ABSTRACT:
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.
KEYWORDS:
1. High-frequency ac (HFAC) microgrids
2. Quasi-resonant switched-capacitor (QRSC)
3. Multilevel inverter (MLI)
4. Self-voltage balancing
SOFTWARE: MATLAB/SIMULINK
BLOCK DIAGRAM:
Fig. 1. Power sources
for a single-phase 500-Hz microgrid.
CIRCUIT
DIAGRAM:
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.
CONCLUSION:
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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