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Impact of uncertainties on resonant overvoltages following transformer energization
Independent Insulation Group, Ludvika, Sweden.
Luleå tekniska universitet, Institutionen för teknikvetenskap och matematik, Energivetenskap.ORCID-id: 0000-0003-4074-9529
2020 (engelsk)Inngår i: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 187, artikkel-id 106503Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [sv]

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 modelling of individual components. This article investigates uncertainties affecting resonant overvoltages caused by transformer energization. Several study cases investigating the impact of different uncertainties on resonances and resonant overvoltages, performed in PSCAD, are presented. The results show that some uncertainties may have a significant impact on the resulting impedance characteristics and on the resulting overvoltage levels.

sted, utgiver, år, opplag, sider
Elsevier, 2020. Vol. 187, artikkel-id 106503
Emneord [en]
Harmonics, Power quality, Power system transmission, Transformer
HSV kategori
Forskningsprogram
Elkraftteknik
Identifikatorer
URN: urn:nbn:se:ltu:diva-80199DOI: 10.1016/j.epsr.2020.106503ISI: 000556736000064Scopus ID: 2-s2.0-85087381229OAI: oai:DiVA.org:ltu-80199DiVA, id: diva2:1453170
Konferanse
13th International Conference on Power Systems Transients (IPST 2019), 16-20 June, 2019, Perpignan, France
Merknad

Godkänd;2020;Nivå 0;2020-08-31 (marisr);Konferensartikel i tidskrift

Tilgjengelig fra: 2020-07-09 Laget: 2020-07-09 Sist oppdatert: 2022-06-28bibliografisk kontrollert
Inngår i avhandling
1. Managing uncertainties through efficient calculation of transients and harmonic propagation in power systems
Åpne denne publikasjonen i ny fane eller vindu >>Managing uncertainties through efficient calculation of transients and harmonic propagation in power systems
2022 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Alternativ tittel[sv]
Metoder för att hantera osäkerheter genom effektiv beräkning av transienter och spridning av övertoner
Abstract [en]

The electric power system is undergoing changes including large-scale introduction of renewable energy sources together with HVDC/FACTS, changes in the network such as increased amount of high voltage cables, industrial electrification, and changes in the load composition. These changes will impact the system in different ways and lead to challenges that must be addressed to facilitate planning, dimensioning, and operation of the system in a secure and economical way. 

The aforementioned changes introduce uncertainties in terms of operational state and modelling of both system and components. One example is the modelling of downstream networks and loads in harmonic propagation studies; the customer impedance may have a significant impact on both the resonance frequency and the damping, but its inclusion remains a challenge due to a lack of knowledge about its behaviour at harmonic frequencies. Another example is the calculation of overvoltages caused by line switching or transformer saturation in different operational states and for varying amounts of underground cable in the network. Methods for calculating overvoltages or harmonic propagation are often based on an assumed complete knowledge of the system under study; uncertainties in the system or components are typically addressed by performing Monte Carlo simulations. However, the use of Monte Carlo methods may be impractical or even unsuitable due to the number of calculations required. Deterministic methods, on the other hand, may provide overly pessimistic results leading to large design margins and high costs. 

This work investigates the application of different methods for managing uncertainties related to the calculation of overvoltages and harmonic propagation. The methods are described, and their advantages and limitations are discussed and illustrated through case studies considering typical uncertainties. Regarding harmonic propagation, two methods are considered: the first method uses copulas to aggregate the harmonic impedance of the downstream network and its loads while retaining its stochastic properties. The method is applied to several medium-voltage and low-voltage networks, and the results show that it is feasible to accurately represent the stochastic behaviour without modelling the downstream network in detail. The second method utilizes the unscented transform together with Cornish-Fisher expansion to calculate the harmonic distortion at the point of connection of a wind farm under different uncertainties. The method is able to estimate the 95% value of individual harmonics accurately when considering variations in emission and impedance, while using a limited number of calculations. 

Regarding overvoltages, three methods are considered: the first method can be used to determine representative fast front overvoltage levels for HVDC cable systems connected to HVDC overhead lines, from a limited number of calculations. The method, applicable to backflashover and shielding failure, accounts for the statistical distribution of lightning current magnitudes, as well as attenuation due to corona discharges on the line. To illustrate the proposed method, it is applied to a case study for a ±525 kV DC system. The second method considers the use of the unscented transform together with Cornish-Fisher expansion to estimate the 2%- value of switching overvoltages from a limited number of calculations. The method is evaluated by considering three-phase energization or reclosing of a line taking into account several aspects such as line length, type of feeding network, impact of trapped charge on the line, and attenuation of the overvoltage level by corona discharges. The method is shown to provide a good approximation of the 2%-value using only about one fifth to one tenth of the number of simulations typically used in traditional methods. The third method makes it possible to estimate a minimum VI-characteristic of surge arresters. This allows for accurate calculation of the absorbed energy when arresters are subjected to resonant overvoltages. 

While many uncertainties may be managed by carrying out a sufficient number of calculations, this may not always be the case. To this end, a method has been proposed to manage uncertainties during system operation, specifically considering the risk of resonant overvoltages due to transformer saturation following the clearing of a nearby line fault. The method utilizes partial disconnection of parallel cables according to a predetermined scheme to shift the system resonance frequency. The method is shown to reduce the duration of the temporary overvoltage and the stress on surge arresters and other equipment.

sted, utgiver, år, opplag, sider
Luleå: Luleå University of Technology, 2022
Serie
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Emneord
Overvoltages, harmonics, uncertainties, stochastic methods
HSV kategori
Forskningsprogram
Elkraftteknik
Identifikatorer
urn:nbn:se:ltu:diva-91960 (URN)978-91-8048-115-1 (ISBN)978-91-8048-116-8 (ISBN)
Disputas
2022-10-21, Hörsal A, Skellefteå, 09:00 (engelsk)
Opponent
Veileder
Tilgjengelig fra: 2022-06-28 Laget: 2022-06-28 Sist oppdatert: 2022-10-03bibliografisk kontrollert

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