Abstract
This paper describes a hybrid modulation scheme for a high-frequency ac-link (HFACL) multistage inverter comprising a front-end dc/ac converter, followed by isolation transformers, an ac/pulsating-dc converter, and a pulsating-dc/ac converter. The hybrid modulation scheme enables 1) removal of the dc-link filter evident in conventional fixed dc-link (FDCL) inverters placed after the ac/pulsating-dc converter stage and before an end stage voltage source inverter and 2) significant reduction in switching loss of the inverter by reducing the high-frequency switching requirement of the pulsating-dc/ac converter by two-third yielding higher efficiency, improved voltage utilization, and reduced current stress. Unlike the FDCL approach, in the HFACL approach, hybrid modulation enables the retention of the sine-wave-modulated switching information at the output of the ac/pulsating-dc converter rather than filtering it to yield a fixed dc thereby reducing the high-frequency switching requirement for the pulsating-dc/ac converter. Overall, the following is outlined: 1) hybrid modulation scheme and its uniqueness, 2) operation of the HFACL inverter using the hybrid modulation scheme, 3) comparison of the efficiency and losses, current stress, and harmonic distortion between the hybrid-modulation-based HFACL inverter and the FDCL inverter, and 4) scaled experimental validation. It is noted that the term hybrid modulation has no similarity with the modulation scheme for a hybrid converter (which are conjugation of two types of converters based on a slow and fast device) reported in the literature. The term hybrid modulation scheme is simply chosen because at any given time only one leg of the inverter output stage (i.e., pulsating-dc/ac converter) switch under high frequency, while the other two legs do not switch. The outlined hybrid modulation scheme is unlike all reported discontinuous modulation schemes where the input is a dc and not a pulsating modulated dc, and at most only one leg stays on or off permanently in a 60° or 120° cycle.
Highlights
S I-DEVICE-BASED high-power inverters usually operate at low switching frequencies owing to higher switching loss and thermal limitations of Si
By using a newly developed technique for reachability analysis of switching power converters using composite Lyapunov function based on a hybrid model, we demonstrate in Fig. 6 that the hybridmodulation-based high-frequency ac-link (HFACL) inverter topology is not a limiting case of an fixed dc-link (FDCL) topology for progressively reducing the dclink capacitance
This paper demonstrates the performance of a hybridmodulation-based HFACL inverter for high-power renewable and alternative-energy-based standalone, and distributed generation systems
Summary
The second topology, which is referred in this paper as the fixed dc-link (FDCL) topology is obtained from Fig. 1 by placing a dc-link capacitor at the output of the ac/pulsating-dc converter Both the topologies have a frontend high-frequency dc/ac converter followed by an ac/pulsatingdc converter and support galvanic isolation. The primary role of the modulation scheme for the single-phase pulsatingdc/ac converter is to demodulate the ac/pulsating-dc converter output on a half-line cycle basis to generate the output sine wave modulation pattern This is different than the hybrid modulation scheme for the HFACL topology (see Fig. 1), where not all the legs modulate uniformly since the output of the ac/pulsating-dc. This effectively yields a two-third reduction in the switching loss of the pulsating-dc/ac converter by modulation and without requiring any complex circuitry
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