Novel methods for earth fault passage indication in non-effectively grounded electricity distribution networks

Electricity distribution networks are commonly subject to supply interruptions and outages caused by faults. This dissertation focuses on medium voltage distribution networks, which typically consist of primary substations having multiple feeders along which secondary substations are located. When a permanent fault occurs on a segment (the part linking two consecutive secondary substations) of a distribution feeder, the faulted segment needs to be identified and isolated. Identifying the faulted segment can be realized through fault passage indicators. This is a straightforward task when the fault type is a short circuit, as these types of faults involve large currents. However, faulted segment identification for earth faults in non-effectively grounded medium voltage distribution networks has remained a challenge as the earth fault current in those networks is typically relatively small. Therefore, the main objective of this dissertation was to develop novel methods for locating single-phase earth faults in medium voltage distribution networks and validating them through simulations and real system measurements. After comprehensive review of state-of-the-art approaches presented in the literature, the dissertation proposes innovative methods for earth fault passage indication aimed at non-effectively grounded urban or rural distribution networks with radial feeders. The proposed methods are underpinned by a theoretical analysis based on the symmetrical components of the currents on a distribution feeder under an earth fault condition. The comparison of the sequence currents collected from various measuring points on the network forms the backbone of the methods. For practical implementation, current measurements need to be transferred to a central location for processing and decision making, but this can be done without accurate time synchronization. The proposed methods were developed and verified through simulations and empirical data. This work is a product of close collaboration between academia and industry that enabled the validation of the proposed methods with the help of empirical data that was provided by system operators and relay manufacturers. The results obtained from simulations and field tests show the efficacy of utilizing sequence current quantities, in the manner proposed in this work, for identifying the passage of earth faults with fault resistances ranging from zero to several kilo-ohms. In practice, the methods are reliable as long as the current measurements are accurate enough.Sähkönjakeluverkoissa esiintyy vikoja, jotka aiheuttavat sähkönjakelun keskeytyksiä eli sähkökatkoja. Tämä väitöskirja käsittelee keskijänniteverkkoja, jotka koostuvat sähköasemista ja niiltä lähtevistä johtolähdöistä. Johtolähtöjen varrella sijaitsevat pienjänniteverkkoa syöttävät muuntamot. Kun jollain johto-osuudella (tässä ns. muuntamovälillä, joka yhdistää kaksi peräkkäistä muuntamoa) ilmenee pysyvä vika, viallinen johto-osuus on tunnistettava ja erotettava. Viallisen johto-osuuden tunnistaminen tapahtuu vianilmaisimien avulla. Viallisen johto-osuuden tunnistaminen on yksinkertaista, kun vikatyyppi on oikosulku, sillä oikosuluille ominaista ovat yleensä suuret vikavirrat. Haasteena on kuitenkin edelleen viallisten johto-osuuksien tunnistaminen maasulkutilanteissa ei-tehollisesti maadoitetuissa keskijänniteverkoissa, joissa maasulkuvirta on tyypillisesti hyvin pieni. Tämän väitöskirjana tavoitteena on ollut kehittää uusia menetelmiä maasulkuvikojen paikantamiseen keskijänniteverkossa ja varmentaa niiden toimivuus simuloinnein ja todellisesta verkosta saatujen mittausten avulla. Kattavan kirjallisuuskatsauksen jälkeen tässä väitöskirjassa esitellään innovatiivisia menetelmiä vikavirran reitin ilmaisuun jakeluverkkojen maasuluissa. Ehdotetut menetelmät tukeutuvat teoreettiseen analyysiin, jossa johtolähdön virrat maasulkutilanteessa on kuvattu symmetristen komponenttien avulla. Menetelmät perustuvat verkon eri pisteissä mitattuihin virran symmetristen komponenttien vertailuun. Käytännön toteutuksessa nämä mittaukset tulee siirtää keskitettyyn järjestelmään prosessointia ja päätöksentekoa varten, mutta tämä voidaan tehdä ilman tarkkaa aikasynkronointia. Ehdotettujen menetelmien kehittämisessä ja testaamisessa hyödynnettiin simulointeja ja kokeellista mittausdataa. Yhteistyö teollisuuden kanssa mahdollisti menetelmien toiminnan todentamisen hyödyntäen todellisista verkoista mitattua dataa, jota saatiin sekä verkkoyhtiöiltä että laitevalmistajilta. Simulointien tulokset ja mittaukset todellisessa verkossa tehdyistä testeistä osoittavat, että virran symmetriset komponentit toimivat hyvin vian paikannuksessa kun vikaresistanssi on nollan ja muutaman tuhannen ohmin välillä. Käytännössä menetelmien luotettavuus riippuu virran mittauksen tarkkuudesta.fi=vertaisarvioitu|en=peerReviewed

Modeling and Simulation of BESS-Upgraded Power Transmission Systems for Frequency Control

This thesis reports on the evaluation of the impact of a battery energy storage system (BESS) on the voltage and frequency stability of a power transmission system. With the increased penetration of renewable energy sources, particularly wind power, into the power system network new challenges regarding the two major concerns in the transportation and distribution of electrical energy, i.e. power quality and system stability, are being experienced. One way to reduce these concerns is the application of MW-level Battery Energy Storage Systems (BESSs). Despite a long-standing interest in the BESS technology, its application in the transmission system had not been seriously considered until relatively recently. Since the main focus for this study is frequency stability, a comprehensive discussion on frequency control, including primary and secondary frequency controls is included. This provides the opening for the evaluation of the impact of the BESS on system frequency. In addition, this thesis outlines several BESS installations from early 1980s to today’s state of the art technology. The thesis conducts research using modeling and simulation in PSCAD and PowerWorld environments to carry out a comprehensive assessment of the impact of the BESS technology to ameliorate the frequency excursions that follow a power load increase in a high-voltage transmission system and providing effective voltage support. In brief, the findings of this thesis indicate the positive influence of the BESS on the power transmission system stability

Non-Directional Earth Fault Passage Indication in Isolated Neutral Distribution Networks

In this paper, two new methods for locating single-phase to ground faults in isolated neutral distribution networks are proposed. The methods are based on the analysis of symmetrical sequence currents. They are solely based on currents, not requiring voltage measurement. The first method employs only the zero sequence current and the second one utilizes the negative sequence current in combination with the zero sequence current. It is revealed why using only zero sequence current with a simple threshold is insufficient and may lead to false results. Using the proposed methods, earth faults with high resistances can be located in isolated neutral distribution networks with overhead lines or cables.© Writers. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license, http://creativecommons.org/licenses/by/4.0/fi=vertaisarvioitu|en=peerReviewed

Intermittent earth fault passage indication in compensated distribution networks

An intermittent or restriking earth fault is a special type of earth fault that is common mostly in compensated cable networks. A great deal of effort has gone into protection against this type of fault. However, locating this fault has not received much attention. Therefore, there is a need to have a reliable method for locating this fault to repair the damaged cable. In this paper, the principles of a new method developed for locating transient intermittent earth faults on distribution networks are presented. The proposed method employs negative and zero sequence currents, and no voltage measurement is required, which means the proposed method has the potential to reduce cost when implemented in practice. It is intended mainly for typical intermittent earth faults in cable distribution networks where the typical fault resistance is in the range of a few ohms. Real data obtained from practical field tests is used to explain the phenomenon. A series of disturbance recordings obtained from field tests validate the proposed method.©2021 Elsevier. This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ This article has been accepted for publication in a future issue of this journal, but has not been fully edited. Content may change prior to final publication.fi=vertaisarvioitu|en=peerReviewed

Earth Fault Location Using Negative Sequence Currents

In this paper, a new method for locating single-phase earth faults on non-effectively earthed medium voltage distribution networks is proposed. The method requires only current measurements and is based on the analysis of the negative sequence components of the currents measured at secondary substations along medium voltage (MV) distribution feeders. The theory behind the proposed method is discussed in depth. The proposed method is examined by simulations, which are carried out for different types of networks. The results validate the effectiveness of the method in locating single-phase earth faults. In addition, some aspects of practical implementation are discussed. A brief comparative analysis is conducted between the behaviors of negative and zero sequence currents along a faulty feeder. The results reveal a considerably higher stability level of the negative sequence current over that of the zero sequence current.fi=vertaisarvioitu|en=peerReviewed

Effective Management of Energy Internet in Renewable Hybrid Microgrids : A Secured Data Driven Resilient Architecture

This paper proposes a two-layer in-depth secured management architecture for the optimal operation of energy internet in hybrid microgrids considering wind turbines, photovoltaics, fuel cell unit, and microturbines. In the physical layer of the proposed architecture, the operation of the grid is formulated as a single objective problem that is solved using teacher learning-based optimization (TLBO). Regarding the cyber layer of the proposed architecture, a two-level intrusion detection system (IDS) is proposed to detect various cyber-attacks (i.e. Sybil attacks, spoofing attacks, false data injection attacks) on wireless-based advanced metering infrastructures. The sequential probability ratio testing (SPRT) approach is utilized in both levels of the proposed IDS to detect cyber-attacks based on a sequence of anomalies rather than only one piece of evidence. The feasibility and performance of the proposed architecture are examined on IEEE 33-bus test system and the results are provided for both islanded and grid-connected operation modes.©2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.fi=vertaisarvioitu|en=peerReviewed

Utilization of a mixture of CTs and current sensors in line differential protection applications

The popularity of Rogowski sensors is increasing due to their advantages over conventional CTs. Consequently, network utilities may sometimes end up facing a question whether it would be possible to use line differential protection on a line where the current measurements at the local end are based on CTs, whereas, the remote end measurements are based on current sensors. The main challenge in the use of such a combination in line differential protection applications is that while CTs may saturate, Rogowski sensors do not. Such issues were studied in a hardware-in-the-loop simulation setup. Based on the studies conducted, it appears that the use of mixture of CTs and current sensors for line differential protection applications is a feasible option provided that the CTs are properly dimensioned and that the protection settings are carefully chosen.fi=vertaisarvioitu|en=peerReviewed

VINPOWER Vaasa innovation platform for future power systems : Final report (summary)

The main objective of the project was to develop smart grid related expertise within the University of Vaasa. While the parallel project (Smart Energy Systems Platform – SESP) focused on the development of the physical research facilities, VINPOWER contributed to the development of the RDI environment by increasing expertise in the selected research areas. Moreover, the project aimed at delivering new knowledge and concepts for the industries, improved cooperation between the project partners and promotion of new technology for enhanced reliability of electricity distribution networks. The research work was divided into four work packages, WP1 Cabled medium voltage networks, WP2 Microgrids, WP3 Big Data Applications, and WP4 Dissemination and networking. In WP1, technologies for fault location, earth fault compensation and reactive power compensation were developed, and concepts for future secondary substation were outlined. In WP2, selected functionalities of microgrids were analysed, and protection and control related functions were developed and demonstrated. The third WP examined how Big Data and FPGA technologies could be utilized for network operation and maintenance needs. A disturbance recording library, intended for national use, was also developed. WP4 complemented the project by dissemination and networking. The main result of the project is the development of the Smart Grid expertise and innovation environment at the University of Vaasa. This provides better basis for future research projects, both nationally and internationally. Examples of more tangible results are the solutions to the compensation of cabled MV networks, developments in protection and control functions of microgrids, the national disturbance recording library, and the comparisons produced in the real world case study of alternatives to improve the reliability of MV feeders. According to the self-evaluation, carried out by the steering group, workshops were the highlight of the project, enabling vivid discussions and promoting networking of all participants.fi=vertaisarvioimaton|en=nonPeerReviewed

Modeling and Simulation of BESS-Upgraded Power Transmission Systems for Frequency Control

This thesis reports on the evaluation of the impact of a battery energy storage system (BESS) on the voltage and frequency stability of a power transmission system. With the increased penetration of renewable energy sources, particularly wind power, into the power system network new challenges regarding the two major concerns in the transportation and distribution of electrical energy, i.e. power quality and system stability, are being experienced. One way to reduce these concerns is the application of MW-level Battery Energy Storage Systems (BESSs). Despite a long-standing interest in the BESS technology, its application in the transmission system had not been seriously considered until relatively recently. Since the main focus for this study is frequency stability, a comprehensive discussion on frequency control, including primary and secondary frequency controls is included. This provides the opening for the evaluation of the impact of the BESS on system frequency. In addition, this thesis outlines several BESS installations from early 1980s to today’s state of the art technology. The thesis conducts research using modeling and simulation in PSCAD and PowerWorld environments to carry out a comprehensive assessment of the impact of the BESS technology to ameliorate the frequency excursions that follow a power load increase in a high-voltage transmission system and providing effective voltage support. In brief, the findings of this thesis indicate the positive influence of the BESS on the power transmission system stability

Technical requirements for practical implementation of fault passage indication

One of the distinctive features of smart grids is feeder automation. Fault location is an important part of this feature and has become an essential function for distribution system operators. Reliable fault location expedites the restoration of power following an outage caused by a permanent fault. The most common type of faults in distribution networks is the single phase to ground fault. To locate earth faults in non-effectively earthed medium-voltage distribution networks, a number of methods have been put forward among which methods that are based on fault passage indication appear to be promising. This paper discusses the technical apparatus required for implementing two FPI-based methods in practice.©2021 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.fi=vertaisarvioitu|en=peerReviewed