Reduced Electrolytic Capacitor Based Single-Phase Converters: Topologies, Control, and Stability

Abstract

Single-phase converters (SPC) find wide applications in different domestic and industrial appliances, ranging from a few hundred Watts to a few Megawatts. Second-harmonic ripple filtering is a critical issue in SPCs, for which Aluminium electrolytic capacitors (Al-caps) are traditionally used due to their low cost and excellent energy density. However, Metallized Polypropylene Film Capacitors (MPPFC) make a better choice in reliability-oriented designs as Al-caps are failure-prone. Considering the volumetric energy densities, one-to-one replacement of Al-caps by MPPFC is impracticable for dc bus filters in SPCs. Hence, active filtering (AF) has become popular to bring down the dc bus capacitance requirement while enhancing the likelihood of employing MPPFCs. In this work, an efficient and compact AF topology named series capacitor stacked buffer (SSB) is modelled to identify its load characteristics, address the control challenges, and determine the stability limits. It enables a fixed switching frequency modulation for implementing close-loop control of SSB to replace existing variable frequency hysteresis control. The dc-link current sensor is eliminated from the existing current-control scheme of SSB by reference current estimation. SSB-based different AF topologies with the minimum switch count are synthesized and compared. Apart from the active solutions, an alternative dc bus filter structure with a second-harmonic tuned LC filter is considered for reducing the capacitance requirement. Despite offering desirable impedance characteristics at the resonance frequency, LC filters fail to ensure consistent filtering performance under frequency and parameter variations. To address this issue, a solid-state tuning restorer (SSTR) is proposed, acting as an electronic inductor or capacitor based on the tuning requirement of the LC filter while ensuring a graceful degradation in the filter characteristics during its failure modes. A dc capacitor-less unified active capacitor and inductor (UACI) is proposed for emulating capacitive and inductive characteristics with a two-terminal converter. Conventionally, an H-bridge-based active capacitor or inductor requires huge dc capacitances to ensure satisfactory input current THD. In the proposed configuration, a dc capacitor-less three-leg converter topology is implemented to emulate a two-terminal unified active capacitor and inductor which smoothly transits from one characteristic to another. The validation of the modelling, operation, and control pertaining to SSB, two-switch SSB, SSTR, and UACI are performed experimentally on the hardware prototypes.