Rozprawy doktorskie na temat „Transformer Taps”

PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO
O fenômeno de estabilidade de tensão vem despertando grande interesse acadêmico e das principais empresas de energia elétrica do mundo desde que começou a ser observado em sistemas reais, no final da década de setenta. Sua ocorrência está relacionada ao carregamento excessivo das linhas de transmissão. Modelar transformador com tap variável adequadamente é fundamental em análises de estabilidade de tensão, tanto no que diz respeito às informações fornecidas ao operador referentes às margens de estabilidade de tensão, quanto aos efeitos de ações de controle de tensão. O modelo de transformador com tap variável utilizado mundialmente consiste de uma impedância, obtida através do ensaio em curto-circuito e com tap nominal, em série com um transformador ideal. Esta tese mostra que, em estudos de estabilidade de tensão, o uso desse modelo leva a resultados qualitativamente errados. Para demonstração, utiliza-se um circuito pequeno e os conceitos de máxima potência transmitida, impedância equivalente da carga, e efeito do controle de tensão. Propõe-se um novo modelo coerente com os resultados obtidos em laboratório, com as leis de circuitos elétricos e com a teoria de estabilidade de tensão. Esse modelo pode ser utilizado em qualquer estudo em regime permanente. Através de diversas simulações computacionais, diferenças quantitativas e principalmente qualitativas foram obtidas comparando-se os resultados dos dois modelos.
The voltage stability phenomenon is of interest since it began to be observed in real systems in the late seventies. It happens due to excessive loading of transmission lines. The modeling of tap-changing transformers is fundamental in voltage stability analysis, in terms of the information provided to the operator about voltage stability margins and the effects of voltage control actions. The model for tap-changing transformers currently in widespread use consists of an impedance, measured in a short-circuit test with a nominal tap, in series with an ideal transformer. The use of this model in voltage stability studies leads to qualitatively incorrect results, as shown in this thesis. For demonstration purpose a small circuit and the concepts of maximum load, equivalent load impedance and voltage control effects are used. An improved model that takes into account laboratory results, circuit laws and voltage stability theory is proposed. This model can be used in any steady-state study. It gives results that are not only more accurate than those obtained with the conventional model, but also, as shown in this thesis, qualitatively different.

Voltage is one of the most important parameters for electrical power networks. The Distribution Network Operators (DNOs) have the responsibility to maintain the voltage supplied to consumers within statutory limits. On-Load Tap Changer (OLTC) transformer equipped with Automatic Voltage Control (AVC) relay is the most widely used and effective voltage control device. Due to a variety of advantages of adding Distributed Generation (DG), more and more distributed resources are connected to local distribution networks to solve constraints of networks, reduce the losses from power supply station to consumers. When DG is connected, the direction of power flow can be reversed when the DG output power exceeds the local load. This means that the bidirectional power flow can either be from power grid towards loads, or vice versa. The connection point of DG may suffer overvoltage when the DG is producing a large amount of apparent power. The intermittent nature of renewable energy resources which are most frequently used in DG technology results in uncertainty of distribution network operation. Overall, conventional OLTC voltage control methods need to be changed when DG is connected to distribution networks. The required voltage control needs to address challenges outlined above and new control method need to be formulated to reduce the limitations of DG output restricted by current operational policies by DNOs. The thesis presents an analysis of voltage control using OLTC transformer with DG in distribution networks. The thesis reviews conventional OLTC voltage control schemes and existing policies of DNOs in the UK. An overview of DG technologies is also presented with their operation characteristics based on power output. The impact of DG on OLTC voltage control schemes in distribution networks is simulated and discussed. The effects of different X/R ratio of overhead line and underground cable are also considered. These impacts need to be critically assessed before any new method implementation. The thesis also introduces the new concepts of Smart Grid and Smart Meter in terms of the transition from passive to active distribution networks. The role of Smart Meter and an overview of communication technologies that could be used for voltage control are investigated. The thesis analyses the high latency of an example solution of which cost and availability are considered to demonstrate the real-time voltage control using Smart Metering with existing communication infrastructures cannot be achieved cost-effectively. The thesis provides an advanced compensation-based OLTC voltage control algorithm using Automatic Compensation Voltage Control (ACVC) technique to improve the voltage control performance with DG penetration without communication. The proposed algorithm is simulated under varying load and DG conditions based on Simulink MATLAB to show the robustness of the proposed method. A generic 11kV network in the UK is modelled to evaluate the correct control performance of the advanced voltage control algorithm while increasing the DG capacity.

On-load tap-changers (OLTC) are devices in the power grid that keeps the voltage level constant for consumers, regardless of the power demand. Hitachi ABB Power Grids, producer of the OLTC family named VUC, guarantees 30 years of lifetime. Such a pledge requires high standard devices. This thesis has analyzed data from routine tests of switching times in the diverter switch of OLTC’s, performed before devices were put in service. The correlation of part switching times for all units leaving the factory during the past year was evaluated by calculating Pearson’s correlation coefficient. A linear trend was fitted to the data, realizing that the prediction errors, as well as the part switching times, were Gaussian distributed. The time while the resistor vacuum interrupter was open could be predicted within the interval of approximately 2ms with 2 standard deviations accuracy. To classify time series from the routine test as expected or unexpected, a model-based algorithm was implemented. The average switching time for all consecutive switches was used to define expected series. A moving average was implemented to neglect outliers and remove oscillating patterns. The majority of all data was classified as expected time series. The ones who did not, still preserved a good correlation between the part switching times. Examining the relationship between part switching times could be a valuable perspective in further classification of expected time series. The possibility of incorporating measurement of part switching times on OLTC’s in normal operation, to use the knowledge gained by this thesis, was investigated. Position sensors were mounted to measure the position of the lifting yokes, opening and closing the vacuum interrupters. The time while the vacuum interrupter contacts were open could be estimated with better accuracy than the position sensor provided. Unfortunately, those sensors cannot be utilized in normal operation. If other possibilities could be found, perhaps a laser position sensor, the implemented algorithm would be valuable.

Distributed generation is transforming the power system grid to decentralized system where separate units like wind power generators or solar panel shall coexist and operate in tandem in order to supplement each other and make one extensive system as a whole so called smart grid. It is utmost important to have a control ability over such units not only on a field level but on a system level as well. To be able to communicate with numerous devices and maintain interoperability universal standard is a must. Therefore, one of the core standards relevant to smart grids is IEC 61850 – Power Utility Automation which comes into assistance and tackles aforementioned challenges. This project uses IEC61850 architecture to implement client/server windows applications for on-load tap changer remote control. The proposed solution and designed applications are tested together with a real time simulator where simple power system is modelled to emulate the system response to control signals in a real time. In this way, the implemented applications can be tried and assessed as if performing in real environment. Consequently, a user of the client application is able to remotely control voltage on the power transformer's secondary side and manipulate the switching equipment simulated in the model.
Distribuerad generation håller på att förändra transmissionsnätet till decentraliserat system där separata enheter som vindkraftverk eller solpanel skall samexistera och fungera tillsammans för att komplettera varandra och att göra ett omfattande system som helhet så kallade smarta elnät. Det är ytterst viktigt att ha en kontroll förmåga över sådana enheter inte bara på ett fältnivå utan även på systemnivå. För att kunna kommunicera med många enheter och bibehålla interoperabiliten som universell standard är ett måste. En av de grundläggande normer som är relevanta för smarta nät är IEC 61850 - Skydd & Automation, som kommer in i bistånd och möter ovan nämnda utmaningar. Detta projekt använder IEC61850-struktur för att implementera klient/server windows applikation för lindningskopplarens fjärrkontroll. Den föreslagna lösningen och utformade applikationer testas tillsammans med en realtidssimulator där enkelt kraftsystem modelleras för att emulera systemets svar på de givna styrsignalerna i realtid. På detta sätt kan de implementerade programmen prövas och bedömas hur de utföras i verklig miljö. Följaktligen kan användare av klientapplikationen fjärrstyra spänningen på transformatorns sekundärsida och manipulera ställverk som simuleras i modellen.

The possibility and feasibility of experimental detection of localized defects in tubes using laser-induced ultrasonic wave approach through Thermo Acousto Photonic Non Destructive Evaluation (TAP-NDE) and Signal processing through wavelet transform is examined in this research. Guided waves in cylindrical surfaces provide solutions for detection of different defects in the material. Several experiments were conducted to this respect. Wave propagation in both axial and circumferential directions was studied. The dispersive wave propagation of ultrasonic waves in hollow cylinders has been investigated experimentally, primarily for use in non-contact and nondestructive inspections of pipes and tubes. The laser ultrasonic waves propagated in cylindrical waveguides are particularly attractive because of their unique characteristics in the applications of nondestructive evaluation (NDE). Contrary to studies making use of only axially symmetric guided waves in hollow cylinders, here are analyzed also nonaxisymmetric waves. The analysis of data is made by using the Gabor wavelet transform. The capability of modeling the guided wave dispersion in hollow cylinders is used in developing guided wave experimental techniques for flaw detection. Good agreement was obtained when comparing the dispersion spectra between theory and experimentation. Measurement of group velocities of guided waves, which are obtained directly from the wavelet transform coefficients, can be used to determine allocation and sizing of flaws.

Thesis (MEng (Electrical Engineering)--Cape Peninsula University of Technology, 2019
The purpose of an electrical power system is to supply electrical energy to the customers. Power transformers are required to transform the system voltage from generation to transmission and distribution levels. Protection and control systems must ensure that power system high voltage equipment such as transformers operate and deliver save, reliable and secure electricity supply. The aim of the project research work is to develop and implement a strategy, methods and algorithms for monitoring, protection and voltage control of parallel power transformers based on IEC 61850-9-2 process bus standard. NamPower is a power utility in Namibia. The IEC 61850 protocol for electrical substation automation system is used for the protection and control of 5 power transformers operated in parallel in an existing substation system. The IEC 61850-9-2 process bus standard is however not used in regards of Sampled Values (SV). Protection and control devices are connected to a substation communication network, routers and switches using fibre optic linked Ethernet. Inductive Current Transformers (CTs) and Voltage Transformers (VTs) secondary circuits are hardwired to Intelligent Electronic Devices (IEDs) and fibre optic links are not used for this purpose at process level communication. The research focuses on the implementation of the IEC 61850 standard with Merging Units (MUs) and sampled values to improve the existing implemented protection and control system at NamPower. This includes substation communication networks and MUs used for transformer protection, voltage regulator control and cooling fan control. At the present the CTs located at the transformer bushings and switchgear and the VTs located at the switchgear are hardwired to the inputs on protection and control IEDs. The research focuses on issues with the copper wires for voltage and currents signals and how these issues can be eliminated by using the MUs and the SV protocol. The MUs which are considered in this Thesis is to improve the voltage regulator control and the control of the cooling fan motors. The voltage regulator control IED is situated at the tap change motor drive of the On-Load Tap Changer (OLTC). The IED of each transformer is required to regulate the voltage level of the secondary side bus bar it is connected to. All the regulating IEDs are required to communicate with each other and collectively to control the bus bar voltage depending on the switching configuration of the parallel transformers. The control circuit for controlling the cooling fan motors is hardwired. Temperature analogue signal input into a programmable automation controller IED can be used for controlling the transformer cooling fans. A strategy, methods and algorithms for transformer protection, voltage regulator control and cooling fan motor control of parallel power transformers need to be developed and implemented based on IEC 61850-9-2 process bus. Power utilities and distributors can benefit from interpretation of the IEC 61850-9-2 standard and implementing MUs and SV in substations. MUs can be included in the power transformer protection, automation and control systems. A cost reduction in high voltage equipment, substation installation and commissioning costs and better performance of protection and control system are anticipated.

This is a bachelor's thesis in Mechanical engineering. The main topic of the thesis is the electrical calculation of the tap changer's shield. Although there are many kinds of tap-changers, this study focuses on VUCL models. The theoretical background of the thesis is that the new shield is suitable and useable in all VUCL models. The primary topic of the thesis is to study that the new shield protects the resistance box in the middle of the tap changers. To do this study, Creo Parametric, Comsol, and SpaceClaim are the programs used.
Detta är en kandidatexamen i maskinteknik. Avhandlingens huvudämne är den elektriska beräkningen av kranbytarens sköld. Även om det finns många olika typer av lindnings fokuserar denna studie på VUCL modeller. Den teoretiska bakgrunden för avhandlingen är att den nya skölden är lämplig och användbar i alla VUCL-modeller. Det primära ämnet för avhandlingen är att studera det nya sköld skyddar motståndet rutan i mitten av lindningskopplare. För att göra denna studie är Creo Parametric, Comsol och SpaceClaim de program som används.

To mitigate high voltages in transmission systems with low demands, traditional solutions often consider the installation of reactive power compensators. The deployment and tuning of numbers of VAr compensators at various locations may not be cost-effective. This thesis presents an alternative method that utilises existing parallel transformers in distribution networks to provide reactive power supports for transmission systems under low demands. The operation of parallel transformers in small different tap positions, i.e. with staggered taps, can provide a means of absorbing reactive power. The aggregated reactive power absorption from many pairs of parallel transformers could be sufficient to provide voltage support to the upstream transmission network. Network capability studies have been carried out to investigate the reactive power absorption capability through the use of tap stagger. The studies are based on a real UK High Voltage distribution network, and the tap staggering technique has been applied to primary substation transformers. The results confirm that the tap staggering method has the potential to increase the reactive power demand drawn from the transmission grid. This thesis also presents an optimal control method for tap stagger to minimise the introduced network loss as well as the number of tap switching operations involved. A genetic algorithm (GA) based procedure has been developed to solve the optimisation problem. The GA method has been compared with two alternative solution approaches, i.e. the rule-based control scheme and the branch-and-bound algorithm. The results indicate that the GA method is superior to the other two approaches. The economic and technical impacts of the tap staggering technique on the transmission system has been studied. In the economic analysis, the associated costs of applying the tap staggering method have been investigated from the perspective of transmission system operator. The IEEE Reliability Test System has been used to carry out the studies, and the results have been compared with the installation of shunt reactors. In the technical studies, the dynamic impacts of tap staggering or reactor switching on transmission system voltages have been analysed. From the results, the tap staggering technique has more economic advantages than reactors and can reduce voltage damping as well as overshoots during the transient states.

Voltage imbalance in low voltage (LV) networks is expected to deteriorate as low carbon technologies, e.g. electric vehicles (EVs) and heat pumps (HPs) are increasingly deployed. The new electrical demand attributable to EVs and HPs would increase the voltage magnitude variation, increasing the possibility of voltages moving outside the statutory LV magnitude limits. Moreover, the single-phase nature of EVs and HPs, which will be connected via a single-phase 'line & neutral' cable to a 3-phase four-wire LV mains cable buried beneath the street, further entangles this voltage management problem; the non-balanced voltage variations in the three phases boost the levels of voltage imbalance. Excessive voltage imbalance and magnitude variation need to be mitigated to limit their adverse effects on the electric network and connected plant. The voltage imbalance in LV networks is conventionally reduced by reinforcing the network, generally at a high cost. Some modern methods for voltage imbalance mitigation have been introduced in recent years. The power electronic converter based methods are inadequate due to the generation of harmonics, significant power losses and short lifetime. Besides, automatic supply phase selection and smart EV charging rely on an advanced smart communication system, which currently is not available. This project aims to develop alternative solutions that mitigate the voltage imbalance seen in LV networks. A voltage balancing method based on Scott transformer (ST) is proposed. This method does not generate harmonics and is independent of the smart communication system. Computer simulations demonstrated the proposed method is able to convert a non-balanced 3-phase voltage into a balanced 3-phase voltage at either a point on the LV feeder or a 3-phase load supply point with the predefined voltage magnitude. Besides, a physical voltage balancing system was created based on the proposed method and it was tested in an LV network in the laboratory. The test results show the balancing system is capable of maintaining a low level of voltage imbalance on the LV feeder by rapidly compensating for the voltage rises and sags caused by single-phase load variations. This voltage balancing method is a potential solution for the network utilities to accommodate the significant penetration of low carbon technologies without breaching the network voltage limits. The impact of EVs and HPs on the LV network voltages is investigated based on a Monte Carlo (MC) simulation platform, which comprises a statistical model of EV charging demand, profiles generators of residential and HP electrical demand, and a distribution network model. The MC simulation indicates the impact of EVs and HPs is related to their distribution; when more than 21EVs and 13HPs are non-evenly distributed on a 96-customer LV feeder, the voltage limits are likely to be violated. Moreover, the effectiveness of the ST based voltage balancing method and the demand response based TOU tariff, implemented either alone or together, in mitigating the impact of EVs and HPs is investigated based on the established MC simulation platform. The results indicate the ST based balancing method alone is able to completely mitigate the voltage limit violations regardless of the penetration levels of EVs and HPs. Moreover, using both of the two investigated methods further enhances the balancing effectiveness of the ST based voltage balancing method.

Renewable energy is harnessed from continuously replenishing natural processes. Some commonly known are sunlight, water, wind, tides, geothermal heat and various forms of biomass. The focus on renewable energy has over the past few decades intensified greatly. This thesis contributes to the research on developing renewable energy technologies, within the wind power, wave power and marine current power projects at the division of Electricity, Uppsala University. In this thesis grid connection of permanent magnet generator based renewable energy sources is evaluated. A tap transformer based grid connection system has been constructed and experimentally evaluated for a vertical axis wind turbine. Full range variable speed operation of the turbine is enabled by using the different step-up ratios of a tap transformer. This removes the need for a DC/DC step or an active rectifier on the generator side of the full frequency converter and thereby reduces system complexity. Experiments and simulations of the system for variable speed operation are done and efficiency and harmonic content are evaluated.  The work presented in the thesis has also contributed to the design, construction and evaluation of a full-scale offshore marine substation for wave power intended to grid connect a farm of wave energy converters. The function of the marine substation has been experimentally tested and the substation is ready for deployment. Results from the system verification are presented. Special focus is on the transformer losses and transformer in-rush currents. A control and grid connection system for a vertical axis marine current energy converter has been designed and constructed. The grid connection is done with a back-to-back 2L-3L system with a three level cascaded H-bridge converter grid side. The system has been tested in the laboratory and is ready to be installed at the experimental site. Results from the laboratory testing of the system are presented.
Wind Power
Wave Power
Marine Currnet Power

This thesis deals with a power flow control in the electric power system. An overview of FACTS devices is introduced; their basic characteristics as well as examples of their application are discussed. A significant part of this thesis investigates phase shifting transformers (PST´s), which seem to be suitable for implementation in the transmission system of the Czech Republic. The PST´s are useful devices that control active power flows on cross-border lines and regulate unwanted and unexpected power flows. Basic types and characteristics of the PST´s are discussed. In chapter 7 is designed laboratory task that should validate the regulatory capabilities of the transformer PST. Designs of models of individual parts of this laboratory task are presented. In the last part of this thesis the regulation effect of the PST is validated in the GLF/AES program.

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In this work a methodology for reduction of maintenance cost in the on-load tap changers (OLTC) of extra high voltage is proposed. The methodology is based on the use of Artificial Neural Networks (ANN) for the intelligent processing of input signals of the commutator. The neural nets adequately trained allow to create an information system and dedicated diagnosis of the OLTC. This system can interpret and diagnosis the components through the real time input signals in order to delay the power transformer maintenance intervals, foreseeing when the OLTC is going to maintenance have intervention based on its condition. It has been adopted a multiperceptron ANN architecture in which the input vector has 22 components and the output considers only one component with the status of the OLTC condition in function of its operation time. This output information is used to determine the periods of maintenance of the commutators. It is reported an application of the proposed system considering the on load tap changer of an autotransformer bank of 500/230/13.8 kV, 600MVA of Centrais Elétricas do Norte do Brasil S/A (ELETRONORTE). The results indicate the advantages of the maintenance based on the condition using ANN.
Neste trabalho é proposta uma metodologia para redução de custo de manutenção nos comutadores de tap sob carga (OLTC) dos transformadores de potência de extra alta tensão. A metodologia está baseada na utilização de redes neurais artificiais (RNA) para o processamento inteligente dos sinais de entrada dos comutadores. As redes neurais adequadamente treinadas permitem criar um sistema de informação e diagnóstico dedicado a OLTC que podem interpretar e diagnosticar os componentes através das entradas em tempo real de forma a, postergar os intervalos de manutenção, prevendo quando o OLTC deverá sofrer intervenção de manutenção baseada na condição do OLTC. Foi adotada uma arquitetura de RNA de multiperceptron na qual a entrada considera um vetor com 22 entrada e apenas uma saída com o status da condição do OLTC em função do tempo de operação. Essa informação de saída é utilizada para determinar os períodos de manutenção dos comutadores de tap. É realizada uma aplicação do sistema proposto considerando o comutador de tap sob carga de um banco de autotransformador de 500/230/13.8kV, 600MVA da Centrais Elétricas do Norte do Brasil S/A( ELETRONORTE) e os resultados indicam as vantagens da manutenção baseada na condição usando RNA.

In this Master thesis, microwave connections between circuit boards are constructed. The primary frequency band is the X-band (8-12 GHz). The purpose of the connections is to enable a more simple and cheaper way of mounting the circuit boards inside a container.

The connections have been designed and evaluated, using different computer programs. A few prototypes have been built and measured.

The main goal of this Master thesis was to design a connection, that would be useful in practice. The connections should be easy to manufacture and have a good performance. They should also have a high tolerance for manufacturing errors.

The main part of this report contains descriptions of different designs. The designs are presented together with simulated and measured results.

The report contains designs based upon coplanar waveguides and a phase shifting technique. The result shows that designs that are using coplanar waveguides are good. The phase shifting technique has some limitations and need to be developed further.

The novel contribution of the thesis is the design and implementation of decentralised output feedback power system controllers for power oscillation damping (POD) over the entire operating regime of the power system. The POD controllers are designed for the linearised models of the nonlinear power system dynamics. The linearised models are combined and treated as parameter varying switched systems. The thesis contains novel results for the controller design, bumpless switching and stability analysis of such switched systems. Use of switched controllers against the present trend of having single controller helps to reduce the conservatism and to increase the uncertainty handling capability of the power system controller design. Minimax-LQG control design method is used for the controller design. Minimax-LQG control combines the advantages of both LQG and H control methods with respect to robustness and the inclusion of uncertainty and noise in the controller design. Also, minimax-LQG control allows the use of multiple integral quadratic constraints to bound the different types of uncertainties in the power system application. During switching between controllers, switching stability of the system is guaranteed by constraining the minimum time between two consecutive switchings. An expression is developed to compute the minimum time required between switchings including the effect of jumps in the states. Bumpless switching scheme is used to minimise the switching transients which occur when the controllers are switched. Another contribution of the thesis is to include the effect of on load tap changing transformers in the power system controller design. A simplified power system model linking generator and tap changing transformer dynamics is developed for this purpose and included in the controller design. The performance of the proposed linear controllers are validated by nonlinear computer simulations and through real time digital simulations. The designed controllers improve power system damping and provide uniform performance over the entire operating regime of the generator.

Este projeto de pesquisa propõe desenvolver um novo modelo e uma nova abordagem para a resolução do problema da otimização Volt/VAr em redes de distribuição de energia elétrica. A otimização Volt/VAr consiste em, basicamente, determinar os ajustes das variáveis de controle tais como bancos de capacitores chaveados, transformadores com comutação de tap sob carga e reguladores de tensão, de modo a satisfazer, simultaneamente, as restrições de carga e de operação para um dado objetivo operacional. Esse problema, matematicamente, foi formulado como um problema de programação não linear, multiperíodo, e com variáveis contínuas e discretas. Algoritmos de programação não linear foram utilizados com o intuito de aproveitar as vantagens das matrizes altamente esparsas montadas ao longo do método de solução. Para utilizar tais algoritmos, as variáveis discretas são tratadas como contínuas por meio da utilização de funções senoidais que penalizam a função objetivo do problema original enquanto estas não convergirem para algum dos pontos pré-definidos no seu domínio. O caráter multiperíodo do problema, contudo, refere-se à consideração de uma restrição que relaciona os ajustes das variáveis de controle para sucessivos intervalos de tempo na medida em que limita o número de operações de chaveamento desses dispositivos para um período de 24-horas. O estudo fundamenta-se, metodologicamente, em métodos do tipo Primal-Dual Barreira-Logarítmica. Para demonstrar a eficiência do modelo proposto e a robustez dessa abordagem, a partir de dados teóricos obtidos de levantamentos bibliográficos, testes foram realizados em sistemas-teste de 10, 69 e 135 barras, e em um sistema de 442 barras do noroeste do Reino Unido. As implementações computacionais foram feitas nos softwares MATLAB, AIMMS e GAMS, utilizando o solver IPOPT como método de solução. Os resultados mostram que a abordagem proposta para a resolução do problema de programação não linear é eficaz para tratar adequadamente todas as variáveis presentes em problemas de otimização Volt/VAr.
This work proposes a new model and a new approach for solving the Volt / VAr optimization problem in distribution systems. The Volt/VAr optimization consists, basically, to determine the settings of the control variables of switched capacitor banks, on-load tap changer transformers and voltage regulators, in order to satisfy both the load and operational constraints, to a given operational objective. The problem is formulated as a nonlinear programming problem, multiperiod, and with continuous and discrete variables. Nonlinear programming algorithms were used in order to take advantage of the highly sparse matrices built along the solution method. The discrete variables are treated as continuous along the solution method by means of the use of sinusoidal functions that penalize the original objective function while the control variables do not converge to any of the predefined discrete points in its domain. The multiperiod, or dynamic, characteristic of the problem, however, refers to the use of a constraint that relates the settings of the control variables for successive time intervals that limits the control devices switching operations number for a period of 24-hours. The study is based, methodologically, on Primal-Dual Logarithmic Barrier method. To demonstrate the effectiveness of the proposed model and the robustness of this approach, the data were obtained from theoretical literature surveys, and tests were performed on test-systems of 10, 69 and 135 buses, and in a 442 buses located in the Northwest of the United Kingdom. The computational implementation was accomplished in the softwares MATLAB, AIMMS and GAMS, using the IPOPT solver as solution method. The results have shown the approach for solving nonlinear programming problems is effective to appropriate cope with all the variables presented in Volt/VAr optimization problems.

The state of a given power system i.e. voltage magnitudes and angles at all the buses can be computed using the Newton-Raphson Load Flow (NRLF) method when active power and reactive power loads are specified at all the buses of the system. This computation can be carried out more e ciently(in terms of computer memory and time) using the Fast Decoupled Load Flow(FDLF) method for a large class of systems. These methods are the most widely used methods in power system studies. NRLF and FDLF methods require modifications, if voltage magnitude is spec-i ed at some of the generator buses instead of the reactive power load. In such situations, generator reactive power outputs(manipulated by adjusting the field excitation) have to be adjusted to meet this specification. This necessitates the determination of the value of these additional control variables. There are some more similar adjustments that are required to be made in a practical load flow. Sometimes, the voltage at some load buses may be specified. They are to be maintained at the scheduled value using the taps on in-phase transformers(OLTC transformers). Similarly, it is possible in some situations that the active power flow in some lines are specified to be kept at a particular value. The device which facilitates such a control is the phase shifting transformer(PSTs) and the PST tap value is the additional control variable to be determined. The other operation of interest in interconnected power systems is the area interchange control(AIC). This requires that the sum of active power flow between two areas of the system is maintained at the specified value. The control variable value that enables this adjustment is the active generation in a particular generator bus in the area referred to as a swing bus. The load flow problem is referred to as a adjusted load flow problem in cases where in, some of these control variables must also be determined in addition to the state of the system. It must be pointed out here that the control variables must be strictly kept within their limits while bringing the controlled variables to their specified values. If a control variable tends to reach a value beyond its limits, then it is to be set at the limit and the corresponding controlled variable will not be at its scheduled value. Adjusted load flow problems generally involve many control variables of the same type or multiple control variables of different types. The challenge in finding adjusted load flow solutions stems from the fact that the relation between the controlling and controlled variables is not one to one; each controlling variable affects many of the controlled variables. The existing approaches of adjusted load flow solutions generally consider only one type of these adjustments. There are only a very few attempts where more than one type of adjustment is considered. The two broad directions pursued for developing algorithms for adjusted solutions, by the earlier researchers are (1) Introducing additional equations in order to include control variable(between iterations) and (2) Adjusting the controlling variables between unadjusted load flow solution iterations based on the local sensitivity of the controlled variable with respect to a particular controlling variable. The schemes in use for finding adjusted load flow solutions have a flavour of trial and error type of algorithms. Their success in any situation is known to depend on specific details of implementation. Implementation details that guarantee success are not in the public domain. Many times they exhibit oscillatory convergence behaviour requiring very large number of iterations or fail to converge. It is also known that in some situations these algorithms could converge to anomalous solutions(solutions that are inconsistent with practical system behaviour). Such limitations of the existing approaches and also the need for developing better methods is well documented in the literature. Some recent work has shown the promise of the formulation of the adjusted load flow problem in the complementarity framework considering a few of the adjustments. This thesis is intended to further explore this promising direction of investigation. In particular, in this thesis, we develop new algorithms in complementarity framework for the following situations and demonstrate their attractive features as compared with the existing approaches. In this thesis, the following algorithms have been proposed, developed, tested and their performance compared with the existing algorithms. . Two algorithms for including OLTC adjustments, in the FDLF method as Mixed Complementarity Problem(MCP) and Non-linear Complementarity(NCP) formulations. In addition, the above algorithms are further extended to incorporate generator bus Q-limit adjustments simultaneously with the OLTC adjustments. Two new algorithms(two each in MCP and NCP formulations) are developed to handle generator Q-limits and OLTC adjustments individually as well as together in the NRLF formulation in rectangular coordinates. Four algorithms(two in MCP and two in NCP) to handle PST constraints in NRLF and FDLF methods. Four algorithms(two in MCP and two in NCP) to handle AIC constraints in NRLF and FDLF methods. In addition, the PST and AIC adjustment algorithms above are combined to simultaneously carry out PST and AIC adjustments in NRLF as well as FDLF methods. Four algorithms(two for NRLF and two for FDLF) to simultaneously incorporate all the four adjustments simultaneously using MCP and NCP formulations. These algorithms are also shown to be capable of incorporating simultaneously any subset of these four adjustments The thesis focusses only on incorporating adjustments in the NRLF and FDLF methods as they are the most widely used schemes in the industry as well as the academia. It is also pointed out that the investigations here consider the adjustment problem in the traditional framework and hence, none of the power electronics based control equipment or the modern distributed generation sources are considered here. Results of extensive computational experiments are presented and the attractive performance of the new algorithms as compared with the traditional ones are high-lighted. All the new algorithms developed here are fundamentally different from the existing adjusted load flow approaches(not based on complementarity framework) in that they meet the specifications on the system variables and limits on the controlling variables automatically; without requiring either heuristic algorithmic choices or problem specific algorithm manipulation - a fairly common feature in all the existing approaches. This extremely desirable feature of the proposed algorithms is due to the fact that the pro-posed formulations for the adjusted load flow problems in complementarity framework, transform these problems to that of solving a fixed set of non-linear equations. The results in the thesis provide strong evidence of the promise of the new methods for adoption into the widely used NRLF and FDLF programs so as to make solving the adjusted load flow problem as simple as solving the unadjusted load flow problem.

Masters Research - Master of Philosophy (MPhil)
One of the most effective and powerful tools for the diagnosis of poor mechanical integrity of transformer windings is Frequency Response Analysis (FRA). For large transformers within a power network, which are often critical and expensive, FRA adds enormous value in the diagnosis of winding defects. Which can easily result in the catastrophic failure of a transformer. FRA due to its non-invasive nature and relatively fast assessment, has become an essential tool for the monitoring of transformer winding structural integrity. Enabling utilities to maintain and monitor power transformer windings in a cost-effective manner to minimize the likelihood of an unexpected and costly outage. Transformers are only designed to withstand certain levels of mechanical forces which can be surpassed during short circuit events, transport mishaps or loss of clamping pressure; which may result in winding displacement and deformation. Defects can also occur due to transformer aging, through short circuits and over voltages. To date, FRA has been almost exclusively utilized off-line, which requires a transformer to be completely isolated from the network. However, there is a need to move from off-line to on-line measurement due to the growing importance in the development of smart grid systems. In this study, FRA sensitivities have been investigated through case studies, which inform interpretation of FRA for both off-line and on-line measurements. Real scenarios such as conducting measurements on GSU transformers at power stations, autotransformers at substations and measurements on a rectifier transformer at an Aluminium smelter company have been investigated. This thesis proposes a bushing tap signal injection method using high frequency Current Transformers (CT). The developed technique has the potential to be used for on-line monitoring of power transformers. The technique uses a wide-band signal coupled into the transformer winding via a high frequency CT. The response signal is also measured through another higher frequency CT at a corresponding phase bushing. This non-invasive monitoring tool provides a safe means to access an in-service power transformer, which makes it feasible for permanent onsite monitoring of power transformers.

碩士
國立臺北科技大學
電機工程系所
104
Transformer tap settings of power plant affects reactive power output and plant equipment performance.It is an important issue. Therefore, this thesis references IEEE C57.116 standard and applys power flow analysis to optimal settings of transformer no voltage tap changer.Because transformers tap are no voltage tap changer and can not switch under the voltage condition, they need to do optimal settings after complete evaluation in every online stage. The thesis derives generator voltage and reactive power relationship by using IEEE C57.116 standard, knowing the system voltage, unit transformer impedance and unit transformer tap is an important factor in the impact of the generator reactive power range,and then applys power flow analysis to reactive power output capability of generator for each tap in the normal system voltage range. Unit auxiliary equipment in power plant is necessary and power supply is powered by a generators.To boost the operating efficiency and stability of the equipment, its voltage must also operate within a limited range by tap setting of unit auxiliary transformer. In this thesis, The common four kinds of power supply mode are used and simulated reactive power range in auxiliary equipment voltage limits.The thesis is based on the maximum absorption of reactive power and proposed optimization setting.The results of analysis can realize that tap -2.5% is maximum absorption of reactive power.Finally, adding loss analysis of power of plant as an additional basis for selecting tap, providing a reference for dispatchers.

碩士
輔仁大學
電機工程學系碩士在職專班
104
The purpose of this paper is to optimize transformer no-voltage tap changer of main generator step up power transformer, start up power transformer, emergency start up power transformer and auxiliary transformer for a steam power plant. This study determines the step up power transformer tap so that the transformer is capable of absorbing or delivering the maximum MVAR output range within the expected system and generator operating voltage variation ranges based on IEEE Std. C57.116. In order to maintain bus voltage of auxiliary transformer loadings within acceptable range because of system voltage deviation and maximum MVAR output range of generator, no-voltage tap changer of auxiliary transformer, start up power transformer and emergency start up power transformer shall be selected through a series of load flow analysis.

The purpose of this dissertation is to develop a predictive maintenance plan to diagnose contact degradation on load tap changers, LTCs, and voltage regulators using pattern recognition techniques The following sensor technologies have been applied to the LTC and voltage regulator. Strain gages were attached to the drive arm of the devices to detect changes in the strain associated with each tap change. Acoustic transducers and accelerometers were used to measure changes in the sound wave and vibration associated with each tap change. Both the load tap changer and the voltage regulator were run with new contacts and repeatable signals or signatures were obtained for each sensor type Burned contacts and failed finger contacts, two common modes of failure for these machines, were obtained from Entergy. The burned contacts were installed on the voltage regulator and on each phase of the LTC and the tests were repeated. These sets of data showed that there exist distinct signatures for each equipment type experiencing this failure mode. The magnitudes of the good and bad contact signals were different enough that classification could be performed A power spectral density was also computed for the two cases, operational contacts and burned contacts. These frequency plots show significant differences in the two modes which provide useful information for classification as well Two classification methods were derived for the two-case study under consideration, to determine whether the unit being tested is in need of maintenance. These are template matching and a linear classifier used to partition the feature space into two non-overlapping areas. Graphical results show the validity of each method The future of the project involves field testing the predictive maintenance program on energized substation equipment for a significant period of time. The predictive maintenance system's performance will be measured by its ability to identify equipment in need of maintenance or repair. The completion of this stage of the project will depend upon future funding and upon the cooperation of Entergy Services Inc
acase@tulane.edu

碩士
國立臺灣科技大學
電機工程系
92
In order to ensure voltage quality during operations in the power systems, the On-Load Tap Changer (OLTC) for transformer were used to keep the voltage output stable. The paper discussed and analyzed the potential distribution along the transformer winding by using an impulse voltage applied across the winding. The distribution of impulse voltage on the OLTC at different taps and connections of 69 kV transformer windings has been simulated by using a voltage oscillation analysis program. The simulation results verify the causes of failures at the different connections for OLTCs of 69/ll.95-23.9 kV, 25 MVA transformers in Taipower’s (Taiwan Power Company’s) secondary substations. In addition, the advantages and disadvantages of OLTCs at different connections were discussed. Moreover, improvement strategies for new and existing OLTCs have been provided, that can avoid OLTC failures which may affect the system reliability if the failure is caused due to potential distribution. Indeed, the proposed solutions are practical in the Taipower system and can reduce the failure rates of line-end OLTCs.

碩士
國立臺北科技大學
電機工程研究所
105
Based on cost considerations, the transformers connected to generators use no voltage tap changers.They are only operated without voltage conditions, so they must be optimized before the on-line phase. This thesis optimizes the setting of no voltage tap changer transformers connected to generators.The optimal goals are the maximum of reactive power absorbtion and the minimum of active power loss. The optimized setting of tap positions can stabilize the system voltage and reduce the active power loss. First, the thesis refers the IEEE C57.116 standard to learn the relationship between the generator voltage and the reactive power, and analysis the range of reactive power output to the system caused by the system voltage, unit transformer impedance and transformer taps. Second, the thesis use the reactive power corresponds to the system voltage and the generator voltage derivatives the currents cross the transformers high side and low side. The result of active power loss is the sum of the copper loss caused by the currents and the iron loss caused by the operation of transformers. In this thesis, the first simulation uses the full load situation to analysis the range of reactive power and active power loss under the conditions of generator voltage, generator excitation limit and system voltage. The second simulation uses actual generation situation to analysis the maximum of reactive power absorbtion and the minimum of active power loss under every tap position by power flow. Finally, the operators can use the result of the simulations in different generation situation to learn the reactive power output and the active power loss under every tap position.

M.Ing.
In this thesis, a new isolated DC-DC converter topology is designed which implements a high frequency primary tap changing transformer. This converter is designed to be implemented into renewable energy applications. Renewable or alternative energy resources are becoming more popular by necessity. The voltage generated by renewable energy resources is dependent on the weather conditions, thus the voltage may vary. This novel topology can maintain a regulated DC output voltage for a wide input voltage range by implementing a high frequency primary tap changing transformer. This converter is thus designed to accommodate the voltage changing conditions of renewable energy resources. This converter only requires output bus capacitance for DC bus stiffening. The overall required bus capacitance is therefore effectively reduced compared to other converter topologies. This isolated topology also improves the fundamental power quality. Other power quality components of this new converter are also analysed.

碩士
國立臺灣科技大學
電機工程系
105
Regarding the case of a power company that has used power transformers with the On-Load-Tap-Changer (OLTC) mechanical device, a mishap attributable to voltage control of power transformers caused system voltage to become too high or too low and resulted in an accident. After reviewing the analysis, it is found that the cause of this kind of accidents is related to an OLTC control system. How to avoid the above accidents and improve the weakness of the current OLTC control system is the purpose of this study. In this study, the research method is to collect the previous representative cases of mishaps involving voltage control of power transformers. After exploring the reasons and in-depth understanding of the OLTC operation control mode, it is proposed that an "auxiliary controller" be added to the current OLTC control system. Using the simple rules of four electrical signals of CTIV monitoring, namely sampling action numbers (C), action time (T), action current (I) and action voltage (V) to correctly monitor the operation of the OLTC, we will be able to improve the weaknesses of the current OLTC control system and avoid accidents related to voltage control. In addition, the design of the "auxiliary controller" has abandoned the current EM circuit, and changed with the application of PLC, PM and MMI interface as the main units, which are already mature in the industry and considered user-friendly. Finally, through laboratory simulation tests and field tests in two substations, it was proved that the "auxiliary controller" can achieve the purpose of enhancing stable power supply and safe operation of the power transformer.

碩士
國立虎尾科技大學
電機工程研究所在職專班
99
Operations of electric arc furnace in steel plants cause serious variation of bus voltage amplitude on primary substation, then On-load Tap Changer (OLTC) of main transformer in the substation happens too many switching times. The switching events affect extreme reliability of the transformer. Based on measured data of the transformer by power SCADA system, the thesis applies Back-propagation neural network method to study the effectiveness of Auto Voltage Regulation (AVR) on OLTC and qualities of secondary bus voltage of the transformer. Besides, the under-voltage electronic fault indictor of AVR is improved also. As a result, acting times of OLTC of the transformer can satisfy a proper operating requirement.

碩士
國立中正大學
電機工程所
98
As the countries all over the world face the demand of economical developments, many large electric loads, such as steel and petrochemical plants, semiconductor parks, and others have been widely operated in the power system network, meanwhile many manufacturers start to increase the productivity by expanding more power-consuming equipment. Among many electric loads, the operations of steel plants play as one of the most important roles since they provide steel materials which are essential to economical developments. Electric arc furnaces (EAFs) are commonly used in steel industries for steel-making; however, the operation of EAFs present violent and random characteristic which may cause considerable power quality problems. One of significant power quality pollution comes from the voltage flicker. In this study, the goal is to investigate the power quality problems that occur in a Taipower Primary substation system, Sinying P/S, where the P/S feeds electric power to several steel-making customers. Recently, due to the operations of these EAF loads, it is observed that serious voltage fluctuations occurred at the bus that connected to the P/S transformers. The on-load tap changers (OLTCs) of the transformers have thus frequently switching on and off, which deteriorate transformers life and also affect the supply power quality. For mitigating such disturbances, it is necessary to estimate if it needs to set up any flicker control devices, such as static var compensators (SVCs). The main studies of this thesis include following tasks: (1) Performing field measurements for observation of the actual power quality problems occurred on Sinying P/S; (2) Review the related standards to assess the voltage flicker index; (3) Investigation of the inter-operation between SVC and OLTC; (4) Providing appropriate improving strategies to deal with voltage flicker or other power quality problems.

This dissertation investigates the feasibility of fixed tap or reduced tap transformers on the Eskom Transmission system, thereby reducing transformer failures and reducing the cost of new or replacement transformers. The main analysis uses Optimal Power Flow (OPF) techniques based on a Siemens PSS/E software platform and a minimum system loss operating point or objective function while reducing tap movements and maintaining prudent system limits but allowing shunt VAr devices to freely compensate for the reduced tapping. Various years and system conditions are analysed.Finally it is shown that replacing some Transmission transformers with fixed-tap transformers is a more cost effective solution. The contribution of this research is two-fold: 1. to show that there is scope for Eskom System Operations to operate its system more efficiently in terms of system losses 2. to show that an OPF methodology is a practical technique to limit transformer tap movements and allow optimal VAr placement while minimising system losses

碩士
國立中正大學
電機工程研究所
99
In this study, the goal is to investigate the power quality problems that occur in a Taipower’s primary substation system (P/S), where the P/S feeds electric power to several steel-making customers. Recently, due to the operations of these EAF loads, it is observed that serious voltage fluctuations occurred at the bus that connected to the P/S transformers. The on-load tap changers (OLTCs) of the transformers have thus frequently switched on and off, which deteriorate the transformer life and also affect the supply power quality. This study analyzes the power quality impact of severely load variations on the feeder under the area where has several steel industry customers. It uses simulation techniques and accurate equipment models to test the simulation, and assess the effects of installing SVC to support the installation of SVC in the existing area. In order to improve the switching situation of OLTC, this study adopts the fuzzy control for OLTC and SVC coordination. And it explores the relation between the OLTC operation and the voltage flicker to clarify the result of OLTC malfunction. Finally, it assesses the power quality of a primary substation with large arc furnace loads.

碩士
國立成功大學
電機工程學系碩士在職專班
104
This thesis proposes alarm equipment for auxiliary system of on-load tap changer of transformers. The proposed system is capable of filtering the abnormal signal and delivering the alarming signal. This design is motivated because the occurrence of abnormal operation of on-load tap changer often results in the abnormal increment or decrement of voltage at the secondary side of transformers. Therefore, the thesis is devoted to designing the alarm equipment for auxiliary system of on-load tap changer based on the number of times of signal changes. From the test result, it was found that the method proposed in this thesis along with the developed system come with the functions of noise detection, noise filtering, and system forewarning. The outcome gained from these tests serve as beneficial references for electric power applications.

碩士
大同大學
電機工程研究所
91
Generally the on-load tap-changer transformers of Taiwan Power Company’s conventional power distribution system only detect and gather the low-voltage side single-phase voltage for reference values. Because the unbalance of distribution system will cause the different three phase voltage that will make the on-load tap-changer transformers inappropriate adjustment of the voltage. The thesis is using DSP collocated wavelet transformation filtering the noise, then calculated the positive voltage accurately as the reference voltage of three-phase voltage transformer to avoid the voltage at the end points of the consumer feeder is too high or too low. The thesis adopts Simulink release 13 to simulate a five bus distribution system. The results of simulation are the comparison between the distribution system voltages that the reference voltages of on-load tap-changer transformer is adopted the single phase voltage and the reference voltages of on-load tap-changer transformer is adopted the positive sequence voltage. According to the results of simulation, using the positive sequence voltage as the reference value of the on-load tap-changer transformer can avoid the inappropriate adjustment of the voltage that was used the single phase as the reference value and the lifetime of tap-changer increase because of the less position shift. Therefore using the positive sequence voltage as the reference value of the on-load tap-changer transformer is the better choice.

碩士
中華大學
電機工程學系碩士班
94
In the field of digital image processing, the JPEG-2000 standard uses the discrete wavelet transform for image compression; hence, the two-dimensional (2-D) forward discrete wavelet transform (FDWT)/ inverse DWT (IDWT) has recently been used as a powerful tool for image coding/decoding systems. 2-D FDWT/IDWT demands massive computations, hence, it requires a parallel and pipelined architecture to implement high-efficiency application-specific integrated circuits (ASIC) or field programmable gate array (FPGA). At the heart of the analysis stage of the system is the FDWT. In the synthesis stage, the IDWT recovers the original image from the coefficients of FDWT. This paper proposes two architectures with computing time for 2-D FDWT and IDWT. The first high-efficiency architecture comprises a transform module, an address generator, and a RAM module. The transform module has uniform and regular structure, simple control flow. The second architecture features parallel and pipelined computation and high throughput, both of the proposed architectures are 100% hardware-utilization and suitable for 2-D digital image processing, such as JPEG-2000. The proposed VLSI architectures are realized with Verilog HDL, synthesized by the Synopsys Design Compiler. Finally, the layouts for those designs are generated with the Synopsys Astro Tools in a 0.18 1P6M CMOS technology.

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.

The objective of an Energy Control Center (ECC) is to ensure secure and economic operation of power system. The challenge to optimize power system operation, while maintaining system security and quality of power supply to customers, is increasing. Growing demand without matching expansion of generation and transmission facilities and more tightly interconnected power systems contribute to the increased complexity of system operation. Rising costs due to inflation and increased environmental concerns has made transmission, as well as generation systems to be operated closure to design limits, with smaller safety margins and hence greater exposure to unsatisfactory operating conditions following a disturbance. Investigations of recent blackouts indicate that the root cause of most of these major power system disturbances is voltage collapse. Information gathered and preliminary analysis, from the most recent blackout incident in North America on 14th August 2003, is pointing the finger on voltage instability due to some unexpected contingency. In this incident, reports indicate that approximately 50 million people were affected interruption from continuous supply for more than 15 hours. Most of the incidents are related to heavily stressed system where large amounts of real and reactive power are transported over long transmission lines while appropriate real and reactive power resources are not available to maintain normal system conditions. Hence, the problem of voltage stability and voltage collapse has become a major concern in power system planning and operation. Reliable operation of large scale electric power networks requires that system voltages and currents stay within design limits. Operation beyond those limits can lead to equipment failures and blackouts. In the last few decades, the problem of reactive power control for improving economy and security of power system operation has received much attention. Generally, the load bus voltages can be maintained within their permissible limits by reallocating reactive power generations in the system. This can be achieved by adjusting transformer taps, generator voltages, and switchable Ar sources. In addition, the system losses can be minimized via redistribution of reactive power in the system. Therefore, the problem of the reactive power dispatch can be optimized to improve the voltage profile and minimize the system losses as well. The Instability in power system could be relieved or at least minimized with the help of most recent developed devices called Flexible AC Transmission System (FACTS) controllers. The use of Flexible AC Transmission System (FACTS) controllers in power transmission system have led to many applications of these controllers not only to improve the stability of the existing power network resources but also provide operating flexibility to the power system. In the past, transmission systems were owned by regulated, vertically integrated utility companies. They have been designed and operated so that conditions in close proximity to security boundaries are not frequently encountered. However, in the new open access environment, operating conditions tend to be much closer to security boundaries, as transmission use is increasing in sudden and unpredictable directions. Transmission unbundling, coupled with other regulatory requirements, has made new transmission facility construction more difficult. In fact, there are numerous technical challenges emerging from the new market structure. There is an acute need for research work in the new market structure, especially in the areas of voltage security, reactive power support and congestion management. In the last few decades more attention was paid to optimal reactive power dispatch. Since the problem of reactive power optimization is non-linear in nature, nonlinear programming methods have been used to solve it. These methods work quite well for small power systems but may develop convergence problems as system size increases. Linear programming techniques with iterative schemes are certainly the most promising tools for solving these types of problems. The thesis presents efficient algorithms with different objectives for reactive power optimization. The approach adopted is an iterative scheme with successive power-flow analysis using decoupled technique, formulation and solution of the linear-programmingproblem with only upper-bound limits on the state variables. Further the thesispresents critical analysis of the three following objectives, Viz., •Minimization of the sum of the squares of the voltage deviations (Vdesired) •Minimization of sum of the squares of the voltage stability L indices (Vstability) •Minimization of real power losses (Ploss) Voltage stability problems normally occur in heavily stressed systems. While the disturbance leading to voltage collapse may be initiated by a variety of causes, the underlying problem is an inherent weakness in the power system. The factors contributing to voltage collapse are the generator reactive power /voltage control limits, load characteristics, characteristics of reactive compensation devices, and the action of the voltage control devices such as transformer On Load Tap Changers (OLTCs). Power system experiences abnormal operating conditions following a disturbance, and subsequently a reduction in the EHV level voltages at load centers will be reflected on the distribution system. The OLTCs of distribution transformers would restore distribution voltages. With each tap change operation, the MW and MVAR loading on the EHV lines would increase, thereby causing great voltage drops in EHV levels and increasing the losses. As a result, with each tap changing operation, the reactive output of generators throughout the system would increase gradually and the generators may hit their reactive power capability limits, causing voltage instability problems. Thus, the operation of certain OLTCs has a significant influence on voltage instability under some operating conditions. These transformers can be made manual to avoid possible voltage instability due to their operation during heavy load conditions. Tap blocking, based on local measurement of high voltage side of load tap changers, is a common practice of power utilities to prevent voltage collapse. The great advantage of this method is that it can be easily implemented, but does not guarantee voltage stability. So a proper approach for identification of critical OLTC s based on voltage stability criteria is essential to guide the operator in ECC, which has been proposed in this thesis. It discusses the effect of OLTCs with different objectives of reactive power dispatch and proposes a technique to identify critical OLTCs based on voltage stability criteria. The fast development of power electronics based on new and powerful semiconductor devices has led to innovative technologies, such as High Voltage DC transmission (HVDC) and Flexible AC Transmission System (FACTS), which can be applied in transmission and distribution systems. The technical and economicalBenefits of these technologies represent an alternative to the application in AC systems. Deregulation in the power industry and opening of the market for delivery of cheaper energy to the customers is creating additional requirements for the operation of power systems. HVDC and FACTS offer major advantages in meeting these requirements. .A method for co-ordinated optimum allocation of reactive power in AC/DC power systems by including FACTS controller UPFC, with an objective of minimization of the sum of the squares of the voltage deviations of all the load buses has been proposed in this thesis. The study results show that under contingency conditions, the presence of FACTS controllers has considerable impact on over all system voltage stability and also on power loss minimization.minimization of the sum of the squares of the voltage deviations of all the load buses has been proposed in this thesis. The study results show that under contingency conditions, the presence of FACTS controllers has considerable impact on over all system voltage stability and also on power loss minimization. As power systems grow in their size and interconnections, their complexity increases. For secure operation and control of power systems under normal and contingency conditions, it is essential to provide solutions in real time to the operator in ECC. For real time control of power systems, the conventional algorithmic software available in ECC are found to be inadequate as they are computationally very intensive and not organized to guide the operator during contingency conditions. Artificial Intelligence (AI) techniques such as, Expert systems, Neural Networks, Fuzzy systems are emerging decision support system tools which give fast, though approximate, but acceptable right solutions in real time as they mostly use symbolic processing with a minimum number of numeric computations. The solution thus obtained can be used as a guide by the operator in ECC for power system control. Optimum real and reactive power dispatch play an important role in the day-to-day operation of power systems. Existing conventional Optimal Power Flow (OPF) methods use all of the controls in solving the optimization problem. The operators can not move so many control devices within a reasonable time. In this context an algorithm using fuzzy-expert approach has been proposed in this thesis to curtail the number of control actions, in order to realize real time objectives in voltage/reactive power control. The technique is formulated using membership functions of linguistic variables such as voltage deviations at all the load buses and the voltage deviation sensitivity to control variables. Voltage deviations and controlling variables are translated into fuzzy set notations to formulate the relation between voltage deviations and controlling ability of controlling devices. Control variables considered are switchable VAR compensators, OLTC transformers and generator excitations. A fuzzy rule based system is formed to select the critical controllers, their movement direction and step size. Results show that the proposed approach is effective for improving voltage security to acceptable levels with fewer numbers of controllers. So, under emergency conditions the operator need not move all the controllers to different settings and the solution obtained is fast with significant speedups. Hence, the proposed method has the potential to be integrated for on-line implementation in energy management systems to achieve the goals of secure power system operation. In a deregulated electricity market, it may not be always possible to dispatch all of the contracted power transactions due to congestion of the transmission corridors. System operators try to manage congestion, which otherwise increases the cost of the electricity and also threatens the system security and stability. An approach for alleviation of network over loads in the day-to-day operation of power systems under deregulated environment is presented in this thesis. The control used for overload alleviation is real power generation rescheduling based on Relative Electrical Distance (RED) concept. The method estimates the relative location of load nodes with respect to the generator nodes. The contribution of each generator for a particular over loaded line is first identified , then based on RED concept the desired proportions of generations for the desired overload relieving is obtained, so that the system will have minimum transmission losses and more stability margins with respect to voltage profiles, bus angles and better transmission tariff. The results obtained reveal that the proposed method is not only effective for overload relieving but also reduces the system power loss and improves the voltage stability margin. The presented concepts are better suited for finding the utilization of resources generation/load and network by various players involved in the day-to-day operation of the system under normal and contingency conditions. This will help in finding the contribution by various players involved in the congestion management and the deviations can be used for proper tariff purposes. Suitable computer programs have been developed based on the algorithms presented in various chapters and thoroughly tested. Studies have been carried out on various equivalent systems of practical real life Indian power networks and also on some standard IEEE systems under simulated conditions. Results obtained on a modified IEEE 30 bus system, IEEE 39 bus New England system and four Indian power networks of EHV 24 bus real life equivalent power network, an equivalent of 36 bus EHV Indian western grid, Uttar Pradesh 96 bus AC/DC system and 205 Bus real life interconnected grid system of Indian southern region are presented for illustration purposes.

The objective of an Energy Control Center (ECC) is to ensure secure and economic operation of power system. The challenge to optimize power system operation, while maintaining system security and quality of power supply to customers, is increasing. Growing demand without matching expansion of generation and transmission facilities and more tightly interconnected power systems contribute to the increased complexity of system operation. Rising costs due to inflation and increased environmental concerns has made transmission, as well as generation systems to be operated closure to design limits, with smaller safety margins and hence greater exposure to unsatisfactory operating conditions following a disturbance. Investigations of recent blackouts indicate that the root cause of most of these major power system disturbances is voltage collapse. Information gathered and preliminary analysis, from the most recent blackout incident in North America on 14th August 2003, is pointing the finger on voltage instability due to some unexpected contingency. In this incident, reports indicate that approximately 50 million people were affected interruption from continuous supply for more than 15 hours. Most of the incidents are related to heavily stressed system where large amounts of real and reactive power are transported over long transmission lines while appropriate real and reactive power resources are not available to maintain normal system conditions. Hence, the problem of voltage stability and voltage collapse has become a major concern in power system planning and operation. Reliable operation of large scale electric power networks requires that system voltages and currents stay within design limits. Operation beyond those limits can lead to equipment failures and blackouts. In the last few decades, the problem of reactive power control for improving economy and security of power system operation has received much attention. Generally, the load bus voltages can be maintained within their permissible limits by reallocating reactive power generations in the system. This can be achieved by adjusting transformer taps, generator voltages, and switchable Ar sources. In addition, the system losses can be minimized via redistribution of reactive power in the system. Therefore, the problem of the reactive power dispatch can be optimized to improve the voltage profile and minimize the system losses as well. The Instability in power system could be relieved or at least minimized with the help of most recent developed devices called Flexible AC Transmission System (FACTS) controllers. The use of Flexible AC Transmission System (FACTS) controllers in power transmission system have led to many applications of these controllers not only to improve the stability of the existing power network resources but also provide operating flexibility to the power system. In the past, transmission systems were owned by regulated, vertically integrated utility companies. They have been designed and operated so that conditions in close proximity to security boundaries are not frequently encountered. However, in the new open access environment, operating conditions tend to be much closer to security boundaries, as transmission use is increasing in sudden and unpredictable directions. Transmission unbundling, coupled with other regulatory requirements, has made new transmission facility construction more difficult. In fact, there are numerous technical challenges emerging from the new market structure. There is an acute need for research work in the new market structure, especially in the areas of voltage security, reactive power support and congestion management. In the last few decades more attention was paid to optimal reactive power dispatch. Since the problem of reactive power optimization is non-linear in nature, nonlinear programming methods have been used to solve it. These methods work quite well for small power systems but may develop convergence problems as system size increases. Linear programming techniques with iterative schemes are certainly the most promising tools for solving these types of problems. The thesis presents efficient algorithms with different objectives for reactive power optimization. The approach adopted is an iterative scheme with successive power-flow analysis using decoupled technique, formulation and solution of the linear-programmingproblem with only upper-bound limits on the state variables. Further the thesispresents critical analysis of the three following objectives, Viz., •Minimization of the sum of the squares of the voltage deviations (Vdesired) •Minimization of sum of the squares of the voltage stability L indices (Vstability) •Minimization of real power losses (Ploss) Voltage stability problems normally occur in heavily stressed systems. While the disturbance leading to voltage collapse may be initiated by a variety of causes, the underlying problem is an inherent weakness in the power system. The factors contributing to voltage collapse are the generator reactive power /voltage control limits, load characteristics, characteristics of reactive compensation devices, and the action of the voltage control devices such as transformer On Load Tap Changers (OLTCs). Power system experiences abnormal operating conditions following a disturbance, and subsequently a reduction in the EHV level voltages at load centers will be reflected on the distribution system. The OLTCs of distribution transformers would restore distribution voltages. With each tap change operation, the MW and MVAR loading on the EHV lines would increase, thereby causing great voltage drops in EHV levels and increasing the losses. As a result, with each tap changing operation, the reactive output of generators throughout the system would increase gradually and the generators may hit their reactive power capability limits, causing voltage instability problems. Thus, the operation of certain OLTCs has a significant influence on voltage instability under some operating conditions. These transformers can be made manual to avoid possible voltage instability due to their operation during heavy load conditions. Tap blocking, based on local measurement of high voltage side of load tap changers, is a common practice of power utilities to prevent voltage collapse. The great advantage of this method is that it can be easily implemented, but does not guarantee voltage stability. So a proper approach for identification of critical OLTC s based on voltage stability criteria is essential to guide the operator in ECC, which has been proposed in this thesis. It discusses the effect of OLTCs with different objectives of reactive power dispatch and proposes a technique to identify critical OLTCs based on voltage stability criteria. The fast development of power electronics based on new and powerful semiconductor devices has led to innovative technologies, such as High Voltage DC transmission (HVDC) and Flexible AC Transmission System (FACTS), which can be applied in transmission and distribution systems. The technical and economicalBenefits of these technologies represent an alternative to the application in AC systems. Deregulation in the power industry and opening of the market for delivery of cheaper energy to the customers is creating additional requirements for the operation of power systems. HVDC and FACTS offer major advantages in meeting these requirements. .A method for co-ordinated optimum allocation of reactive power in AC/DC power systems by including FACTS controller UPFC, with an objective of minimization of the sum of the squares of the voltage deviations of all the load buses has been proposed in this thesis. The study results show that under contingency conditions, the presence of FACTS controllers has considerable impact on over all system voltage stability and also on power loss minimization.minimization of the sum of the squares of the voltage deviations of all the load buses has been proposed in this thesis. The study results show that under contingency conditions, the presence of FACTS controllers has considerable impact on over all system voltage stability and also on power loss minimization. As power systems grow in their size and interconnections, their complexity increases. For secure operation and control of power systems under normal and contingency conditions, it is essential to provide solutions in real time to the operator in ECC. For real time control of power systems, the conventional algorithmic software available in ECC are found to be inadequate as they are computationally very intensive and not organized to guide the operator during contingency conditions. Artificial Intelligence (AI) techniques such as, Expert systems, Neural Networks, Fuzzy systems are emerging decision support system tools which give fast, though approximate, but acceptable right solutions in real time as they mostly use symbolic processing with a minimum number of numeric computations. The solution thus obtained can be used as a guide by the operator in ECC for power system control. Optimum real and reactive power dispatch play an important role in the day-to-day operation of power systems. Existing conventional Optimal Power Flow (OPF) methods use all of the controls in solving the optimization problem. The operators can not move so many control devices within a reasonable time. In this context an algorithm using fuzzy-expert approach has been proposed in this thesis to curtail the number of control actions, in order to realize real time objectives in voltage/reactive power control. The technique is formulated using membership functions of linguistic variables such as voltage deviations at all the load buses and the voltage deviation sensitivity to control variables. Voltage deviations and controlling variables are translated into fuzzy set notations to formulate the relation between voltage deviations and controlling ability of controlling devices. Control variables considered are switchable VAR compensators, OLTC transformers and generator excitations. A fuzzy rule based system is formed to select the critical controllers, their movement direction and step size. Results show that the proposed approach is effective for improving voltage security to acceptable levels with fewer numbers of controllers. So, under emergency conditions the operator need not move all the controllers to different settings and the solution obtained is fast with significant speedups. Hence, the proposed method has the potential to be integrated for on-line implementation in energy management systems to achieve the goals of secure power system operation. In a deregulated electricity market, it may not be always possible to dispatch all of the contracted power transactions due to congestion of the transmission corridors. System operators try to manage congestion, which otherwise increases the cost of the electricity and also threatens the system security and stability. An approach for alleviation of network over loads in the day-to-day operation of power systems under deregulated environment is presented in this thesis. The control used for overload alleviation is real power generation rescheduling based on Relative Electrical Distance (RED) concept. The method estimates the relative location of load nodes with respect to the generator nodes. The contribution of each generator for a particular over loaded line is first identified , then based on RED concept the desired proportions of generations for the desired overload relieving is obtained, so that the system will have minimum transmission losses and more stability margins with respect to voltage profiles, bus angles and better transmission tariff. The results obtained reveal that the proposed method is not only effective for overload relieving but also reduces the system power loss and improves the voltage stability margin. The presented concepts are better suited for finding the utilization of resources generation/load and network by various players involved in the day-to-day operation of the system under normal and contingency conditions. This will help in finding the contribution by various players involved in the congestion management and the deviations can be used for proper tariff purposes. Suitable computer programs have been developed based on the algorithms presented in various chapters and thoroughly tested. Studies have been carried out on various equivalent systems of practical real life Indian power networks and also on some standard IEEE systems under simulated conditions. Results obtained on a modified IEEE 30 bus system, IEEE 39 bus New England system and four Indian power networks of EHV 24 bus real life equivalent power network, an equivalent of 36 bus EHV Indian western grid, Uttar Pradesh 96 bus AC/DC system and 205 Bus real life interconnected grid system of Indian southern region are presented for illustration purposes.