A novel voltage sag approach during unintentional islanding scenarios: A survey from real recorded events

In this paper, a new voltage sag approach is analytically modelled and validated using both simulation and field measurements. The main aspect of this approach is to propose a new voltage sag feature which appears during unintentional islanding operations (IOs). The unintentional IO occurs when an induction motor is removed from the main utility following a circuit breaker (CB) clearing, transiently, the induction motors (IMs) are acting as generators maintaining the affected distribution feeder with voltage until it is reconnected. The voltage sag modelled in the current article follows an exponential form, as a matter of fact, here it will be demonstrated that the proposed model reaches satisfactorily the field measurements and evidences the dependability of the model adopted. Furthermore, it is worthwhile to note that this novel power quality (PQ) event has not been investigated yet and enhances voltage sag studies. Lastly, it is crucial to point out that all recorded events and a large amount of data needed so as to validate this transient, has been measured in a distribution network (DN) located in Spain.Postprint (published version

A novel voltage sag approach during unintentional islanding scenarios: A survey from real recorded events

In this paper, a new voltage sag approach is analytically modelled and validated using both simulation and field measurements. The main aspect of this approach is to propose a new voltage sag feature which appears during unintentional islanding operations (IOs). The unintentional IO occurs when an induction motor is removed from the main utility following a circuit breaker (CB) clearing, transiently, the induction motors (IMs) are acting as generators maintaining the affected distribution feeder with voltage until it is reconnected. The voltage sag modelled in the current article follows an exponential form, as a matter of fact, here it will be demonstrated that the proposed model reaches satisfactorily the field measurements and evidences the dependability of the model adopted. Furthermore, it is worthwhile to note that this novel power quality (PQ) event has not been investigated yet and enhances voltage sag studies. Lastly, it is crucial to point out that all recorded events and a large amount of data needed so as to validate this transient, has been measured in a distribution network (DN) located in Spain.Postprint (published version

Power quality disturbances assessment during unintentional islanding scenarios. A contribution to voltage sag studies

This paper presents a novel voltage sag topology that occurs during an unintentional islanding operation (IO) within a distribution network (DN) due to large induction motors (IMs). When a fault occurs, following the circuit breaker (CB) fault clearing, transiently, the IMs act as generators due to their remanent kinetic energy until the CB reclosing takes place. This paper primarily contributes to voltage sag characterization. Therefore, this novel topology is presented, analytically modelled and further validated. It is worth mentioning that this voltage sag has been identified in a real DN in which events have been recorded for two years. The model validation of the proposed voltage sag is done via digital simulations with a model of the real DN implemented in Matlab considering a wide range of scenarios. Both simulations and field measurements confirm the voltage sag analytical expression presented in this paper as well as exhibiting the high accuracy achieved in the three-phase model adopted.Postprint (published version

A novel assessment of unintentional islanding operations in distribution networks

This thesis aims to investigate an unexpected islanding operation (IO) which has been identified in a real distribution network. The process of recording and processing the data obtained from the field measurements in the distribution network (DN) has been the starting point of this research. It has to be underlined that this IO raised a problem and became a major challenge for the distribution operator. Therefore, the aim of this thesis is twofold; solving a real problem as well as further enhance the current research studies about IOs in DNs. IOs have been object of study during the recent years due to the rapid proliferation of the distributed generation (DG) within the so-called smart grids (SGs). Commonly, the power of these DG resources ranges between hundreds of kW and few MW and are allocated at either low voltage or medium voltage levels. One of the significant issues that these resources are raising is, undoubtedly, the IOs. These situations occur when a portion of the grid operates in parallel with the main grid following a disconnection. Thereby IOs, where the DG is energising the grid after a CB opening, must be identified and tripped in the minimum time possible. Failure to do so, the list of hurdles may include; power quality (PQ) disturbances (e.g., frequency and voltage out of range), a safety hazard for the network personnel or out-of-phase reclosings. That is the reason why the research towards the anti-islanding protection methods has elicited great interest. Fundamentally, the substantial improvement of this thesis lies in the fact that, in this IO, there are no DG resources, but large induction motors. In fact, the grid remains energised after the CB disconnection due to the induction motors (IMs) which transiently, act as generators. The island begins with the CB operation and ends when the CB recloses the circuit to restore the electrical supply. This rapid reclosing operation is widely adopted in DNs to avoid manual operations in self-extinguished faults and typically ranges between 0.5 and 1s. Given the fact that usually IOs are originated in the presence of DG, indeed, this IO is utterly unexpectedly for the DSO. Due to the phenomenon mentioned above, the specific goals of this thesis are described down below: 1. The first goal of this thesis focuses on developing a model suitable for validation purposes. To make a proper model validation, the simulations results obtained with this model will be compared with those obtained from field measurements. Thus, once the model has been validated, a thorough investigation regarding the most influential factors will be carried out. 2. The second goal of this thesis falls within the scope of the PQ. During the IO mentioned above, a new voltage sag topology is observed. Consequently, the efforts will be focused on modelling this new type of sag. 3. The third goal of this thesis emerges from the protective point of view. Once the IO has been defined and characterised, the need for identifying and preventing it becomes the main concern. In such a way, the third pillar of the thesis is targeted at implementing a suitable tool to prevent this particular IO. Besides, this new tool will be compared with the currently available methods for ID developed for scenarios with DG.Aquesta tesi te com a objectiu investigar una operació en illa no intencional, que ha set identificada en una xarxa de distribució real. El procés de registre i processament de les dades obtingudes a partir de les mesures de camp en la xarxa de distribució, ha estat el punt de partida d’aquesta investigació. Cal subratllar que aquesta operació en illa va plantejar un problema i es va convertir en un repte important per l’operador de distribució. Per tant, l'objectiu d'aquesta tesi és doble; resoldre un problema real, així com millorar els estudis de recerca actuals sobre les illes no intencionals en xarxes de distribució elèctrica. El fenomen de les illes dins una xarxa elèctrica, han estat objecte d’estudi durant els darrers anys a causa de la ràpida proliferació de la generació distribuïda. Habitualment, la potència d’aquests recursos distribuïts oscil·la entre centenars de kW i pocs MW i s’assignen a nivells de baixa tensió o mitja tensió. Una de les qüestions importants que plantegen aquests recursos és, sens dubte, les illes. Aquestes situacions es produeixen quan una part de la xarxa elèctrica funciona en paral·lel amb la xarxa principal després d’una desconnexió. Per això, les illes no intencionals es donen quan la generació distribuïda energitza la xarxa després de la obertura d’un interruptor. Principalment, l’objectiu es identificar aquesta situació i desconnectar dites fonts en el mínim temps possible. En el cas de que això no succeeixi, els següents disturbis poden produir-se; pertorbacions de la qualitat de potència (PQ) (per exemple, freqüència i tensió fora del rang), un perill per a la seguretat del personal de la xarxa o bé reconnexions fora de fase. Aquesta és la raó per la qual la investigació vers els mètodes de protecció “anti-islanding” han despertat un gran interès. Essencialment, la millora substancial d’aquesta tesi rau en el fet que, en aquesta illa, no hi ha recursos energètics distribuïts, sinó grans motors d’inducció. Així, la xarxa elèctrica continua energitzada després de la desconnexió del interruptor a causa dels motors d’inducció, que actuen de forma transitòria com a generadors. L’illa comença amb l’obertura del interruptor i finalitza quan aquest tanca el circuit per restablir el subministrament elèctric. Aquesta operació de reconnexió ràpid es freqüent en xarxes de distribució per evitar operacions manuals en faltes temporals i generalment oscil·la entre 0,5 i 1s. Tenint en compte que generalment les illes tenen l'origen en presència de generació distribuïda , realment, la illa elèctrica objecte d’aquesta tesi és inesperada per l’operador de distribució. A causa del fenomen esmentat anteriorment, els objectius específics d'aquesta tesi es descriuen a continuació: 1. El primer objectiu d'aquesta tesi se centra a desenvolupar un model adequat per la validació. Per fer una validació adequada del model, es compararan els resultats de les simulacions obtinguts amb aquest model amb els obtinguts de les mesures de camp. Així, un cop validat el model, es durà a terme una investigació completa sobre els factors més influents. 2. El segon objectiu d'aquesta tesi entra dins de l'àmbit d'aplicació del PQ. Durant l’esmentada illa, s’observa una nova topologia de forat de tensió. En conseqüència, els esforços se centraran en modelar aquest nou tipus de forat. 3. El tercer objectiu d'aquesta tesi s’emmarca en el punt de vista de proteccions. Un cop definida i caracteritzada l’illa, la necessitat d’identificar-la i prevenir-la esdevé la principal preocupació. D’aquesta manera, el tercer pilar de la tesi té com a objectiu la implementació d’una eina adequada per prevenir aquesta particular illa. A més, es compararà aquesta nova eina amb els actuals mètodes utilitzats per a identificar les illes en escenaris amb generació distribuïda

A novel assessment of unintentional islanding operations in distribution networks

This thesis aims to investigate an unexpected islanding operation (IO) which has been identified in a real distribution network. The process of recording and processing the data obtained from the field measurements in the distribution network (DN) has been the starting point of this research. It has to be underlined that this IO raised a problem and became a major challenge for the distribution operator. Therefore, the aim of this thesis is twofold; solving a real problem as well as further enhance the current research studies about IOs in DNs. IOs have been object of study during the recent years due to the rapid proliferation of the distributed generation (DG) within the so-called smart grids (SGs). Commonly, the power of these DG resources ranges between hundreds of kW and few MW and are allocated at either low voltage or medium voltage levels. One of the significant issues that these resources are raising is, undoubtedly, the IOs. These situations occur when a portion of the grid operates in parallel with the main grid following a disconnection. Thereby IOs, where the DG is energising the grid after a CB opening, must be identified and tripped in the minimum time possible. Failure to do so, the list of hurdles may include; power quality (PQ) disturbances (e.g., frequency and voltage out of range), a safety hazard for the network personnel or out-of-phase reclosings. That is the reason why the research towards the anti-islanding protection methods has elicited great interest. Fundamentally, the substantial improvement of this thesis lies in the fact that, in this IO, there are no DG resources, but large induction motors. In fact, the grid remains energised after the CB disconnection due to the induction motors (IMs) which transiently, act as generators. The island begins with the CB operation and ends when the CB recloses the circuit to restore the electrical supply. This rapid reclosing operation is widely adopted in DNs to avoid manual operations in self-extinguished faults and typically ranges between 0.5 and 1s. Given the fact that usually IOs are originated in the presence of DG, indeed, this IO is utterly unexpectedly for the DSO. Due to the phenomenon mentioned above, the specific goals of this thesis are described down below: 1. The first goal of this thesis focuses on developing a model suitable for validation purposes. To make a proper model validation, the simulations results obtained with this model will be compared with those obtained from field measurements. Thus, once the model has been validated, a thorough investigation regarding the most influential factors will be carried out. 2. The second goal of this thesis falls within the scope of the PQ. During the IO mentioned above, a new voltage sag topology is observed. Consequently, the efforts will be focused on modelling this new type of sag. 3. The third goal of this thesis emerges from the protective point of view. Once the IO has been defined and characterised, the need for identifying and preventing it becomes the main concern. In such a way, the third pillar of the thesis is targeted at implementing a suitable tool to prevent this particular IO. Besides, this new tool will be compared with the currently available methods for ID developed for scenarios with DG.Aquesta tesi te com a objectiu investigar una operació en illa no intencional, que ha set identificada en una xarxa de distribució real. El procés de registre i processament de les dades obtingudes a partir de les mesures de camp en la xarxa de distribució, ha estat el punt de partida d’aquesta investigació. Cal subratllar que aquesta operació en illa va plantejar un problema i es va convertir en un repte important per l’operador de distribució. Per tant, l'objectiu d'aquesta tesi és doble; resoldre un problema real, així com millorar els estudis de recerca actuals sobre les illes no intencionals en xarxes de distribució elèctrica. El fenomen de les illes dins una xarxa elèctrica, han estat objecte d’estudi durant els darrers anys a causa de la ràpida proliferació de la generació distribuïda. Habitualment, la potència d’aquests recursos distribuïts oscil·la entre centenars de kW i pocs MW i s’assignen a nivells de baixa tensió o mitja tensió. Una de les qüestions importants que plantegen aquests recursos és, sens dubte, les illes. Aquestes situacions es produeixen quan una part de la xarxa elèctrica funciona en paral·lel amb la xarxa principal després d’una desconnexió. Per això, les illes no intencionals es donen quan la generació distribuïda energitza la xarxa després de la obertura d’un interruptor. Principalment, l’objectiu es identificar aquesta situació i desconnectar dites fonts en el mínim temps possible. En el cas de que això no succeeixi, els següents disturbis poden produir-se; pertorbacions de la qualitat de potència (PQ) (per exemple, freqüència i tensió fora del rang), un perill per a la seguretat del personal de la xarxa o bé reconnexions fora de fase. Aquesta és la raó per la qual la investigació vers els mètodes de protecció “anti-islanding” han despertat un gran interès. Essencialment, la millora substancial d’aquesta tesi rau en el fet que, en aquesta illa, no hi ha recursos energètics distribuïts, sinó grans motors d’inducció. Així, la xarxa elèctrica continua energitzada després de la desconnexió del interruptor a causa dels motors d’inducció, que actuen de forma transitòria com a generadors. L’illa comença amb l’obertura del interruptor i finalitza quan aquest tanca el circuit per restablir el subministrament elèctric. Aquesta operació de reconnexió ràpid es freqüent en xarxes de distribució per evitar operacions manuals en faltes temporals i generalment oscil·la entre 0,5 i 1s. Tenint en compte que generalment les illes tenen l'origen en presència de generació distribuïda , realment, la illa elèctrica objecte d’aquesta tesi és inesperada per l’operador de distribució. A causa del fenomen esmentat anteriorment, els objectius específics d'aquesta tesi es descriuen a continuació: 1. El primer objectiu d'aquesta tesi se centra a desenvolupar un model adequat per la validació. Per fer una validació adequada del model, es compararan els resultats de les simulacions obtinguts amb aquest model amb els obtinguts de les mesures de camp. Així, un cop validat el model, es durà a terme una investigació completa sobre els factors més influents. 2. El segon objectiu d'aquesta tesi entra dins de l'àmbit d'aplicació del PQ. Durant l’esmentada illa, s’observa una nova topologia de forat de tensió. En conseqüència, els esforços se centraran en modelar aquest nou tipus de forat. 3. El tercer objectiu d'aquesta tesi s’emmarca en el punt de vista de proteccions. Un cop definida i caracteritzada l’illa, la necessitat d’identificar-la i prevenir-la esdevé la principal preocupació. D’aquesta manera, el tercer pilar de la tesi té com a objectiu la implementació d’una eina adequada per prevenir aquesta particular illa. A més, es compararà aquesta nova eina amb els actuals mètodes utilitzats per a identificar les illes en escenaris amb generació distribuïda.Postprint (published version

A novel assessment of unintentional islanding operations in distribution networks

This thesis aims to investigate an unexpected islanding operation (IO) which has been identified in a real distribution network. The process of recording and processing the data obtained from the field measurements in the distribution network (DN) has been the starting point of this research. It has to be underlined that this IO raised a problem and became a major challenge for the distribution operator. Therefore, the aim of this thesis is twofold; solving a real problem as well as further enhance the current research studies about IOs in DNs. IOs have been object of study during the recent years due to the rapid proliferation of the distributed generation (DG) within the so-called smart grids (SGs). Commonly, the power of these DG resources ranges between hundreds of kW and few MW and are allocated at either low voltage or medium voltage levels. One of the significant issues that these resources are raising is, undoubtedly, the IOs. These situations occur when a portion of the grid operates in parallel with the main grid following a disconnection. Thereby IOs, where the DG is energising the grid after a CB opening, must be identified and tripped in the minimum time possible. Failure to do so, the list of hurdles may include; power quality (PQ) disturbances (e.g., frequency and voltage out of range), a safety hazard for the network personnel or out-of-phase reclosings. That is the reason why the research towards the anti-islanding protection methods has elicited great interest. Fundamentally, the substantial improvement of this thesis lies in the fact that, in this IO, there are no DG resources, but large induction motors. In fact, the grid remains energised after the CB disconnection due to the induction motors (IMs) which transiently, act as generators. The island begins with the CB operation and ends when the CB recloses the circuit to restore the electrical supply. This rapid reclosing operation is widely adopted in DNs to avoid manual operations in self-extinguished faults and typically ranges between 0.5 and 1s. Given the fact that usually IOs are originated in the presence of DG, indeed, this IO is utterly unexpectedly for the DSO. Due to the phenomenon mentioned above, the specific goals of this thesis are described down below: 1. The first goal of this thesis focuses on developing a model suitable for validation purposes. To make a proper model validation, the simulations results obtained with this model will be compared with those obtained from field measurements. Thus, once the model has been validated, a thorough investigation regarding the most influential factors will be carried out. 2. The second goal of this thesis falls within the scope of the PQ. During the IO mentioned above, a new voltage sag topology is observed. Consequently, the efforts will be focused on modelling this new type of sag. 3. The third goal of this thesis emerges from the protective point of view. Once the IO has been defined and characterised, the need for identifying and preventing it becomes the main concern. In such a way, the third pillar of the thesis is targeted at implementing a suitable tool to prevent this particular IO. Besides, this new tool will be compared with the currently available methods for ID developed for scenarios with DG.Aquesta tesi te com a objectiu investigar una operació en illa no intencional, que ha set identificada en una xarxa de distribució real. El procés de registre i processament de les dades obtingudes a partir de les mesures de camp en la xarxa de distribució, ha estat el punt de partida d’aquesta investigació. Cal subratllar que aquesta operació en illa va plantejar un problema i es va convertir en un repte important per l’operador de distribució. Per tant, l'objectiu d'aquesta tesi és doble; resoldre un problema real, així com millorar els estudis de recerca actuals sobre les illes no intencionals en xarxes de distribució elèctrica. El fenomen de les illes dins una xarxa elèctrica, han estat objecte d’estudi durant els darrers anys a causa de la ràpida proliferació de la generació distribuïda. Habitualment, la potència d’aquests recursos distribuïts oscil·la entre centenars de kW i pocs MW i s’assignen a nivells de baixa tensió o mitja tensió. Una de les qüestions importants que plantegen aquests recursos és, sens dubte, les illes. Aquestes situacions es produeixen quan una part de la xarxa elèctrica funciona en paral·lel amb la xarxa principal després d’una desconnexió. Per això, les illes no intencionals es donen quan la generació distribuïda energitza la xarxa després de la obertura d’un interruptor. Principalment, l’objectiu es identificar aquesta situació i desconnectar dites fonts en el mínim temps possible. En el cas de que això no succeeixi, els següents disturbis poden produir-se; pertorbacions de la qualitat de potència (PQ) (per exemple, freqüència i tensió fora del rang), un perill per a la seguretat del personal de la xarxa o bé reconnexions fora de fase. Aquesta és la raó per la qual la investigació vers els mètodes de protecció “anti-islanding” han despertat un gran interès. Essencialment, la millora substancial d’aquesta tesi rau en el fet que, en aquesta illa, no hi ha recursos energètics distribuïts, sinó grans motors d’inducció. Així, la xarxa elèctrica continua energitzada després de la desconnexió del interruptor a causa dels motors d’inducció, que actuen de forma transitòria com a generadors. L’illa comença amb l’obertura del interruptor i finalitza quan aquest tanca el circuit per restablir el subministrament elèctric. Aquesta operació de reconnexió ràpid es freqüent en xarxes de distribució per evitar operacions manuals en faltes temporals i generalment oscil·la entre 0,5 i 1s. Tenint en compte que generalment les illes tenen l'origen en presència de generació distribuïda , realment, la illa elèctrica objecte d’aquesta tesi és inesperada per l’operador de distribució. A causa del fenomen esmentat anteriorment, els objectius específics d'aquesta tesi es descriuen a continuació: 1. El primer objectiu d'aquesta tesi se centra a desenvolupar un model adequat per la validació. Per fer una validació adequada del model, es compararan els resultats de les simulacions obtinguts amb aquest model amb els obtinguts de les mesures de camp. Així, un cop validat el model, es durà a terme una investigació completa sobre els factors més influents. 2. El segon objectiu d'aquesta tesi entra dins de l'àmbit d'aplicació del PQ. Durant l’esmentada illa, s’observa una nova topologia de forat de tensió. En conseqüència, els esforços se centraran en modelar aquest nou tipus de forat. 3. El tercer objectiu d'aquesta tesi s’emmarca en el punt de vista de proteccions. Un cop definida i caracteritzada l’illa, la necessitat d’identificar-la i prevenir-la esdevé la principal preocupació. D’aquesta manera, el tercer pilar de la tesi té com a objectiu la implementació d’una eina adequada per prevenir aquesta particular illa. A més, es compararà aquesta nova eina amb els actuals mètodes utilitzats per a identificar les illes en escenaris amb generació distribuïda

Volatility risk premium: new insights into the systematic edge in the market for option sellers

Financial options have been at the forefront of financial innovation. Their value depends significantly on volatility, one of the most studied variables of financial markets for a long time. Our study provides empirical evidence on the dynamics of the relationship between realized volatility of asset returns and implied volatility extracted from option prices, and on the so-called "volatility risk premium". We confirm that the volatility risk premium is still present in today’s market. Most literature looks at the volatility risk premium as a tool to investigate other financial dynamics. We innovate by investigating the patterns of volatility risk premium singularly as a time series, and so assess whether this is a stationary series, as well as if it presents signs of quarterly seasonality and/or marginal effects from two explanatory variables. We find a clear indication of stationarity and valuable marginal effects by financial variables such as realized volatilities. We also update the literature on the comparison between indices and individual equities surrounding the volatility risk premium; our results confirm that previous findings of a larger premium in individual equities are still applicable nowadays. Lastly, by analyzing option prices, we empirically confirm that volatility risk premium provides a monetary option mispricing. The results we obtain are supported by daily observations, from 2000 to 2020, on three equity indices, five individual equities, one commodity exchange-traded fund, one currency exchange-traded fund, and one emerging-market exchange-traded fund.A volatilidade tem sido, desde há muito tempo, uma das variáveis mais estudadas nos mercados financeiros. Para além disso, os produtos financeiros derivados, mais especificamente as opções financeiras, têm desempenhado um papel importante na inovação financeira. Como tal, pretendemos fornecer nova pesquisa académica relativamente às dinâmicas na relação entre volatilidade realizada e volatilidade implícita nos preços das opções, e acerca do denominado "prémio de risco de volatilidade". Confirmamos empiricamente que o prémio de risco de volatilidade está presente nos mercados à data de hoje. Introduzimos alguma inovação ao investigar o prémio de risco de volatilidade singularmente, procurando concluir se é uma série estacionária, bem como se apresenta padrões sazonais numa base trimestral e/ou correlação com outras variáveis financeiras. Neste sentido, concluímos que a série é estacionária, e encontramos correlações acentuadas com outras variáveis, tais como a volatilidade realizada. Procuramos também atualizar a literatura na comparação do prémio de risco de volatilidade entre índices de ações e ativos individuais, e confirmamos as conclusões da literatura anterior sobre o maior prémio de risco presente nos ativos individuais. Por fim, analisamos também os preços de opções, a fim de testar empiricamente se o prémio de risco de volatilidade distorce os preços das opções a um nível monetário material, concluindo que este é precisamente o caso. O estudo abrange o período de 2000 a 2020, com períodos variáveis consoante o ativo em questão. Os dados envolvem três índices de ações, cinco ações individuais e três ETF's. Os dados seguem uma frequência diária

Optimal tuning of fractional order controllers for dual active bridge-based DC microgrid including voltage stability assessment

In this article, three evolutionary search algorithms: particle swarm optimization (PSO), simulated annealing (SA) and genetic algorithms (GA), have been employed to determine the optimal parameter values of the fractional-order (FO)-PI controllers implemented in the dual active bridge-based (DAB) DC microgrid. The optimum strategy to obtain the parameters of these FO-PI controllers is still a major challenge for many power systems applications. The FO-PI controllers implemented in the DAB are used to control the DC link voltage to the desired value and limit the current flowing through the converter. Accordingly, the investigated control system has six parameters to be tuned simultaneously; Kp1, Ki1, ¿1 for FO-PI voltage controller and Kp2, Ki2, ¿2 for FO-PI current controller. Crucially, this tuning optimization process has been developed to enhance the voltage stability of a DC microgrid. By observing the frequency-domain analysis of the closed-loop and the results of the subsequent time-domain simulations, it has been demonstrated that the evolutionary algorithms have provided optimal controller gains, which ensures the voltage stability of the DC microgrid. The main contribution of the article can be considered in the successful application of evolutionary search algorithms to tune the parameters of FO-based dual loop controllers of a DC microgrid scheme whose power conditioner is a DAB topology.Peer ReviewedPostprint (published version

Assessment of unintentional islanding operations in distribution networks with large induction motors

This paper is aimed at assessing the impact of unintentional islanding operations (IOs) in the presence of large induction motors (IMs) within distribution networks (DNs). When a fault occurs,followingthecircuitbreaker(CB)faultclearing,theIMsacttransientlyasgenerators,duetoits inertia, until the CB reclosing takes place. The present work is the outcome of a project carried out in a small DN, where ¿eld measurements were recorded over two years. This paper provides a detailed description of the test system, a selected list of ¿eld measurements, and a discussion on modeling guidelinesusedtocreatethemodeloftheactualpowersystem. Themaingoalistovalidatethesystem model by comparing ¿eld measurements with simulation results. The comparison of simulations and ¿eld measurements prove the appropriateness of the modeling guidelines used in this work and highlight the high accuracy achieved in the implemented three-phase Matlab/Simulink modelPostprint (published version

FNAC Portugal in 2013 : how to react to the changes in the market?

The markets are continuously changing and the markets of technological and cultural products are no exception. When FNAC opened its first store in Portugal, in 1998, the market was quite different from how it is nowadays. The client motivations as well as expectations have been drastically changing and the access to certain products is now easier than ever, especially due to the Internet. This Thesis exposes the recent policies of FNAC: the new product categories, the bet on multichannel strategy, the new store formats and the new business model. All these are part of FNAC new strategic plan – FNAC 2015. These new policies seem to be the right answer to the problems that emerged from the evolution of the markets where FNAC operates. The key markets for FNAC have been dropping in the last years and the clients’ profile has changed. At the same time that FNAC wants to overpass these challenges it wants to expand its business in Portugal. The company should not only look at the bright side of these new policies but also look at the possible negative outcomes and be prepared for them. These policies can have some disadvantages to the brand and it’s up to it to overcome them and become a stronger brand.Os mercados estão continuamente a mudar e os mercados de produtos tecnológicos e culturais não exceção. Quando a FNAC abriu a sua primeira loja em Portugal, em 1998, o mercado era bastante diferente do que é hoje em dia. As motivações dos clientes e as suas expectativas têm mudado drasticamente e o acesso a determinados produtos é agora mais fácil do que nunca, especialmente devido à Internet. Esta tese expõe as recentes políticas da FNAC: as novas categorias de produto, a aposta na estratégia de multicanal, os novos formatos de loja e o novo modelo de negócio – que fazem parte do novo plano estratégico da FNAC: FNAC 2015. Estas novas políticas parecem ser a resposta correta para os problemas que surgiram da evolução dos mercados onde a FNAC opera. Estes mercados-chave para a FNAC têm caído nos últimos anos e o perfil de cliente mudou. Ao mesmo tempo que a FNAC quer ultrapassar estes desafios quer também expandir o seu negócio em Portugal. A empresa não deve olhar apenas para o lado positivo destas novas políticas mas deve também olhar para os possíveis resultados negativos e estar preparada para os mesmos. Estas novas políticas podem ter algumas desvantagens para a marca e cabe à FNAC ultrapassa-las e tornar-se uma marca mais forte