Abstract
A single-phase symmetric-bipolar-type high-frequency isolated buck-boost ac–ac (SBT-HFI-BBAC) converter is proposed. The converter is composed of two full-bridge inverter modules, input/output <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> filters, and a high-frequency transformer (HFT) with series capacitors. It can provide identical noninverting and inverting buck-boost output voltages with symmetric-bipolar voltage gains of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$ \pm D/({1 - D})$</tex-math></inline-formula> . The frequency of the output voltage can also be adjusted in discrete steps, similar to single-phase matrix converters (MCs). It supplies continuous input and output currents and works well with nonunity power factor loads. The magnetizing current of the HFT has no line frequency component due to the presence of the series blocking capacitors. Moreover, due to the absence of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ac</i> switches, the converter has a simple commutation process, unlike conventional ac-ac/MCs. Two switching strategies are proposed; Strategy-I with fewer high-frequency switches and Strategy-II with reduced switch currents, providing more flexibility in switch power loss optimization. The proposed SBT-HFI-BBAC converter is well suited for application as a dynamic voltage restorer, compensating a wide range of grid voltage sags and swells, without the need for a low-frequency voltage injection transformer. Its step-changed frequency operation can also find use in various applications, such as high-gain ac–dc converters, traction systems, etc. A comprehensive description of circuit operation is presented along with component design guidelines and appropriate comparisons. Experimental verification results are provided using a 400-W laboratory test circuit.
Highlights
A C–AC power converters are extensively utilized in industrial applications, flexible ac transmission system (FACTS) devices for grid control, traction systems, etc
The symmetric NIBB/inverting buck-boost (IBB) and step-changed frequency operations with buck-boost voltage capability are the main features of the proposed SBT-HFI-BBAC converter which are not reported by any of the counterpart converters [26]–[31]
NIBB and IBB operations can generate positive and negative series voltages to mitigate a broad range of grid voltage sags and swells, respectively
Summary
A C–AC power converters are extensively utilized in industrial applications, flexible ac transmission system (FACTS) devices for grid control, traction systems, etc. All of the above described bipolar ac–ac converters and MCs are nonisolated, and they must incorporate an isolation stage for applications with an ac grid interface (such as DVR [10]) and traction systems [11], to avoid fault propagation [20] This is usually accomplished through an external line frequency transformer [21], which adds significant weight and volume due to its low-frequency operation (50 or 60 Hz), and suffers from high power losses, high starting in-rush currents, and dc magnetic flux bias [22]. Impedance source (ZS) network-based HFI bipolar ZS buck-boost ac–ac converters [26] and their modified versions with reduce passive components [27] have been proposed They lack noninverting buck voltage and step-changed frequency operations. Experimental verification results are provided, obtained using a 400-W laboratory prototype of the proposed converter
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