| 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. |
- Ilves, Kalle, et al.
(författare)
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A submodule implementation for parallel connection of capacitors in modular multilevel converters
- 2013
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Ingår i: Power Electronics and Applications (EPE), 2013 15th European Conference on.
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Konferensbidrag (refereegranskat)abstract
- The modular multilevel converter is a suitable converter topology for high-voltage high-power applications and consists of series-connected submodules. Typically, these submodules are half-bridges with dc capacitors. A voltage ripple in the submodule capacitors is inevitable due to the current flowing in the arms. The converter should therefore be controlled in such a way that the capacitor voltages are kept balanced and close to their nominal values over time. This paper presents a new submodule circuit which alleviates the balancing of the capacitor voltages. The proposed submodule circuit consists of two capacitors and eight switches, forming a three-level submodule. Ideally, the voltage and current rating of the switches can be chosen such that the combined power rating of the semiconductors is the same as for equivalent half-bridge submodules. The proposed submodule circuit provides the possibility of connecting the two capacitors in parallel when the intermediate voltage level is used. This will reduce the capacitor voltage ripple, especially at low switching frequencies and thus allow for a reduction of the size, weight, and cost of the submodule capacitors. The proposed submodule circuit is validated by both simulation results and experiments on a laboratory prototype. It is found that the parallel connection of the submodule capacitors will, in fact, significantly improve the balancing of the capacitor voltages.
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| 6. |
- Ilves, Kalle, et al.
(författare)
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Analysis and operation of modular multilevel converters with phase-shifted carrier PWM
- 2013
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Ingår i: 2013 IEEE Energy Conversion Congress and Exposition, ECCE 2013. - : IEEE. - 9781479903351 ; , s. 396-403
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Konferensbidrag (refereegranskat)abstract
- The modular multilevel converter is a suitable topology for high-voltage high-power applications since the cascaded submodules can generate high-voltage waveforms with excellent harmonic performance at low switching frequencies. Many publications have been presented on the modulation and control of this converter type, some of which are based on phase-shifted carrier modulation. This paper presents an analysis of how the switching frequency affects the capacitor voltages, circulating current and alternating voltage at phase-shifted carrier modulation. It is found that integer multiples of the fundamental frequency should be avoided as they can cause the capacitor voltages to diverge. Suitable switching frequencies are then defined for which the arm and line quantities will be periodic and symmetric in the upper and lower arms. The theoretical results are then validated by both simulations and experimental results.
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