Comprehensive Analysis and Design of a Quasi Two-Level Converter Leg

Milovanović, Stefan; Dujić, Dražen

doi:10.24295/cpsstpea.2019.00018

Cited by 7 publications

(10 citation statements)

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“…Employing SiC MOSFETs allows for a low step time (i.e. the time each intermediate voltage level appears during the staggered switching transition, see Figure 1) of

t_{\rm step}=500\nobreakspace \,{\rm ns}

(several

\upmu

s would be required in the case of IGBTs due to the slower switching speeds and hence longer interlock delay times [27, 30]). Further, considering a permissible maximum peak‐to‐peak capacitor voltage variation of

\mathrm{\Delta}{U}_{\text{C,pp}}=400\hspace*{0.20202em}\, \mathrm{V}

(20% of the nominal peak cell voltage

\hat{u}_{\rm Mt1,a}

), a small cell capacitance of about

C_{\rm M}=1\nobreakspace \,\upmu {\rm F}

results.…”

Section: Design Aspectsmentioning

confidence: 99%

New single‐stage bidirectional three‐phase ac‐dc solid‐state transformer

Kopacz,

Menzi,

Krismer

et al. 2024

Electronics Letters

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High‐power applications with low‐voltage (LV) dc loads, for example, fast charging stations for electric vehicles (EVs), are typically supplied from the medium‐voltage (MV) grid. Aiming for low volume, MVac‐LVdc solid‐state transformers (SSTs) that provide galvanic separation with medium‐frequency transformers (MFTs) are thus considered, which are conventionally realized as two‐stage systems that consist of an ac‐dc power‐factor‐correction (PFC) rectifier and an isolated dc‐dc converter. This letter extends a new single‐stage isolated bidirectional PFC rectifier concept to MV levels, resulting in an SST that performs MVac‐LVdc conversion in a single converter stage and, unlike many modular SST concepts, employs only a single MFT. The SST's primary side operates modular‐multilevel‐converter (MMC) bridge legs, whose input voltages contain large ac components, in the quasi‐two‐level mode to minimize the cell capacitors. The secondary side employs a standard LV three‐phase rectifier, and a dual‐active‐bridge‐(DAB)‐like modulation strategy allows power flow regulation and ensures sinusoidal grid currents. The concept is explained and validated using detailed circuit simulations of an exemplary 1‐MW system operating between a 10‐kV three‐phase grid and an 800‐V dc output. The component stresses are evaluated, and an efficiency of 98.1% and a power density of up to 0.6 kW/dm3 are estimated.

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“…Employing SiC MOSFETs allows for a low step time (i.e. the time each intermediate voltage level appears during the staggered switching transition, see Figure 1) of

t_{\rm step}=500\nobreakspace \,{\rm ns}

(several

\upmu

\mathrm{\Delta}{U}_{\text{C,pp}}=400\hspace*{0.20202em}\, \mathrm{V}

(20% of the nominal peak cell voltage

\hat{u}_{\rm Mt1,a}

), a small cell capacitance of about

C_{\rm M}=1\nobreakspace \,\upmu {\rm F}

results.…”

Section: Design Aspectsmentioning

confidence: 99%

New single‐stage bidirectional three‐phase ac‐dc solid‐state transformer

Kopacz,

Menzi,

Krismer

et al. 2024

Electronics Letters

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“…Alternatively, bridge-legs for single-cell SSTs can be realized with a series connection of semiconductors [17,18], super-cascode configurations [19,20], or multi-level converter structures, i.e., modular multi-level converter (MMC) [21][22][23][24][25] and flying capacitor converter (FCC) structures [18,[26][27][28]. However, due to unavoidable differences (manufacturing tolerances etc.)…”

Section: Introductionmentioning

confidence: 99%

“…With Q2L modulation, the intermediate voltage levels are only used during the switching transitions. The bridgeleg's output voltage thus transitions between the two DC voltage levels (positive and negative) in a staggered fashion [22][23][24][28][29][30][31][32][33][34]. Note that these staggered transitions of the the Q2L-MMC and Q2L-FCC topologies feature lower average dv/dt compared to the (MV) 2-level converters, which is beneficial for the design of EMI filters and magnetics such as medium-frequency transformers, and lowers the stress of the electric insulation [29,[35][36][37][38].…”

Section: Introductionmentioning

confidence: 99%

Load-Independent Voltage Balancing of Multi-Level Flying Capacitor Converters in Quasi-2-Level Operation

et al. 2021

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Quasi-2-level (Q2L) operation of multi-level bridge-legs, especially of flying-capacitor converters (FCC), is an interesting option for realizing single-cell power conversion in applications whose system voltages exceed the ratings of available power semiconductors. To ensure equal voltage sharing among a Q2L-FCC’s switches, the voltages of a Q2L-FCC’s minimized flying capacitors (FCs) must always be balanced. Thus, we propose a concept for load-independent FC voltage balancing: For non-zero load current, we use a model predictive control (MPC) approach to identify the commutation sequence of the individual switches within a Q2L transition that minimizes the FC or cell voltage errors. In case of zero load current, we employ a novel MPC-based approach using cell multiple switching (CMS), i.e., the insertion of additional zero-current commutations within a Q2L transition, to exchange charge between the FCs via the charging currents of the switches’ parasitic capacitances. Experiments with a 5-level FCC half-bridge demonstrator confirm the validity of the derived models and verify the performance of the proposed load-independent balancing concept.

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“…The paper concludes that the branch inductance of the converter should be minimized for the best MMC design and studies the selection of the module capacitances. Milovanović et al provide a more profound analytic study of the converter behavior in [23]. However, the findings and the analysis done for dc-dc applications cannot be directly applied to drive applications, since in drive applications, unlike in dc-dc applications, the output current is almost constant during a modulation period and the duty cycle generally varies in a wide range.…”

Section: Introductionmentioning

confidence: 99%

“…However, the findings and the analysis done for dc-dc applications cannot be directly applied to drive applications, since in drive applications, unlike in dc-dc applications, the output current is almost constant during a modulation period and the duty cycle generally varies in a wide range. Hence, the resonant behavior has to be described in a different manner than in [22], [23] and the design process has to be modified as well. Beside the basic design rules proposed in [10], there is no unified methodology for the design of the quasi-two-leveloperated MMCs that do not control internal currents of the converter (first group of approaches) for drive applications.…”

Section: Introductionmentioning

confidence: 99%

Designing a Passively Damped Quasi-Two-Level-Operated Modular Multilevel Converter for Drive Applications

Kucka¹,

Mertens²

2020

IEEE Access

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This paper concentrates on the simplest quasi-two-level PWM operation mode for modular multilevel converters, where the internal currents within the converter are not controlled. The model of the converter is derived and the properties of the inherent resonance circuit within the converter are discussed. The paper proposes an optimized design approach for the converter parameters and shows several challenges in the practical design. The main problem of the studied converter operation mode is the dependency of the converter behavior on many parasitic parameters that can significantly vary in the converter production. Moreover, the paper shows that when the converter losses are low, the optimized converter inductances are below the values expected for realistic converter construction. Consequently, the module capacitances have to be significantly increased or the converter internal currents reach exceedingly high values. Furthermore, a comparison is drawn to the quasi-two-level PWM operation mode in which the leg currents and branch energies are controlled, showing several crucial disadvantages of the studied passively damped operation mode. The utilized models and the basic concepts are validated experimentally on a downscaled converter prototype.INDEX TERMS Modular multilevel converter, passive damping, quasi-two-level.

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Comprehensive Analysis and Design of a Quasi Two-Level Converter Leg

Cited by 7 publications

References 0 publications

New single‐stage bidirectional three‐phase ac‐dc solid‐state transformer

New single‐stage bidirectional three‐phase ac‐dc solid‐state transformer

Load-Independent Voltage Balancing of Multi-Level Flying Capacitor Converters in Quasi-2-Level Operation

Designing a Passively Damped Quasi-Two-Level-Operated Modular Multilevel Converter for Drive Applications

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