Browsing by Author "Lehtonen, Matti, Prof., Aalto University, Department of Electrical Engineering and Automation, Finland"

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  • Advanced Earth Fault Mitigation Using Virtual Air Gap Reactors
    (2024) Sevsek, David
     School of Electrical Engineering | Doctoral dissertation (monograph)
    The shift towards electrification of energy sectors and global climate change has increased the demand for reliable and safe electricity networks. This has posed a significant challenge for distribution system operators (DSOs), especially when dealing with single-line-to-earth (SLTE) faults, the most common type of faults in distribution networks. Traditional arc suppression coils (ASCs) used to limit SLTE fault currents are often limited by operational constraints. A novel type of ASC, the virtual air gap (VAG) reactor, may help to overcome this issue. These reactors use a set of auxiliary coils controlled by power electronics that can change their inductance within milliseconds. This quick inductance-changing capability could make VAG reactors a valuable asset when used as ASCs in compensated networks. To investigate the potential of VAG reactors, a dynamic time-domain model of VAG reactors was developed and validated by comparing it with a low-voltage (LV) VAG reactor. Based on the dynamic model, two distinct control approaches for VAG reactors were developed with the objective of using the reactor as an ASC in a compensated medium voltage (MV) distribution network. One controller has active harmonic mitigation, while the other requires less power but cannot control harmonics. The combination of the dynamic VAG reactor model along with the two distinct controllers was then compared to a traditional ASC in a series of time-domain MATLAB/Simulink simulations of a compensated MV distribution network. Furthermore, a series of laboratory tests were conducted to investigate the duration of an arc before its self-extinction, depending on the magnitude of the arcing current, the rate of rise of transient recovery voltage (RRTRV), and the length of the initial spark gap. The results of the simulations showed that the residual fault current can be effectively reduced during a SLTE fault by changing the inductance of the reactor to its optimal operating point. Both control approaches demonstrated the typical features of traditional ASCs, including the slowly increasing recovery voltage after fault extinction. The laboratory arc test showed that the duration of an arc before self-extinction depends more on the RRTRV than on the magnitude of the arcing current for currents at 10 A or below. Furthermore, it was found that smaller spark gaps result in longer burning arcs due to their more stable characteristics. The combination of the results of the laboratory arc test with the simulation results of the VAG reactors in the network simulation led to the conclusion that VAG reactors can be a viable alternative to traditional ASCs and help enhance the reliability of electricity networks.
  • Analysing flexibility in energy system investment planning - Impact of variable renewable energy, temporal structures and operational constraints
    (2024) Helistö, Niina
     School of Electrical Engineering | Doctoral dissertation (article-based)
    The proliferation of wind and solar energy increases the flexibility needs of power systems on multiple temporal and spatial scales. While various technologies exist and are being developed that can provide flexibility, exploring the interactions and roles of new and existing technologies in flexibility provision requires investment planning models which can correctly capture temporal, spatial, sectoral and technological diversity, and detail. Being aware of the most important details and the state-of-the-art methods for modelling them will facilitate higher quality planning results and help avoid misguided investment decisions. This dissertation focuses on developing and exploring methods and frameworks for assessing the need for and provision of flexibility when planning energy system investments. The methods should be able to capture important temporal variations, as well as the necessary operational constraints of energy systems. In addition, a number of case studies were carried out to explore the sensitivity of electricity prices, the role of conventional thermal power plants and the benefits of different energy technologies in future energy systems. The case studies provide insight into the type of power system flexibility needed with increasing shares of wind and solar energy, as well as insight into further modelling needs. The temporal representation of the investment planning model is shown to significantly impact the total system costs resulting from the planning outcome. Correctly capturing extreme periods and interannual variations in weather are key to enhancing resource adequacy considerations. Similarly, intra-annual variations need to be captured using, for example, appropriately selected representative days or weeks. According to the results, the best selection method and the sufficient number of selected periods depend on system characteristics. The results also suggest that the modelling of power plant start-ups and shutdowns, ramp rates, as well as simplified stability requirements and reserve products generally has less impact on total costs than the various possible temporal representations. However, correctly capturing the flexibility of sector-coupling technologies is demonstrated to have a significant impact. Investment planning capabilities and additional features related to flexibility were included in Backbone, an adaptable energy system modelling framework, which is also available as opensource software. Backbone can be utilised to create models for studying the design and operation of high-level large-scale and fully detailed smaller-scale energy systems from various perspectives.
  • Assessing the Flexibility of Demand Response and Sector Coupling for Efficient Power System Integration of Variable Renewable Generation
    (2022) Bashir, Arslan Ahmad
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Due to the rising concerns on global warming, renewable electricity generation is expected to form a considerable share in the future generation assortment worldwide. Given the fluctuating nature of renewable energy sources (RESs), their integration would adversely affect the power system balance. Addressing this challenge requires smart and flexible solutions. Demand response (DR) is a cost-effective load shaping tool that can follow the intermittent generation profile. The associated benefits can be stepped up by using excess renewable generation for the cross-sectoral integration, such as the power to heat (P2H) coupling that has the advantage of mitigating carbon emissions in both electricity and district heat (DH) sub-sectors. The objective of this dissertation is to assess the potential advantages offered by DR of thermostatically controlled loads (TCLs), P2H coupling and their tandem for efficient RESs integration. The goal of the dissertation is segmented into four major tasks. A comprehensive realization of the up- and down-ramping of TCLs is studied in the first task considering comfort priorities. The second task proposes an aggregator-oriented framework directed towards minimizing power imbalances and operating costs in a microgrid through thermostatic load management. The third task establishes a unique tool to optimize P2H coupling by harnessing the DR of HVAC loads in a housing community. Finally, the last task proposes a generic framework for the system-wide coupling among electricity, DH, and transport sub-sectors by utilizing the flexibility of the DH system to mitigate carbon emissions. To fulfil the objectives, the proposed models are simulated considering Finnish systems. The following deductions can be drawn from the results. The flexibility of TCLs is constrained by the load ratings, thermal comfort choices and respective demands. Activating DR substantially reduces power imbalances and operational costs. It is also revealed that mitigating curtailments and emissions by employing a deep borehole for P2H coupling is value-added under DR, as it also enables DH customers to participate in DR program despite flat tariff. Additionally, utilizing the deep borehole together with a small thermal storage can attain a carbon-free DH system. Finally, the inherent flexibility of the DH system has the potential to integrate a greater share of RESs, which enables utilizing the excess renewable generation in other sub-sectors, such as partial electrification of the DH and transport sub-sectors. The developed frameworks can be utilized by load aggregators, system operators, and policymakers to get a deep insight into the DR opportunities, the advantages of cross-sectoral integration for better integration of RESs to accomplish carbon-neutral energy systems as emphasized in the European Union climate strategy.
  • Connectivity for smart grids: Novel communications solutions in evolving electrical grids
    (2024) Borenius, Seppo
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Increasing electrification and power grid evolution to allow integrating large amounts of renewable generation will be key enablers for creating a sustainable, carbon-neutral energy system. The expected increased demand for electricity will result from efforts to reduce the use of fossil fuels in the industry, heating, and transport sectors. The larger share of intermittent generation from renewable sources, such as wind and solar, as well as the decreasing number of traditional controllable inertia-providing generators will lead to higher system volatility. This volatility challenge is worsened by the lack of seasonal system-level electric energy storage capacity. In distribution grids, the volatility challenge and increased power system dynamics will necessitate expanded automation, which in turn will require enhanced connectivity solutions. This thesis contributes first by taking a forward-looking, system-level view in exploring possible power grid futures and then by identifying approaches for integrating electric power grids with Information and Communications Technologies (ICT) in these futures. The overall research problem of the thesis is defined as follows: How can communications solutions support the creation of sustainable resilient power grids by the 2030s? The research extensively utilises expert panels, formal scenario planning and value networks based on the Finnish power grid context as a case example. The thesis proceeds in three stages. The thesis first establishes multiple scenarios, i.e. possible power grid futures. These describe the potential evolution from the perspective of grid management and the services offered to customers. Thereafter, in the second stage, the thesis explores the role of both the latest as well as anticipated new communication technologies, the feasibility of applying these in future distribution grids, and the potential impact of softwarisation on power grid architectures. The more extensive use of ICT gives rise to new attack points for malicious actors and consequently increases the vulnerability of the electric energy system. The thesis continues by identifying the most significant cybersecurity risks and trends, followed by an examination of how well these risks and trends are currently analysed and understood in academia and industry. In the third stage, the thesis shifts the focus to the business level. The opportunities for various actors are explored by identifying the potential industry (business) architectures for the communications solutions required to manage future distribution grids. The results of this thesis should help stakeholders, such as actors within the energy and ICT sectors as well as regulators and politicians, to consider alternative futures in order to make correct decisions on which businesses to be in, how to invest until the 2030s, as well as how to ensure the reliability and cost efficiency of the electric power system.
  • Distribution Automation and Self-Healing Urban Medium Voltage Networks
    (2016) Siirto, Osmo
     School of Electrical Engineering | Doctoral dissertation (article-based)
    In this dissertation, Distribution automation (DA) and self-healing methods for urban medium voltage (MV) networks are thoroughly investigated. DA is one of the key elements in mitigating the inconvenience caused by power outages. DA is especially targeted at shortening the interruption time once the fault has occurred. The research begins with the studies on customer interruption costs (CIC). Customer group and CIC variation in different urban areas are analysed. As a part of this research, also a postal survey to estimate the CIC of commercial customer was carried out. The idea is to define, where customers are mostly affected by interruptions and then utilize this information in DA optimization. It was shown, that CIC increases as the energy density increases and that CICs are highest in urban core. A general optimization method was developed and presented in this work. The utilized planning approach relies on the comparison between CICs and the lifetime costs associated with DA. Also the effects of substation outages to the benefit of DA are evaluated. A new general and flexible method of DA optimization is developed. The correlation between an optimal DA coverage and an urban area type is studied. Also the effect of substation outages on the benefit of distribution automation is evaluated. The results show, that focusing DA in an urban core is more beneficial than implementing DA evenly in MV network. Besides a new general layout, a new detailed DA optimization method is formulated. The new detailed model informs the exact location of the automation and takes into account the impact of earth fault events and their required time-consuming fault management process. Owing to the obtained results, ignoring the impact of earth faults and the fault management procedures, may lead to suboptimal DA optimization. After the novel general and detailed DA optimization models, practical self-healing implementations are presented. Neutral current compensation is an effective method of mitigating the interruptions. However the sustained operation during earth faults, enabled by compensating the neutral current, increases the risk of an earth fault evolving into multiple faults. To decrease this risk, the isolation of the fault should be rapid. This leads to a need of a fast fault management automation. New fault management features and functions for fast fault location, isolation and recovery are created and presented in this work. The need for comprehensive and inexpensive fault indications was described in this work. There is the need for a simple but a reliable earth fault indicator which is based on current measurements solely. Three new fault indication solutions are proposed. Finally a feasibility test of the methods presented in this work was performed in a real urban distribution network.
  • Earth Fault Distance Computation Methods Based on Transients in Power Distribution Systems
    (2014) Adzman, Mohd Rafi
     School of Electrical Engineering | Doctoral dissertation (monograph)
    The most common fault type in MV distribution network is single line to earth fault. The initial transients of earth faults are important especially for unearthed and compensated neutral networks. The earth fault transient signals consist of many different frequency components, which result from charging and discharging of the network capacitances. The transient components provide valuable information for fault location purposes. The charging component has higher amplitude and lower frequency than the discharge component and hence is more suitable to be used for fault location purposes. In this thesis, we discuss algorithms to locate an earth fault in unearthed or a compensated neutral MV networks using the information of the measured transient signal. The networks considered are assumed to be radially operated and they are modeled using Electromagnetic Transient Program-Alternative Transient Program (EMTP-ATP). Five types of fault location algorithms have been developed which are called general model (GM) algorithm, exact model (EM) algorithm, continuous wavelet transform (CWT) based method, multiple regression analysis (MRA) based method and artificial neural network (ANN). GM algorithm is developed based on a simplified model of symmetrical components while EM algorithm is developed with exact "pi"-model of symmetrical components. Both algorithms utilize the frequency of charging transient to estimate the fault distance. CWT based algorithm requires both voltage and current of transient signals to estimate the fault path inductance. MRA and NN algorithms were developed using the transient signal measured from the secondary side of the MV/LV distribution transformer. In addition, an algorithm to find a correct path towards the position of fault in network which has many branches is presented. The results from intensive simulations and experiments in actual distribution networks are also presented in this thesis. The results are analyzed using signal processing techniques. The algorithms apply continuous wavelet transform (CWT) to locate the dominant charge transient frequency and extract the specific coefficient corresponding to the charge transient frequency. In this thesis, the properties of Hilbert transformation (HT) are used to estimate the damping attenuation of the transient signal. Finally the performance of the proposed fault location algorithms is evaluated and the results are compared. Based on the simulation results, it is found that the proposed algorithms work at a reasonable level of accuracy. The results from real experiment data show that both CWT and GM algorithms have a comparable result.
  • Electricity distribution system planning considering incentive reliability regulations
    (2020) Jooshaki, Mohammad
     School of Electrical Engineering | Doctoral dissertation (monograph)
    Electricity distribution systems are dynamically expanded in anticipation of new demands. Cost and reliability are the most important factors in finding the optimal plans for the network expansion. Recently, implementation of incentive reliability regulations has accentuated the role of reliability considerations in distribution systems studies. This is because such incentive schemes create a direct link between distribution companies' revenues and their service reliability. Thus, these schemes have changed the role of reliability from a technical constraint to an economic factor. In this occasion, distribution companies require new techniques to incorporate the effects of incentive reliability regulations in their planning studies to get the most benefits. Motivated by these points, in this dissertation, various mathematical models are developed to incorporate the incentive reliability regulations into the planning studies of electricity distribution networks. Due to the rapid pace of technological advances in distribution systems, the proposed models consider a wide range of possible scenarios for future networks such as energy hubs, preventive maintenance scheduling, interconnection with natural gas distribution networks, and integration of various distributed generation technologies. In this respect, mathematical models for co-expansion planning of integrated electricity and natural gas networks, optimal scheduling of preventive maintenance actions, and optimal energy hub planning are devised. In addition, various frameworks are proposed to investigate the impacts of parameters of the incentive reliability schemes on distribution companies performance and potential outcomes of the implementation of the incentive regulations. In order to incorporate the reliability regulations into the panning studies, reliability indices must be quantified. Nonetheless, conventional reliability assessment techniques are not applicable due to their limitations. Novel reliability evaluation techniques are proposed in this thesis to address this issue. The proposed reliability models are cast as mixed-integer linear programming expressions which facilitates their integrations into standard mathematical programming models for distribution system planning and operation. It is worth mentioning that the reliability assessment models developed in this thesis can be applied to both passive and active distribution networks.
  • Harnessing Demand Response for Power System Flexibility - Residential Heating and Thermostatically Controlled Loads
    (2017) Alahäivälä, Antti
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Power systems are required to achieve a balance between generation and consumption in order to function. This task has conventionally been performed mainly by generators following the electricity demand. However, such a paradigm is changing along with the emerging of variable power generation. While a part of the generation becomes more variable, the rest of the power plants are left with increased responsibility to maintain the power balance. To assist with the balance maintenance, the electricity consumption can also be employed to provide the power system with flexibility. This demand-side flexibility, or demand response (DR), is the focus in this thesis. The aim of the dissertation is to propose a framework for the harnessing of DR for power system flexibility with a particular focus on residential heating loads. The topic is divided into three objectives, which are covered in this thesis and the publications. Firstly, the dissertation focuses on load aggregation whose purpose is to make the distributed and small flexibility resources visible and controllable. A central part of the aggregation approach is an aggregator, an entity acting as an intermediary between the loads and power system markets. The aggregator's daily operation is assisted by a virtual power plant, which automatically operates and responds to different situations in the maintenance of power system balance. Secondly, the thesis proposes three coordination strategies for the scheduling and control of DR. The strategies focus on different markets and parts of the power balance maintenance process, including a day-ahead electricity market, a balancing power market, and the manipulation of consumption patterns. At the same time, they provide alternative approaches for the interaction between the aggregator and consumers. The first of the introduced strategies emphasizes local decision-making and control, the second centralized, and the third is a combination of local and centralized. The functionality of the proposed strategies is tested with simulation studies. Thirdly, the thesis considers the utilization of DR in the power system frequency control. This application enables the loads to contribute to the power balance in the cases of disturbances and other unforeseen imbalances. Within the proposed framework, thermostatically controlled loads are able to react to the system frequency locally and to provide frequency containment reserves (FCR). Furthermore, the aggregator can centrally control loads in order to assists in frequency restoration. It is also argued with simulations that with a proper frequency coordination, the potential of DR to provide FCR can be improved.
  • Influence of Ground Electrical Properties on Lightning-Electromagnetic Fields for Wind Farms and Overhead Lines - Applications of the FDTD Method
    (2016) Rizk, Mohammad
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Direct and indirect lightning events result in transient disturbances in electric power systems that may cause damages of the components of the power system and interruptions of the power supply. The resistivity and permittivity of the ground have a significant influence on these transient disturbances due to both lightning events. In order to study this influence, five research tasks have been implemented using the finite-difference time-domain method. The 1st and 2nd tasks are relevant to indirect lightning problems where lightning-induced voltages (LIVs) on overhead lines have been investigated due to nearby return strokes on the ground. The 3rd and 4th tasks are related to direct lightning problems where the electromagnetic fields have been studied due to direct stroke to a grounding system and a grounded wind turbine (WT) considering the impact of the horizontal stratification of the ground. The 5th one includes both direct and indirect lightning problems where the electromagnetic fields are studied for a grounded WT struck by a return stroke beside a nearby overhead line. For a proper design of the overhead line insulation, it is required to accurately compute LIVs. The influences of the ground resistivity, permittivity and the rate of rise of lightning current are investigated on the lightning electromagnetic fields and LIVs impinging overhead lines. A formula has been proposed to calculate the peak values of LIVs due to the typical first and subsequent strokes in the IEEE Standard 1410-2010 considering high values of ground resistivity. The proposed formula is applied to calculate the indirect lightning performance in terms of an annual number of flashovers per 100 km of the overhead line. The grounding system is essential for effective lightning protection against direct strokes so the effect of ground resistivity and permittivity on the impedance of grounding systems has been studied. Such study is extended to investigate the lightning-electromagnetic fields on the cable sheath in a wind farm due to a lightning stroke striking a grounded WT. The impact of connecting the grounding systems of the WTs on mitigating these electromagnetic fields has been also investigated. For the horizontally stratified ground case, it is found that the reflections at the boundary between the two layers affect these electromagnetic fields significantly. Since overhead lines may exist nearby WTs, the influence of ground resistivity and permittivity on LIVs and electric fields impinging the line is studied due to a return stroke striking a nearby grounded WT. The first and subsequent strokes are adopted for this study besides the consideration of the horizontally stratified ground case.
  • Investigation of Lightning-Initiated Flashover Faults in Medium Voltage Overhead Lines - Modelling and Experimental Evaluation
    (2016) Mahmood, Farhan
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Lightning can cause flashovers on Medium Voltage (MV) lines from both direct and nearby strikes. The work presented in this dissertation thoroughly investigates the characteristics of lightning-initiated flashover faults, expected overvoltage stress and the performance of different surge protection schemes to mitigate the lightning overvoltages. A full-scale experimental set-up is established to investigate the nature of lightning-initiated flashover faults in MV lines. The flashover voltage of the overhead line is determined by changing the type (metallic/wood) and the configuration (grounded/ungrounded) of the cross arm. On the other hand, the instant at which the flashover occurs is determined by the actual volt-time curve of the line insulation whereas the arcing phenomenon is represented by the dynamic arc model. A complete model of the experimental set-up is developed in Alternative Transients Program–Electromagnetic Transients Program (ATP-EMTP) and the experimental results have been reproduced with reasonable accuracy. The lightning performance of a typical MV unearthed network due to both first and subsequent direct strokes is also analyzed. The transient overvoltages at the MV terminals of distribution transformer are determined from ATP-EMTP simulations. Accordingly, the effectiveness of different lightning protection schemes based on of spark gaps and surge arresters is also assessed. The volt-time curves and the flashover probability distributions of different types of insulation gaps subjected to positive and negative standard and short tail impulse voltages were established. The study was further extended by testing the insulation gaps with combined AC and lightning impulse voltages. The experimental flashover probability distributions are then compared with those predicted by the modified Gaussian cumulative distribution function of the insulation flashover under combined voltages. A statistical method of insulation coordination based on probabilistic risk assessment is also introduced in this work to evaluate the lightning performance of MV lines. In this regard, the probabilistic model of insulation flashover is experimentally validated to predict the probability of single-phase, two-phase, and three-phase flashover of insulators. Accordingly, the effect of combined AC and lightning-induced overvoltages on the risk of flashovers above perfectly conducting and lossy ground is also investigated. Finally, the risk-based insulation coordination method is applied to determine the optimum insulation level and spacing between the consecutive surge arresters for the mitigation of lightning-induced overvoltages.
  • Magnetically Controlled Reactor - a Novel Component in Smart Grid
    (2018) Penttonen, Jyrki
     School of Electrical Engineering | Doctoral dissertation (article-based)
    There is currently rapid increase of solar and wind generation in the grid and growing emphasis on the reliability and availability of electrical supply.  These changes have triggered a need to have a more flexible grid, which can adapt instantly to a different load, production and failure situations. First part of this thesis was to investigate ways to create grid elements presenting instantly controllable inductance, which would fulfill many Smart Grid needs such as compensating reactive power, controlling voltage in a distribution grid or compensating earth fault currents. A reactor topology was identified, modeled and validated in laboratory in which inductance could be controlled quickly, linearly and with no moving parts using an optimized virtual air gap. One promising application for this new type of instantly controllable and linear reactor is in compensating earth faults in resonant earthed distribution networks. Typically arc suppression coils (ASC) are not tuned to exact resonance due to practicalities related to current technology whereas the new magnetically controlled reactor developed in this work is fast and thus allows full resonance tuning. In a second part of this thesis earth fault arcs were modeled using black box models and it was established, that tuning ASC to full resonance improves the earth fault extinguishment probability. This result further indicates, that magnetically controlled reactor is a good replacement for current mechanically operated ASC. In a typical earth fault compensation arrangement the substation transformer provides delta connection to the medium voltage side thereby lacking neutral connection. Arc suppression coil is connected between earth and neutral, which means that normally an earthing transformer is needed to provide connection point for arc suppression coil. Third contribution of this thesis was to model and simulate a novel earth fault compensating reactor, in which the earthing transformer and arc suppression coil functions could be integrated into a same space saving structure, while providing instant control in zero sequence impedance using the magnetically controlled reactor developed in this work.
  • Methods for Arc-Flash Prediction in Medium Voltage and Low Voltage Switchgear
    (2015) Hussain, Ghulam Amjad
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Nowadays we are highly dependent on the electricity. It has become a need of daily life including domestic, commercial, transformational, industrial, health care and telecommunication purposes. Switchgear forms an integral part of the distribution network in power system. Hence, the occurrence of an electrical fault in switchgear causes interruption of electricity to the end users. Being highly dependent on electricity, such unexpected interruptions are unbearable by the modern society. Arc-flash is the unintentional discharge of electricity through air which produces very high temperature (19,600 ºC), which is hotter than the surface of the sun and a force equivalent to being hit by a hand grenade. Occurrence of an arc-flash in switchgear not only causes interruption of electric supply but damage to the equipment and personnel working in the vicinity. In the worst cases, damage is extended to the whole substation, hence maintenance duration can last upto few weeks. In USA alone, five to ten explosions occur due to arc-flashes every day. Due to the importance of the matter, arc-flash protection has been highly demanded for last two decades. Several reactive protection techniques have been introduced in the literature and some of them were implemented commercially. Continuous monitoring of the equipment and earlier detection of potential failures can facilitate a more proactive and a comprehensive arc-flash prevention system. This thesis mainly focuses on the pre-emptive detection of potential arc-flash in medium voltage (MV) and low voltage (LV) switchgear using online condition monitoring techniques. Three non-intrusive sensors i.e. D-dot sensor, Rogowski coil and thermal ionization detector have been designed, implemented and tested in switchgear to detect partial discharge, low power arcing and hotspots due to bad connections. Assessment of reliability, sensitivity and operability of the mentioned sensors is done in the laboratory. D-dot sensor is also tested on-site in operational MV switchgear for partial discharge. An effective de-noising technique based on discrete wavelet transform (DWT) is presented in this thesis. Moreover a novel idea for the detection of multiple faults in a panel based on correlation between the cumulative energy and apparent charge of discharge events is also presented. Hybrid low cost and non-intrusive solution integrated to the supervisory control and data acquisition (SCADA) system capable of continuous monitoring of switchgear and indicating any potential arc faults in switchgear before they lead to severe damage to the equipment and vicinity will be a breakthrough solution in minimizing arc-flash accidents, emergency interruptions and saving precious lives.
  • Methods for Monitoring Electromechanical Oscillations in Power Systems
    (2017) Seppänen, Janne
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Electromechanical oscillations are an inherent property of power systems and the damping of the oscillations is the limiting factor for the transmission capacity of certain transmission corridors. In the most severe situations, unstable oscillations may lead to blackouts. Thus, it is important to monitor the characteristics of the oscillations. The oscillations can be monitored for example by using phasor measurement units (PMU). The development of wide area monitoring systems (WAMS) consisting of several PMUs has enabled the use of multiple synchronized measurement signals received from several locations in the power system to be used for the monitoring and analysis of the oscillatory modes. This thesis presents four new multivariate methods (i.e. use several measurements from different locations of the grid) for the monitoring of the electromechanical modes. The methods are able to continuously identify electromechanical modes using ambient oscillations, which are mainly excited by load variations and are constantly present in power systems. The performance, characteristics and limitations of the methods are studied using simulated data as well as real measured data. This thesis also presents comparisons of different modal identification methods and illustrates additional analysis tools that can be used to support the modal identification in real power systems. This thesis shows that the proposed methods are functional for monitoring of electromechanical modes. Due to certain limitations in modal identification methods, the thesis also highlights the need of using additional tools, such as spectral analyses, which may significantly help the interpretation of modal identification results. The methods presented in this thesis can be used as building blocks for transmission system operators (TSO) to create functional applications for real-time and offline modal analysis of power systems. Consequently, the information given by the methods may be used to improve the security and reliability of power systems.
  • Modelling Escalator Power Consumption and Demand Response Potential
    (2019) Uimonen, Semen
     School of Electrical Engineering | Doctoral dissertation (article-based)
    The power system faces a major transformation due to the increased penetration of renewable energy sources. Uncertainties, as a result of the intermittent-nature of renewable power generation, require the power system to be proportionally flexible. Maintaining the balance between the supply and demand while providing enough stability in the grid has become a challenge that requires participation of the demand-side. Engagement of the consumer-side can be passive, through utilizing energy efficient technologies, and active, through a mechanism called demand response. This dissertation focuses on escalator potential in demand response and its energy efficiency. More specifically the thesis provides an angle and a framework for escalator technology to participate in the balancing of the power system grid by providing flexibility. Developed approaches can be used as a supportive tool in decision-making processes for various stakeholders, such as building designers, managers and investors. The description of how escalators can be utilized in demand response is separated into two main topics. First, the escalator power consumption measurements and modelling approaches are used to develop a simulation tool that allows modelling high-resolution power consumption profiles. The main modelling approach is based on simulating the impact of passenger traffic on the power consumption profiles. Long-term power consumption measurements indicate that passenger traffic is recurring which allows to simulate the power consumption of escalators throughout the year. The approach also allows to simulate large numbers of escalator units and the aggregate of power consumption. Second, the developed simulation tool is utilized to model the available flexibility for demand response. The selected approach to reducing the escalator power consumption is through speed reduction. One of the consequences of the method is that speed reduction causes delays to the passenger travel time and might increase the queuing time. The simulation results indicate that the proposed approach for power curtailment meets the technical requirements for participation in the incentive-based and price-based demand response programs specified by the energy markets. In the thesis, the flexibility modelling process progresses from simulation-based models to statistical models which focus on predicting the possible power curtailment of escalator units for frequency containment. As an example of utilizing the statistical approach, the thesis proposes a solution for selecting the best fitting escalators for a task that requires smaller than available target of power curtailment. The statistical model allows to greatly reduce the computation time, which allows frequent short-term predictions.  
  • Models for Evaluating the Power Consumption of Elevators - The Perspective of Power Systems and Demand Response
    (2019) Tukia, Toni
     School of Electrical Engineering | Doctoral dissertation (article-based)
    The field of power systems is experiencing multiple changes that affect the designing and operation of power generation and delivery. The most significant drivers of the transition are climate change and urbanization. The climate change has accelerated the installation of intermittent, renewable power sources, which has increased the need for demand-side flexibility, or demand response (DR). On the other hand, the accelerating urbanization alters the demand of power, especially the distribution of load types, necessitating updates to load modeling. This dissertation focuses on developing power consumption models for an electric load type which has a rapidly increasing installed base due to the accelerating urbanization – the elevators. The main objective is to provide models which are relatively simple to adopt while providing adequate accuracy. The modeling has been divided into four stages.  The first stage of the modeling begins by assessing the diurnal, weekly, and seasonal energy consumption patterns of elevators with the help of literature and measurements. The results of the first stage indicate that elevator energy consumption is strongly recurring and the consumption patterns correlate with the day types, which can be obtained from the calendar. Furthermore, the intraday power consumption profiles are shown to result from the experienced passenger traffic patterns. The second stage of the modeling depicts an elevator model capable of providing the power consumption in high-resolution as a result of the simulated passenger traffic. The elevator model entails multiple layers. First, a collective group control algorithm is employed to minimize the waiting time of passengers in multi-unit elevator groups. Second, the power consumption profiles of each resulting trip are modeled by considering the mechanical and electrical properties of the elevator, the speed and direction of the trip, and the concurrent loading caused by the passengers. In addition, the stationary (standby) power demand is modeled to occupy the time between the trips. The third stage of the modeling employs the created elevator model to simulate the power consumption profile of a large elevator fleet with varying characteristics. The dissertation then assesses the power system-specific characteristics of the aggregated elevator power consumption in dense, urban areas with a high concentration of elevators. The fourth stage of the modeling is focused on evaluating the potential of elevators in demand response. With the combined simulation of elevator passenger traffic and the resulting power consumption, the model enables a detailed view of the performance of different control methods in terms of the obtained power change against the delay experienced by the passengers. Therefore, the approach can be employed to compare various elevator setups and apply DR actions only to the most favorable units to optimize the selection process of DR participation.
  • Novel Methods for Arcing Fault Detection and Location in Power Distribution Systems
    (2017) Zoko Ble, Frank
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Conventional power system protection functions mainly deal with currents and voltages associated with faults and most protection devices monitor current, voltage, or impedance against critical setting values. However, in many arc fault conditions of earlier stage, the arc associated current amount is under the setting of relays, and subsequently the arc continues to burn undetected. Power arc generates electromagnetic signals. Radiation producing power arcs can originate from different sources and causes, like due to tree leaning on energized conductors, downed conductor or broken insulators. This thesis first investigates the characteristics of these electromagnetic signals and then examines algorithms for arc fault location using laboratory produced arcs. Using strategically placed antennas the electric arc electromagnetic (EM) radiation can be detected at remote distance and, to some extent, the arc source location can also be determined. This work investigates feasibility of detecting and locating the actual arc source point in 3D Cartesian plane through the multiple radiation detection and location methods. In this study the following six different arcing fault detection and location methods are compared in pinpointing electric arc electromagnetic (EM) sources in power systems: cross-correlation method (XCORR), first peak of arrival method (FPA), leading edge of first arrival peak method (LEFAP), energy attenuation method (ENERGY) or inverse square method, angle of arrival method (AoA), and wavelet analysis (WAVELET). It is evident that the interpretation of the results based on these six methods depends to some extent on statistical methods; therefore, the methods undergo statistical analysis evaluation in order to determine a suitable power arc location algorithm which can be used in real field measurements. Specifically, analysis of variance (ANOVA) and multiple linear regressions (MLR) are applied for statistical estimation and for comparison of the results with the actual source point. The results show that it is feasible for the six methods, using the radiated signals from arcing fault, to detect and locate the arcing source with a reasonable accuracy. The cross-correlation among these proposed methods shows a better potential in clarifying the electric arc source position when compared with other algorithms.
  • Optimal Market Strategies in Energy Systems with High Penetration of Wind Power
    (2022) Tavakkoli, Mehdi
     School of Electrical Engineering | Doctoral dissertation (article-based)
    The ever-increasing penetration of renewable energy resources (RERs), specifically wind power generation (WPG) into the power grid has significantly changed the operation of power systems. Integration of large-scale WPG creates different challenges for the power systems, hence obliging them to provide sufficient flexibility to accommodate uncertainties arisen from the unpredictable nature of WPG. To this end, providing higher operating reserve capacity for the power system is motivated for large scale integration of intermittent WPG. The other alternative for addressing this issue is demand response management, an economically and practically efficient way for providing flexibility, which has emerged as a key concern in practice and has become the focus of many works. This thesis proposes a novel method to incorporate the reserve capacity for the wind power producers (WPP) in the electricity market. At the next step, it presents a novel and comprehensive model for demands to investigate their strategic behavior in the electricity market while there is a large share of WPG in the system.
  • Photovoltaic Hosting Capacity of Distribution Networks
    (2024) Püvi, Verner
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Due to an increasing share of renewable energy sources and widespread adoption of distributed photovoltaics (PV), the ability of the distribution networks to reliably interconnect new PV installations without hindering the power system's operation gained a lot of interest in the recent decade. Political support and falling prices of the photovoltaics make the panels available for as small as single household installations and some of the network operators have already reported the power quality issues caused by the PVs. To address this issue, this thesis investigates the PV hosting capacity (HC) of low-voltage distribution networks. The thesis is split into two main contributions centered around the power quality limitations of the HC and network structure influence on the HC. The thesis starts with a review of the HC definitions, its most common limiting factors, and reports on the results of a measurement campaign of low-voltage substations. In the first part, a Monte Carlo-based HC evaluation methodology is presented, which is used for the PV-only and energy storage-augmented scenarios. Alongside the single-phase PV installations, a voltage unbalance (VU) mitigation methodology is presented. Despite the VU being a very strict limit, it can be mitigated by relatively low power injections. Moreover, a comparison of PV curtailment and network reinforcement is presented to find the break-even points of the costs of the two. The second part of the thesis presents the distribution network's structure impact on the hosting capacity. A fixed set of customers is simulated with multiple feeding substations and a varying number of PV plants. The slime mold algorithm was proposed to be employed for generating numerous network topologies and its advantages over other algorithms were shown. The results revealed that around one-third of the customers can have PV installations until the HC is depleted. Voltage control can increase the HC, however remains the risk of possible need to change residential PV policies to sustain the current pace of PV installations. Finally, the thesis explores the practical side of the HC and analyzes the accuracy of distribution network state estimation. A PV safety margin is proposed, that represents an equivalent PV power that has to be curtailed in order to keep the estimated values below the actual values of the states.
  • Real-time thermal state and component loading estimation in active distribution networks
    (2015) Degefa, Merkebu
     School of Electrical Engineering | Doctoral dissertation (article-based)
    Highly stochastic loading and distributed generation in the emerging active distribution networks means that electric utilities need to deploy intelligent network management tools in order to use their assets to the fullest. Real-Time Thermal Rating (RTTR) provides the possibility for short term and even real-time active distribution network management, enabling the network to run closer to an overload state without damage. In this dissertation, pertinent developments and proposals are presented in three stages on the path towards the development of a real-time monitoring and operation system for active distribution networks. The first stage is the development of distribution network component thermal models for real time implementation. In this dissertation, a numerical model of the air-gap convective heat transfer for underground cable installations inside unfilled conduit is developed. In addition, a dynamic thermal model is developed for prefabricated secondary substation cabins. The most dominant but difficult to solve heat transfer mechanism, natural convection, is modelled by introducing the stack effect principle into the problem. Measurements from a scaled model of prefabricated substations, measurements from actual cabins and 3D Finite Element Method (FEM) simulations are used to validate the numerical model. In the second stage, this dissertation explores the usability of customer level automatic meter reading (AMR) measurements for distribution network state estimation and for load forecasting applications. A method to forecast substation level loads with their respective confidence intervals using hourly AMR metered customer level consumptions is presented. The forecasting and monitoring of a distribution network in real-time can be met with the modeling of classified type load classes. However, it requires careful incorporation of the necessary factors, such as within-group and between-group correlations of customer classes. Binding the aforementioned findings, in the third stage, a framework for day-ahead hour-by-hour thermal state forecasting and thermal ratings of distribution network components is proposed and studied. This work has demonstrated that up to three hours ahead thermal state forecasting of an active distribution network can be achieved with an acceptable level of accuracy. In this dissertation, the benefits and practical implications of the real-time thermal rating are investigated. The introduction of real-time thermal rating in an active distribution network management system enhances the loading capacity significantly compared to static rating. This has been revealed through an increased utilization of installed DGs and through better integration potential of additional DGs.
  • Statistical Methods for Variable Renewable Energy Generation Modelling
    (2018) Ekström, Jussi
     School of Electrical Engineering | Doctoral dissertation (article-based)
    One significant way to mitigate global warming is to replace energy generation based on fossil fuels with CO2 emission free energy sources. In electricity generation, these kinds of energy sources are, e.g., hydro, nuclear and variable renewable energy (VRE) sources, such as wind and solar energy. The installed capacities of wind power and photovoltaic panels are growing globally at an increasing pace. However, the increase of the installed variable generation capacity can cause problems for the power systems as it also increases the variability of the power generation. To ensure reliable operation of the power systems and large scale integration of VRE generation, the behaviour of VRE generation has to be modelled and understood both in short and long terms.  This thesis focuses on developing statistical modelling methodologies applicable in Monte Carlo simulations for the long term modelling of wind and solar power generation in new non-measured generation locations. Modelling methodologies are developed for the modelling of new wind power plants (WPPs) and new photovoltaic power plants (PVPs) in non-measured locations, and for the joint modelling of both generation types. Furthermore, an approach to model wind direction in new WPPs is developed and a methodology for the modelling of changing installed capacity of wind generation in large geographical areas is proposed.  The proposed methodologies are able to model both temporal and spatial dependency structures in multiple new generation locations and can be applied in the analysis of the variability of the generation and power ramps. The main modelling approach in the proposed methodologies is based on time series models combined with data transformations comparable to copula modelling. The developed methodologies are verified against out-of-sample test data, consisting of measurements which were left out from the model selection and estimation processes.  In addition, the effects of the geographical distribution of the VRE generation locations to the aggregated power generation are studied with WPPs and PVPs. The observed and quantified impact of the negative correlation between WPPs and PVPs is analysed jointly with geographical distributions of the locations.