Abstract
This paper proposes a hybrid modulation strategy for a medium-voltage hybrid seven-level (7-L) converter, which consists of a three-phase silicon (Si) IGBT-based active-neutral-point-clamped (ANPC) stage and a cascaded silicon carbide (SiC) MOSFET-based H-bridge (HB) stage in each phase. Within the framework of the proposed modulation strategy, three floating HB converters are controlled using a high-frequency three-level (3-L) space-vector-modulation (SVM), while a low-frequency modulation is applied to the ANPC converter. To regulate the DC voltages across floating capacitors, a zero-sequence voltage (ZSV) injection method is adopted. The proposed method possesses three major advantages. First, it enables the simplification from a 7-L to a 3-L SVM, which significantly reduces the computational burden. Second, with all the high-frequency switching actions assigned to SiC devices in the HB stage, the power loss of the hybrid 7-L converter is reduced. Third, the hybrid modulation together with the ZSV injection enables a wider stable operating range compared to the existing 7-L SVM method. Finally, both simulation and experimental results performed on the scale-down and the full-scale 1 MVA prototype validate the effectiveness of the proposed hybrid modulation strategy.
Highlights
This paper proposes a hybrid modulation strategy for a medium-voltage hybrid seven-level (7-L) converter, which consists of a three-phase silicon (Si) IGBT-based active-neutral-point-clamped (ANPC) stage and a cascaded silicon carbide (SiC) MOSFET-based H-bridge (HB) stage in each phase
In [21], instead of using all-SiC switches in a traditional three-phase current source inverter, Si IGBTs are adopted in six switch positions, while an auxiliary SiC MOSFET is employed to increase the overall switching frequency and lower the power losses
A hybrid modulation method combining a 3-L space vector modulation (SVM) and a phase-shifted pulse width modulation (PS-PWM) [29] is proposed in [30] for a modified five-level (5-L) ANPC converter, where the four high voltage power devices operate at fundamental frequency, while the other LV power devices operate at high switching frequency
Summary
To increase the power density of the HMCs, pushing up the switching frequency is an effective solution It is still quite challenging for the state-of-the-art highpower silicon (Si) devices to operate beyond the kilohertz range, due to high switching losses. A 1-MW three-level (3-L) ANPC in a novel two-stage structure is proposed in [27], which uses Si IGBTs at high-voltage (HV) stage and SiC MOSFETs at LV stage, such that the converter can achieve both low cost and low power losses. As it can be seen from the above literature review, to fully exploit the benefits of the hybrid Si/SiC HMC, sophisticated modulation strategies still need to be developed It is an attractive solution, in terms of cost and efficiency, to apply the hybrid Si/SiC configuration to the aforementioned HMC topology, which consists of an ANPC stage with cascaded H-bridges. Comprehensive simulation results and experimental results on both scale-down and full-scale prototypes are presented to validate the effectiveness of the proposed hybrid modulation strategy
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