Dissertations / Theses on the topic 'Transformer voltage control'

The research programme at the University of Canterbury includes the development and applications of partial core inductors and transformers for high voltage testing of generator insulation. Unlike a conventional full core transformer, a partial core transformer has no limbs and yokes. A partial core transformer is a compromise between a full core and coreless transformer. It is superior to its full core counterpart as far as cost, weight and ease of transportation are concerned. Partial core transformers have a low magnetising reactance and hence draw a high magnetising current. This characteristic makes them a perfect fit in applications where the load is capacitive in nature, such as a.c. power frequency high voltage testing of generator insulation and cable testing etc. The work carried out for this thesis focuses on automatically controlling the amount of reactive power on the supply side of a partial core transformer. The considered design includes a third winding around the existing two windings. A power electronic controller is connected to the third winding, which modifies the VAr absorption characteristics of the magnetically coupled supply winding. Two options are considered to achieve continuous reactive power control in the partial core transformer as explained below. First, a thyristor controlled reactor (TCR) is proposed as the VAr controller. It is modelled using PSCAD/EMTDC software. Simulations reveal the design criteria, overall performance and the limitations of the suggested proposal. The TCR connected tertiary winding takes the capacitive burden of the supply. The model demonstrates the ability of the automatically controlled TCR to provide a continuous variation of reactive power without significant under or over compensation. This feature limits the supply current to its real component only, so the supply provides only the losses of the system. Second, a voltage source converter is considered as the VAr controller. This is modelled in PSCAD/EMTDC and a hardware prototype is designed and built. Based on the analysis, the control algorithm (including a digital PI controller) is implemented using an 8 bit micro-controller, PIC18LF4680. The prototype is tested in the laboratory for both active and inductive load conditions as seen from the supply side. Performance of the hardware prototype is discussed in detail. The PSCAD/EMTDC model and the hardware prototype successfully demonstrate the feasibility of a STATCOM controlled partial core transformer. The proposed system is capable of compensating a wide range of capacitive loads as compared with its TCR counterpart. It is proved that the system is very robust and remains dynamically stable for a large system disturbance such as change in load from full capacitive to inductive and vice versa. This confirms that the system is capable of providing continuous VAr control.

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.

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.

This work focuses on the safe and stable operation of an autonomous power system interconnecting an AC source with various types of power electronic loads. The stability of these systems is a challenge due to the inherent nonlinearity of the circuits involved. Traditionally, the stability analysis in this type of power systems has been approached by means of small-signal methodology derived from the Nyquist stability criterion. The small-signal analysis combined with physical insight and the adoption of safety margins is sufficient, in many cases, to achieve a stable operation with an acceptable system performance. Nonetheless, in many cases, the margins adopted result in conservative measures and consequent system over designs. This work studies the system stability under large-perturbations by means of three different tools, namely parameter space mapping, energy functions, and time domain simulations. The developed parameters space mapping determines the region of the state and parameter space where the system operation is locally stable. In this way stability margins in terms of physical parameters can be established. Moreover, the boundaries of the identified stability region represent bifurcations of the system where typical nonlinear behavior appears. The second approach, based on the Lyapunov direct method, attempts to determine the region of attraction of an equilibrium point, defined by an operation condition. For this a Lyapunov function based on linear matrix inequalities was constructed and tested on a simplified autonomous system model. In Addition, the third approach simulates the system behavior on a computer using a detailed system model. The higher level of model detail allows identifying unstable behavior difficult to observe when simpler models are used. Because the stability of the autonomous power system is strongly associated with the characteristics of the energy source, an improved voltage controller for the generator is also presented. The generator of an autonomous power system must provide a good performance under a wide variety of regimes. Under these conditions a model based controller is a good solution because it naturally adapts to the changing requirements. To this extent a controller based on the model of a variable frequency synchronous generator has been developed and tested. The results obtained show a considerable improvement performance when compared to previous practices.
Ph. D.

A repetitive control method for output voltage control of a three phase uninterruptible power supply (UPS) with isolation transformer is investigated. In the method voltage control loop is employed in the stationary dq frame. The controller eliminates the periodic errors on the output voltages due to inverter voltage nonlinearity and load disturbances. The controller design and implementation details are given. The controller is implemented on a 5-kVA UPS prototype which is constructed in laboratory. Linear and nonlinear loads for balanced and unbalanced load operating conditions are considered. The steady-state and dynamic performance of the control method are investigated in detail. The theory of the control strategy is verified by means of simulations and experiments.

The future substation depends on finding a way to mitigate the effects of the drawbacks of the conventional legacy by employing the efficiency of the solid state switches in light of changing the loading features by time such as Electrical Vehicles (EV) and Photo-voltaic (PV) cells. In distribution transformers the ratio between the primary voltage and the secondary voltage cannot be changed, and the use of the on-load taps changers are limited. Poor voltage regulation and reactive power transmission is a direct reason for losses and shortening the life of several devices. This research discusses the considerations of applying Power Electronics (PE) approaches and designs that provide additional functions in regulating the voltage and controlling the reactive power that is injected in the distribution network, using embedded fractional rated converters attached partially with the windings of the LV transformer. These approaches studies the possible considerations that have the potentials to enhance the unit with more flexibility in controlling the voltage and reactive power at the last mile of the network, in order to decrease the losses and meet the future expectations for low voltage networks modifications, and that by using a Power Electronic (PE) approach has less losses and more functionality depending on the reliability of transformer and intelligence of PE solutions. The approach of a hybrid distribution transformer is introduced and its functionality in regulating the voltage and injecting reactive power is illustrated. A back-to-back converter is controlled according to the immediate need for voltage control and reactive power in Low Voltage (LV) networks, and for the purpose of controlling three unbalanced phases using two control strategies; resonant controller and vector control. The overall controller adds or decreases voltage (10%-20%) to/from the total output voltage in order to control the whole output voltage of the transformer. In addition, some loads need high amount of reactive power at last mile of the network, therefore the consideration of using switched capacitors technique is introduced to serve at the end user side whereby its ability to provide automatic variable reactive power compensation in a closed loop system is illustrated. The considerations results indicate significant potentials for deploying PE in the last mile of the network by using innovative designs and suitable control functions with less losses and costs.

Series Active Filters (SAF) are designed for harmonic isolation and load voltage regulation of single-phase and three-phase voltage harmonic source type nonlinear loads. The novel Absolute Value Method (AVM) for load voltage harmonic extraction is proposed and applied in the control algorithm of SAF. The SAF compensated systems are represented by simplified linear models such that SAF controller gains can be easily determined. Harmonic isolation and load voltage regulation performances of 2.5 kW single-phase and 10 kW three-phase SAF compensated systems are evaluated by detailed simulations. Laboratory prototype single-phase and three-phase SAFs and loads are designed and manufactured. Digital signal processor based control platform is employed. Exclusive laboratory tests are conducted. Via laboratory experiments and simulations it is shown that AVM yields superior harmonic isolation and load voltage regulation performance compared to the conventional low/high pass filtering method. Theory, simulations, and experiments are well correlated and illustrate the feasibility of the proposed method.

This diploma thesis discusses two major topics. The first one is the control size of the voltage in LV networks in regard to the increase in distributed generation concered to renewable energy sources. The study contains a review focused on the current state of low voltagegrid followed by a proposal for the solution of the oncoming state. The solution is identified as a deploying OLTC distribution transformer. In the case of more complex topology is deployed a series voltage transformer. Both methods are part of the Smart Grid. The thesis also analyzes the issue of the data measurement and data transmission. The second part of the thesis consists of the description of selected control strategies and their simulations. The design of individual system elements in the PSCAD is described. From these elements, a test network was constructed and tested the individual simulation scenarios.

In this thesis a two-stage AC/DC/DC power converter is designed and implemented. The AC/DC input stage of the converter consists of the two&
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phase interleaved boost topology employing the average current mode control principle. The output stage consists of a zero voltage switching phase shifted full bridge (ZVS&
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PS&
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FB) DC/DC converter. For the input stage, main design goals are obtaining high input power factor, low input current distortion, and well regulated output dc voltage, and obtaining these attributes in a power converter with high power density. For the input stage, the interleaved structure has been chosen in order to obtain reduced line current ripple and EMI, reduced power component stresses, and improved power density. The control of the pre&
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regulator is provided by utilizing a new commercial monolithic integrated circuit, which provides interleaved continuous conduction mode power factor correction (PFC). The output stage is formed by utilizing the available prototype hardware of a ZVS&
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PS&
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FB DC/DC converter and mainly the system integration and controller design and implementation studies have been conducted. The converter small signal model is derived and utilizing its transfer function and employing voltage loop control, the output voltage regulator has been designed. The output voltage controller is implemented utilizing a digital signal processor (DSP). Integrating the AC/DC preregulator and DC/DC converter, a laboratory AC/DC/DC converter system with high overall performance has been obtained. The overall system performance has been verified via computer simulations and experimental results obtained from laboratory prototype.

A measurement tool has been developed to estimate errors in relaying current transformers and voltage transformers. The tool has been developed to collect data in a substation and send it to a remote location over a telephone line. Different schemes were evaluated and tested in the laboratory. The final choice was made on the basis of the hardware and transmission cost constraints. The measurement unit was developed using hardware similar to that used in a computer relay. The signals from the current and voltage transducers were sampled using a microprocessor and an analog to digital converter in real-time. The measurement device has been installed in the field. The data from the field was collected remotely and analyzed in the Virginia Tech Power Systems laboratory. The analysis of the data is presented at the end.
Master of Science

Orientador: Júlio Borges de Souza
Resumo: O transformador de estado sólido tem se apresentado como uma ferramenta indispensável na construção das novas redes elétricas inteligentes, uma vez que essa nova estrutura de rede altera o layout tradicional, viabilizando a conexão de fontes de energia descentralizadas. Contudo, essa conexão de sistemas de geração distribuída na rede originou a bidirecionalidade do fluxo de potência, resultando em um novo panorama para as atividades de operação e manutenção das redes para as distribuidoras de energia. A análise dos possíveis impactos técnicos gerados na rede de distribuição deve ser realizada, com o intuito de garantir um nível de qualidade energética dentro dos padrões estabelecidos pela Agência Nacional de Energia Elétrica – ANEEL. Dentre os impactos, destacam-se a elevação do nível de tensão e a alteração do fator de potência, ambos, produzidos pelo excesso de potência injetada na rede por este novo cenário de geração de energia. Nesse contexto, esta dissertação tem como principal objetivo analisar o comportamento de uma rede de distribuição genérica com penetração de geração distribuída e avaliar o perfil de tensão diante de diferentes níveis de inserção dessa geração na rede. As características de ajuste de tensão instantânea e capacidade de interação com sistemas de armazenamento que o transformador de estado sólido possui foram empregadas no auxílio da regulação dos níveis de tensão que se apresentaram fora dos padrões determinados pela resolução 794/2018 da ANEEL, qua... (Resumo completo, clicar acesso eletrônico abaixo)
Abstract: The solid state transformer has been presented as an essential tool in the construction of new intelligent electric grids, since this new network structure changes the traditional layout, making possible the connection of decentralized energy sources. However, this connection of distributed generation systems in the network created the bidirectionality of the power flow, resulting in a new panorama for the power distributors concerning the networks activities of operation and maintenance. The analysis of the possible technical impacts generated in the distribution network should be carried out with the purpose of guaranteeing a level of energy quality within the standards established by the National Electric Energy Agency (ANEEL). Among the impacts, we highlight the elevation of the voltage level and the change in the power factor, both produced by the excess power injected into the grid by this new energy generation scenario. In this context, the main objective of this dissertation is to analyze the behavior of a generic distribution network with distributed generation penetration and to evaluate the voltage profile before different levels of insertion of this generation in the network. The solid state transformer has instantaneous voltage adjustment characteristics and interaction capacity with storage systems, which were used to aid in the regulation of voltage levels that were out of the standards determined by ANEEL resolution 794/2018, when a high level of distributed g... (Complete abstract click electronic access below)
Mestre

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.

The aim of this master’s thesis is provide detailed information about car battery’s charger with a switching power supply. The kind of this power supply is one-pulsed conducting inverter with transformer. The charger’s rates of nominal voltage are 6 and 12 V. The method of charging is charging by constantly voltage with a current limitation. The values of a current limitations are 0.5A, 5A and 20A. The charger’s maximal current is 50A. This current can help to the accumulator with starting of a vehicle. This device is equipped by indicators which indicate the end of charging.

The diploma thesis is describing how forward converter works. The diploma thesis presents the function of forward converter with demagnetizing winding and presents the function of two-switched forward converter. The diploma thesis descibes the behaviour of continuous current mode and discontinuous current mode. The diploma thesis explains the reasons for implementation feedback and presents the basic types of compensations. The project deals with AC analysis of two-switched forward converter with continuous peak current mode control. The Analog prototyping metod is used for digital control design. The function of the converter was tested in laboratory. The laboratory results have been compared with the theoretical and the simulation results.

The aim of this work is optimalisation of induction furnace in foundry for company PROMET FOUNDRY a.s. The company has two induction furnances. There are installed as identical construction. They have 2 modes of operation. First mode is founding and second mode is mode, where is temperature in maintain mode. Only one induction furnance can work in the founding mode at a time though. Inducion furnances are in the single-phase connection and they cause unbalance in the distribution network. Near the foundry there is a small network area whitch it is operated by company Zásobování teplem Vsetín a.s. The consumption of electrical energy in foundry so big, that in the year 2009 was made elaborate for Zásobování teplem Vsetín a.s. It was write at Laboratoře diagnostiky výkonů (Laboratory of performance diagnostics), which is a part of Electrotechnic Department at Technical Univarsity of Ostrava. The ordered study was named “Verification of causes of increased reactive energy consumption during transition from electricity delivery to electricity consumption”. The conclusion of this assignment confirms that in distribution network in the Jiráskova area in Vsetín there is unbalance of electrical energy and there is high part of reactive power. The next conclusion is crucial to find the customer who made the unbalance and to set relevant remedy. The last step will be the identification whether such device can actually be effectively balanced. It was subsequently proved that the Promet Foundry was causing the unbalance and that balanced consumption would be reasonable. Promet Foundry thus addressed Autel a.s. company with an inquiry to make a study of removing the causes of the unbalance which is caused by current induction furnaces operation at a minimum possible cost, least possible influence on the performance and minimum construction changes concerning the building. In this thesis there will be some topics. The result of which will be introducing of used heating technology, introducing of company and of effective plant performance and subsequent suggestions of possible unbalance removal or reactive power decrease. Several ways which are being implemented in the industry in order to balance consumption will be described. A suitable balancing plant will be subsequently chosen and its parameters will be calculated.

The thesis discuses an integration of photovoltaic power stations to electric network. The first part describes connecting conditions of small sources to distribution system, including administrative requirements, feasibility study, and requirements to the energy meters, measuring, control devices, switching devices and protection. The second part is aimed to describe problems of the photovoltaic system. Solar radiation generating and reducing of its intensity incident upon the earth surface are described in this part. The quantum of produced electric power depends on climatic conditions in the fixed area, seasons, etc. This work also discusses the types of photovoltaic cells and their actual efficiency. Inverters are further important components of the photovoltaic system. The parameters of the inverters have a great influence on the total actual efficiency of the photovoltaic system. Different methods of the photovoltaic panels’ connection with the inverters and their advantages and disadvantages are also mentioned. The supporting structure of the photovoltaic panels and eventually transformer are further important components of photovoltaic system. The work also analyze the methods of connection of the photovoltaic power station to distributive low voltage and medium voltage network, electric energy accumulation and possibilities of the sale of produced electric energy. The large number of the connected photovoltaic power stations has negative influences to electric network. The third part contains the design of a photovoltaic power plant with a capacity of 516,24 kWp on the scoped area in southern Bohemia. The project documentation for the location where the power plant is designed is also made. It contains the design of photovoltaic panels, the design of the inverters to get an optimal power load. This part also contains a calculation of the photovoltaic system losses and the design of transformer and the cable junction calculation of the distributive system. The feasibility study of the power plant connected to distributive system is also conducted. Its delivery rate will be connected to the distribution point Řípov (110/22 kV). The calculation results show us that this photovoltaic power plant can be linked to the distribution system. The final part of this paper contains an economic estimate of the photovoltaic power plant operating and the calculation of the return. An Economic return is influenced by the wide range of values that affect the total return rate. The calculation of an operating economy is made for several variants. The return rate in refer to contemporary redemption price for 2010 with no consideration for a bank loan is 7 years. If we consider the bank loan it would be 12 years. The penetrative reduction of the redemption price is expected for 2011. Calculation works with the decline of 30 %. It would extend the rate of return to 11 years without a bank loan or to 22 years with the bank loan. The bank loan is considered to cover 80 % of the investment.

Wound Field Synchronous Generators provide more than 95% of the electricity need worldwide. Their primacy in electricity production is due to ease of voltage regulation, performed by simply adjusting the direct current intensity in their rotor winding. Nevertheless, the rapid progress of power electronics devices enables new possibilities for alternating current add-ins in a more than a century long DC dominated technology. Damping the rotor oscillations with less energy loss than before, reducing the wear of the bearings by actively compensating for the mechanic unbalance of the rotating parts, speeding up the generator with no need for additional means, these are just few of the new applications which imply partial or total alternated current supplying of the rotor winding. This thesis explores what happens in a winding traditionally designed for the direct current supply when an alternated current is injected into it by an inverter. The research focuses on wound field salient pole synchronous machines and investigates the changes in the field winding parameters under AC conditions. Particular attention is dedicated to the potentially harmful voltage surges and voltage gradients triggered by voltage-edges with large slew rate. For this study a wide frequency band simplified electromagnetic model of the field winding has been carried out, experimentally determined and validated. Within the specific application of the fast field current control, the research provides some references for the design of the rotor magnetic circuit and of the field winding. Finally the coordination between the power electronics and the field winding properties is addressed, when the current control is done by means of a long cable or busbars, in order to prevent or reduce the ringing.

Les dispositifs DBD se répandent dans un grand nombre d’applications industrielles. Utilisés depuis plus de 150 ans pour la production d’ozone afin de décontaminer l’eau à grande échelle, ils ont depuis la fin du XXème siècle investi les domaines du traitement de surface polymère, du dépôt de couche mince sur substrat et de l’émission lumineuse pour la décontamination ainsi que la médecine. Ces dispositifs sont mis en oeuvre avec un générateur électrique dont les caractéristiques impactent fortement la qualité de la décharge. Ce travail s’inscrit en partie dans le cadre du développement d’une application de traitement de surface à pression atmosphérique. Il aborde la problématique de l’augmentation de la vitesse de dépôt de couche mince au travers des paramètres de l’alimentation électrique. Plus précisément, ce travail s’intéresse aux apports d’une alimentation en courant rectangulaire et aborde également les problématiques liées à la conception et à la fabrication de ce convertisseur. En particulier, une grande attention est portée sur l’étude du transformateur élévateur, car au travers de ses éléments parasites capacitifs, ce dernier peut limiter le transfert de puissance entre la source électrique et le dispositif DBD. Un deuxième aspect de cette étude consiste à entrevoir l’intérêt que revêtent deux convertisseurs statiques dédiés à l’alimentation de dispositifs DBD. Le premier consiste en une alimentation résonante en régime de conduction discontinue dont la particularité est de posséder trois degrés de liberté (fréquence, tension d’entrée et largeur d’impulsion), ce qui lui confère un intérêt exploratoire. Le second convertisseur consiste en une alimentation résonante haute tension et haute fréquence permettant l’éviction du transformateur élévateur, et mettant en oeuvre des interrupteurs au nitrure de gallium (GaN) afin d’atteindre une fréquence de fonctionnement supérieure au mega-Hertz avec un faible niveau de pertes.

This Diploma work is concerned with the design of project documentation for discharging shakeout machine electric installation. This work continues in the Semestral project 2 in which the background of a new machine was made. Then, this work deals with the replacement of a switch room with required devices including the design of a new connecting cables and power streams. In the first part, the work deals with the control part of the device and the design of control and signal cables including the product documentation of the device. In the second part, the work deals with the proposal of building electro installation in which a new lighting and its use is described. The last part focuses on the coordination during the realization of a suggested device. In this part, all problems coming from the particular working procedures including a proposal of possible solutions are described. According to the project documentation and following realization, the documentation of final concept reflecting the real status of the device is described.

碩士
國立交通大學
電子工程系所
97
Design of low-voltage, low-power CMOS Voltage-Controlled Oscillator(VCO) will be investigate in this thesis. We will discuss how to use transformer feedback to enhance transconductance to reduce power consumption and achieve high output swing at the same time. After using forward body bias, we improve the structure further. The differential VCO is implemented using TSMC 0.18um process. The EM simulation results with ADS momentum. A designed 10GHz VCO achieves a turning range of 9.5GHz, and phase noise about -112dBc at 1MHz offset frequency with power consumption 1.3 mW.

碩士
國立臺灣科技大學
電機工程系
105
This thesis investigates the voltage control method and strategy of the main transformer feeding area in a secondary substation with a large amount of solar power. Considering the voltage variation caused by loads and incorporation of solar energy, a representative example system is established based on the structure of the main transformer feeding area in secondary substation in simulation and analysis. If the existing voltage control methods cannot solve the voltage variation problem, the feeder voltage regulator (FVR) is considered to install to improve furthermore. Finally, it is to integrate four devices, which are on-load tap changer (OLTC), shunt capacitor (SC) in the substation, feeder voltage regulator, and smart inverter (SI), to propose the overall voltage control strategies and discuss the coordination among these devices. In this study, the MATLAB/Simulink is used to establish the distribution system model to analyze the voltage control and equipment action in the situation of light, medium, and heavy system loads and different solar power generation. The situations of summer residential and industrial hourly load and solar power are under consideration as well. At last, it is to use the MATLAB/GUI to develop the user-friendly voltage control interface. The overall voltage control strategy can be set to operate in automatic or manual mode, which is expected to provide recommendations for the operating personnel.

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.

This research work deals with the application of the phase shifting transformer in improving the steady state performance and voltage stability of transmission network that has transmission lines at different voltage levels running in parallel to each other. Transmission power system networks are usually developed using lines built at a certain voltage level initially. As power demand requirements increase, building of the new lines at the same voltage level becomes necessary. However, lesser and lesser improvements in transfer capacity are realised when the additional lines are built. This prompts utilities to consider higher voltages for future lines as these have a higher transfer capacity. Utilities usually lay, i.e., they build in parallel, newer, higher voltage transmission lines along side the existing lower voltage ones. Power flow in power system is mainly influenced by impedances of equipment. If the combined impedance of the existing, lower voltage transmission system is relatively less than the impedance of the newer, higher voltage ones, power may primarily flow through it rather than via the newer, parallel higher voltage transmission network. This may lead to a serious underutilisation of the newer infrastructure with a higher transmission capacity. Transmission networks similar to the one described above are common throughout the world. This study was undertaken towards finding solutions to the problem of under utilisation of such transmission lines. The study was performed by first reviewing the literature on the use of phase shifting transformers to redirect power flow in transmission networks throughout the world. This was followed by analysis of the theory on how and what determines the power flow in power networks. Several simulations of varying the phase of the phase shifting transformer were performed on the Cape network, as a case study, to investigate the impact on the power flow distribution and voltage stability performance of the 765 kV and 400 kV transmission lines carrying power to the Western Cape. In this dissertation, it has been demonstrated that a phase shifting transformer can be used to alter the power flow patterns so that power flows are restructured or redistributed, such that power which originally flowed via the low impedance, lower voltage system is transferred to the parallel higher voltage transmission system of lines. It is shown that once the power flows are redistributed, steady state and voltage stability performance of the total system can be enhanced and an increase in its power transfer capacity can be realised.
Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2011.

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.