Abstract
An AC-AC converter with a high-frequency link employing an LLC resonant converter operating in the vicinity of the resonance frequency was studied, in which a single output power stage is used, formed by a high-frequency AC-AC converter employing four-quadrant switches. The topology, its operation and the modulation strategy are described. The high-frequency stage switches located on the primary side of the transformer operate with soft switching of the ZVS type, while the four-quadrant switches that form the output stage operate with soft switching of the ZCS type. Experimental data obtained with a 1.5 kW experimental prototype (input $220~V_{RMS}$ , output $220~V_{RMS}$ and switching frequency 40 kHz), which was designed, built and tested in the laboratory, are reported herein. This converter can be considered a candidate for the building block of medium voltage solid-state transformers (SST) for power distribution systems.
Highlights
The isolated AC-AC converter with a high-frequency link is the basic building block of solid-state transformers [1]-[10]
The AC input voltage Vin is chopped by the four-quadrant switches S1 and S2, generating a high frequency amplitude modulated voltage, which is applied to the high frequency transformer
A low frequency sinusoidal voltage Vout is reconstructed at the output terminals by the four-quadrant switches S3 and S4, modulated by phase shift and synchronized with respect to the sinusoidal input voltage
Summary
The isolated AC-AC converter with a high-frequency link is the basic building block of solid-state transformers [1]-[10]. A low frequency sinusoidal voltage Vout is reconstructed at the output terminals by the four-quadrant switches S3 and S4, modulated by phase shift and synchronized with respect to the sinusoidal input voltage. Despite its simplicity, this topology is limited in its practical use, as it presents commutation difficulties, low efficiency and subjects the semiconductors to instantaneous voltages equal to at least twice the peak value of the input voltage. The second stage consists of an LLC resonant converter [11]-[15], operating at the resonant frequency, a condition that ensures soft commutation of the switches. An in-depth analysis of the converter is presented, with the inclusion of experimental results obtained from an experimental prototype designed, built and tested in the laboratory, with the purpose of verifying its performance and its potential to be used as a building block in solid-state transformers
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