| 1. |
- Ahmed, Noman, et al.
(författare)
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A computationally efficient continuous model for the modular multilevel converter
- 2014
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Ingår i: IEEE Journal of Emerging and Selected Topics in Power Electronics. - : IEEE. - 2168-6777. ; 2:4, s. 1139-1148
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Tidskriftsartikel (refereegranskat)abstract
- Simulation models of the modular multilevel converter (MMC) play a very important role for studying the dynamic performance. Detailed modeling of the MMC in electromagnetic transient simulation programs is cumbersome, as it requires high computational effort and simulation time. Several averaged or continuous models proposed in the literature lack the capability to describe the blocked state. This paper presents a continuous model, which is capable of accurately simulating the blocked state. This feature is very important for accurate simulation of faults. The model is generally applicable, although it is particularly useful in high-voltage dc applications.
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| 2. |
- Antonopoulos, Antonios, et al.
(författare)
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Modular multilevel converter AC motor drives with constant torque from zero to nominal speed
- 2014
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Ingår i: IEEE transactions on industry applications. - 0093-9994 .- 1939-9367. ; 50:3, s. 1982-1993
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Tidskriftsartikel (refereegranskat)abstract
- Modular multilevel converters are shown to have a great potential in the area of medium-voltage drives. Low-distortion output quantities combined with low average switching frequencies for the semiconductor devices create an ideal combination for very high-efficiency drives. However, the large number of devices and capacitors that have to conduct the fundamental-frequency current require more complex converter control techniques than its two-level counterpart. Special care needs to be taken for starting and operation at low speeds, where the low-frequency current may cause significant unbalance between the submodule capacitor voltages and disturb the output waveforms. In this paper, principles for converter operation with high torque in the whole speed range are investigated. Experimental results from a down-scaled 12-kVA prototype converter running a loaded motor at various speeds between standstill and the rated speed are also provided.
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| 3. |
- Norrga, Staffan, 1968-, et al.
(författare)
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Decoupled steady-state model of the modular multilevel converter with half-bridge cells
- 2012
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Konferensbidrag (refereegranskat)abstract
- Modular multilevel converters, based on cascading of halfbridge converter cells, can combine low switching frequency with low harmonic interference. They can be designed for high operating voltages without direct series connection of semiconductor elements. This has led to a rapid adoption within high-power applications such as HVDC, STATCOM and railway interties. Analysing the operation of these converters in the frequency domain poses a few challenges due to the presence of significant low-order harmonic voltages in the cell capacitors. This paper presents a frequency-domain model of the MMC converter with halfbridge cells, based on a two-stage approach. First, the circuit equations are decoupled by a simple linear transformation, whereby the circuit schematic can be separated into a dc-side and an ac-side part. Second, the switching operation within the phase arms is modelled in the frequency domain by iterated convolution. The model is verified against a timedomain simulation of a converter with ratings valid for HVDC applications. It is shown that the proposed methodology, where all calculations are made in the frequency domain, can accurately reproduce the results from the simulation.
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| 4. |
- Norrga, Staffan, 1968-, et al.
(författare)
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Frequency-Domain Modeling of Modular Multilevel Converters With Application to Maximizing the Operating Region
- 2012
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Ingår i: IECON 2012 - 38th Annual Conference on IEEE Industrial Electronics Society. - : IEEE. - 9781467324212 ; , s. 4967-4972
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Konferensbidrag (refereegranskat)abstract
- Modular multilevel converters (MMC), based on cascading of half-bridge converter cells, can combine low switching frequency with low harmonic interference. They can be designed for high operating voltages without direct series connection of semiconductor elements. This has led to a rapid adoption within high-power applications such as high voltage direct current transmission, railway interties and medium voltage industrial motor drives. Analyzing the operation of these converters in the frequency domain poses a few challenges due to the presence of significant low-order harmonic voltages in the cell capacitors. This paper treats a frequency-domain methodology for computing inner variables of the MMC with half-bridge cells, based on a two-stage approach. First, the circuit equations are decoupled by a simple linear transformation, whereby the circuit schematic can be separated into a dc-side and an ac-side part. Second, the variables of the cell strings are computed in the frequency domain by iterated convolution. It is shown that the proposed methodology, where all calculations are made in the frequency domain, can accurately reproduce the results from a PSCAD simulation. Furthermore, the model is successfully employed as part of an optimization algorithm for maximizing the power handling capability of the converter by appropriately controlling the circulating current and zero-sequence ac-side voltage. Results from this work point to significant possibilities for improvement.
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| 5. |
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| 6. |
- Bakas, Panagiotis, et al.
(författare)
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Design considerations and comparison of hybrid line-commutated and cascaded full-bridge converters with reactive-power compensation and active filtering capabilities
- 2019
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- This paper compares two hybrid topologies that combine the line-commutated converter (LCC) with cascaded full-bridge (FB) converters. The latter are utilized for compensating the reactive power and filtering the current harmonics of the LCC. The method that was developed for dimensioning these hybrid topologies is presented in detail. This method is utilized for calculating the arm voltage and current waveforms, which are used to estimate other important quantities, such as conduction losses and energy variations. Finally, the studied converters are compared in terms of voltage/current ratings, semiconductor requirements, conduction losses, and energy variations.
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| 7. |
- Bakas, Panagiotis, et al.
(författare)
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Hybrid alternate-common-arm converter with director thyristors - Impact of commutation time on the active-power capability
- 2019
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Ingår i: 2019 21st European Conference on Power Electronics and Applications, EPE 2019 ECCE Europe. - Genova, Italy : Institute of Electrical and Electronics Engineers Inc.. - 9789075815313 - 9781728123615
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Konferensbidrag (refereegranskat)abstract
- This paper investigates the impact of the thyristor commutation time on the peak currents and the active-power capability of the hybrid alternate-common-arm converter (HACC). This converter employs director thyristors for the alternate connection of a common arm in parallel to the main arms. The parallel connection enables current sharing among the arms, which allows the HACC to transfer higher output power without increasing the peak arm current. It is shown that the active-power capability of the HACC is doubled for a certain current-sharing factor, which, however, is altered by the thyristor commutation time. Therefore, the impact of the commutation time on the active-power capability of the HACC is investigated theoretically. Finally, this analysis is verified by simulation results.
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| 8. |
- Bakas, Panagiotis, et al.
(författare)
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Hybrid alternate-common-arm converter with high power capability : Potential and limitations
- 2020
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Ingår i: IEEE transactions on power electronics. - : IEEE. - 0885-8993 .- 1941-0107. ; 35:12, s. 12909-12928
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Tidskriftsartikel (refereegranskat)abstract
- This paper studies a new hybrid converter thatutilizes thyristors and full-bridge (FB) arms for achieving higherpower capability than the full-bridge (FB) modular multilevel converter (MMC) with reduced semiconductor requirements. The study covers the theoretical analysis of the energy balancing,the dimensioning principles, the maximum power capability, and the limitations imposed by the discontinuous operation of theconverter. Based on the analysis of these aspects, the theoretical analysis is concluded by identifying the design constraints that need to be fulfilled for achieving the maximum power capabilityof the converter. It is concluded that the maximum power capability can be achieved for a certain range of modulation indices and is limited by both the commutation time of the thyristors andthe power angle. Finally, simulation and experimental results that confirm the theoretical analysis and the feasibility of the studied converter are presented and discussed.
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| 9. |
- Bakas, Panagiotis, et al.
(författare)
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Hybrid Converter With Alternate Common Arm and Director Thyristors for High-Power Capability
- 2018
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Ingår i: 2018 20th European Conference on Power Electronics and Applications (EPE’18 ECCE Europe).
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Konferensbidrag (refereegranskat)abstract
- This paper presents the basic operating principles of a new hybrid converter that combines thyristors and full-bridge (FB) arms for achieving high active-power capability. This converter consists of a modular multilevel converter (MMC) equipped with additional common arms, which alternate between the upper and lower dc poles. This alternation is achieved by the thyristors that are utilized as director switches and allow the parallel connection of the common arms and the arms of the MMC. The main contributions of this paper are the analysis of the operating principles, the simulation verification of the functionality of the proposed converter, and the comparison of the latter with the full-bridge modular multilevel converter (FB-MMC).
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| 10. |
- Bakas, Panagiotis
(författare)
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Hybrid Converters for HVDC Transmission
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The line-commutated converter (LCC) and the voltage-source converter (VSC) are the two main converter technologies utilized in high-voltage direct current (HVDC) transmission applications. Depending on the application requirements, one technology might be more advantageous than the other. On the one hand, the LCC features technological maturity, high efficiency, and high power-transfer capability, but it lacks the ability to independently control active and reactive power and to ride through ac faults. On the other hand, the VSC overcomes the shortcomings of the LCC and offers more functionality, as it features the ability to independently control active and reactive power, ac-fault ride through capability, black-start capability, and superior harmonic performance. Yet, it is less mature, less efficient, and has lower power-transfer capability than the LCC. Thus, the combination of the LCC and the VSC topologies could yield hybrid converters that leverage the complementary characteristics of both technologies and thus are optimized for HVDC applications. Therefore, the main objective of this thesis is to investigate existing and derive new hybrid converters that combine the complementary characteristics of the LCC and VSC technologies.The hybrid converters investigated in this thesis are divided in two main categories, namely: (a) current-source; and (b) voltage-source hybrid converters. The former category includes hybrid converters that are based on the LCC structure and utilize a VSC part either for compensating the reactive power consumed by the LCC, or for active filtering of the LCC current harmonics, or for independently controlling active and reactive power, or for achieving a combination of these functionalities. Four different current-source hybrid converters have been investigated and compared in terms of functionality, conduction losses, and semiconductor requirements.The second category includes more complex circuits that combine thyristors and modular VSC elements in ways that enable these hybrid converters to operate as VSCs and to achieve high active-power capability. Two new voltage-source hybrid converters are analyzed and compared in terms of active-power capability, semiconductor requirements, and controllability. This study reveals that the hybrid alternate-common-arm converter (HACC) is the most interesting circuit; thus, an in-depth analysis is performed for this converter. The theoretical analysis shows that, under certain operating conditions, the HACC can transfer twice the active power of the full-bridge modular multilevel converter (FB-MMC) with lower semiconductor rating per unit of active power. Yet, if the total commutation time of the thyristors and/or the power angle are increased beyond certain values, the active-power capability of the HACC is reduced. Finally, simulation and experimental results are provided in order to verify the theoretical analysis and prove the feasibility of the HACC.
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