| 1. |
- Barakou, Fani, et al.
(författare)
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Impact of Load Modeling on the Harmonic Impedance seen from the Transmission Network
- 2016
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Ingår i: 17th International Conference on Harmonics and Quality of Power. - Piscataway, NJ. - 9781509037926 ; , s. 283-288
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Konferensbidrag (refereegranskat)abstract
- Traditionally when studying harmonics at transmis-sion system level the medium and low voltage grid is modeledwith either an equivalent short-circuit impedance or a fixed PQload (based on the powerflow) directly at the secondary side ofthe transmission transformer. However the recent replacement ofconventional loads with electronic loads as well as the extendeduse of power electronics in LV level will result in reductionof damping and increase of household capacitance which maychange the harmonic impedance seen from the upstream network(MV or HV) both in the dominant frequency and magnitude.In this paper a HV feeder is modeled in detail starting fromthe transmission transformer up to the household equipmentto investigate whether changes in the type of load affect theimpedance seen from the HV. The simulations are performedusing software package PSCAD/EMTDC.
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| 2. |
- Bollen, Math, et al.
(författare)
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Different Fault Types and Voltage Dips in relation to Shielding of Subtransmission Lines
- 2016
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Ingår i: 2016 17th International Conference on Harmonics and Quality of Power. - Piscataway, NJ. - 9781509037926 ; , s. 518-523
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Konferensbidrag (refereegranskat)abstract
- This paper presents a method for considering the voltage-dip performance of the power system in the design of subtransmission lines. The shielding performance of the lines is expressed in terms of the number of faults for different number of phases involved in the fault. Relations between fault type and voltage dip at the terminals of sensitive equipment are used to determine the resulting number of equipment trips. The method is illustrated by applying it to three different tower designs used in the Swedish subtransmission networks. It is shown that the installation of shield wires significantly reduces the number of equipment trips, especially in combination with the installation of additional capacitance in adjustable-speed drives.
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| 3. |
- Etherden, Nicholas, et al.
(författare)
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The transparent hosting-capacity approach – overview, applications and developments
- 2015
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Konferensbidrag (refereegranskat)abstract
- This paper summarizes the hosting capacity approach and gives some recent developments: including uncertainty in location and size of production units; curtailment to connect more production than according to the initial hosting capacity. For both developments it is shown that the transparency of the approach still holds but also that the results may be strongly location dependent. It is however also shown that the hosting-capacity approach can be used to obtain rough estimations, rules-of-thumbs, and to make a first assessment in case more detailed studies are not possible for example because insufficient data is available.
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| 4. |
- Lennerhag, Oscar, 1989-, et al.
(författare)
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A power system model for resonance studies
- 2019
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Ingår i: CIRED 2019 Proceedings. - : AIM.
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Konferensbidrag (refereegranskat)abstract
- This article presents a publicly available power system model suitable for studying switching transients considering the effect of resonances, as well as the spread of harmonics. The model covers voltage levels from 400 kV to 400 V, with all relevant parameters included. The system is characterized by a large share of cables at transmission level, with a first resonance frequency between 100 and 150 Hz. This article describes the overall layout of the model as well as the modelling of different components. Application examples are given, including the study of transfer impedances and transformer energization transients. Limitations of the model and further developments of the model are provided.
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| 5. |
- Lennerhag, Oscar, 1989-, et al.
(författare)
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A Stochastic Aggregate Harmonic Load Model
- 2020
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Ingår i: IEEE Transactions on Power Delivery. - : IEEE. - 0885-8977 .- 1937-4208. ; 35:5, s. 2127-2135
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Tidskriftsartikel (refereegranskat)abstract
- This article presents a stochastic aggregate harmonic load model that can be used to accurately replicate the stochastic behavior of the network impedance downstream of a point of aggregation. The method has been applied to three low voltage networks and the results show that it is able to accurately represent their stochastic behavior while significantly reducing the computational burden compared to modelling the downstream network in detail.
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| 6. |
- Lennerhag, Oscar, 1989-, et al.
(författare)
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An Improved Statistical Method for Calculating Lightning Overvoltages in HVDC Overhead Line/Cable Systems
- 2019
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Ingår i: Energies. - : MDPI. - 1996-1073. ; 12:16
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Tidskriftsartikel (refereegranskat)abstract
- HVDC cable systems connected to HVDC overhead lines are subject to fast front overvoltages emanating from the line when lightning strikes a shield wire (backflashover) or a pole conductor (shielding failure). Representative fast front overvoltage levels for HVDC cable systems are usually established without considering their statistical characteristics. A statistical method to determine overvoltages related to the acceptable mean time between failure (MTBF) for the cable system was developed previously. The method accounts for the statistical distribution of lightning current magnitudes as well as the attenuation of the overvoltage wave due to corona discharges on the line, since this effect dominates for system voltages up to about ±320 kV. To make the method suitable for higher system voltages as well, this article introduces an improved statistical method which also accounts for surge attenuation through resistive effects, soil ionization, and statistical treatment of overvoltages due to shielding failures. To illustrate the improved method, it is applied to a case study for a ±525 kV DC line.
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| 7. |
- Lennerhag, Oscar, 1989-, et al.
(författare)
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Application of a stochastic aggregate load model to study harmonic propagation in distribution networks
- 2021
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Ingår i: CIRED 2021 - The 26th International Conference and Exhibition on Electricity Distribution. - : Institution of Engineering and Technology. ; , s. 733-737
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Konferensbidrag (refereegranskat)abstract
- This paper has applied a method for aggregating downstream networks while retaining their stochastic properties, for use in harmonic propagation studies in distribution networks. The method has been applied to two Swedish medium-voltage networks where the connected low-voltage networks, including customer loads, are represented by stochastic aggregated harmonic load models. The results show that the studied medium-voltage networks exhibit a relatively low resonance frequency for their first resonance point. In the frequency range around the first resonance, the uncertainty in the impedance was found to be high, whereas for higher frequencies the uncertainty in the impedance characteristic was much smaller.
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| 8. |
- Lennerhag, Oscar, 1989-, et al.
(författare)
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Application of Unscented Transform for Efficient Calculation of Statistics for Electromagnetic Transients
- 2022
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Ingår i: IEEE Transactions on Power Delivery. - : IEEE. - 0885-8977 .- 1937-4208. ; 37:5, s. 3469-3476
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Tidskriftsartikel (refereegranskat)abstract
- 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 Cornish-Fisher expansion to estimate the 2%-value of switching overvoltages using only about one tenth the number of calculations as a Monte Carlo method, with similar accuracy. The uncertainties considered are the energizing instants of the three phases. The method is illustrated through EMT simulations in PSCAD and it is shown to provide a good approximation of the 2%-value in most of the studied cases.
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| 9. |
- Lennerhag, Oscar, 1989-
(författare)
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Calculation Methods for Power Systems Facing Large Uncertainties
- 2020
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- The electric power system is facing a number of changes as part of the transition to a more sustainable energy system, including a large-scale integration of renewable energy sources, changes in the grid such as the introduction of more cables at higher voltage levels, and changes in the load composition, e.g. due to a transition to energy-efficient appliances. These changes will lead to new technical challenges in transmission, subtransmission and distribution networks that need to be addressed in order to facilitate planning, dimensioning and operation of the power system in an economical and secure way.Resonances affect both overvoltages and harmonic propagation and there are a number of trends that affect resonances, where the general trend is towards resonances at lower frequencies and with less damping. Another aspect that should be considered is an increased uncertainty in both resonances and harmonic sources, caused by the introduction of renewable energy sources, energy efficiency measures and an increase in flexibility. Calculation methods used for calculating overvoltages or harmonic propagation are often based on an assumed complete knowledge of the system under study. Uncertainties, e.g. in the operational conditions of the system or in the modeling of components, are addressed e.g. by performing Monte Carlo simulations. However, depending on the scale of the problem, this may not be practical or even possible. At the other end of the spectrum are deterministic methods, but in case of large uncertainties such methods may give overly pessimistic results. As a consequence, large design margins may be used which can result in barriers for the integration of renewable energy sources. To overcome these issues, there is a need for stochastic methods which can be used to estimate statistical values while reducing the required number of simulations.Downstream networks and loads are important to consider in studies on harmonic propagation or resonant overvoltages since they can have a significant impact on both the frequency and damping of resonances. However, their inclusion remains a challenge for several reasons, e.g. due to a lack of knowledge about the customer impedance at harmonic frequencies. For the purpose of studying the impact of existing and future loads on the spread of harmonics and resonant overvoltages, there is a need for stochastic models, both at the individual customer level and aggregated. A method was developed which makes it possible to aggregate a given downstream network into equivalent impedances, whilst preserving its stochastic behavior. The aggregated model can then be used to represent the impact of the downstream network accurately in studies whilst significantly reducing the computational burden compared to modeling the downstream network in detail. The method has been illustrated by applying it to three Swedish LV networks.More and more HVDC systems take the form of mixed cable/overhead line systems. In such a configuration, the cable system will be subject to overvoltages originating from lightning strikes to the line. Representative fast front overvoltage levels for HVDC cable systems connected to overhead lines are typically determined in a deterministic fashion without considering the statistical characteristics of the associated lightning overvoltages. As a consequence, this may lead to unnecessarily conservative dimensioning of the cable insulation. A statistical method for determining representative fast front overvoltage levels for the cable system has been developed previously based on the statistical concept for overvoltage protection of substations. The method focuses on backflashovers and accounts for the statistical distribution of the lightning current magnitude as well as attenuation of the overvoltage due to corona discharges. An improved statistical method has been developed which improves upon the original method by considering surge attenuation due to resistive effects as well as the influence of soil ionization. The improved method is also applicable to shielding failures. The goal of the method is to minimize the number of calculations required to determine overvoltage levels in the cable system related to the acceptable mean time between failure. The method was demonstrated by applying it to a case study for a ±525 kV dc cable system.
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| 10. |
- Lennerhag, Oscar, 1989-, et al.
(författare)
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Efficient Calculation of Switching Overvoltages Considering Corona Attenuation
- 2023
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Ingår i: IEEE Transactions on Power Delivery. - : IEEE. - 0885-8977 .- 1937-4208. ; 38:4, s. 2735-2741
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Tidskriftsartikel (refereegranskat)abstract
- 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 Carlo methods through simulations in PSCAD. The method is shown to be able to estimate the 2%-value with comparable accuracy to methods used in industry today, but with only one fifth of the number of calculations.
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