ABSTRACT Power transformers are key components for electrical energy transfer in a power system. Stability and security of transformer protection are important to system operation. Many maloperation cases of transformer differential protection are caused by inrush current problems. The phenomenon of transformer inrush current has been widely discussed in literature. Therefore, this research only discussed and analyzed inrush current problems to transformer differential protection. To understand the inrush current problems on differential protection, transformer simulation models were presented using Matlab/Simulink. Differential relay simulations for internal and external faults, and relay performance during current transformer saturation were performed using PSCAD. Recommendations were made on methods of reducing the blocking time of differential protection during inrush conditions.

Microgrid deployment in electric grids improves reliability, efficiency, and quality, as well as the overall sustainability and resiliency of the grid. Specifically, microgrids alleviate the effects of power outages. However, microgrid implementations impose additional challenges on power systems. Microgrid protection is one of the technical challenges implicit in the deployment of microgrids. These challenges occur as a result of the unique properties of microgrid networks in comparison to traditional electrical networks. Differential protection is a fast, selective, and sensitive technique. Additionally, it offers a viable solution to microgrid protection concerns. The differential zone protection scheme is a cost-effective variant of differential protection. To implement a differential zone protection scheme, the network must be split into different protection zones. The reliability of this protection scheme is dependent upon the number of protective zones developed. This thesis proposes a new differential zone protection scheme using a graph partitioning algorithm. A graph partitioning algorithm is used to partition the microgrid into multiple protective zones. The IEEE 13-node microgrid is used to demonstrate the proposed protection scheme. The protection scheme is validated with MATLAB Simulink, and its impact is simulated with DIgSILENT PowerFactory software. Additionally, a comprehensive comparison was made to a comparable differential zone protection scheme. / Master of Science / A microgrid is a group of connected distributed energy resources (DERs) with the loads to be served that acts as a local electrical network. In electric grids, microgrid implementation enhances grid reliability, efficiency, and quality, as well as the system's overall sustainability and resiliency. Microgrids mitigate the consequences of power disruptions. Microgrid solutions, on the other hand, bring extra obstacles to power systems. One of the technological issues inherent in the implementation of microgrids is microgrid protection. These difficulties arise as a result of microgrid networks' distinct characteristics as compared to standard electrical networks. Differential protection is a technique that is fast, selective, and sensitive. It also provides a feasible solution to microgrid protection problems. This protection scheme, on the other hand, is more expensive than others. The differential zone protection scheme is a cost-effective variation of differential protection that lowers protection scheme expenses while improving system reliability. The network must be divided into different protection zones in order to deploy a differential zone protection scheme. The number of protective zones generated determines the reliability of this protection method. Using a network partitioning technique, this thesis presents a new differential zone protection scheme. The microgrid is divided into various protection zones using a graph partitioning algorithm. The proposed protection scheme is demonstrated using the IEEE 13-node microgrid. MATLAB Simulink is used to validate the protection scheme, while DIgSILENT PowerFactory is used to simulate its impact. A comparison of a similar differential zone protection scheme was also done.

Master of Science / Department of Electrical and Computer Engineering / Noel Schulz / Transmission networks are an important part of an electric power system. They help transfer power from the point of generation (power plants) to the substation. In order to minimize losses during power transfer, the lines are operated at high voltages. The high voltage lines not only have a high power transmission capacity, but they are also prone to faults of larger magnitudes. Thus the occurrence of such faults results in a need for the faults to be cleared quickly in order to limit damage caused to the system. Hence, relays are installed at the Buses to provide protection to the lines. Transmission lines in a power system are most commonly protected by distance relays that use directional comparison schemes. However, due to the simplicity of line differential schemes, there has been an increase in the use of differential relays for complex networks. Moreover, since the relays require only current as the operating parameter, their settings can be determined easily. This report discusses the design of a line current differential protection scheme for a transmission line using SEL 311L relays. The relay settings have been determined and then tested for seven fault scenarios, three internal fault points and four external fault points. To set and test the relays, AcSELerator Quickset, SEL 5030 and PowerWorld programs have been used. Real life power system is simulated with the help of SEL AMS (Adaptive Multichannel Source) that acts as the source to provide the required data to the relays. The relays trip and open the breaker contacts for an internal fault. During an external fault, the relays do not trip and the breaker contacts remain closed. The response of the relays in case of communication failures has been discussed.

Differential protection is the unit protection system which is applied to protect a particular unit of power systems. Unit is known as zone in protection terminology which is equivalent to simple electrical node. In recent time, low impedance current differential protection schemes based on percentage restrained characteristics are widely used in power systems to protect busbar systems. The main application issue of these schemes is mis-operation due to current transformer (CT) saturation during close-in external faults. Researchers have suggested various solution of this problem; however, individually they are not sufficient to puzzle out all mis-operational scenarios. This thesis presents a new bus differential algorithm by defining alternative partial operating current characteristics of a differential protection zone and investigating its performance for all practical bus faults. Mathematical model of partial operating current and operating principle of the proposed bus differential relay are described in details. A CT saturation detection algorithm which includes fast and late CT saturation detection techniques is incorporated in relay design to increase the sensitivity of partial operating current based internal-external fault discriminator for high impedance internal faults. Performance of the proposed relay is validated by an extensive test considering all possible fault scenarios.

Current differential protection principle is superior in terms of sensitivity and speed of operation in comparison with other protection principle used in power systems. From the last five decades, various current differential protection schemes are widely used to protect busbars, transformers, and short-transmission lines. The deployment of high capacity microwave and optical fiber technologies redefined the line protection systems by facilitating the use of current differential protection schemes for long transmission lines. The common application issue of these schemes is mis-operation due to current transformer (CT) saturation during close-in external faults. Moreover, transformer differential protection schemes face mis-trip due to inrush current during energization. The techniques presented in the literature to address those issues, de-sensitize protection function and increase the time of operation. A comprehensive fault discrimination algorithm and an inrush current detection algorithm are highly demanded for current differential protection schemes. The purpose of this dissertation is to optimize the performance of differential schemes applied to protect busbar, transformer and line. This research derives the mathematical model of saturated secondary current of CT and introduces the concept of Partial Operating Current (POC). Based on these mathematical developments, the characteristics of POC are identified for all three types of differential zones like busbar, transformer and line protection. A new inrush current blocking algorithm is developed for transformer differential protection. A new time-domain CT saturation detection algorithm is also proposed. Based on these new developments, three separate differential schemes are designed for busbar, transformer, and line protection, respectively. The proposed schemes provide complete immunity against the mis-operations due to CT saturation during close-in external faults and transformer inrush current without sacrificing the sensitivity for internal faults. The speed of operation is also improved. The model for each scheme is built in Matlab platform and the performance is validated using the test system simulated in Electro-Magnetic Transient Program (EMTP) for all possible fault scenarios. Documented results show the improved performance of the proposed schemes when compared to traditional differential schemes in terms of reliability, sensitivity, selectivity, and speed

abstract: A robust, fast and accurate protection system based on pilot protection concept was developed previously and a few alterations in that algorithm were made to make it faster and more reliable and then was applied to smart distribution grids to verify the results for it. The new 10 sample window method was adapted into the pilot protection program and its performance for the test bed system operation was tabulated. Following that the system comparison between the hardware results for the same algorithm and the simulation results were compared. The development of the dual slope percentage differential method, its comparison with the 10 sample average window pilot protection system and the effects of CT saturation on the pilot protection system are also shown in this thesis. The implementation of the 10 sample average window pilot protection system is done to multiple distribution grids like Green Hub v4.3, IEEE 34, LSSS loop and modified LSSS loop. Case studies of these multi-terminal model are presented, and the results are also shown in this thesis. The result obtained shows that the new algorithm for the previously proposed protection system successfully identifies fault on the test bed and the results for both hardware and software simulations match and the response time is approximately less than quarter of a cycle which is fast as compared to the present commercial protection system and satisfies the FREEDM system requirement. / Dissertation/Thesis / M.S. Electrical Engineering 2014

UHV transmission lines have several advantages when it comes to transferring of bulk amount ofpower in long distances. Although the technology is available from 1980’s, the number of UHVtransmission lines around the world has been increased in the beginning of this century in orderto supply the high demand of electricity. The number is going to increase even more in the nextfew years. Thus, it is important to study the UHV line characteristics and update the powersystem equipment as well as the operation procedure accordingly.UHV transmission lines exhibits some distinct phenomena which are not present in thetransmission lines with lower voltage levels e.g. high amount of charging current, non-linearincrease of apparent fault impedance with the increase in fault distance, longer time constant forthe DC component in fault current etc. Some of these are very important for different protectionschemes of the transmission line. The aim of this thesis work is to study the impact of UHV linecharacteristics on line differential protection and propose solutions to overcome them.Line differential protection is popular for its good selectivity and simplicity as long as there is adependable reliable communication system between the two ends of the line. High amount ofcapacitive charging current and large phase shift between sending and receiving end currents aretwo important characteristics of UHV lines which have severe impact on the line differentialprotection. It becomes very critical for the protection scheme to maintain good sensitivity andsecurity at the same time. As a result, compensation of the charging current becomes essential.The fixed compensation method is used to compensate the charging current of long lines withlower voltage levels. However, it cannot satisfy the sensitivity requirements for line differentialprotection scheme on UHV lines. Hence, it is necessary to search for other compensationmethods.In this thesis, a solution related with charging current compensation method is proposed with theuse of adaptive phase shift compensation. Several simulations have been done to examine thecharacteristics of the developed method in the worst case scenarios. It is found that the methodexhibits very good sensitivity as well as security for line differential protection on UHV lines. / UHV-transmissionsledningar har ett antal fördelar när det gäller överföringskapacitet av effektöver långa avstånd. Även om tekniken har varit tillgänglig sedan 1980-talet är det inte förränunder det senaste årtiondet som utbyggnaden har tagit fart ordentligt. Detta har skett för att mötaden höga efterfrågan på elektricitet. De närmaste åren väntas utbyggnaden öka ytterligare. Motdenna bakgrund är det relevant att studera karaktäristiken av UHV samt att uppdaterakraftsystemutrustning såväl som driften.UHV-tramsmissionslinor är förenade med vissa fenomen som inte förekommer förtransmissionsledningar med lägre spänning. Vissa av dessa fenomen är har stor inverkan påskyddsutrustning för transmissionslinorna. Syftet med denna uppsats är att studera inverkan frånUHV-transmissionsledningar på differentialskydd samt att föreslå lösningar för att överkommainverkan.Differentialskydd är populärt tack vare goda selektiva egenskaper och enkelhet så länge det finnsett pålitligt kommunikationssystem. Hög kapacitans och stor fasskiftning mellan strömmen påavsändar- och mottagarsidan är två viktiga egenskaper hos UHV-transmissionsledningar vilkahar stor inverkan på differentialskyddet. Det är väldigt viktigt att skyddsutrustningen kanupprätthålla god sensitivitet samt säkerhet. Av denna anledning är kompensation för den högakapacitansen viktig. Konstant kompensation används för att kompensera för hör kapacitans hoslånga transmissionslinor med lägre spänning. Denna metod är däremot inte fungerande för UHV.Det är därför nödvändigt att söka efter en annan lösning.I denna uppsats föreslås en lösning på behovet att kompensera för hög kapacitans genom attanvända adaptiv fasskiftkompensation. Flertalet simuleringar har genomförts för att utvärderakaraktäristiken av den utvecklade metoden. Det konstateras att metoden väldigt god känslighetoch säkerhet för differentialskydd av UHV-transmissionsledningar.

A qualidade e a continuidade do fornecimento de energia elétrica aos consumidores são fatores muito importantes quando da avaliação da eficiência de um sistema elétrico de potência. Nesse contexto, os transformadores são equipamentos muito importantes e demandam especial atenção quando do projeto do esquema de proteção. Apesar do crescente desenvolvimento das metodologias de proteção de transformadores trifásicos, alguns aspectos ainda não foram totalmente solucionados. Um desses diz respeito à proteção diferencial de transformadores de potência, a qual apresenta vários problemas na discriminação de faltas internas ao transformador. A geração de correntes diferenciais provocada por fenômenos transitórios, como a energização do transformador, produz a incorreta operação do relé, ocasionando uma queda na eficiência do esquema de proteção diferencial. Assim sendo, o presente trabalho apresenta uma nova metodologia de proteção diferencial de transformadores trifásicos, a qual utiliza a transformada wavelet para extrair os sinais transitórios dominantes induzidos pelas faltas internas. A transformada wavelet é uma eficiente ferramenta utilizada no estudo de sinais não-estacionários e de rápida transição. De forma a atender os principais problemas do esquema convencional de proteção, a transformada wavelet discreta é utilizada para decompor os sinais de corrente diferencial em várias faixas de freqüências. Após essa decomposição, a variação de energia espectral dos coeficientes de detalhe wavelet é analisada pelo algoritmo proposto, e assim uma discriminação entre faltas internas e correntes de magnetização, ou correntes inrush, é feita. Usando um modelo elaborado de um sistema elétrico de transmissão são efetuadas rigorosas simulações computacionais para avaliar o desempenho do algoritmo proposto. Os resultados obtidos nessas simulações mostram que a metodologia de proteção diferencial de transformadores trifásicos baseada na variação de energia espectral dos coeficientes wavelets apresenta um ótimo desempenho quando comparada com a metodologia de proteção convencional. / Power supply quality and continuity are very important aspect when assessing the efficiency of an electric power system. In this context, the transformers are key equipments that require special attention during the protection scheme design. Despite the increasing development of methodologies for three-phase transformers protection, some aspects have not yet been fully studied. One of these aspects concerns to the differential protection of power transformers, which presents several restrictions regarding the characterization of internal faults. The observation of differential currents caused by transient phenomena such as transformer energization, produces an incorrect operation of protective relaying, causing a drop in the protection scheme efficiency. Therefore, this work presents a new methodology for differential protection of three-phase transformers using the wavelet transform to extract the transient signals induced by the dominant internal faults. The wavelet transform is an efficient tool in the study of non-stationary signals with fast transients. In order to overcome the main problems of the traditional protection scheme, the discrete wavelet transform is used to decompose the differential current signals into several bands of frequencies. After this decomposition, the spectral energy variation of the wavelet detail coefficients is analyzed by the proposed algorithm and, thus, classification between internal faults, external faults and inrush currents is performed. Using a transmission system model, accurate simulations are performed to evaluate the computational performance of the proposed protection algorithm. The results obtained in these simulations show that the proposed methodology has a great performance when compared with traditional protection philosophies.

Este trabalho fundamenta-se no desenvolvimento e aprimoramento de uma metodologia de proteção diferencial de Transformadores de Potência. A metodologia desenvolvida avalia eventos transitórios que dificultam a operação correta de relés diferenciais aplicados à proteção de transformadores. O estudo concentra-se no estabelecimento de contribuições ao estado da arte associadas à análise de sinais de corrente diferenciais geradas tanto por faltas internas e externas quanto por distúrbios transitórios. A concepção da metodologia proposta baseou-se na quantificação da energia espectral gerada a través dos coeficientes de detalhe da Transformada Wavelet Discreta. A metodologia de proteção proposta foi desenvolvida em ambiente MATLAB® e testada por meio de simulações realizadas através do software ATP/EMTP (Alternative Transients Program/Electromagnetic Transients Program). Os resultados da pesquisa mostram a aplicabilidade do algoritmo de proteção, mesmo nas condições mais adversas, como na ocorrência da saturação dos transformadores de corrente. / This work is based on the development and improvement of a methodology to differential protection of power transformer. The proposed methodology evaluates transient events that difficult the correct operation of differential relays applied to transformer protection. The study establishes contributions to the state of the art related to differential current analysis generated by internal and external faults and transient disturbance. The conception of the proposed methodology was based on the spectral energies variation generated by each event and calculated through the detail coefficient of Discrete Wavelet Transform. The proposed methodology was developed in MATLAB® environment and tested through several simulations performed with the ATP/EMTP software (Alternative Transients Program / Electromagnetic Transients Program). The results of the research show the applicability of the protection algorithms, even in adverse conditions, such as saturation of current transformers.

A qualidade e a continuidade do fornecimento de energia elétrica aos consumidores são fatores muito importantes quando da avaliação da eficiência de um sistema elétrico de potência. Nesse contexto, os transformadores são equipamentos muito importantes e demandam especial atenção quando do projeto do esquema de proteção. Apesar do crescente desenvolvimento das metodologias de proteção de transformadores trifásicos, alguns aspectos ainda não foram totalmente solucionados. Um desses diz respeito à proteção diferencial de transformadores de potência, a qual apresenta vários problemas na discriminação de faltas internas ao transformador. A geração de correntes diferenciais provocada por fenômenos transitórios, como a energização do transformador, produz a incorreta operação do relé, ocasionando uma queda na eficiência do esquema de proteção diferencial. Assim sendo, o presente trabalho apresenta uma nova metodologia de proteção diferencial de transformadores trifásicos, a qual utiliza a transformada wavelet para extrair os sinais transitórios dominantes induzidos pelas faltas internas. A transformada wavelet é uma eficiente ferramenta utilizada no estudo de sinais não-estacionários e de rápida transição. De forma a atender os principais problemas do esquema convencional de proteção, a transformada wavelet discreta é utilizada para decompor os sinais de corrente diferencial em várias faixas de freqüências. Após essa decomposição, a variação de energia espectral dos coeficientes de detalhe wavelet é analisada pelo algoritmo proposto, e assim uma discriminação entre faltas internas e correntes de magnetização, ou correntes inrush, é feita. Usando um modelo elaborado de um sistema elétrico de transmissão são efetuadas rigorosas simulações computacionais para avaliar o desempenho do algoritmo proposto. Os resultados obtidos nessas simulações mostram que a metodologia de proteção diferencial de transformadores trifásicos baseada na variação de energia espectral dos coeficientes wavelets apresenta um ótimo desempenho quando comparada com a metodologia de proteção convencional. / Power supply quality and continuity are very important aspect when assessing the efficiency of an electric power system. In this context, the transformers are key equipments that require special attention during the protection scheme design. Despite the increasing development of methodologies for three-phase transformers protection, some aspects have not yet been fully studied. One of these aspects concerns to the differential protection of power transformers, which presents several restrictions regarding the characterization of internal faults. The observation of differential currents caused by transient phenomena such as transformer energization, produces an incorrect operation of protective relaying, causing a drop in the protection scheme efficiency. Therefore, this work presents a new methodology for differential protection of three-phase transformers using the wavelet transform to extract the transient signals induced by the dominant internal faults. The wavelet transform is an efficient tool in the study of non-stationary signals with fast transients. In order to overcome the main problems of the traditional protection scheme, the discrete wavelet transform is used to decompose the differential current signals into several bands of frequencies. After this decomposition, the spectral energy variation of the wavelet detail coefficients is analyzed by the proposed algorithm and, thus, classification between internal faults, external faults and inrush currents is performed. Using a transmission system model, accurate simulations are performed to evaluate the computational performance of the proposed protection algorithm. The results obtained in these simulations show that the proposed methodology has a great performance when compared with traditional protection philosophies.