Discrimination of PD Signal using Wavelet Transform for Insulation Diagnosis of GIS under HVDC

중전기 산업에서 부분방전의 검출 및 분석 기술은 전력설비의 상태진단 및 자산관리를 위한 가장 효과적인 방법으로 간주되어 왔다. 그러나 검출의 감도 및 정확도는 현장 노이즈에 영향을 받아 위험도 평가, 결함 판별 또는 위치 추정의 오류를 유발한다. 교류전압에서 부분방전 신호의 노이즈 제거는 활발히 연구되었지만, 최근 이슈가 되고 있는 HVDC에서 관련 연구는 미흡한 실정이다. HVDC 기술이 급속히 발전되면서 관련 전력설비 진단을 위하여, HVDC에서 부분방전 신호의 노이즈를 제거할 필요가 있다. 이들 배경으로 본 논문에서는 HVDC 가스절연구조에서 절연진단의 감도 및 정확도를 향상할 목적으로 웨이블릿 변환을 이용하여 부분방전 신호를 식별하였다. 직류에서 부분방전 신호를 발생하기 위하여 실험계를 구축하였다. HVDC는 몰드변압기, 고압 다이오드 및 커패시터로 구성된 정류회로로 발생시켰다. 가스절연구조에서 발생하는 절연결함을 모의하기 위하여 도체돌출, 외함돌출, 자유입자 및 절연물 크랙 4종의 전극계를 제작하였다. 전극계는 SF6 가스를 0.5MPa로 충진하였으며, 차폐함을 사용하여 외부 노이즈의 영향을 최소화하였다. 4종의 모의결함에서 부분방전 단일펄스를 검출하여 HVDC에서 부분방전을 분석하기 위한 웨이블릿 변환 기술을 최적화하였다. 상관계수 및 동적시간워핑 법을 이용하여 부분방전 펄스와 다양한 모웨이블릿의 유사성을 비교하였다. 결과로부터 동적시간워핑 법에 의해 선정된 모웨이블릿 bior2.6이 HVDC에서 부분방전 신호 분석에 가장 적합하였다. 최적의 문턱함수 및 문턱값을 선정하기 위하여 감쇠 지수 펄스 및 감쇠 진동 펄스를 모의하였으며, 신호-잡음비, 상관계수, 크기 변화를 비교한 결과, 중간 문턱함수-자동 문턱값이 최적의 조합으로 선정되었다. 실제 부분방전 분석 및 평가 시 단일 펄스가 아닌 펄스 시퀀스가 사용되기 때문에, 최적화된 웨이블릿 변환 기술을 이용하여 모의결함으로부터 검출된 부분방전 신호를 식별하였으며, 그 효과를 고역 통과 필터와 비교하였다. 결과로부터, 부분방전 신호 식별 시 고역통과필터에 비해 웨이블리 기술이 잡음 감소와 상관계수가 높게, 크기 변화가 낮게 나타났다. 뿐만 아니라 웨이블릿 방법은 배경 잡음, 진폭 변조 전파 장해, 비정현 잡음 및 스위칭 임펄스로 간섭된 부분방전 신호를 식별하는 데 효과적이었다. 본 논문에서 제안한 웨이블릿 변환 기술은 현장의 노이즈로부터 부분방전 신호를 성공적으로 식별하였다. 향후 HVDC에서 가스절연구조의 부분방전 검출 및 분석에 적용될 것으로 예상되며, 부분방전 검출, 위험도 평가, 결함 판별 및 위치 측정의 정확도가 향상될 수 있을 것으로 기대된다.Contents ⅰ Lists of Figures and Tables ⅲ Abstract ⅵ Chapter 1 Introduction 1 1.1 Research Background 1 1.2 Dissertation Outline 5 Chapter 2 Partial Discharge Review 7 2.1 Mechanism and Recurrence 7 2.2 Detection and Measurement 12 2.3 Analysis Methods 23 Chapter 3 Experiment and Optimization 45 3.1 Experimental Setup 45 3.2 Optimization of Wavelet Transform 49 Chapter 4 Discrimination of PD Sequences 66 4.1 DEP-type Pulse Sequence 70 4.2 DOP-type Pulse Sequence 79 Chapter 5 Conclusions 89Docto

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

On the feasibility of unlicensed communications in the TV white space: Field measurements in the UHF band

In practical unlicensed communications in TV band, radio devices have to identify, at first, the transmission opportunities, that is, the portion of the spectrum licensed for broadcasting services unoccupied in a certain region at certain time, that is, the so-called TV white space. In this paper the outcome of field measurements in the UHF TV band (470-860 MHz) conducted in EU is presented. To obtain empirical values for the parameters upon which unlicensed radio devices are able to distinguish in a real scenario between empty and occupied TV channels, signal power measurements have been performed in Italy, Spain, and Romania on rural, suburban, and urban sites, at different heights over the ground by using different analysis bandwidths. The aim of this work is to provide a set of practical parameters upon which harmless unlicensed communication in the UHF TV white space is feasible. The results have been analyzed with respect to the hidden node margin problem, spectrum sensing bandwidth, and occupancy threshold

Design of 6U Nanosatellites in Formation Flying for the Laser Crosslink Mission

With a recent growth in the volume of spaceborne data, free space optical (FSO) or laser communication systems are attracting attention, as they can enable super-high data rates faster than 1 Gbps. The Very high-speed Inter-satellite link Systems using Infrared Optical terminal and Nanosatellite (VISION) is a technical demonstration mission to establish and validate laser crosslink systems using two 6U nanosatellites in formation flying. The final goal is to achieve a Gbps-level data rate at a distance of thousands of kilometers. To establish space-to-space laser communication, the payload optical axes of each satellite should be precisely aligned during the crosslink. The payload is the laser communication terminal (LCT) including the deployable space telescope (DST), which improves optical link performances. The 6U nanosatellite bus is designed with commercial off-the shelf-(COTS) components for agile systems development. For precise formation flying, the bus is equipped a with relative navigation system with a GNSS receiver and RF crosslink, star tracker, 3-axis reaction wheels (RWs), and propulsion system. This proposed concept of the laser crosslink systems will contribute to the construction of the LEO communication constellation with high speed and secure links in future

Analysis on Statistical Characteristics of Partial Discharges in SF6 Gas under HVDC

With the rapid development of HVDC technology and the issue of smart grid, it is a new challenge to monitor and diagnose performance of the related power facilities. This thesis dealt with the statistical characteristics of partial discharge(PD) pulse in SF6 gas under HVDC in terms of discharge inception voltage(DIV) and discharge extinction voltage(DEV), discharge magnitude, and pulse count as well as the statistical characteristics extracted from the discharge distribution and density function. To simulate the typical insulation defects in gas insulated switchgear(GIS), electrode systems such as a protrusion on conductor(POC), a protrusion on enclosure(POE), a free particle(FP), a void inside spacer(Void), and a crack inside spacer(Crack) were fabricated. All of them were filled with SF6 gas in ranges from 0.1MPa to 0.5MPa. A HVDC source was generated by a rectifying circuit which is composed of a 100kV diode, and a 0.5μF capacitor. PD signal produced from the electrode systems was detected through a 50Ω non-inductive resistor and was analyzed by a digital storage oscilloscope(DSO) with a sampling rate of 5GS/s and a DAQ system based on LabVIEW program. The DIV and DEV in POC, POE, and Crack increased with the gas pressure. The gas pressure did not strongly affect the DIV and DEV in FP. The DIV and DEV in Void were almost similar to the increase of the SF6 gas pressure. The maximum discharge magnitude Qmax and pulse count in 5 seconds of each electrode system increased as the applied voltage was raised. However, the mean discharge magnitude Qmean did not change significantly. For each electrode system, the discharge distribution and density function presented distinguishable patterns. Therefore, it is possible to identify the type of defects in gas insulated equipment operated under HVDC by analysing statistical characteristics extracted from the discharge distribution and density function.Contents Contents ⅰ Lists of Figures and Table ⅲ Abstract ⅴ Chapter 1 Introduction 1 Chapter 2 Theory 3 2.1 Partial discharge 3 2.2 Detection and analysis methods 9 2.2.1 Detection methods 10 2.2.2 Analysis methods 14 Chapter 3 Experiment and Analysis 18 3.1 Experiment 18 3.1.1 Insulation defects 18 3.1.2 Measurement system 21 3.1.3 Experimental setup 23 3.2 Results and Analysis 24 3.2.1 DIV and DEV 24 3.2.2 Discharge magnitude and pulse count 27 3.2.3 Statistical characteristics 30 Chapter 4 Conclusions 39 References 4

The Insulation of HVDC Extruded Cable System Joints. Part 2: Proposal of a New AC Voltage PD Measurement Protocol for Quality Control during Routine Tests

The review of materials, design and testing of joints for HVDC extruded cable systems provided in previous Part 1 paved the way to this Part 2 position paper by the DEIS HVDC Cable Systems Technical Committee, whose aim is to remedy the scarcity of existing standardized tests on joints. After a sound analysis, here routine tests are identified as the first practical target for the onset of new testing procedures, AC-PD measurements as the readily-available measurement from manufacturers’ experience for quality control of joints during routine tests and VHF/UHF wireless sensors as the best tool for such measurements in the noisy environment of factories. Thereby, a novel protocol for PD measurement using AC voltages and VHF/UHF electromagnetic sensors, for quality control during routine tests on HVDC extruded joints, is proposed

Hybrid Satellite-Terrestrial Communication Networks for the Maritime Internet of Things: Key Technologies, Opportunities, and Challenges

With the rapid development of marine activities, there has been an increasing number of maritime mobile terminals, as well as a growing demand for high-speed and ultra-reliable maritime communications to keep them connected. Traditionally, the maritime Internet of Things (IoT) is enabled by maritime satellites. However, satellites are seriously restricted by their high latency and relatively low data rate. As an alternative, shore & island-based base stations (BSs) can be built to extend the coverage of terrestrial networks using fourth-generation (4G), fifth-generation (5G), and beyond 5G services. Unmanned aerial vehicles can also be exploited to serve as aerial maritime BSs. Despite of all these approaches, there are still open issues for an efficient maritime communication network (MCN). For example, due to the complicated electromagnetic propagation environment, the limited geometrically available BS sites, and rigorous service demands from mission-critical applications, conventional communication and networking theories and methods should be tailored for maritime scenarios. Towards this end, we provide a survey on the demand for maritime communications, the state-of-the-art MCNs, and key technologies for enhancing transmission efficiency, extending network coverage, and provisioning maritime-specific services. Future challenges in developing an environment-aware, service-driven, and integrated satellite-air-ground MCN to be smart enough to utilize external auxiliary information, e.g., sea state and atmosphere conditions, are also discussed