A Study on the Phase-asynchronous PD Diagnosis Method for Gas Insulated Switchgears

Gas-insulated switchgear(GIS) is one of the most important power facilities and a valuable asset in a power system for providing stable and reliable electrical power. It has been in operation for more than 45 years due to its high reliability with low failure rate. Although GIS has a low-maintenance requirement, its failure caused by partial discharge(PD) leads to considerable financial loss. The ultra-high frequency(UHF) method is an effective tool to detect insulation defects inside GIS and widely used for on-line and on-site diagnosis. It is also less sensitive to noise as well as better for PD detection compared to other measurement methods. Most of utilities, laboratories, and countries perform the PD detection using narrow-band or wide-band frequency ranges and classify types of PDs by conventional methods with a phase angle of the voltage applied to power equipment. In many cases of on-site PD measurement in the field, however, it is difficult to classify types of PDs due to the phase-asynchronous PD signals. This thesis described a new method of PD diagnosis which can classify types of PDs without phase information of the voltage applied to GIS. The 327 cases of on-site measurement data were collected from 2003 to 2015. The statistical analysis of collected on-site measurement data was performed according to voltage classes, maintenance results, defect causes, and defect locations. From the statistical analysis, the most frequent PD and noise types were a floating element and an external interference, respectively. To develop the new method of PD diagnosis which is applicable to the on-site PD diagnosis without phase synchronization, the features were extracted to classify defect types using the representative data of 82 cases, including 66 PD and 16 noise cases. The features consisted of 5 frequency and 6 phase parameters. The 5 frequency parameters were the number of distribution ranges, maximum value, ranges of first and second peak value, peak differences between first and second peak value, and density levels. 6 phase parameters were the number of phase groups, overall distribution ranges or not, the distribution ranges of each group, density levels, peak differences between first and second group, and shapes. 82 cases of representative data were selected through the review of data validation and analyzed using the designed 11 feature parameters, from which 5 effective parameters were extracted to identify the defect types using the decision tree-based technique by 4 steps: the number of groups in phase parameters(first step), shapes in phase parameters(second step), the number of distribution ranges & density levels in frequency parameters(third step), and ranges of first and second peak value in frequency parameters(fourth step). As a result, the decision tree-based diagnosis algorithm was able to classify types of 6 PDs and 4 noises and 77 of 82 cases were exactly classified. The diagnosis performance of new method proposed in this thesis therefore had an accuracy rate over 94% and was able to diagnose almost every type of defect. The new method also was applied to on-site GIS diagnosis in South Korea and Malaysia to verify its reliability. In two cases, portable and on-line UHF PD systems were installed without phase synchronization, and the defect cause and location inside GISs were inspected visually by on-site engineers after on-site PD measurement. The two cases were analyzed by the new method based on decision-tree based diagnosis algorithm and results of the new method were identical to results of internal inspection. From the results, the new method of PD diagnosis proposed in this thesis is quite useful to classify various defect types using the phase-asynchronous PD signals in the on-site measurement.Contents ⅰ Lists of Figures and Tables ⅲ Abstract ⅶ Chapter 1 Introduction 1 Chapter 2 Partial Discharges 8 2.1 PD Classification 8 2.2 Typical PD sources in GIS 17 2.3 Technical methods and strategies for PD diagnosis 23 2.4 PD analysis methods 33 Chapter 3 Data Acquisition and Analysis 39 3.1 Statistical analysis 41 3.2 Feature extraction 50 Chapter 4 New Method of PD Diagnosis 84 4.1 New PD diagnostic algorithm 84 4.2 Case studies in Korea and Malaysia 86 Chapter 5 Conclusions 95 References 98Docto

성긴 지수 이산대수 문제

학위논문 (박사)-- 서울대학교 대학원 : 수리과학부, 2012. 8. 천정희.이산대수 문제는 현대 공개키 암호에 있어 가장 중요한 수학적 기반 문제의 하나이다. 수많은 암호 시스템과 프로토콜들이 이산대수가 어렵다는 가정하게 설계 및 제안되고 있으며 이러한 연구는 활발하게 진행되고 있다. 이산대수 기반 암호 시스템의 효율성은 지수승 연산 속도에 직결된다. Hoffstein과 Silverman은 이산대수 문제가 정의된 군에서 빠른 지수승과 안전성을 보장하기 위해 해밍 웨이트가 작은 지수들의 곱(성긴 지수 곱)을 사용할 것을 제안하였다. 특히 GF(2^n)에서의 제곱연산 그리고 Koblitz 타운 곡선에서의 두 배 연산은 각각의 군 연산보다 훨씬 빠르기 때문에 성긴 지수 곱을 사용하면 연산을 매우 가속화시킬 수 있다. 본 학위 논문에서는 성긴 지수 곱 이산대수 문제의 안전성을 분석한다. 현재의 성긴 지수 곱 이산대수 문제의 안전성 분석은 성긴 지수 이산대수 문제의 분석 기법에 의존하고 있는데 이로부터는 본래 문제의 정확한 안전성을 측정할 수 없다. 본 논문에서는 성긴 지수 곱 이산대수 문제의 안전성을 분석하기 위해 매개화된 분할 시스템을 이용하여성긴 지수 곱 이산대수 문제를 공격하는 효율적인 알고리즘을 제안한다. 제안 알고리즘은 현재까지 알려진 알고리즘 중 가장 빠른 시간 안에 성긴 지수 곱 이산대수 문제의 해를 찾는다. 실증적인 예로써 Coron, Lefranc 그리고 Poupard가 CHES 2005에서 제안한 GPS 인증 스킴의 비밀키와 Hoffstein과 Silverman이 제안한 (2,2,11)-지수에 대해 제안 알고리즘을 적용하여 각각에 대해 2^{61.82} 그리고 2^{53.02} 번의 군 연산을 사용하여 비밀키를 복구할 수 있음을 보인다.The discrete logarithm problem is one of the most important underlying mathematical problems in contemporary public key cryptography. Under the assumption that the problem is infeasible, a great number of cryptosystems have been constructed and researches in this area are still underway actively. The efficiency of cryptosystems based on the discrete logarithm problem primarily relies on the speed at which exponentiation can be performed. On this line of research to address the issue Hoffstein and Silverman suggested the use of low Hamming weight product exponents to accelerate group exponentiation while maintaining the security level. Taking low Hamming weight product exponents, computation costs on GF(2^n) or Koblitz elliptic curves can be reduced significantly, where the cost of squaring and elliptic curve doubling is much lower than that of multiplication and elliptic curve addition, respectively. In the thesis we focus our concern on the security analysis of the discrete logarithm problem of low Hamming weight product exponents. The current estimate on the security of the problem mainly depends on the approaches for the case of low Hamming weight exponents, which does not fit into the product form well. We come up with parameterized splitting systems to resolve this problem. We show that it yields an efficient algorithm for the discrete logarithm problem of low Hamming weight exponents with lower complexity than that of any previously known algorithms. To demonstrate its application, we attack the GPS identification scheme modified by Coron, Lefranc, and Poupard in CHES 2005 and Hoffstein and Silverman's (2,2,11)-exponents. The time complexity of our key recovery attack against the GPS scheme is 2^{61.82}, which was expected to be 2^{78}. Hoffstein and Silverman's (2,2,11)-exponent can be recovered with a time complexity of 2^{53.02}, which is the lowest among the known attacks.1. Introduction 2. The Low Hamming Weight Discrete Logarithm Problem 3. The Low Hamming Weight Product DLP 4. Parameterized Splitting Systems 5. A New Algorithm from Parameterized Splitting Systems 6. Cryptanalysis 7. Conclusion and Open ProblemsDocto

A Study on the Acoustic Detection of Partial Discharge in Insulation Oil

This thesis describes the frequency spectrum, positioning and propagation characteristics of the acoustic signals produced by the partial discharge(PD) in applying an acoustic detection technique for on-line insulation diagnosis of oil filled transformers. Some defects were simulated by needle-to-plane, plane-to-plane and floating electrode systems. The acoustic signals were analyzed using a wideband AE sensor with frequency ranges of 100[kHz]∼1[MHz] and narrowband AE sensor with resonant frequency of 140[kHz]. A decoupler and an amplifier were designed to detect and amplify only the acoustic signals. The decoupler separates the acoustic signal from the DC source without any distortion, and the amplifier produces a gain of 40[dB] in frequency ranges of 11[kHz]∼4[MHz] at -3[dB]. From the experimental results, it was confirmed the frequency spectrums of the acoustic signals were 50[kHz]∼180[kHz] for the needle-to-plane, 50[kHz]∼200[kHz] for the plane-to-plane and 50[kHz]∼150[kHz] for the floating electrode system. Using the three AE sensors, the positioning error of the PD was calculated to be within 3[%]. Also, output voltage, as a function of applied discharge magnitude, was linear, while the measured sensitivity of this experimental configuration was 23.65[mV/pC]. Therefore, this paper concludes that the acoustic detection technique for on-line insulation diagnosis of oil filled transformers could be used reliably.목 차 ⅰ 그림 목차 ⅲ Abstract ⅴ 제 1 장 서 론 1 1.1 연구배경 1 1.2 연구목적 및 내용 2 제 2 장 이 론 4 2.1 절연열화 및 부분방전 4 2.2 절연진단기술 11 제 3 장 실험장치 19 3.1 전극계 19 3.2 검출회로 21 제 4 장 실험 및 분석 25 4.1 주파수 스펙트럼 25 4.2 부분방전의 위치표정 29 4.3 음향신호의 전파특성 33 제 5 장 결 론 39 참 고 문 헌 41 그림 목차 그림 2.1 내부방전 6 그림 2.2 연면방전 7 그림 2.3 코로나방전 7 그림 2.4 부분방전의 등가회로 8 그림 2.5 연면방전의 발생 8 그림 2.6 부분방전 발생패턴 9 그림 2.7 유중가스분석법 13 그림 2.8 전기적 검출법 14 그림 2.9 음향검출법 15 그림 2.10 음파의 입사파, 반사파 및 굴절파 18 그림 3.1 전극계의 구성 20 그림 3.2 역결합회로 21 그림 3.3 역결합회로의 주파수 응답 22 그림 3.4 증폭회로 22 그림 3.5 증폭회로의 주파수 응답 23 그림 3.6 음향검출시스템 24 그림 4.1 음향검출 실험계의 구성 25 그림 4.2 음향신호의 예 27 그림 4.3 주파수 스펙트럼 28 그림 4.4 AE센서의 위치 30 그림 4.5 부분방전의 위치표정 31 그림 4.6 음향신호의 검출 32 그림 4.7 교정실험계의 구성 33 그림 4.8 교정펄스파형의 예 34 그림 4.9 방전전하량과 출력전압의 관계 35 그림 4.10 실험계의 구성 35 그림 4.11 거리에 따른 유중 음향신호 측정 36 그림 4.12 검출파형의 예 37 그림 4.13 거리에 따른 음향신호의 감쇄 3

POSTECH Molecular graphics system development and Ab initio studies of water clusters

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새만금 간척사업의 경제적 타당성 재평가 : 내부매립으로 멸실될 산림생태계의 간접사용가치를 중심으로

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(A) study of Benjamin Briffen`s song cycle, [Seven sonnets of Michelangelo]

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Pseudomonas putida Biotype B detosteroid isomerase의 and characterization of ketosteroid isomerase from pseudomonas putida biotype B

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Bayesian single change point detection in a sequence of multivariate normal observations

A Bayesian method is used to see whether there are changes of mean, covariance, or both at an unknown time point in a sequence of independent multivariate normal observations. Noninformative priors are used for all competing models: no-change model, mean change model, covariance change model, and mean and covariance change model. We use several versions of the intrinsic Bayes factor of Berger and Pericchi (Berger, J.O. and Pericchi, L.R., 1996, The intrinsic Bayes factor for model selection and prediction. Journal of the American Statistical Association, 91, 109-122 Berger, J.O. and Pericchi, L.R., 1998, Accurate and stable Bayesian model selection: the median intrinsic Bayes factor. Sankkya Series B, 60, 1-18.) to detect a change point. We demonstrate our results with some simulated datasets and a real dataset.This work was supported by Grant No. R02-2000-00024 from the Basic Research Program of the Korea Science Engineering Foundation

연구개발비 지출액이 기업가치에 미치는 영향에 관한 연구

학위논문(석사)--서울대학교 대학원 :경영학과 경영학전공,2000.Maste