Abstract
A high-frequency isolated multilevel cascaded-type bipolar direct PWM ac-ac converter is proposed. By cascading N-unit cells, the converter can produce symmetric bipolar voltage gain of ±ND. It does not require a separate ac source for each unit cell, and can provide higher ac output voltages with multiple levels using low-voltage rating devices, decreasing output voltage harmonics and filtering requirements. Moreover, phase-shift PWM switching is employed, increasing the effective frequency of the output filter, further reducing its requirements. The converter can be utilized as a dynamic voltage restorer (DVR) for grid voltage regulation, with the following features: No need for an external dc source, separate ac sources, and a line-frequency voltage injection transformer. Injection of higher voltage magnitudes than the line voltage, mitigating deep voltage sags. Injection of a negative series voltage to provide compensation for voltage swells. The detailed circuit operation is explained together with component design guidelines and application as DVR. Finally, experimental results are provided using an 800-W scale-down laboratory prototype.
Highlights
D UE to the integration of renewable energy sources and diversified load conditions, grid voltages are vulnerable to power quality issues including, voltage fluctuations, unequal three-phase voltages, and voltage sags and swells
The most common dynamic voltage restorer (DVR) consists of a voltage
The output current lags the output voltage and both are purely sinusoidal without any distortion, proving the compatibility of the proposed converter with reactive or nonunity power factor loads
Summary
D UE to the integration of renewable energy sources and diversified load conditions, grid voltages are vulnerable to power quality issues including, voltage fluctuations, unequal three-phase voltages, and voltage sags and swells. Conventional buck and boost direct ac–ac converters [5] are able to compensate only voltage sags (or swells) of limited magnitude They require external line-frequency voltage injection (and isolation) transformers. These provide bipolar buck–boost voltage operation to compensate a wide magnitude range of voltage sags and swells These topologies are nonisolated and research works in [10]–[12] proposed ac–ac converters with high-frequency transformer isolation. They have all the benefits of multilevel cascaded-type converters based DVRs (as explained earlier) while solving the commutation problems of direct ac–ac converters They have some limitations, as explained below: 1) They require a large number of coupled-inductors (or simple inductors) inserted in each phase-leg to prevent shoot-through issues, greatly increasing their magnetic component requirement. Experimental results are provided using an 800-W laboratory prototype circuit
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