Power system harmonics are always an important issue in power networks as they can cause many negative impacts, such as equipment thermal stress, on installations within power networks. Recently, with the increasing connections of power electronic devices based Renewable Energy Source (RES) and High Voltage Direct Current (HVDC) transmission applications, harmonics in power networks, especially high frequency harmonics (>50th order or 2.5 kHz) are on the rise. Currently, the majority of conventional VTs, such as Wound-type Voltage Transformers (WVT) and Capacitor Voltage Transformers (CVT), are widely installed and used in High Voltage (HV) and Extra High Voltage (EHV) power networks for voltage measurement. Since most of them were mainly designed to measure voltage with the required accuracy at the fundamental frequency (i.e. 50Hz in the UK), they are limited to measuring high frequency harmonics due to the coupling of their internal inductive and capacitive elements. To achieve high frequency harmonic measurements, voltage measurement devices with wide frequency bandwidths are required. Recently, non-conventional VTs, such as optical voltage transducers, are commercially available, which could provide accurate voltage measurements over a wide range of frequency. However, before they can be considered by any power utilities, their frequency response performances must be tested at a rated fundamental voltage with required minimum harmonic injections from 100Hz to 5 kHz. This must require a test system which should be capable of providing a rated fundamental voltage up to 400kV with controllable harmonic injections at required levels from 100Hz to 5 kHz. Therefore, the objective of this project is to design and implement such a test system in the National Grid (NG) HV laboratory at the University of Manchester. However, the design and the implementation of such a test system bring many challenges; for instance, a lack of adequate equipment and considerable power to provide the required harmonic injections above 0.5% to the test object.In this thesis, an Instrument Voltage Transformer Frequency Response (VTFR) test system with three different voltage power source designs is presented; The voltage power source designs are: (i) Design 1 is based on a single power source inductive coupling method to provide both a rated fundamental voltage and controllable harmonics; (ii) Design 2 is based on two separate voltage power sources inductive coupling method to provide both a rated fundamental voltage and controllable harmonics; and (iii) Design 3 is based on two separate voltage power sources capacitive coupling method to provide both the rated fundamental voltage and controllable harmonics. A hybrid approach, which combines the VTFR test system with both the voltage power sources Design 2 and 3, is proposed for testing the frequency response of any type of VTs at their rated fundamental voltages with 1% harmonic injections from 100Hz to 5 kHz. The proposed VTFR test system with voltage power source designs were firstly validated at a relatively low voltage of 33kV in the HV laboratory. Then three different VTFR test systems were constructed based on available equipment for testing VTs from 11kV to 400kV. An 11kV, a 33kV WVT and a 400kV WVT and a 275kV CVT were tested. The test results were analyzed, compared and discussed. The models of the test systems were also established and simulated. Simulation results were analysed, compared and discussed.

M. Tech. (Power Engineering Department, Faculty of Engineering and Technology) -- Vaal University of Technology / This dissertation is devoted to the study of partial discharge evaluation of a high voltage transformer. The 400 V/300 000 V (300 kV) high voltage transformer in the high voltage laboratory was manufactured in 1967. Given the old age of the transformer and the crucial importance of insulation systems, it is vital to assess the condition of its insulation to ensure the effectiveness and the reliability of the transformer as well as the safety of the personnel using it. In order to achieve that, it is important to evaluate the partial discharge in the insulation system as this is the main cause of destruction of insulation. The phase-resolved partial discharge method was the main method used to perform the partial discharge measurements in this research. Partial discharge measurements were performed on a faulty 11 kV voltage transformer. Defects were also created in samples of solid insulation at predetermined locations with different shapes and sizes. The measurements taken on the 11 kV voltage transformer and samples of solid insulation formed a basis to prove the validity of the assessment methods on the 300 kV transformer. Using the method mentioned above, partial discharges were recorded with respect to the phase of the applied voltage with the aid of a commercial instrument which complies with IEC 60270, (ICM monitor - partial discharge detector from Power Diagnostix Systems GmbH). The observations from this study have furthered the understanding of partial discharge processes. The patterns recorded were analysed in order to conclude about the condition of the transformer. From the partial discharge pattern, the type and location of partial discharges were concluded. The patterns obtained from the transformer reveal that the device is healthy and can be operated up to 200 kV.

Power systems are protected by numerical relays that detect and isolate faults that may occur on power systems. The correct operation of the relay is very important to maintain the security of the power system. <p>Numerical relays that use voltage measurements from the power system provided by coupling capacitor voltage transformers (CCVT) have sometimes difficulty in correctly identifying a fault in the protected area. The fundamental frequency voltage phasor resulting from these CCVT measurements may result in a deviation from the true value and therefore may locate this phasor temporarily in the incorrect operating region. This phasor deviation is due to the CCVT behavior and the CCVT introduces spurious decaying and oscillating transient signal components on top of the original voltage received from the power system in response to sudden voltage changes produced during faults. Most of the existing methods for estimating the voltage phasor do not take advantage of the knowledge of the CCVT behavior that can be obtained from its design parameters.<p>A new least squares error method for phasor estimation is presented in this thesis, which improves the accuracy and speed of convergence of the phasors obtained, using the knowledge of the CCVT behavior. The characteristics of the transient signal components introduced by the CCVT, such as frequencies and time constants of decay, are included in the description of the curve to be fitted, which is required in a least squares fitting technique. Parameters such as window size and sampling rate for optimum results are discussed.<p>The method proposed is evaluated using typical power systems, with results that can be compared to the response if an ideal potential transformer (PT) were used instead of a CCVT. The limitations of this method are found in some specific power system scenarios, where the natural frequencies of the power system are close to that of the CCVT, but with longer time constants. The accuracy with which the CCVT parameters are known is also assessed, with results that show little impact compared to the improvements achievable.

Power systems are protected by numerical relays that detect and isolate faults that may occur on power systems. The correct operation of the relay is very important to maintain the security of the power system. <p>Numerical relays that use voltage measurements from the power system provided by coupling capacitor voltage transformers (CCVT) have sometimes difficulty in correctly identifying a fault in the protected area. The fundamental frequency voltage phasor resulting from these CCVT measurements may result in a deviation from the true value and therefore may locate this phasor temporarily in the incorrect operating region. This phasor deviation is due to the CCVT behavior and the CCVT introduces spurious decaying and oscillating transient signal components on top of the original voltage received from the power system in response to sudden voltage changes produced during faults. Most of the existing methods for estimating the voltage phasor do not take advantage of the knowledge of the CCVT behavior that can be obtained from its design parameters.<p>A new least squares error method for phasor estimation is presented in this thesis, which improves the accuracy and speed of convergence of the phasors obtained, using the knowledge of the CCVT behavior. The characteristics of the transient signal components introduced by the CCVT, such as frequencies and time constants of decay, are included in the description of the curve to be fitted, which is required in a least squares fitting technique. Parameters such as window size and sampling rate for optimum results are discussed.<p>The method proposed is evaluated using typical power systems, with results that can be compared to the response if an ideal potential transformer (PT) were used instead of a CCVT. The limitations of this method are found in some specific power system scenarios, where the natural frequencies of the power system are close to that of the CCVT, but with longer time constants. The accuracy with which the CCVT parameters are known is also assessed, with results that show little impact compared to the improvements achievable.

Phasor Measurement Units (PMUs) are widely acknowledged as one of the most significant developments in the field of real-time monitoring of power system. By aligning time stamps of voltage and current phasor measurements, which are consistent with Coordinated Universal Time (UTC), a coherent picture of the power system state can be achieved through either direct measurements or simple linear calculations. With the growing number of PMUs installed or planned to be installed in the near future, both utilities and research institutions are looking for novel applications of synchrophasor measurements from these widely installed PMUs. In this dissertation, the author proposes two new PMUs measurements applications: three-phase instrument transformer calibration, and three-phase line parameter calculation with instrument transformers. First application is to calibrate instrument transformers. Instrument transformers are the main sensors used in power systems. They provide isolation between high voltage level of primary side and metering level of the secondary side. All the monitoring and measuring systems obtain input signals from the secondary side of instrument transformers. That means when instrument transformers are not accurate, all the measurements used in power system are inaccurate. The most important job of this dissertation is to explore a method to automatically calibrate all the instrument transformers in the power system based on real-time synchrophasor measurements. The regular instrument transformer calibration method requires the instrument transformer to be out of service (offline) and calibrated by technicians manually. However, the error of instrument transformer changes when environment changes, and connected burden. Therefore, utilities are supposed to periodically calibrate instrument transformers at least once a year. The high labor and economic costs make traditional instrument transformer calibration method become one of the urgent problems in power industry. In this dissertation we introduce a novel, low cost and easy method to calibrate three-phase instrument transformers. This method only requires one three-phase voltage transformer at one bus calibrated in advance. All other instrument transformers can be calibrated by this method as often as twice a day, based on the synchrophasor measurements under different load scenarios. Second application is to calculate line parameters during calibrating instrument transformers. The line parameters, line impedance and line shunt admittance, as needed by utilities are generated by the computer method. The computer method is based on parameters, such as the diameter, length, material characteristics, the distance among transmission line, the distance to ground and so on. The formulas to calculate line parameters have been improved and re-modeled from time to time in order to increase the accuracy. However, in this case, the line parameters are still inaccurate due to various reasons. The line parameters errors do affect the instrument transformers calibration results (with 5% to 10% error). To solve this problem, we present a new method to calculate line parameters and instrument transformers in the same processing step. This method to calibrate line parameter and instrument transformers at the same time only needs one pre-calibrated voltage transformer and one pre-calibrated current transformer in power system. With the pre-calibrated instrument transformers, the line parameter as well as the ratio correction factors of all the other instrument transformers can be solved automatically. Simulation results showed the errors between calculated line parameters and the real line parameter, the errors between calibrated ratio correction factors and the real ratio correction factors are of the order of 10e-10 per unit. Therefore, high accuracy line parameters as well as perfectly calibrated instrument transformers can be obtained by this new method. This method can run automatically every day. High accuracy and dynamic line parameters will significantly improve power system models. It will also increase the reliability and speed of the relay system, enhance the accuracy of power system analysis, and benefit all other researches using line parameters. New methods of calculating line parameter and the instrument transformer calibrations will influence the whole power industry significantly. / Ph. D.

Power transformers are one of the most important components in the electrical energy network. Extending transformer life is very economically valuable due to power outage. Therefore the development of instruments to monitor the transformer condition is of great interest. Detection of partial discharges (PDs) in power transformers is an effective diagnostic because it may reveal and quantify an important aging factor and provide information on the condition of the transformer. However, partial discharge diagnostics are still not effectively used for online monitoring of transformers because of the complexity of PD measurements and difficulties of discriminating of PDs and other noise sources. This thesis presents a further study of detection and location of partial discharges in power transformers based on previous work conducted at the Center for Photonics Technology (CPT) at Virginia Tech. The detection and positioning system consists of multiple extrinsic Fabry-Parot interferometric (EFPI) fiber acoustic sensors which can survive the harsh environment of oil-filled transformers. This thesis work is focused on optimal arrangement of multiple sensors to monitor and locate PD activities in a power transformer. This includes the following aspects. First, the sensor design requirements are discussed in order to successfully detect and accurately position the PD sources. In the following sections, Finite Element Method (FEM) is used to model the EFPI sensor fabricated at CPT. Experiments were conducted to measure the angular dependence of the frequency response of the sensor. It is shown that within the range of ±45º incident angles, the sensitivity varies by 3-5dB. Finally, the thesis demonstrates a PD positioning experiment in a 500 gallon water tank (R à H = 74" à 30" cylinder) using a hyperbolic positioning algorithm and time difference of arrival (TDOA). Finally we demonstrated that 100% of the positioning data is bounded by a 22.7à 4.1à 5.3 mm₃ cube, with a sensing range of 810 mm using the leading edge method with FIR filtering. / Master of Science

A partial discharge (PD) is the dissipation of energy caused by the buildup of localized electric field intensity. In high voltage devices such as transformers, this buildup of charge and its release can be symptomatic of problems associated with aging, such as floating components and insulation breakdown. This is why PD detection is used in power systems to monitor the state of health of high voltage transformers. If such problems are not detected and repaired, the strength and frequency of PDs increases and eventually leads to the catastrophic failure of the transformer, which can cause external equipment damage, fires and loss of revenue due to an unscheduled outage. Reliable online PD detection is a critical need for power companies to improve personnel safety and decrease the potential for loss of service. The PD phenomenon is manifested in a variety of physically observable signals including electric and acoustic pulses and is currently detected using a host of exterior measurement techniques. These techniques include electrical lead tapping and piezoelectric transducer (PZT) based acoustic detection. Many modern systems use a combination of these techniques because electrical detection is an older and proven technology and acoustic detection allows for the source to be located when several sensors are mounted to the exterior of the tank. However, if an acoustic sensor could be placed inside the tank, not only would acoustic detection be easier due to the increased signal amplitude and elimination of multipath interference, but positioning could also be performed with more accuracy in a shorter time. This thesis presents a fiber optic acoustic sensing system design that can be used to detect and locate PD sources within a high voltage transformer. The system is based on an optical acoustic (OA) sensor that is capable of surviving the harsh environment of the transformer interior while not compromising the transformer's functionality, which allows for online detection and positioning. This thesis presents the theoretical functionality and experimental validation of a band-limited OA sensor with a usable range of 100-300 kHz, which is consistent with the frequency content of an acoustic pulse caused by a PD event. It also presents a positioning system using the time difference of arrival (TDOA) of the acoustic pulse with respect to four sensors that is capable of reporting the three-dimensional position of a PD to within ±5cm on any axis. / Master of Science

It is known from the literature that conventional voltage and current transformers measure inaccurate values for voltage and current harmonics which are parts of power quality. Maximum bandwidth of conventional current transformers, which are used in electricity transmission and distribution systems, is 1.5-2 kHz and it is lower for conventional voltage transformers. Also, it is known that / voltages in some frequency spectrum are measured higher and voltages in another frequency spectrum are measured lower by the conventional voltage transformers. Furthermore, because of the phase shift of fundamental component caused by the conventional current and voltage transformers, losses and efficiency can not be calculated accurately. In this work, through the simultaneous measurements taken at the same feeder by both conventional transformers and new technology measurement transformers / amplitude and phase shift errors which are caused by conventional transformers depending on frequency and so harmonics, are examined and evaluated. Amplitude coefficients and phase shifts are determined for different types of conventional transformers to be able to calibrate measurement deviation. Through this work, measured data by conventional transformers will be accurate and realistic in terms of harmonic components. This matter is important to determine whether the accurate limits which will be set in the future possibly concerning with harmonics and interharmonics, are surpassed or not / also for punitive sanction.

Στην παρούσα διπλωματική εργασία γίνεται αναλυτική περιγραφή της λειτουργίας της διάταξης του Μεταγωγέα Τάσης Υπό Φορτίο ενός μετασχηματιστή υψηλής τάσης 150 kV/ 21 kV. Επίσης, περιγράφονται και αναλύονται σφάλματα που εμφανίζονται σε μετασχηματιστές με Μεταγωγέα Τάσης Υπό Φορτίο με χρήση εμπειρικών δεδομένων από την εταιρεία Δ.Ε.Η. Α.Ε. και τη θεωρητική ανάλυση και εξήγησή τους μέσα από βιβλιογραφική μελέτη . Συγκεκριμένα, περιγράφονται τα αίτια που δημιουργούν αυτά τα σφάλματα, τα συμπτώματα που εμφανίζονται στο μετασχηματιστή και το μεταγωγέα τάσης , οι επιπτώσεις των σφαλμάτων αυτών στο μετασχηματιστή και τη διάταξη του μεταγωγέα τάσης . Επιπλέον, γίνεται αναφορά στα συστήματα προστασίας που διαθέτει ο μετασχηματιστής και ο μεταγωγέας τάσης προκειμένου να προστατευθεί από τις επιπτώσεις αυτών των σφαλμάτων. Στη συνέχεια, μελετάται συγκεκριμένο σφάλμα, που παρουσιάστηκε στο Μ/Σ 150 kV/21 kV Νο1 του Υ/Σ 150 kV/21 kV των Ελληνικών Ναυπηγείων Σκαραμαγκά, και καταγράφεται η αντιμετώπισή του βήμα προς βήμα σε συνεργασία με την εταιρεία ΠΑΡΑΛΟΣ ΤΕΧΝΙΚΗ Α.Ε., στην οποία ανατέθηκε η επίλυση του σφάλματος. Τέλος, μέσα από την προσομοίωση ενός Μ/Σ υψηλής τάσης με μεταγωγέα τάσης υπό φορτίο με χρήση του λογισμικού πακέτου MatLab/SimuLink κατανοούμε σε βάθος τη λειτουργία του μεταγωγέα τάσης υπό φορτίο. Η προσομοίωση περιλαμβάνει την εφαρμογή διαταραχών της τάσης του δικτύου στο Μ/Σ στην περίπτωση ορθής και εσφαλμένης λειτουργίας του οργάνου του ρυθμιστή τάσης, καθώς η εσφαλμένη λειτουργία του ρυθμιστή τάσης ήταν και η αιτία του σφάλματος στο Μ/Σ του Υ/Σ 150 kV/21 kV των Ελληνικών Ναυπηγείων Σκαραμαγκά. / The particular project deals with the detailed description of the layout and the operation of the on load tap changer belonging to a transformer of high tension 150kV/21 kV. Furthermore, it describes and analyzes faults that occurs to transformers with on load tap changer using empirical data from DEI S.A. (Public Power Corporation) as well as theoretical analysis and explanation based on bibliography. In particular, there are described the reasons behind these faults , the symptoms that appear to the transformer and the tap changer and the consequences to the transformer and the layout of the tap changer. Moreover, there is reference to the protection systems that the transformer and the tap changer have in order to be protected from the consequences of the faults. Further on, a particular troubleshooting is studied that occurred at the transformer 150 kV/21 kV no1 of the high voltage substation 150 kV/21 kV at the Hellenic Shipyards of Skaramagas and its handling is reported step by step in association with PARALOS TECHNICAL S.A. , the company which was assigned to solve the troubleshooting. Finally, the operation of the on load tap changer is studied in depth through simulation of a high voltage transformer with on load tap changer using MatLab/SimuLink software package. The simulation includes the application of voltage variations at the transformer, occurred to the high voltage network in the case of orderly or disorderly operation of the voltage regulator, as the faulty operation of the voltage regulator was the reason behind the troubleshooting of the transformer of the substation of 150 kV/ 21 kV at the Hellenic Shipyards of Skaramagas.

This dissertation proposes a modeling methodology for Inductive and Capacitive Voltage Transformers (IVT and CVT) for high-frequency electrical transients analysis, up to 3 MHz. The model construction starts from the VT s sweep frequency test, carried out by a commercial Sweep Frequency Response Analyzer (SFRA) with special connections, in order to obtain its short-circuit Admittance Matrix. The SFRA is designed for another purpose, to detect displacement of windings or fault in the magnetic core of power transformers based on voltage transfer analysis. Therefore, due to this fact, the measured data needs to be corrected and recalculated, through and external routine, to adequately represent the proper and mutual admittance values of the Matrix. After its correction, the Admittance Matrix serves as input data to the Matrix Fitting method, a frequency-response approximation tool by means of modified rational functions, which is part of the Vector Fitting (VF) routine, freely available for non-commercial purposes in MATLAB®. The results are an approximated rational function, in state-space or pole-residue model, and a RLC equivalent network proper to be used in electromagnetic transients analysis software, such as the Alternative Transients Program (ATP). The models validation is accomplished based on the comparison between the transient responses of the created model, through simulation in ATP/EMTP, and the IVTs and CVTs laboratory tests when both are submitted to a step function excitation. / Esta dissertação propõe uma metodologia para a modelagem de Transformadores de Potencial Indutivos (TPIs) e Capacitivos (TPCs) para estudos de transitórios eletromagnéticos em altas frequências, na ordem de até 3 MHz. A obtenção dos modelos inicia através do ensaio de varredura em frequência dos TPs utilizando um Sweep Frequency Response Analyzer (SFRA) comercial por meio de conexões especiais de modo a obter as Matrizes de Admitâncias de curto-circuito. Os fabricantes do SFRA projetaram este equipamento com outro objetivo, o da realização de estudos de transferência de tensão, para detectar possíveis deformações mecânicas ou falhas internas em transformadores de potência. Desse modo, os dados obtidos dessa medição necessitam ser corrigidos e recalculados, através de uma rotina externa, a fim de representar adequadamente os valores das admitâncias próprias e mútuas desta Matriz. Uma vez corrigida, essa Matriz de Admitâncias servirá como entrada para inicializar o método do Ajuste Matricial (Matrix Fitting), que é uma ferramenta fundamental para aproximação da resposta em frequência por funções racionais modificadas que integra a rotina de Ajuste Vetorial, disponível livremente para fins não comerciais no MATLAB®. Como resultado, são obtidas sua função racional aproximada, em modelos de espaço de estados ou de polo-resíduo, e uma rede RLC equivalente complexa, propícia para o uso direto em programas de análises de transitórios eletromagnéticos, como o Alternative Transients Program (ATP). A validade dos modelos criados foi verificada por meio da comparação das respostas transitórias dos equivalentes gerados, através de simulações no EMTP/ATP, com os ensaios feitos em laboratório e em campo em TPIs e TPCs quando submetidos a uma excitação com uma função degrau.