Fault Location in Transmission Systems Using Synchronized Measurements

Compared with conventional measurements from supervisory control and data acquisition (SCADA) system, phasor measurement units (PMUs) provide time-synchronized and direct measurements of phasors. The availability of synchronized phasor measurements can significantly improve power system protection and analysis. This dissertation is specifically committed to using synchronized measurements for estimation of fault locations in transmission systems. Transmission lines are prone to various short-circuit faults. Accurate fault location is critical for rapid power recovery. Chapter 2 proposes a new fault location method based on sparse wide area measurements. One distinguishing feature of this method is its applicability to both transposed and untransposed transmission lines. In addition, the method is developed based on sparse-wide area measurement that may be taken far away from the faulted line. Shunt capacitances of transmission lines are also fully considered by the algorithm. Moreover, when synchronized measurements from multiple buses are available, an optimal estimator can be used to make the most use of measurements, and to detect and identify potential bad measurements. Most of the existing fault location literatures discuss common shunt faults, including single line-to-ground faults, line-to-line faults, line-to-line-to-ground faults, and three-phase faults. However, in addition to common shunt faults, some complex faults may also occur in power systems. Among these complex faults, evolving fault and inter-circuit fault are two typical examples. Chapter 3 extends the method developed in Chapter 2 to deal with evolving faults. The proposed wide-area fault location methods are immune to fault type evolution, and are applicable to both transposed and untransposed lines. Chapter 4 discusses location of inter-circuit faults. Inter-circuit fault is a type of simultaneous fault, and it is the most common simultaneous fault type. Inter-circuit faults between each circuit in a double-circuit line is the most common inter-circuit fault. A fault location method for inter-circuit faults on double-circuit lines are developed and evaluated in Chapter 4. Chapter 5 puts forward a fault location algorithm, which does not require line parameters information, for series-compensated transmission lines. Two-end synchronized voltage and current measurements are utilized. The proposed method is independent of source impedance and fully considers shunt capacitances of transmission lines

Management and Protection of High-Voltage Direct Current Systems Based on Modular Multilevel Converters

The electrical grid is undergoing large changes due to the massive integration of renewable energy systems and the electrification of transport and heating sectors. These new resources are typically non-dispatchable and dependent on external factors (e.g., weather, user patterns). These two aspects make the generation and demand less predictable, facilitating a larger power variability. As a consequence, rejecting disturbances and respecting power quality constraints gets more challenging, as small power imbalances can create large frequency deviations with faster transients. In order to deal with these challenges, the energy system needs an upgraded infrastructure and improved control system. In this regard, high-voltage direct current (HVdc) systems can increase the controllability of the power system, facilitating the integration of large renewable energy systems. This thesis contributes to the advancement of the state of the art in HVdc systems, addressing the modeling, control and protection of HVdc systems, adopting modular multilevel converter (MMC) technology, with focus in providing services to ac systems. HVdc system control and protection studies need for an accurate HVdc terminal modeling in largely different time frames. Thus, as a first step, this thesis presents a guideline for the necessary level of deepness of the power electronics modeling with respect to the power system problem under study. Starting from a proper modeling for power system studies, this thesis proposes an HVdc frequency regulation approach, which adapts the power consumption of voltage-dependent loads by means of controlled reactive power injections, that control the voltage in the grid. This solution enables a fast and accurate load power control, able to minimize the frequency swing in asynchronous or embedded HVdc applications. One key challenge of HVdc systems is a proper protection system and particularly dc circuit breaker (CB) design, which necessitates fault current analysis for a large number of grid scenarios and parameters. This thesis applies the knowledge developed in the modeling and control of HVdc systems, to develop a fast and accurate fault current estimation method for MMC-based HVdc system. This method, including the HVdc control, achieved to accurately estimate the fault current peak value and slope with very small computational effort compared to the conventional approach using EMT-simulations. This work is concluded introducing a new protection methodology, that involves the fault blocking capability of MMCs with mixed submodule (SM) structure, without the need for an additional CB. The main focus is the adaption of the MMC topology with reduced number of bipolar SM to achieve similar fault clearing performance as with dc CB and tolerable SM over-voltage

Modern Applications in Optics and Photonics: From Sensing and Analytics to Communication

Optics and photonics are among the key technologies of the 21st century, and offer potential for novel applications in areas such as sensing and spectroscopy, analytics, monitoring, biomedical imaging/diagnostics, and optical communication technology. The high degree of control over light fields, together with the capabilities of modern processing and integration technology, enables new optical measurement systems with enhanced functionality and sensitivity. They are attractive for a range of applications that were previously inaccessible. This Special Issue aims to provide an overview of some of the most advanced application areas in optics and photonics and indicate the broad potential for the future

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

Effects of Distorted Voltages on the Performance of Renewable Energy Plant Transformers

The significant global growth in renewable energy production has led to increasing concerns about the problems associated with electrical equipment in power plants connected with this type of energy. The crucial electrical components of renewable energy generation are step-up transformers, with respect to which, gassing problems and premature insulation failures have been extensively reported in recent years. One of the factors related to the reported problems is the presence of high-frequency high-dV/dt voltages that are created by switching operations in wind energy plants. Addressing this challenge necessitates the investigation of transformer insulation systems under high-dV/dt pulse voltages. This thesis presents research undertaken in order to examine the performance of wind turbine step-up transformers under distorted voltages, with consideration of internal resonance phenomena and high-frequency dielectric effects. To evaluate the effect of distorted voltage on the acceleration of the ageing process in transformers, model transformers have been aged under distorted power converter voltage as well as under pure sinusoidal power grid voltage. Relevant parameters are monitored as indicators of the condition of the transformer insulation, and the results are compared throughout the ageing period. In addition, to study the transformer behavior under high-frequency high-dV/dt voltages, a detailed high-frequency model is necessary. High frequency modeling of large power transformers is complex and time-consuming due to their sizes. Therefore, as an alternative approach, this work proposes a modeling method that considers the high-frequency behavior of a scale down model transformer, and then relates it to the behavior of the actual size transformer. To verify the proposed modelling method, an experimental study investigates the correlations between the frequency responses of the two model transformers of different power ratings and sizes. Comparison of the frequency responses of the scaled and original transformers validates the proposed approach of scaling transformers for high-frequency study. High-frequency measurements are performed on an actual wind turbine transformer to represent a linear wideband black-box model of wind turbine step-up transformer in an electromagnetic transient study. Using the simulation results for switching impulsive waveforms imposed on wind turbine transformers due to the adjacent breakers operations, this work evaluates the effects of impulsive voltage parameters such as rate of rise and repetition frequency on inception voltage and intensity of partial discharge, generally assumed to be the main long-term cause of insulation deterioration. Partial discharge parameter measurement under high-dV/dt voltages is challenging due to interferences from fast oscillations, and difficulties of PD energy measurements. To avoid such issues, which are related to electromagnetic detection methods under pulse energization, this work uses a chemical approach to compare PD, based on the rate of hydrogen generation in a controlled test chamber with oil/paper samples. Gas monitoring of the oil containing impregnated paper samples show good linear correlation between the amount of hydrogen detected and PD energy level. Transformers installed in renewable energy plants require specific design considerations in order to protect them from the adverse effects of abovementioned voltage distortions. A number of manufacturers practice the implementation of electrostatic shields in transformer windings to filter the transferred voltages. Although this method has shown some improvements, effectiveness of the electrostatic shielding for a broad frequency range requires further studies. The effectiveness of electrostatic shielding in alleviating the transfer of high-frequency distortions from LV winding to HV winding and vice versa is evaluated with an experimental study. To compare the internal field enhancement at different frequencies in the presence and absence of an electrostatic shield, the frequency response of the voltage distribution inside the transformer’s winding is also measured and analyzed. Internal short circuit is one of the far-reaching incidents that has been recently reported for many wind-farm transformers. Detecting the location of an internal short circuit in a transformer winding is beneficial in improving future designs by defining the critical spots for target oriented insulation reinforcement. In an effort to identify trends with inter-turn fault locations and frequency responses, this research investigates the effect of the location of deliberately initiated internal faults on parameters such as transfer voltages and input impedance by means of frequency response analysis. Finally, alternative approaches are suggested for wind-farm design based on a method for recognizing the transformer compatibility with its surrounding devices such as power converters, breakers and cables

Electrical transmission systems for large offshore wind farms

Simulations of switching transients were carried out in EMTP-RV. Overvoltages in offshore wind farms ranged from temporary over voltages to very fast front transients. Transient Recovery Voltages of the offshore circuit breakers exceeded IEC 62271 requirements in some situations. The disconnection of an array produced the most severe overvoltages, exceeding IEC 60071 requirements.EThOS - Electronic Theses Online ServiceGBUnited Kingdo