The Eskom Transmission Network experiences an exceptionally high number of line faults, the cause of which, may not be correctly identified. This thesis analyses a number of all the possible factors responsible for causing these faults. The objective is to assign probable causes of these faults and that the correct preventative or corrective measures may be planned. The percentage of unexplained line faults is estimated to be 35 % of the total system faults. It is important for the Transmission Group of Eskom to minimise the number of faults. Major efforts to minimise identified faults such as bird streamers, veld fires, sugar cane fIfes, lightning and a hypothesised light pollution, light wetting mechanism has been undertaken by the transmission grid authority. This thesis presents an analysis of the statistical data of the unknown faults (unknown faults is defined as lines that trip due to a reason which could not be identified) that has been undertaken. This analysis takes into account a number of categories of causes of line faults. The period, for which the performance of the lines was analysed for was the years inclusive of 1993 to 1997. The investigation has focused on the identification of the under-performing lines of the main Transmission Network. The identified poorly performing lines have been compared with each other from the perspective ofthe following variables: • Region • Voltage (System Voltage) • Climatic Data Line faults - Time ofDay analysis • Line Faults - Time of Year analysis. The analysis indicates that the majority of unexplained flashovers occur between 22 :00 and 07:00 the following morning (Britten et al, 1999). Almost all of the under performing lines in South Africa fall in the sub-tropical/humid climatic area. All the lines studied are insulated with standard glass disc insulators. The analysis indicated that most of the unexplained line faults occur during the months when the seasons change, e.g. from autumn to winter. The analysis further indicates that most unexplained line faults occurred during the months of April to May and August to September. Of note is that during the period of this investigation bird guarding was performed on some lines. Installing bird guards may reduce those line faults that are caused by bird streamers. However, the bird pollution (deposited on glass disc insulators) that is not washed off at the same time as the bird guard installation may cause the line to trip due to the combination of the pollution and wetting resulting in a pollution type flashover. This is a possible cause of some unexplained line faults that occur from April to May. Bird streamers are also identified as the most probable cause of the unexplained faults which occur during the late evening periods (22:00 - 00:00). Pollution (with wetting) during the early morning periods may result in faults for the period 00:00 to 02:00. Line faults in the early morning periods (04:00 - 7:00) could be due to bird streamers or pollution and wetting, depending on the time of year in which the faults occurs. / Thesis (M.Sc.)-University of Natal, Durban, 2003.

The purpose of the proposed research is to design a Decentralized Voltage/Stability Monitoring and Control System to counteract voltage violations and the impact of disturbances/contingencies on power system voltage stability. A decentralized voltage and stability control system is designed to coordinate the controls of the local secondary voltage control devices and necessary load shedding without requiring information about the rest of the system. The voltage/stability control can be formulated as a multi-objective optimization problem. The control objectives include, but are not limited to: minimization of system active/reactive losses; maximization of the system stability margin; and minimization of the control actions. The constraints of the optimization problem depend on the specifications of the actual system components. For the first time, margin sensitivities of the control actions are included in the control formulation. The concept of using margin sensitivity to evaluate the post-control load margin is presented as a fast and accurate way to assess potential voltage and stability control options. A system decomposition procedure is designed to define the disturbance-affected zone as an independent control subsystem. A normal constraint algorithm is adopted to identify the most suitable control solution in a shorter timeline than the typical utility voltage-control practice. Both steady-state and dynamic simulations are performed to compare the proposed system with typical utility control practices.

Thesis (M.S.)--Mississippi State University. Department of Electrical and Computer Engineering. / Title from title screen. Includes bibliographical references.

Thesis (M.S.) -- Mississippi State University. Department of Electrical and Computer Engineering. / Title from title screen. Includes bibliographical references.

Orientador : Vivaldo Fernando da Costa / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação / Made available in DSpace on 2018-08-04T02:42:32Z (GMT). No. of bitstreams: 1 Domingues_AdrianaFavaro_D.pdf: 789245 bytes, checksum: 9ca2b124a01cd871aa7f4bbb4c63c57a (MD5) Previous issue date: 2005 / Resumo: Este trabalho aborda o problema das oscilações eletromecânicas de baixa freqüência de modo interárea em sistemas de energia elétrica através de duas metodologias: primeiramente, através do método convencional da análise modal linear e, posteriormente, através da aplicação do método da forma normal dos campos vetoriais como ferramenta para viabilizar o acréscimo dos termos não lineares de segunda ordem resultantes da expansão em série de Taylor. Neste caso, o método da forma normal é aplicado a um Modelo de Sensibilidade de Potência com representação de segunda ordem, para investigar os efeitos das interações não lineares entre os modos naturais de oscilação de sistemas de energia elétrica. São consideradas, em ambas as análises, a inclusão de dispositivos FACTS e da modelagem dinâmica das cargas. As simulações são realizadas para um sistema simétrico de duas áreas e para o Sistema Equivalente Sul-Sudeste Brasileiro. A metodologia de análise proposta mostra-se bastante satisfatória como alternativa à simulação não linear no domínio do tempo e à análise modal convencional / Abstract: In this work, the analysis of power systems inter-area mode oscillations is performed by the application of two different methodologies: first, the linear modal analysis, and then the analysis including second order nonlinear terms from a Taylor series expansion, with the application of the method of normal forms of vector fields. In this case, the method of normal forms is applied to a Power Sensitivity Model including second order nonlinear terms, in order to investigate the effects of nonlinear interactions between system modes. Both methodologies consider the inclusion of FACTS devices and dynamic load model. Simulations are performed for a symmetric two-area test power system and for the Equivalent South-Southeast Brazilian system. The results obtained show that the methodology proposed is very effective as an alternative to linear modal analysis and timedomain simulation in the performance of inter-area mode oscillations analysis / Doutorado / Energia Eletrica / Doutor em Engenharia Elétrica

No description available.

Orientador: Vivaldo Fernando da Costa / Dissertação (mestrado) - Universidade Estadual de Campinas, Faculdade de Engenharia Eletrica e de Computação / Made available in DSpace on 2018-08-14T11:14:17Z (GMT). No. of bitstreams: 1 Paccini_RodrigodeOliveira_M.pdf: 1988341 bytes, checksum: 7be3a801c003202ec884c8313b60ac5f (MD5) Previous issue date: 2009 / Resumo: Quando um sistema interligado de corrente alternada sofre variações de carga, sua freqüência pode exibir um comportamento oscilatório. Se este mesmo sistema possuir um elo de corrente contínua em paralelo com as linhas de intercâmbio, esse efeito poderá ser atenuado devido à atuação do controlador de corrente do elo. Esta tese apresenta um estudo que avalia a eficiência de um elo de CC para o amortecimento de oscilações de freqüência de intercâmbio sob condições de pequenas perturbações, operando o sistema com Controle Automático de Geração (CAG). Um modelo de injeção de potência utilizado para representar o elo CC é implementado no Modelo de Sensibilidade de Potência (MSP). Uma das vantagens desse modelo é permitir o desacoplamento da rede em duas partes distintas: o balanço ativo e o balanço reativo, em que as duas podem ser tratadas juntas ou separadamente. Com a escolha desse modelo, tornou-se possível implementar toda a rede no formato de diagrama de blocos. Além disso, as barras de carga do sistema são acessíveis através do diagrama de blocos, o que torna possível variar também a característica da carga. Através dessa representação, tornou-se viável a inserção do elo de CC no diagrama de blocos, pois o mesmo foi modelado como uma injeção de potência nas barras terminais nos balanços ativo e reativo, fechando um novo balanço de potência. As análises no domínio do tempo foram realizadas no programa Matlab Simulink®. Também utilizando este software, um controlador de amortecimento de oscilações de potência (POD-Power Oscillation Damping Controller) foi projetado para a modulação da potência do elo de CC. Os resultados obtidos mostram que o elo de CC possui um grande potencial para a manutenção do amortecimento de oscilações de freqüência de modo inter-área, quando equipados com controladores POD. / Abstract: When an interconnected alternate-current power system suffers variations of loads, its frequency can exhibit an oscillatory behavior. If this system possesses a direct-current link in parallel with the inter-tie lines, this effect could be decreased, due to the link current controller action. This work presents a study that evaluates the efficiency of DC link for the damping of interarea frequency oscillations under small disturbances, operating the system with automatic generation control (AGC). A DC power injection model is implemented in the Power Sensitivity Model (PSM). One of the advantages of this model is to allow the detach of the network in two distinct parts: the active and reactive balances, where the two can be dealt together or separately. With the choice of this model, it's possible to implement the complete network in the block diagram format. Moreover, the network load bus are accessible through the block diagram, which this are possible to vary the load characteristic. Through this representation, the insertion of DC link in the block diagram became viable, therefore it was modeled as a power injection in the terminals bus in the active and reactive balances, closing a new power balance. The time domain analysis is carried out the software Matlab Simulink®. Also using this software, a POD controller (Power Oscillation Damping Controller) was designed for the DC power modulation. The results show that DC link comprises a great potential for the maintenance of frequency oscillations damping in interarea mode, when coupled to POD controllers. / Mestrado / Energia Eletrica / Mestre em Engenharia Elétrica

Abstract not available

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.

Reactive power is critical to reliable operation of the modern AC power system. There is a plethora of motor-loads, transformers, and power-electronic loads connected to the power grid, which consume reactive power for normal operation. Transmission lines also consume reactive power when they are loaded above their surge impedance loading (SIL). Reactive power can exact opportunity cost due to reduced capacity of the lines to carry real power, which in turn lowers revenue. Most transmission owners (TOs) levy large penalties against load serving entities (LSEs), industrial facilities, and other end-use customers, who consume more than their allotted amount, as measured by their power factor. These penalties are to incentivize their customers to meet their reactive power needs locally as well as to recuperate the TOs' financial losses. Harmonic pollution is another factor that prevents the optimal operation of the grid and the connected loads. Harmonics are attributable to proliferation of the diode-rectifier- or thyristor-rectifier-interfaced loads such as variable speed ac drives and power supplies in server farms, electric arc furnaces, and other non-linear loads, which are widely employed by the industrial sector. With wider adoption of harmonic-rich loads by the consumer sector as well, such as HDTVs and compact fluorescent lamps (CFLs), greater level of triplen harmonics associated with single-phase loads are also increasingly seen on the distribution grid. The increasing penetration of renewable resources and electrification of light-duty vehicles are expected to further aggravate the stresses and congestion on the utility grid. Reactive power compensation is necessary for supporting the AC grid and maintaining a healthy voltage stability margin. Compensation can also enhance the utilization of system capacity, lower system losses, provide fault ride-through, and enable a quick fault recovery. Existing VAR and harmonic compensation technologies are either too expensive or inadequate to meet the dynamic needs of the modern and the future power system. This dissertation presents a novel class of Dynamic VAR and Harmonic Compensators (DVHCs) for supplying or absorbing reactive power and providing harmonic filtering, where the compensation is in shunt with the line and the load. The underlying concept is based on augmenting a static or passive component like a capacitor or an inductor with a direct AC converter and imbuing the passive component with dynamic properties. The direct AC converter can be configured as a buck, a boost, or a buck-boost. A `fail-normal' switch is an integral part of the DVHCs that bypasses the converter when it fails, preserving the original functionality and the reliability of the passive component. The DVHCs are modular and scalable such that they can be employed in applications ranging from residential and industrial with voltages less than 480 V, to power distribution level with voltages as high as 35 kV. The Dynamic Inductor (D-IND) and the Dynamic Capacitor (D-CAP) are subclasses of the DVHCs. As the applications for supplying leading VARs are more prevalent, the primary focus of this work is on the buck, the boost, and the buck-boost configurations of the D-CAP. To understand the characteristics and operation of the DVHCs, this work has developed time-domain models for analyzing the transient and dynamic behavior; frequency-domain models for understanding the harmonic interactions and the steady-state relationships between switch duty and current harmonics; and small-signal models for studying the dynamics of the converter due to various perturbations. The small-signal models also enable extraction of transfer functions in designing controllers and assessing stability margins. Control architectures and techniques are presented for effectively controlling the D-CAP when commutating the semiconductor devices with both high and low switching frequencies. In modularly scaling the DVHCs to higher voltages, three medium-voltage topologies are discussed. They are based on series-connecting fractionally-rated devices, AC flying capacitors, and series cascading multiple two-level cells. These implementations allow direct connect to the medium-voltage grid, thereby obviating the use of transformers, and subsequently reducing the losses, cost, complexity, and footprint. A novel AC snubber concept is proposed to provide safe commutation of the AC switches, fault tolerance by managing the energy trapped in parasitics and filters, and to enable dynamic and static voltage sharing when integrated around the series-connected devices. Design equations for selecting and rating the devices and components in the buck, the boost, and the buck-boost configurations of the D-CAP are presented. Three sets of example designs, with one at low-voltage and two at medium-voltage, are discussed to demonstrate the typical size and ratings of the various components under realistic operating conditions. Measurements and the related discussions of a 40 kVA buck D-CAP prototype built to validate the effectiveness of the proposed concepts are presented.