게 예측하기 위한 기술들을 소개하고자 한다.
Effect of beam slope on the static aerodynamic response of edge-girder bridge-deck
Lee, Hoyeop,Moon, Jiho,Chun, Nakhyun,Lee, Hak-eun Techno-Press 2017 Wind and Structures, An International Journal (WAS Vol.25 No.2
2-edge box girder bridges have been widely used in civil engineering practice. However, these bridges show weakness in aerodynamic stability. To overcome this weakness, additional attachments, such as fairing and flap, are usually used. These additional attachments can increase the cost and decrease the constructability. Some previous researchers suggested an aerodynamically stabilized 2-edge box girder section, giving a slope to the edge box instead of installing additional attachments. However, their studies are limited to only dynamic stability, even though static aerodynamic coefficients are as important as dynamic stability. In this study, focus was given to the evaluation of static aerodynamic response for a stabilized 2-edge box girder section. For this, the slopes of the edge box were varied from $0^{\circ}$ to $17^{\circ}$ and static coefficients were obtained through a series of wind tunnel tests. The results were then compared with those from computational fluid dynamics (CFD) analysis. From the results, it was found that the drag coefficients generally decreased with the increasing box slope angle, except for the specific box slope range. This range of box slope varied depending on the B/H ratio, and this should be avoided for the practical design of such a bridge, since it results in poor static aerodynamic response.