Hydropower plants contains a lot of vital electrical power technology such as generators and transformers. To prevent damage to this equipment due to overvoltage, they are often protected by surge arrester and in some cases also by surge capacitors. The design and dimension of these components vary between different sites and regions and are often done using a template without considering the actual conditions of the site. In this thesis Simulink and Simscape Power System are used to study the case when a hydropower station is exposed to overvoltage. One of Fortum’s hydropower stations is simulated when being exposed to lightning and switching overvoltage to see how the existing overvoltage protection handles this stress. The results show that the surge arresters mounted in front of the generator managed to keep it safe for all overvoltage scenarios it is exposed to. They also shows that the surge capacitors mounted in front of the generator reduces the gradient of the overvoltage surge. However, as the surge arresters already keeps the overvoltage at safe levels, there is no need of reducing the overvoltage gradient and the surge capacitors becomes excessive in this particular hydropower plant. / Vattenkraftverk innehåller många vitala komponenter så som generatorer och transformatorer. För att skydda dessa oftast väldigt dyra komponenter mot överspänningar, används ventilavledare och ibland också överspänningskondensatorer. Utformning och dimensionering av skydden varierar mellan olika anläggningar och regioner. Dimensioneringen av skydden görs ofta schablonmässigt, utan någon väsentlig anpassning till de faktiska förhållandena. I detta examensarbete används simuleringsverktyget Simulink tillsammans med tillägget Simscape Power Systems för att simulera vad som händer när ett vattenkraftverk utsätts för olika överspänningar. Ett av Fortums vattenkraftverk studeras när det utsätts för blixt- och kopplingsöverspänningar för att se vad ventilavledarna samt överspänningskondensatorerna i denna anläggning fyller för funktion. Resultaten visar att ventilavledarna monterade framför generatorn klarar av att hantera alla överspänningar de utsätt för under simuleringarna. Resultaten visade också att överspänningskondensatorerna monterade framför generatorn sänker spänningsgradienten, men eftersom ventilavledarna redan håller spänningen på en säker nivå blir överspänningskondensatorerna överflödiga i denna station.

abstract: Prior work in literature has illustrated the benefits of using surge arrester as a way to improve the lighting performance of the substation and transmission line. Installing surge arresters would enhance the system reliability but it comes with an extra capital expenditure. This thesis provides simulation analysis to examine substation-specific applications of surge arrester as a way of determining the optimal, cost-effective placement of surge arresters. Four different surge arrester installation configurations are examined for the 500/230 kV Rudd substation which belongs to the utility, Salt River Project (SRP). The most efficient configuration is identified in this thesis. A new method “voltage-distance curve” is proposed in this work to evaluate different surge arrester installation configurations. Simulation results show that surge arresters only need to be equipped on certain location of the substation and can still ensure sufficient lightning protection. With lower tower footing resistance, the lightning performance of the transmission line can typically be improved. However, when surge arresters are installed in the system, the footing resistance may have either negative or positive effect on the lightning performance. Different situations for both effects are studied in this thesis. This thesis proposes a surge arrester installation strategy for the overhead transmission line lightning protection. In order to determine the most efficient surge arrester configuration of transmission line, the entire transmission line is divided into several line sections according to the footing resistance of its towers. A line section consists of the towers which have similar footing resistance. Two different designs are considered for transmission line lightning protection, they include: equip different number of surge arrester on selected phase of every tower, equip surge arresters on all phases of selected towers. By varying the number of the towers or the number of phases needs to be equipped with surge arresters, the threshold voltage for line insulator flashover is used to evaluate different surge arrester installation configurations. The way to determine the optimal surge arresters configuration for each line section is then introduced in this thesis. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2018

Submitted by Johnny Rodrigues (johnnyrodrigues@ufcg.edu.br) on 2018-04-21T14:21:51Z No. of bitstreams: 1 GERALDO BEZERRA LIMA JÚNIOR - DISSERTAÇÃO PPGEE 2014..pdf: 4130681 bytes, checksum: 7aa2b6dbcd9d18892be46ec5c14536b6 (MD5) / Made available in DSpace on 2018-04-21T14:21:51Z (GMT). No. of bitstreams: 1 GERALDO BEZERRA LIMA JÚNIOR - DISSERTAÇÃO PPGEE 2014..pdf: 4130681 bytes, checksum: 7aa2b6dbcd9d18892be46ec5c14536b6 (MD5) Previous issue date: 2014-09-02 / Nos dias atuais a confiabilidade e a continuidade no fornecimento de energia elétrica são cada vez mais exigidos. Os para-raios tem uma importante função neste processo que é de proteger os demais equipamentos integrantes da rede elétrica de sobretensões atmosféricas e de manobra. Quando falham, normalmente provocam o desligamento de linhas de transmissão ou de transformadores, penalizando severamente a concessionária pela indisponibilidade ou por multas impostas pela agência reguladora. Como todo equipamento importante para o sistema elétrico, os para-raios necessitam de técnicas eficazes de monitoramento. Na literatura e na indústria podem ser encontradas diversas técnicas, procedimentos ou equipamentos destinados ao seu monitoramento. Todos os procedimentos ou equipamentos possuem suas limitações, vantagens e desvantagens, sejam elas técnicas, econômicas ou operacionais. Visando suplantar algumas das limitações existentes nas técnicas e procedimentos normalmente empregados no setor elétrico, encontrase proposto neste trabalho uma metodologia de análise da condição operacional de para-raios em campo, com base em medições da corrente de fuga total. Com esta metodologia pretende-se minimizar as dificuldades operacionais relacionadas às atividades de monitoramento, uma vez que será medida apenas a corrente de fuga total, diferentemente de técnicas e procedimentos usuais que requerem também a medição da tensão aplicada. Um banco de dados de correntes de fuga foi obtido com medições realizadas em 05 (cinco) subestações. As medições foram agrupadas por modelo/fabricante e tempo de operação dos para-raios. O instrumento de medição utilizado é composto de um amperímetro do tipo alicate e um osciloscópio digital portátil. O processo de medição em campo mostrou-se efetivo e prático devido à facilidade de manuseio do instrumento utilizado. Ensaios em laboratório foram realizados para avaliação da metodologia e dos instrumentos de medição. A partir da base de dados produzida, foi possível com emprego de técnicas de processamento de sinais e estatística, observar o comportamento gaussiano dos dados e o crescimento do valor médio da componente de 3ª harmônica da corrente total em função do tempo de operação, evidenciando, assim, a correlação entre a elevação da corrente de fuga e a diminuição da vida útil dos para-raios. / Nowadays the reliability and continuity in the electricity supply are increasingly required. The surge arresters has an important function in this process, which is to protect the other utility equipment against lightning and switching surges. When they fail, usually causing the shutdown of transmission lines or transformers, severely penalizing the power utility for the unavailability or regulator fines. As any important equipment for the electrical system, surge arresters need effective monitoring techniques. In the literature and industry several techniques, procedures or equipment for their monitoring can be found. All procedures or devices have their limitations, advantages and disadvantages, whether technical, economic or operational. Aiming to overcome some of the limitations in existing techniques and procedures commonly employed in the electrical industry, a methodology for analyzing the surge arrester operational condition based on measurements of total leakage current is proposed in this paper. With this methodology is intended to minimize operational difficulties associated with monitoring activities, since it will only measure the total leakage current unlike the usual techniques and procedures, which require the measurement of the applied voltage, too. A database of leakage currents with measurements performed in five (05) power substations was obtained. Measurements were grouped by arresters model/manufacturer and operation time. The measuring instrument used consists of a clamp type ammeter and a portable digital oscilloscope. The process of field measurement was effective and practical due to the ease of handling of the used instrument. Laboratory tests were performed to evaluate the methodology and measuring instruments. From the database produced, it was possible through the use of techniques of signal processing and statistical observe the Gaussian behavior of the data and the average increase of the 3rd harmonic component of the total current as a function of operating time, which highlights thus, the correlation between the increase of the leakage current and the decrease of the lifetime of the surge arrester.

Abstract Adding a dc circuit to an existing transmission line is one method of significantly increasing the power transfer capability of a transmission corridor. The resulting hybrid system has significant coupling between the ac and dc circuits, not only because of the proximity of the circuits, but also from the fact that they may share the same sending end or receiving end ac systems. The resultant interaction produces overvoltages on the dc system which can be somewhat higher than for a conventional dc scheme. This thesis investigates the overvoltages on a hybrid ac-dc transmission system and suggests some design considerations which could be taken into account to reduce stresses on certain critical components which result from such an arrangement. Blocking filters consisting of a parallel L-C combination in series with the dc converter were included to limit the flow of fundamental frequency current in the dc line. This thesis also investigates the proper blocking filter configuration to be used as an incorrectly chosen blocking filter can cause resonance overvoltages on the dc line at fundamental frequency. A method of eliminating dc components of the currents in the transformer windings of a dc converter is presented. The method uses the technique of firing angle modulation. It is shown that merely eliminating the fundamental frequency component on the dc side may not remove this dc component. The impact of such control action at one converter on the other converters in the dc transmission system is also presented. It is also shown that the undesirable side effects of such a scheme include increased generation of non-characteristic harmonies on both the ac and dc sides. The study is performed using an electromagnetic transients simulation program and theoretical calculations.

Abstract Adding a dc circuit to an existing transmission line is one method of significantly increasing the power transfer capability of a transmission corridor. The resulting hybrid system has significant coupling between the ac and dc circuits, not only because of the proximity of the circuits, but also from the fact that they may share the same sending end or receiving end ac systems. The resultant interaction produces overvoltages on the dc system which can be somewhat higher than for a conventional dc scheme. This thesis investigates the overvoltages on a hybrid ac-dc transmission system and suggests some design considerations which could be taken into account to reduce stresses on certain critical components which result from such an arrangement. Blocking filters consisting of a parallel L-C combination in series with the dc converter were included to limit the flow of fundamental frequency current in the dc line. This thesis also investigates the proper blocking filter configuration to be used as an incorrectly chosen blocking filter can cause resonance overvoltages on the dc line at fundamental frequency. A method of eliminating dc components of the currents in the transformer windings of a dc converter is presented. The method uses the technique of firing angle modulation. It is shown that merely eliminating the fundamental frequency component on the dc side may not remove this dc component. The impact of such control action at one converter on the other converters in the dc transmission system is also presented. It is also shown that the undesirable side effects of such a scheme include increased generation of non-characteristic harmonies on both the ac and dc sides. The study is performed using an electromagnetic transients simulation program and theoretical calculations.

Any switching action or lightning stroke on a transmission system causes excessive of voltage, which are propagated through the equipment. The withstand capability of the electrical equipment to these overvoltages is dependent on the rate of rise, peak value and the duration of these overvoltages and are especially critical to information technology (IT) equipment. Whenever there is a probability of these overvoltages appearing across the electrical equipment, they should be limited to a safe value by the application surge arresters of the metal oxide type. However, surge arresters are expensive, an optimal location for these should be found so as to minimise the number of surge arresters used.

abstract: With the penetration of distributed renewable energy and the development of semiconductor technology, power electronic devices could be utilized to interface re- newable energy generation and the distribution power grid. However, when directly connected to the power grid, the semiconductors inside the power electronic devices could be vulnerable to the power system transient, especially to lightning strikes. The work of this research focuses on the insulation coordination of power elec- tronic devices connected directly to the power distribution system. The Solid State Transformer (SST) in Future Renewable Electric Energy Delivery and Management (FREEDM) system could be a good example for grid connected power electronic devices. Simulations were conducted in Power Systems Computer Aided Design (PSCAD) software. A simulation done to the FREEDM SST showed primary re- sults which were then compare to simulation done to the grid-connected operating Voltage Source Converter (VSC) to get more objective results. Based on the simulation results, voltage surges caused by lightning strikes could result in damage on the grid-connected electronic devices. Placing Metal Oxide Surge Arresers (MOSA, also known as Metal Oxide Surge Varistor, MOV) at the front lter could provide eective protection for those devices from power transient. Part of this research work was published as a conference paper and was presented at CIGRE US National Conference: Grid of the Future Symposium [1] and North American Power Symposium [2]. / Dissertation/Thesis / Masters Thesis Engineering 2017

Os sistemas elétricos de potência estão sujeitos a diversos tipos de sobretensões que podem causar desligamentos ou avarias nos equipamentos elétricos que o compõe, comprometendo a continuidade do fornecimento de energia elétrica aos consumidores e impactando negativamente nos índices de qualidade. Um dos dispositivos mais importantes utilizados na proteção destes equipamentos são os pára-raios, os quais podem ser classificados, no sistema elétrico brasileiro, em função de sua tecnologia: a primeira, mais antiga, utiliza carboneto de silício (SiC), a segunda e mais atual utiliza óxido de zinco (ZnO) em seus elementos internos. Os primeiros estão instalados há muitos anos, até mesmo acima de sua vida útil, sendo assim, podem apresentar falhas em campo durante a sua operação. Os pára-raios de SiC estão sendo gradativamente substituídos pelos de ZnO, pois seria economicamente inviável a sua substituição imediata. Por conseguinte, torna-se imprescindível o desenvolvimento de metodologias para avaliação do seu estado de degradação, a fim de evitar-se que estes equipamentos falhem no campo. A medição da corrente de fuga, analisando-se a sua componente de terceira harmônica, mostrou que podem obtidas informações importantes a respeito dos pára-raios de SiC, fornecendo subsídios para a elaboração de um programa de manutenção que priorize a retirada daqueles mais degradados. / Power systems are subject to many types of overvoltages that may provoke interruptions or damage in electrical equipments, compromising the continuity of electrical energy supply to consumers and impacting negatively the power quality indices. One of the most important device used to protect the electric equipments are the surge arresters, which may be classified, in Brazilian power system, by its technology: the first, more antiquated, using gapped silicon-carbide (SiC) surge arrester, and secondly and newer, zinc-oxide (ZnO) surge arrester. The devices of the first type are installed for many years, even above their lifetime, presenting, in such cases, failures in site while operating. The gapped silicon-carbide surge arresters have been substituted gradually by zinc-oxide surge arresters, because it would be economically impracticable to replace them immediately. Therefore, the development of methodologies to evaluate their degradations condition is necessary, in order to avoid the failure of these equipments in site. Measurements of leakage current with third order harmonic analysis showed that important information regarding gapped silicon carbide surge arresters may be obtained, providing subsidies to elaborate a program of maintenance that prioritize the replacement of the most degraded ones.

Os sistemas elétricos de potência estão sujeitos a diversos tipos de sobretensões que podem causar desligamentos ou avarias nos equipamentos elétricos que o compõe, comprometendo a continuidade do fornecimento de energia elétrica aos consumidores e impactando negativamente nos índices de qualidade. Um dos dispositivos mais importantes utilizados na proteção destes equipamentos são os pára-raios, os quais podem ser classificados, no sistema elétrico brasileiro, em função de sua tecnologia: a primeira, mais antiga, utiliza carboneto de silício (SiC), a segunda e mais atual utiliza óxido de zinco (ZnO) em seus elementos internos. Os primeiros estão instalados há muitos anos, até mesmo acima de sua vida útil, sendo assim, podem apresentar falhas em campo durante a sua operação. Os pára-raios de SiC estão sendo gradativamente substituídos pelos de ZnO, pois seria economicamente inviável a sua substituição imediata. Por conseguinte, torna-se imprescindível o desenvolvimento de metodologias para avaliação do seu estado de degradação, a fim de evitar-se que estes equipamentos falhem no campo. A medição da corrente de fuga, analisando-se a sua componente de terceira harmônica, mostrou que podem obtidas informações importantes a respeito dos pára-raios de SiC, fornecendo subsídios para a elaboração de um programa de manutenção que priorize a retirada daqueles mais degradados. / Power systems are subject to many types of overvoltages that may provoke interruptions or damage in electrical equipments, compromising the continuity of electrical energy supply to consumers and impacting negatively the power quality indices. One of the most important device used to protect the electric equipments are the surge arresters, which may be classified, in Brazilian power system, by its technology: the first, more antiquated, using gapped silicon-carbide (SiC) surge arrester, and secondly and newer, zinc-oxide (ZnO) surge arrester. The devices of the first type are installed for many years, even above their lifetime, presenting, in such cases, failures in site while operating. The gapped silicon-carbide surge arresters have been substituted gradually by zinc-oxide surge arresters, because it would be economically impracticable to replace them immediately. Therefore, the development of methodologies to evaluate their degradations condition is necessary, in order to avoid the failure of these equipments in site. Measurements of leakage current with third order harmonic analysis showed that important information regarding gapped silicon carbide surge arresters may be obtained, providing subsidies to elaborate a program of maintenance that prioritize the replacement of the most degraded ones.