머신러닝 모델을 사용한 2002~2020년 한국의 O3, NO2, CO 농도의 고해상도 추정

학위논문(석사) -- 서울대학교대학원 : 보건대학원 보건학과, 2023. 2. 김호.Backrgound : Long-term exposure to ozone (O3), nitrogen dioxide (NO2), and carbon monoxide (CO) is known to cause various diseases and increase mortality. For that reason, estimating ground-level O3, NO2, and CO concentrations with a high spatial resolution is crucial for assessing the health effects associated with these air pollutants. However, related studies are limited in South Korea. This study aimed to develop machine learning-based models to predict the monthly O3 (average of daily 8-hour maximums), NO2, and CO at a spatial resolution of 1 km × 1 km across South Korea from 2002 to 2020. Methods : Approximately 80% of the monitoring stations were used to train the three machine learning models (random forest, light gradient boosting, and neural network) with a 10-fold cross-validation, and 20% of the monitoring stations were used to test the model performance. The author also applied ensemble models to integrate the variation in predictions among the models. Multiple predictors with satellite-based remote sensing data, inverse distance weighted ground-level air pollutants, land use variables, reanalysis datasets for meteorological variables, and regional socioeconmoic variables collected from various databases were included in the prediction model. Results : For O3, the overall R2 of the ensemble model was 0.841 during the entire study period. Urban areas showed a better model performance (R2 = 0.845) than rural areas (R2 = 0.762). For NO2, the highest overall R2 was 0.756, which best fit in autumn (R2 = 0.768). For CO, the overall R2 value was 0.506. This study provides high spatial resolution monthly average O3 and NO2 estimates with excellent performance (R2 > 0.75). Conclusion : The authors predictions can be used to analyze the spatial patterns in pollutants in relation to population characteristics and studies on the health effects of long-term exposure to air pollution using geocode-based health information and local health data.연구배경 : 오존(O3), 이산화질소(NO2), 일산화탄소(CO)에 장기간 노출되면 각종 질병을 유발하고 사망률을 높이는 것으로 알려져 있다. 그렇기에, 고해상도로 지표면 O3, NO2, CO 농도를 추정하는 것은 이러한 대기오염물질과 관련된 건강 영향을 평가하는 데 매우 중요하다. 하지만, 장기간에 걸쳐 고해상도로 가스상 대기오염물질(O3, NO2, CO)를 추정한 연구는 국내에서 아직 진행된 바가 없다. 따라서, 본 연구는 2002년부터 2020년까지 대한민국 전역에서 1km × 1km의 공간해상도로 월별 O3(일평균 8시간 최대치), NO2, CO를 머신러닝 기반 모델 및 그들의 앙상블 모형을 통해 예측하고자 한다. 연구방법 : 3가지 머신러닝 모델(랜덤 포레스트, 라이트 그래디언트 부스팅, 신경망)의 최적의 파라미터를 찾기 위해 모니터링 스테이션의 약 80%를 훈련 데이터로 사용하였고, 10-fold 교차검증을 통해 훈련 데이터 내에서 훈련/평가 단계를 거쳤으며, 나머지 모니터링 스테이션의 20%를 모델 평가에 사용하였다. 여기에 추가로 머신러닝 모델 간의 예측 변동을 통합하기 위해 앙상블 모델을 적용했다. 데이터에는 위성 기반 원격 감지 데이터, 역거리 가중치 기반 대기오염농도, 토지 이용 변수, 기상 재분석 자료, 다양한 데이터베이스에서 수집된 지역 사회경제적 변수 등이 포함되었다. 연구결과 : O3의 경우, 전체 연구 기간 동안 앙상블 모델의 R2가 0.841을 기록했으며, 도시 지역이 농촌 지역(R2 = 0.762)보다 우수한 예측 성능(R2 = 0.845)을 보였다. NO2의 경우, 앙상블(평균) 모델의 R2가 0.756으로 가장 높았으며, 계절로 보면 가을에 예측 성능이 가장 높았다(R2 = 0.768). CO의 경우, R2가 0.506 을 기록했다. 본 연구는 O3 및 NO2 에서 R2 > 0.75 으로 높은 예측력의 고해상도 월평균 추정치를 제공한다. 결론 : 본 연구에서 얻어진 대기오염 추정 결과는 인구 특성과 관련된 가스상 대기오염물질의 공간 패턴을 분석하거나, 위치 기반 건강 정보와 행정구역 단위 건강 데이터와 엮여서 장기간 대기오염 노출의 건강 영향을 평가하는 연구에 사용될 수 있을 것으로 기대된다.Chapter 1. Introduction 1 Chapter 2. Materials and Methods 6 2.1. Study area 6 2.2. Air pollution monitoring data 6 2.3. Satellite-based remote sensing data 7 2.3.1. Meteorological data 7 2.3.2. Land-use data 10 2.3.3. Surface reflectance 11 2.4. Regional socioeconomic predictors 12 2.5. Modeling procedures 13 2.5.1. Data Preprocessing 14 2.5.2. Machine learning-based model 15 2.5.3. Ensemble Model 16 2.5.4. Model Prediction 17 Chapter 3. Results 19 Chapter 4. Discussion 29 Chapter 5. Conclusion 34 Supplementary materials 47 국문 초록 82 Tables Table 1. Model performance for O3, NO2, and CO overall and in three- and four-year periods 21 Table S1. Detailed information about data sources 61 Table S2. Variables sorted by % missing values 65 Table S3. Results of parameter grid search using 10-fold cross-validation for O3, NO2 and CO 68 Table S4. Yearly ensemble (GAM) performance for O3, NO2, and CO 70 Table S5. Model performances for O3, NO2, and CO by season and urbanity 71 Table S6. Number of monitoring stations by year for O3, NO2 and CO in urban and rural areas 73 Figures Fig. 1. Flowchart of the modeling process. GEE: Google Earth Engine, SEDAC: Socioeconomic Data and Applications Center, RSD: Regional Socioeconomic Database from Korean Disease Control and Prevention Agency 18 Fig. 2. Density scatter plot for monthly averages of the monitored and predicted concentrations of O3, NO2, and CO 26 Fig. 3. Maps of monitored and predicted O3, NO2 and CO during 2002~2020 27 Fig. 4. Percentage decrease in R2 when excluding grouped variables from each machine learning model of O3, NO2, and CO. The closer the color is to red, the greater the effect of the variables on the model performance 28 Fig. S1. Urban/Rural and Metropolitan (Metro) area for entire contiguous regions of South Korea 74 Fig. S2. Distribution maps of predicted O3 (ppb) by year and season for contiguous South Korea 75 Fig. S3. Distribution maps of predicted NO2 (ppb) by year and season for contiguous South Korea 76 Fig. S4. Distribution maps of predicted CO (ppm) by year and season for contiguous South Korea 77 Fig. S5. Monthly fluctuations in the number of monitoring stations for O3, NO2, and CO between 2002 and 2020 78 Fig. S6. Density scatter plot for monthly averages of the monitored and predicted concentrations of O3, NO2, and CO with seasonal discrimination 79석

계통손실 감소를 위한 빌딩 내 PHEV 충방전 스케줄링

학위논문 (석사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2012. 8. 문승일.지구온난화에 의한 이상기후현상이 심해지면서 이산화탄소의 배출이 전혀 없는 전기자동차에 대한 관심이 높아지고 있다. 특히 밤시간 동안 다수의 Onboard Charger PHEV 충전으로 인해 부하가 증가하게 되면 배전단의 전압이 떨어질 수 있고 off peak가 발생할 수 있으며 전력 손실도 증가하게 되어 배전계통에 악영향을 미칠 수 있다. 또한 선로용량의 제약에 걸려 선로용량의 증가를 위해 비용이 들거나 일부의 자동차는 충전을 못하고 기다려야 하는 문제점이 발생한다. 그리고 건물에 다수의 PHEV가 각각 건물의 3상 중 하나의 상에 무작위로 연결되어 충전을 하면 빌딩 부하의 3상 불균형 정도가 심해질 수 있다. 이에 본 논문에서는 배전계통의 모든 모선의 전압을 규정 범위 내에서 유지하면서 사용자가 원하는 시간까지 충전을 완료하며 PHEV 충전으로 인한 선로 용량의 증가를 감소시키면서 빌딩의 3상 부하 불평형을 줄이고 계통 전쳬의 유효전력 손실을 감소 시키기 위한 PHEV의 유효전력과 무효전력 제어 방안을 제안한다. 이와 같은 스케줄링을 위해서 본 논문에서는 Linear Programming(LP)을 사용하였다. 아울러 본 논문에서 제안된 PHEV 유효전력, 무효전력 제어 방안을 이론적으로 분석하였고, 빌딩 부하가 3상 평형인 경우, 3상 불평형인 경우, 자동차의 대수가 변하는 경우 등의 사례 연구를 통해 제안된 방법을 검증하였다.국문 초록......................................................................................................................................ⅰ 목차.................................................................................................................................................ⅱ 그림목차.........................................................................................................................................ⅳ 표목차.............................................................................................................................................ⅴ 제 1 장 서론.................................................................................................1 1.1 연구 배경 및 연구 내용............................................................................................1 1.2 논문의 구성 및 개요..................................................................................................4 제 2 장 PHEV P, Q 스케줄링을 통한 계통손실최소화...................6 2.1 계통손실최소화를 위한 PHEV P, Q 협조 스케줄링의 타당성......................6 2.2 이론적 분석을 통한 PHEV의 무효전력 공급 효과 고찰.................................8 제 3 장 LP를 이용한 PHEV의 P, Q 스케줄링.................................13 3.1 문제의 정식화.............................................................................................................13 3.1.1 목적함수............................................................................................................14 3.1.2 제약조건............................................................................................................15 3.2 PHEV의 유효전력, 무효전력의 스케줄링을 위한 LP......................................16 제 4 장 사례 연구.....................................................................................20 4.1 모의 계통의 구성......................................................................................................20 4.2 사례 연구 결과..........................................................................................................23 4.2.1 빌딩 부하가 3상 평형인 경우....................................................................23 4.2.2 빌딩 부하가 3상 불평형인 경우................................................................25 4.2.3 시간 마다 PHEV 대수가 바뀌는 경우....................................................27 제 5 장 결론................................................................................................30Maste

Development of adiabatic demagnetization refrigerator (ADR) using passive heat switch

학위논문(석사) - 한국과학기술원 : 기계공학과, 2019.2,[iv, 67 p. :]자기열량효과 (Magnetocaloric effect, MCE)는 외부 자기장의 변화에 따라 물질의 온도나 엔트로피가 변화하는 현상으로, 자성 물질로 불리는 일부 물질에서 크게 발현된다. 단열 탈자 냉동기 (Adiabatic Demagnetization Refrigerator, ADR)는 이러한 자기열량효과를 이용한 자기 냉동의 한 방식이다. 단열 탈자 냉동기는 자기장 변화를 제공하는 초전도 자석, 외부 자기장 변화에 따라 온도가 변화하는 자성 물질, 그리고 이 자성 물질의 온도에 따라 적절하게 고온부 또는 저온부와 자성물질을 열적으로 연결했다가 탈착하는 열스위치가 필요하다. 기존의 열스위치는 사이클 과정에 맞추어 적절히 ON/OFF 되도록 능동적인 제어가 필요하고, 따라서 시스템의 복잡성이 증가하게 된다. 본 연구에서는 자기장에 따라 열전도율이 변화하는 초전도체와 응축부 온도에 따라 열전달 능력이 변하는 열사이펀을 이용하여 수동적으로 작동하는 열스위치가 적용된 단열 탈자 냉동기를 개발하고자 한다. 초전도체는 순수한 납 와이어, 열사이펀 작동 유체는 헬륨을 사용하였다. 1차원 수치해석을 통한 예측모델을 개발하여 냉동기의 작동 가능성을 확인하였다. 이를 바탕으로 실험 장치를 제작하였고 냉동 사이클 실험을 수행하였다. 냉동기의 고온부는 2단 G-M 냉동기에 의해 6 K으로 유지 된다. 실험에서 사용된 고온 초전도 자석은 자성 물질에 0~3 T의 자기장 변화를 제공한다. 실험 결과 사이클의 일부 과정에서 열사이펀의 온도가 고온부 6 K 보다 낮은 4.4 K으로 유지가 되었고 이를 통해 열사이펀 내부에 있는 헬륨의 응축을 확인하였다. 본 연구는 단열 탈자 냉동기에서 고온부와 저온부의 열스위치가 모두 수동적으로 작동함으로써 시스템을 단순화시킨데 의의가 있다. 또한 고온부 온도를 4 K으로 더 낮출 수 있다면 저온부가 2 K으로 유지되어 초유체 (superfluid helium)의 액화가 가능할 것이다.한국과학기술원 :기계공학과

A New Control Scheme and Modeling Method for Enhancing Normal Operation and Abnormal Estimation of LCC HVDC System

학위논문 (박사)-- 서울대학교 대학원 : 공과대학 전기·컴퓨터공학부, 2018. 2. 문승일.A new modeling method for high voltage direct current (HVDC) systems and associated controllers is presented for the Power System Simulator for Engineering (PSS/E) simulation environment. The aim is to improve the estimation of transient DC voltages and currents during temporary AC line-to-ground faults. The proposed method consists mainly of three interconnected modules for (a) equation conversion, (b) control-mode selection, and (c) DC-line modeling. Simulation case studies were carried out using PSS/E and a PSCAD/EMTDC model of the Jeju-Haenam HVDC system in Korea. Moreover, a new control method for a line-commutated converter-based (LCC) high-voltage direct-current (HVDC) system is presented and compared to a conventional strategy. In the proposed method, both the DC voltage and current of an LCC HVDC system are regulated to increase the short-term operating margin of DC power transfer and improve transient responses to DC power references. In particular, an increased operating margin of DC power transfer is achieved via the DC voltage regulation method. To verify the effectiveness of the proposed method, a state space model of an LCC HVDC system is developed considering DC voltage and current references as input variables and analyzed for various values of the DC line inductance and converter controller gains. The state space model can be used for time-efficient analyses of the dynamic characteristics of an LCC HVDC system. Simulation case studies are performed using MATLAB, where the state space model of the Jeju-Haenam HVDC system is implemented as a test case and compared to its comprehensive PSCAD model. The simulation results are compared with real operational data and the PSCAD/EMTDC simulation results of the HVDC system during single-phase and three-phase line-to-ground faults, respectively. These comparisons show that the proposed PSS/E modeling method resulted in improved estimation of the dynamic variations in the DC voltage and current for AC network faults, with significant gains in computational efficiency, making it suitable for real-time analysis of HVDC systems. Another the case study results suggest that the proposed method increases the short-term operating margin and speeds up the transient response of the HVDC system. Therefore, it will effectively improve the real-time grid frequency regulation.Chapter 1 Introduction 1 1.1 Motivations and purposes 1 1.2 Highlights and contributions 7 1.3 Dissertations organization 9 Chapter 2 Modeling Methods of LCC HVDC for PSS/E 10 2.1 PSS/E simulation environment 10 2.2 Proposed modeling method for PSS/E 15 2.2.1 Equation conversion 17 2.2.2 Control-mode selection 21 2.2.3 DC-line modeling 25 Chapter 3 Proposed Control Scheme of an LCC HVDC System 27 3.1 Proposed control of an LCC HVDC system 27 3.1.1 State space model for the proposed control method 28 3.1.2 Shortterm operating margin of DC power transfer 39 3.1.3 Root locus analysis of the state space model 38 Chapter 4 Case Study 43 4.1 Test system and simulation conditions of PSS/E model 43 4.1.1 Test system and simulation condition 43 4.1.2 Case 1-comparisons with the real HVDC system 45 4.1.3 Case 2-comparisons with the PSCAD model 48 4.1.4 Case 3-considering the DC-line models and Rcc 51 4.2 Test system and simulation conditions of proposed model 53 4.2.1 Test system and simulation conditions 53 4.2.2 Comparisons of step responses for control methods 55 4.2.3 Comparisons of step responses for DC line parameters 62 4.2.4 Responses to contimuous time-varying reference 64 4.2.5 Frequency regulation using proposed HVDC system 67 Chapter 5 Conclusions and Further Studies 78 References 80 Abstract 89Docto

Numerical Analysis on Quench in a High Temperature Superconducting (HTS) Magnet during Current Dump Process

Quench phenomenon was detected while the conduction-cooled HTS magnet was generating 4 T magnetic field. Current dump process was initiated to discharge the magnet energy to the external resistor when the one section of the magnet voltage taps exceeded the voltage criteria. The magnet, however, was permanently damaged during this event. The damaged spot is located where the highest perpendicular field, generated by the magnet, is exposed. As the result of the magnetic energy dissipation in that small spot, the temperature of the conductor increased extremely and reached the melting point of the conductor. The quench spot, however, did not coincide with the section that triggered the current dump process. There was no pre-quench signal from the damaged spot. Non-reversible voltage rise of the damaged spot, however, occurred during the dump process while the current was discharged from the magnet. To investigate the hostile thermal condition of the conduction-cooled magnet, we undertook the simple numerical analysis to calculate the conductor temperature variation during the dump process. The results show that the trends of the temperature change (higher than 300 K or lower than 100 K after the dump process) depending on the initial temperature of the conductor before the current starts to dump. We speculate that even the current-sharing spot was already formed before the dump process, this resistive voltage associated with it was not detected because of long length of the examined conductor section. Both the experimental and calculated results are presented and fully discussed

Development of Sorption Cooler for 1 K Adiabatic Demagnetization Refrigerator

The sorption cooler, which utilizes helium-4 as a working fluid, is developed for virtual precooling purpose of the 1 K ADR (Adiabatic Demagnetization Refrigerator) and tested in this study. The system consists of an adsorption pump, a thermosyphon and a tin strap. The cooler rejects heat to a 4.5 K heat sink, which is made by a commercial 2-stage Gifford-McMahon cryocooler. The adsorption pump contains activated charcoal of 5.2 g, and thermally connected to the heat sink through a tin strap. Its temperature in controlled from 5 to 30 K by a heater installed on the pump. The thermosyphon is filled with various helium filling ratios while the pump maintains its temperature at 30 K. In this operational stage, the assembled precooling system just works as a passive thermosyphon. When the heater on the pump turns off and the temperature of the pump decreases, helium in the thermosyphon is spontaneously transferred to the pump, and the sorption cooling starts. The cooling capacity of the cooler is measured by a heater installed on the evaporator of the thermosyphon. In a typical experimental condition, the temperature of the evaporator drops under 2.17 K, and the production of the superfluid is confirmed. This cooler is planned to be assembled with the 1 K ADR, to constitute the cascade refrigeration system which can obtain sub-kelvin temperature (<1 K)

Development and validation of a risk scoring model for postoperative adult moyamoya disease

OBJECTIVE The current grading system for moyamoya disease (MMD) is focused on angiographic studies with limited clinical application. The authors aimed to determine relevant factors that may impact postoperative outcome and establish a scoring system to predict the functional outcome. METHODS Adult patients with MMD who underwent treatment between 1998 and 2016 were included. Factors such as age, sex, comorbidity, smoking, MMD family history, initial presentation, multimodal imaging modalities, and types of surgical revascularization were thoroughly reviewed. These factors were analyzed to determine possible risk factors related to unfavorable 6-month postoperative outcomes using the modified Rankin Scale (mRS) (unfavorable: mRS score ≥ 3). A scoring system was developed using these independent risk factors to predict the outcome and validated using prospectively collected data from multiple centers between 2017 and 2018. RESULTS Of 302 patients for whom applications were submitted, 260 patients (321 hemispheres) met the diagnostic criteria. In multivariate analysis, hyperlipidemia, smoking, cerebral infarction on preoperative CT or MRI, and moderately to severely reduced regional cerebrovascular reserve results from Diamox SPECT were significantly related to unfavorable outcome. The authors developed a scoring system and stratified patients into risk groups according to their scores: low-risk (score 0?3), intermediate-risk (score 4?6), and high-risk (score 7?9) groups. This model demonstrated both good discrimination and calibration using C-statistics and the Hosmer-Lemeshow goodness-of-fit test showing 0.812 (95% CI 0.743?0.881) (p = 0.568) for the development and 0.954 (95% CI 0.896?1) (p = 0.097) for the temporal and external validation cohort. CONCLUSIONS The authors’ scoring system is readily adoptable to predict the postoperative outcome for MMD. Their data revealed the importance of smoking and hyperlipidemia, which were the only modifiable factors included in the scoring system. The authors validated their scoring system both internally and externally and maintained good performance, highlighting the system’s generalizability and reliability