In this paper, a quasi-resonant switched-capacitor (QRSC) multilevel inverter (MLI) is proposed with self-voltage balancing for single-phase high-frequency ac (HFAC) microgrids. It is composed of a QRSC circuit (QRSCC) in the frontend and an H-bridge circuit in the backend. The input voltage is divided averagely by the series-connected capacitors in QRSCC, and any voltage level can be obtained by increasing the capacitor number. The different operational mechanism and the resulting different application make up for the deficiency of the existing switched-capacitor topologies. The capacitors are connected in parallel partially or wholly when discharging to the load, thus the self-voltage balancing is realized without any high-frequency balancing algorithm. In other words, the proposed QRSC MLI is especially adapted for HFAC fields, where fundamental frequency modulation is preferred when considering the switching frequency and the resulting loss. The quasi-resonance technique is utilized to suppress the current spikes that emerge from the instantaneous parallel connection of the series-connected capacitors and the input source, decreasing the capacitance, increasing their lifetimes, and reducing the electromagnetic interference, simultaneously. The circuit analysis, power loss analysis, and comparisons with typical switched-capacitor topologies are presented. To evaluate the superior performances, a nine-level prototype is designed and implemented in both simulation and experiment, whose results confirm the feasibility of the proposed QRSC MLI.
- High-frequency ac (HFAC) microgrids
- Quasi-resonant switched-capacitor (QRSC)
- Multilevel inverter (MLI)
- Self-voltage balancing
Fig. 1. Power sources for a single-phase 500-Hz microgrid.
Fig. 2. Circuit of the proposed QRSC MLI when outputting 2n+1 levels.
EXPECTED SIMULATION RESULTS
Fig. 3. Simulation waveforms of the output voltages and currents under different load-types. (a) Vin = 100 V, fo = 500 Hz, ZL = 24 . (b) Vin = 100 V, fo = 500 Hz, ZL = 7.4+j11.3 (|ZL| = 13.5
Fig. 4. (a) Simulation waveforms of the voltages on capacitors C1~C4. (b) Simulation frequency spectrum of the staircase output.
Fig. 5. Simulation waveforms of the capacitors’ charging currents. (a) With quasi-resonant inductor. (b) Without quasi-resonant inductor.
To make up for the deficiency that existing SC MLIs are inappropriate for the preferred series-connected input occasions like mode 2 in Fig. 1, a novel SC MLI is proposed in this paper with different structure and operational mechanism from the traditional ones, and to suppress the current spikes caused by the capacitors’ instant charging from the input source, a quasi-resonant inductor is embedded into the capacitors’ charging loop, reducing the EMI and longing the capacitors’ lifetimes. Meanwhile, the proposed QRSC MLI combines the advantages of the traditional SC MLI, such as self-voltage balancing under FFM and smaller voltage ripples for capacitors when used as HF power conversion, thus, especially adapted for HFAC microgrids. The circuit configuration and the power loss analysis of the proposed QRSC MLI have been presented in this paper, as well as the comparisons with typical SC topologies. Lastly, a nine-level prototype is designed and implemented in both simulation and experiment. The results have validated the superior performances of the proposed topology.
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