| 1. |
- Abrahamsson, Lars, et al.
(författare)
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High-voltage DC-feeder solution for electric railways
- 2012
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Ingår i: IET Power Electronics. - : Institution of Engineering and Technology. - 1755-4535 .- 1755-4543. ; 5:9, s. 1776-1784
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Tidskriftsartikel (refereegranskat)abstract
- For AC railway power supply systems with a different frequency than the public grid, high-voltage AC transmission lines are common, connected to the catenary by transformers. This study suggests an alternative design based on an high-voltage DC (HVDC)-feeder, which is connected to the catenary by converters. Such an HVDC line would also be appropriate for DC-fed railways and AC-fed railways working at a public-grid frequency. The converter stations between the public grid and the HVDCfeeder can be sparsely distributed, not denser than on 100 km distances, whereas the converters connecting the HVDC-feeder to the catenary are distributed denser. Their ratings can be lower than present-day substation transformers or converters, since the power flows can be fully controlled. Despite a relatively low-power rating, the proposed converters can be highly efficient because of the use of medium frequency technology. The proposed feeding system results in lower material usage, lower losses and higher controllability compared with the present solutions. Simulations of the proposed solution show clear advantages regarding transmission losses and voltages compared with conventional systems, especially for cases with weak feeding, and when there are substantial amounts of regeneration from the trains.
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| 2. |
- Abrahamsson, Lars, et al.
(författare)
-
HVDC Feeder Solution for Electric Railways
- 2012
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Ingår i: IET Power Electronics. - 1755-4535 .- 1755-4543.
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Tidskriftsartikel (refereegranskat)abstract
- The railway power supply systems in many sparsely populated countries are relatively weak. Weak railway power supply systems causes problems with power quality, voltage drops, and high transmission losses.For AC railway power supply systems with a different frequency than the public grid, high-voltage AC (HVAC) transmission lines are common, connected to the catenary by transformers.In this paper an alternative design based on an HVDC feeder is suggested. The HVDC feeder is connected to the catenary by converters. Such an HVDC line would also be appropriate for DC-fed railways and AC-fed railways working at public frequency. The converter stations between the public grid and the HVDC feeder can be sparsely distributed, in the range of 100 km or more, whereas the converters connecting the HVDC feeder to the catenary are distributed with a much closer spacing. Their ratings can be lower than substation transformers or electro-mechanical converters, since the power flow can be fully controlled.Despite a relatively low power rating, the proposed converters can be highly efficient due to the use of medium frequency technology. The HVDC-based feeding system results in lower material usage, lower losses and higher controllability compared to present solutions.Simulations of the proposed solution show clear advantages regarding transmission losses and voltages compared to conventional systems, especially for cases with long distances between feeding points to the catenary, and when there are substantial amounts of regeneration from the trains.
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| 3. |
- Kjaer, P. C., et al.
(författare)
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Estimation of field current in vector-controlled synchronous machine variable-speed drives employing brushless asynchronous exciters
- 2005
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Ingår i: IEEE transactions on industry applications. - 0093-9994 .- 1939-9367. ; 41:3, s. 834-840
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Tidskriftsartikel (refereegranskat)abstract
- In wound-rotor synchronous machine variable-speed drives, accurate information about the field winding current is paramount to obtain high dynamic and steady-state performance, such as unity power factor. When brushless excitation is employed and no direct measurement of field winding current is available, it must be estimated. This is a topic which has not received much attention in the literature, despite its importance in high-power synchronous motor drives. This paper presents two different methods for field current estimation, applied in a vector-controlled voltage-source converter-fed drive. Experimental results illustrate the steady-state accuracy and dynamic performance.
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| 4. |
- Kjellqvist, Tommy, et al.
(författare)
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Active snubber circuit for source commutated converters utilizing the IGBT in the linear region
- 2007
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Ingår i: 2007 EUROPEAN CONFERENCE ON POWER ELECTRONICS AND APPLICATIONS, VOLS 1-10. - PISCATAWAY : IEEE SERVICE CTR. ; , s. 2679-2688
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Konferensbidrag (refereegranskat)abstract
- This paper describes a gate control method where an IGBT is controlled in its linear region by means of a closed control loop in order to regulate the voltage slope during turn-on and to clamp the voltage of an anti-parallel diode in a source commutated converter. Controlling the voltage slope may be necessary in a high voltage converter to avoid emission of EMI or to avoid triggering oscillations which may cause insulation failure. Controlling the switching trajectory without influence from the device characteristics is important where series-connection is necessary to increase the overall blocking voltage. The control method has been verified by means of a prototype.
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| 5. |
- Kjellqvist, Tommy, et al.
(författare)
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Active Snubber Circuit for Source Commutated Converters Utilizing the IGBT in the Linear Region
- 2008
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Ingår i: IEEE transactions on power electronics. - : IEEE. - 0885-8993 .- 1941-0107. ; 23:5, s. 2595-2601
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes a gate control method where an IGBT is controlled in its linear region by means of closed loop control in order to regulate the voltage slope during turn-on and to clamp the voltage of an anti-parallel diode in a source commutated converter. Controlling the voltage slope may be necessary in a high voltage converter to avoid emission of EMI or to avoid triggering oscillations which may cause insulation failure. Controlling the switching trajectory without influence from the device characteristics is important where series-connection is necessary to increase the overall blocking voltage. The control method has been verified by means of a prototype.
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| 6. |
- Kjellqvist, Tommy, 1978-, et al.
(författare)
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Design Considerations for a Medium Frequency Transformer in a Line Side Power Conversion System
- 2004
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Ingår i: Proceedings of the 35th Annual IEEE Power Electronics Specialist Conference, PESC 2004. - : IEEE. - 0780383990 ; , s. 704-710
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Konferensbidrag (refereegranskat)abstract
- Design considerations are presented for a medium frequency transformer in a line side power conversion system for electric railway traction. The system employs soft commutation in order to reduce switching losses and allow for high operating frequency of the transformer. Two different designs are evaluated; The first one is dry insulated and the second one is liquid-immersed. Calculations indicate that a 1 MVA transformer operating at 4 kHz could be realized with an active weight of below 150 kg for both dry and liquid-immersed transformer concepts. The transformers are modeled magnetically, electrically and thermally, and a geometric optimization procedure based on a cost function is applied to achieve an optimum design.
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| 7. |
- Kjellqvist, Tommy, et al.
(författare)
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Harmonic Mitigation in Single Phase Mutually Commutated Converter Systems
- 2006
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Ingår i: Proceedings of the 37th Annual IEEE Power Electronics Specialists Conference, PESC 2006. - NEW YORK, NY : IEEE. - 0780397169 ; , s. 1542-1548
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Konferensbidrag (refereegranskat)abstract
- An AC/DC converter comprising a cycloconverter and a capacitively snubbered voltage source converter (VSC), coupled by a medium frequency transformer, offers significant advantages. Bidirectional AC/DC power flow as well as voltage transformation and isolation can be achieved. By mutual commutation of the converters, soft switching can be achieved for all semiconductors, thus, link frequency can be increased, allowing smaller transformer and reduced filter components. However, the mutual commutation scheme imposed by the soft switching requirement puts constraints on the PWM pattern causing low frequency harmonics in certain operating points. It is shown that the low frequency harmonics can be eliminated for all points of operation, both in a single bridge configuration and interlaced configuration, common in AC-fed propulsion. A sub-optimal harmonic mitigation method based on a regularly sampled modulator is proposed. By proper oversampling of the reference waveforms substantial reduction of the low frequency harmonic contents can be achieved. The mitigation method is verified by circuit simulation and the feasibility to propulsion is demonstrated.
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| 8. |
- Kjellqvist, Tommy, 1978-
(författare)
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On Design of a Compact Primary Switched Conversion System for Electric Railway Propulsion
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this thesis, a compact and light primary switched conversion system for AC-fed railway propulsion is investigated. It is characterized by soft switching of all converter stages and a source commutated primary converter comprising series connected valves. Both weight and volume of the conversion system are reduced significantly compared to a conventional system with a low frequency transformer. The conversion system is made up of N isolated AC/DC conversion cells, each comprising a cycloconverter and a voltage source converter (VSC) coupled by a medium frequency transformer. The cells are series connected on the AC side and connected to a common DC-link. Thus, 2N+1 voltage levels can be synthesized at the AC terminal and the voltage stress on the transformer and line filter is reduced compared to a one cell solution. Series connection of semiconductor valves allows independent choice of blocking voltage and number of converter cells. Choosing two converter cells is an attractive compromise. Five level output reduces the harmonic distortion and simplify transformer and line filter design while keeping the complexity of the conversion system low. The mutually commutated converter (MCC) allows a transformer frequency in the range of 4 to 8 kHz without derating the line side converter due to zero voltage switching of the VSC. Modern magnetic materials, like high silicon steel, amorphous and noncrystalline materials allow design of the transformer with high efficiency at elevated frequencies. In a 15 kV system, the peak voltage at the catenary is typically beyond 32 kV which is far beyond the voltage capability of currently available semiconductors. Therefore, several semiconductors are connected in series. Favourable commutation conditions and a new gate drive arrangement allow snubberless commutation of the primary converter stage. Thus, the primary converter can be highly integrated, reducing both weight and volume. The conversion system can be placed on the roof or in the underframe without compromising efficiency or vehicle performance. The feasibility of the conversion concept has been demonstrated by means of a down-scaled prototype. Snubberless commutation of series connected valves is demonstrated.
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| 9. |
- Kjellqvist, Tommy, et al.
(författare)
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Switching Frequency Limit for Soft-Switching MF Transformer System for AC-fed Traction
- 2005
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Ingår i: Proceesings of the 36th Annual IEEE Power Electronics Specialist Conference, PESC 2005. - : IEEE. - 0780390334 ; , s. 1678-1684
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Konferensbidrag (refereegranskat)abstract
- The migration to line side power conversion topologies comprising a medium frequency transformer in AC-fed propulsion systems may reduce size and weight of the conversion system. The switching frequency is a crucial factor affecting the viability of such a conversion system. An increased switching frequency of a mutually commutated conversion system comprising a medium frequency transformer employing soft switching is verified by single pulse measurements. A test bench to characterize standard IGBT modules under soft switching has been build and loss measurements on a 3.3 kV IGBT module are presented for hard and soft switching. A loss model of the investigated topology is developed and the maximum switching frequency for a converter equipped with the investigated module is estimated for hard and soft switching.
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| 10. |
- Kjellqvist, Tommy, et al.
(författare)
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Thermal Evaluation of a Medium Frequency Transformer in a Line Sider Conversion System
- 2009
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Ingår i: Proceedings of the 13th International European Electronics Conference and Exhibition, EPE '09. - : IEEE. - 9781424444328 ; , s. 2517-2526
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Konferensbidrag (refereegranskat)abstract
- A central part of a medium frequency conversion system is the transformer. A medium frequency transformer for railway propulsion may be stressed beyond 30kV with voltage slopes around 50kV/mu s depending on topology while the geometric dimensions are small. A 170kVA transformer prototype operated at 4 kHz has been developed and tested.A transient thermal model of the transformer is developed and verified experimentally. The transient thermal behavior is important in propulsion applications where the conversion system is often heavily overloaded. Simulations indicate that the overloading capability of the transformer is comparable to a conventional traction transformer. Experimental results are in good agreement with the model.
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