Math Bollen

Luleå University of Technology | LTU

Alternative solutions, instead of building new transmission lines, are needed to enable the fast electrification of sectors such as industry, transportation, and heating. This includes smart-grid technology allowing for an increase in hosting capacity without the need for building new transmission lines. The increase in hosting capacity beyond the...

This paper utilizes the Unscented Transform together with Cornish-Fisher expansion to estimate the 2%-value of switching overvoltages when considering the impact of trapped charge and corona attenuation. Simulations were performed using a piecewise linear corona model for twin- and triple-conductor lines. The proposed method was compared to Monte C...

The ability of the distribution network to host electric vehicle (EV) charging might be limited by the harmonic voltages due to their harmonic emission. Network harmonic impedance seen from the point of connection of the charging station plays an important role in harmonic voltage calculation. In this paper, the hosting capacity is estimated for a...

This item starts with a general discussion on power quality, including definitions, terminology, and requirements. It further contains sections on the importance of power quality (which in the end all comes down to reducing the likelihood of interference, i.e., equipment not functioning as it should); some of the subjects that have been historicall...

This paper aims to introduce deep learning to the power quality community by reviewing the latest applications and discussing the open challenges of this technology. Publications covering deep learning to power quality are stratified in terms of application, type of data, and learning technique. This work shows that the majority of the deep learnin...

The harmonic emission from an electric vehicle fast charger depends on factors like charger topology, EV type, initial state of charge of EV battery, as well as supply voltage and background distortion. This paper presents the results from harmonic current measurement of a fast charger for a period of one month in Sweden that has charged a variety...

This paper addresses the issue of seeking sub-10-min patterns in fast rms voltage variations from time-limited measurement data at multiple locations worldwide. This is a rarely considered time scale in studies that could be important for the incorrect operation of end-user equipment. Moreover, measurements from multiple locations could be signific...

With the increasing deployment of solar power, high photovoltaic (PV) penetration is expected to adversely impact the distribution grid. One of the challenges relates to the power flow and transfer capacity of the distribution transformers. Transformers might not have sufficient capacity to accommodate for all the downstream PV to feed back to a hi...

Dynamic line rating (DLR) is a grid enhancing technology to enable a more effective use of transmission capacity of existing infrastructure. The growth in load consumption along with a high integration of electric vehicles (EV) highlights the potential of DLR utilization for reducing the congestion costs and overloading risks. Selecting the proper...

This prestudy has been carried out by RISE Research Institute of Sweden and Luleå University of Technology together with additional 24 stakeholders active in a hydrogen energy system in northern Sweden. Actors and investments have been mapped via interviews within and outside of the project consortium, and techno-economic and regulatory analyses ha...

This work shows that transients at voltage dips starting impact fault-ride-through of wind turbines. For fault-ride-through studies and manufacturer tests, it is therefore important to consider these transients and their magnification from the transmission grid through the collection grid to the wind turbines. Fault-ride-through studies in the lite...

The increased complexity of the transmission grid can endanger the operational security of the grid. Operational risk assessment, a stochastic tool, helps to enhance security. Contingency analysis and its impact quantification are the main constituents of operational risk assessment. In this study, different graphical methods are proposed to visual...

Operational risk assessment is a stochastic approach to quantify the operational security of power systems. In this study, the interrelation between severity factor and operational risk, two important parameters in operational risk assessment, is analyzed. Four different definitions of severity factor, based on the results from network studies, are...

This paper proposes a number of deterministic and stochastic approaches to quantify the hosting capacity of the distribution network for solar photovoltaics (PV) units when that hosting capacity is limited by the loadability of feeder cables or distribution transformers. Such approaches should be used as input to investment and planning decisions b...

Recognition and analysis of voltage sags (dips) allow network operators to predict and prevent problems in real-life applications. Clearing the voltage sag source by direction detection methods is the most effective way to solve and improve the voltage sags and their related problems. However, the existing analytical methods use single or two input...

This paper proposes an unsupervised learning schema for seeking the patterns in rms voltage variations at the time scale between 1 s and 10 min, a rarely considered time scale in studies but could be relevant for incorrect operation of end-user equipment. The proposed framework employs a Kernel Principal Component Analysis (KPCA) followed by a k-me...

This paper proposes a machine-learning-based framework for voltage quality analytics, where the space phasor model (SPM) of the three-phase voltages before, during, and after the event is applied as input data. The framework proceeds along with three main steps: (a) event extraction, (b) event characterization, and (c) additional information extrac...

Light-emitting diode (LED) lighting has, compared to other types of lighting, a significantly lower energy consumption. However, the perceived service life is also important for customer satisfaction and here there is a discrepancy between customers’ experience and manufacturers’ statements. Many customers experience a significantly shorter service...

The increasing demand of the data center's computational capacity in recent years has introduced new data center operational challenges among others to maintain the service level agreements (SLA) and quality of services (QoS), while at the same time limiting energy consumption. In this paper, a stochastic operational risk assessment approach is pre...

Harmonic modeling of low-voltage networks with many devices requires simple but accurate models. This paper investigates the advantages and drawbacks of such models to predict the current harmonics created by single-phase full-bridge rectifiers. An overview is given of the methods, limiting the focus to harmonic analysis. The error of each method,...

Uncertainties in power system studies are typically managed by Monte Carlo methods or by applying some worst case assumptions. As the number of uncertainties increases, there is a need for methods that can estimate statistical parameters from a limited number of calculations. This paper utilizes a method called the Unscented Transform together with...

In the presence of multiple power converters in a power system the total current may exhibit zero-crossing distortion in the form of recurrent damped oscillations with high magnitude. These oscillations are synchronized with the power system frequency. This paper presents a comprehensive analysis of the input current of single-phase ac-dc converter...

Enhancing the efficiency and the reliability of the data center are the technical challenges for maintaining the quality of services for the end-users in the data center operation. The energy consumption models of the data center components are pivotal for ensuring the optimal design of the internal facilities and limiting the energy consumption of...

This paper presents results of harmonic voltage measurements at multiple locations in a low voltage network. Measurements were performed in different residential, industrial and commercial urban areas in the north of Sweden. This paper shows existing level of individual harmonics in these locations as a sample of an urban load and compares the harm...

This work presents measurements on how the emission from a lighting installation, in the frequency range up to 2 kHz, increases with the number of lamps. Measurements are performed on an installation with lamps of the same brand and type. Two indicators are used to quantify the deviation of a linear increase: harmonic deviation factor; aggregation...

The capacity of a power network to host new distributed generation units and non-linear loads like PVs and EVs can be studied from different aspects, such as over voltage, unbalance and harmonic distortion. This paper proposes a new approach for estimating a safe hosting capacity in terms of harmonic distortion, considering standard limits for harm...

Harmonic emission at any point of connection (PoC) depends on the loads connected there and the background voltage distortion at that point. Under balanced supply voltage and load, the harmonics adhere to the rule that third harmonics are zero sequence, fifth harmonic are negative sequence, etc. However, under unbalance in either load or supply vol...

Several tools are available for reducing large quantities of data in order to obtain indices and reporting formats, like IEC 61000-4-30 and the recommendations by CIGRE C4.112. But those methods often suffer from a surplus of information or a lack of information. In this context, this paper introduces a number of alternative graphical methods and v...

Harmonic emission from a wind park to the grid and the interaction between individual turbines within a wind park are among the power quality challenges that have been studied for many years, as part of the connection of wind parks to the grid. Different methods are proposed and various simulation models are developed under some assumptions. This p...

The data centers host sensitive electronic devices like servers, memory, hard disks, network devices, etc., which are supplied by the power supply units. The regulated direct current (DC) output of the power supply units fluctuates with input voltage variation since they typically contain single phase switch-mode power supplies. The voltage dips ca...

Residual current devices (RCDs) are used in low voltage installations to disconnect the circuit in case of fault when the leakage current exceeds a reference magnitude. The magnitude of reference leakage current is based on the magnitude of current that human body can withstand without getting shocked. This reference current is defined based on the...

Residual current devices (RCDs) are used in low voltage installations to disconnect the circuit in case of fault when the leakage current exceeds a reference magnitude. The magnitude of reference leakage current is based on the magnitude of current that the human body can withstand without getting shocked. This reference current is defined based on...

This paper presents a visualisation method, based on deep learning (DL), to assist power engineers in the analysis of large amounts of power-quality data. The method assists in extracting and understanding daily, weekly and seasonal variations in harmonic voltage. Measurements from 10 kV and 0.4 kV in a Swedish distribution network are applied to m...

This work aims to verify the fault-ride-through (FRT) of a wind turbine (WT) by considering two distinct characterization approaches to obtain synthetic dips from a real measurement. One of the approaches considers a detailed characterization to generate the synthetic dip based on the checklist developed by CIGRE working group C4.110. The other app...

This work applies an unsupervised deep feature learning to finding patterns of interharmonics. The main objectives of this work are to provide an additional graphical tool to handle two distinct data inputs: (a) individual interharmonics components in time-series; (b) broadband spectrum by employing spectrograms. Both data inputs are analysed emplo...

Residual current devices (RCDs) are used in low voltage installations to disconnect the circuit in case of fault when the leakage current exceeds a reference magnitude. The magnitude of reference leakage current is based on the magnitude of current that human body can withstand without getting shocked. This reference current is defined based on the...

This paper presents a stochastic approach to single-phase and three-phase EV charge hosting capacity for distribution networks. The method includes the two types of uncertainties, aleatory and epistemic, and is developed from an equivalent method that was applied to solar PV hosting capacity estimation. The method is applied to two existing low-vol...

In this paper, primary and secondary emissions in wind power plants are studied by using transfer admittance and current transfer functions between turbines and the public grid. The use of such transfer functions allows harmonic propagation studies without knowledge of the emission from individual turbines or the background voltage distortion. The...

This paper proposes a stochastic method, ''mixed aleatory-epistemic“, for estimating solar PV hosting capacity (HC) of low-voltage (LV) distribution networks. The approach treats the aleatory and epistemic uncertainties in a different way. The HC is estimated by applying the transfer impedance matrix, 'which is only calculated once', and the superp...

This paper is an overview of the components and failure modes that should be considered when studying the reliability of dynamic thermal rating (DTR) in the power system. A reliability study should include failure of a measurement device, failure of the communication, model inaccuracy, and failure of the control system. The presence of the DTR syst...

With a noticeable increase in the number of electric vehicles (EVs) in the coming years, the lack of grid capacity may become a barrier to the electrification of transportation. This study presents the application of dynamic rating to increase the available transfer capacity and to enable more increase in consumption. Through stochastic calculation...

This paper gives a detailed analysis of the radiation terms in the heating balance for overhead conductors. The terms in the radiation balance determine the rating of an overhead line for use in dynamic line rating schemes. These radiation terms are often not considered and in which there is a relatively big uncertainty. The available knowledge and...

This paper proposes a deep learning (DL) method for the identification of spectral patterns of timevarying waveform distortion in photovoltaic (PV) installations. The PQ big data with information on harmonic and/or interharmonics in PV installations is handled by a deep autoencoder followed by feature clustering. Measurements of voltage and current...

This paper presents the role of the medium-voltage network and low-voltage loads in harmonic voltages with low-voltage customers and harmonic propagation. A general model, as well as a detailed transfer function-based model, are used. By applying them to an existing network, it is shown that remote low-voltage loads have a significant impact on the...

This paper presents the statistical analysis of variations in harmonic voltage in the time scale between 1 second and 10 min, a time scale that is rarely considered in studies but that is of relevance for incorrect operation of end-user equipment. The paper also introduces a set of novel power-quality indices quantifying these variations and it giv...

Supraharmonics (SH) have proliferated in low-voltage (LV) and medium-voltage (MV) grids due to the increasing use of technologies emitting distortion in the range 2–150 kHz. Currently, no recommended practices to assess the effects of SH on the electrical system exist. The propagation of SH through LV and MV grids leads to interference with the ele...

This paper presents an uncertainty analysis of lines equipped with dynamic line rating (DLR) that are exposed to operational overloading. Multiple sources of uncertainties are taken in to account to model the line rating probabilistically. The superiority of dynamic rating above static rating is confirmed in this paper. However, it is shown in this...

This paper addresses the issue of automatically seeking and identifying daily, weekly and seasonal patterns in harmonic voltage from measurement data at multiple locations. We propose a novel framework that employs deep autoencoder (DAE) followed by k-mean clustering. The DAE is used for extracting principal features from time series of harmonic vo...

Changes in the power system such as an increased use of cables in transmission systems lead to lower resonance frequencies, a possible consequence being an increased risk for resonant overvoltages. This paper presents a method for protecting the power system from resonant overvoltages caused by transformer energization or fault clearing in the vici...

Sufficient fault-ride-through (FRT) of large wind power plants (WPPs) is essential for ensuring transmission-grid operational security. The majority of FRT studies do not include all disturbances originated in the transmission grid or include disturbances not relevant for operational security. Using knowledge of power quality, this paper provides a...

The energy demand of data centers is increasing globally with the increasing demand for computational resources to ensure the quality of services. It is important to quantify the required resources to comply with the computational workloads at the rack-level. In this paper, a novel reliability index called loss of workload probability is presented...

The number of data centers and the energy demand are increasing globally with the development of information and communication technology (ICT). The data center operators are facing challenges to limit the internal power losses and the unexpected outages of the computational resources or servers. The power losses of the internal power supply system...

Swedish government’s target is to have 100% renewable electricity production by 2040. Currently, hydropower contributes the majority of renewable electricity generation of the country. The wind power capacity has increased significantly in the past decade. In this paper, practical data is used to study the possibility of reaching the 100% renewable...

Power converters with high switching frequency used to integrate renewable power sources to medium and low voltage networks are sources of emission in the supraharmonic range (2 to 150 kHz). When such converters are connected to a medium voltage (MV) network these supraharmonics propagate through the MV network and can impact network and customer e...

A techno-economic energy flow model for a standalone microgrid was developed to investigate the investment risks related to consumption changes and compare the results to a conventional grid-connection in Sweden. Two different design strategies for a standalone microgrid was used, one with the objective to minimize the life-cycle cost and the other...

The harmonic interaction mechanism in a wind park is examined in this paper. The paper investigates the feasibility of a solution to the yet challenging harmonic contribution estimation from multiple sources in a wind park with limited available information, via an extension of a simple modeling approach as well as from detailed analysis of field m...

More and more devices connected to low voltage networks inject currents in the frequency range above 2 kHz, so called supraharmonics. Supraharmonics can appear as synchronized or non-synchronized to the power system frequency. A device exposed to a fixed supraharmonic frequency are here shown to generate low order interharmonics when the power syst...

Dynamisk belastningsförmåga är att sätt att möjliggöra en högre användning av överföringskapacitet av en luftledning utan att det finns en ökad risk att ledningen blir överbelastad. Från prognosen eller mätning av väderparametrar görs det en uppskattning av hur mycket ström en luftledning klarar. Metoden har studerats förut och det pågår en hel del...

This paper gives an overview of the components and failure modes that should be considered when studying the reliability of grid-size Battery Energy Storage System (BESS). Next to failures of the primary component, a reliability study should consider the failure of the protection, failure of the communication, and failure of the control system. Aft...

Solar photovoltaics in electricity distribution networks is often limited by the rise in voltage magnitude. The pre-connection voltage magnitude is an important factor that determines the hosting capacity. This paper studies to which extent details of the pre-connection voltage magnitude impact the hosting capacity. Extensive measurements of voltag...

Hyper-scale data centers are used to host cloud computing interfaces to support the increasing demand for storage and computational resources. For achieving specific service level agreements (SLA), this infrastructure demands highly available cloud computing systems. It is necessary to analyze the server failure incidents to determine the way of im...

Due to the increasing penetration of wind power, grid codes require that the wind farms remain connected to the grid during voltage dips. The harmonic distortion might change during the short-term reduction in the voltage magnitude. Although the distortion only occurs during the short period of the voltage dip, high levels of harmonic might cause m...

This work compares the response of DFIG-based wind turbines under voltage dips caused by transformer-energizing and caused by an asymmetrical fault. Voltage dips with similar retained voltage and duration and the two distinct causes are applied to an equivalent model of a DFIG-based wind park. The dips are different in terms of harmonic content and...

A number of changes in the power system have increased the risk for more serious resonances in the harmonic frequency range. The changes also result in an increased uncertainty with regard to the frequency and damping of those resonances. Uncertainties could be related to variations with time, uncertain future developments in the grid, and the mode...

This work assesses the low voltage ride-through requirements and the dynamic behavior of a mixed PMSG/DFIG wind power plant. Previous works in the literature applied voltage dips seen at the Point of Common Coupling (PCC) of the wind power plant (WPP) directly, which means that the propagation from the transmission grid to the low-voltage level was...

Electric vehicle chargers and solar photovoltaic inverters are two types of household loads that can potentially impact the power quality of the grid. This paper presents a view of the consequences that the connection of these two nonlinear loads into a low-voltage installation can create on voltage harmonic distortion. The analysis considers the c...

Presentation of our paper at the conference ICREPQ'20. The paper is freely available at: http://www.icrepq.com/icrepq20/208-espin.pdf /Ángela Espín-Delgado

This paper shows a study of the variations of the supraharmonic distortion emitted by an LED lamp during and after its thermal stabilization. The study is aimed to contribute to map the uncertainties existing when quantifying supraharmonics. Measurements of the current drawn by five lamps of the same type and manufacturer are performed. Then, analy...

This paper proposes a novel scheme for analyzing power system measurement data. The main question we seek answers in this study is on “whether one can find some important patterns that are hidden in the large data of power system measurements such as variational data”. The proposed scheme uses an unsupervised deep feature learning approach by first...

Photovoltaic (PV) systems, Electric vehicles (EV) and LED lamps have gained significant popularity in our current society. It is therefore common to find customer installations with all three operating together. PV and EV are known sources of voltage variations on the grid. The impact of these voltage variations on LED lamps situated in close proxi...