A Novel Single-Phase Transformerless TC-UPQC With Reduced DC-Link Voltage

A novel single-phase transformerless three-capacitor unified power quality conditioner (TC-UPQC) is proposed to manage grid current and stabilize load voltage. It consists of two active capacitors and one buffer capacitor. One active capacitor is applied for resisting grid voltage fluctuations and the other is connected in series with a buffer capacitor to supply current compensating. The voltages of active capacitors are regulated by internal half-bridge voltage-source converters (VSCs) and the buffer capacitor bears the remaining voltage imposed by load voltage. Thus, the magnitude difference between the two active capacitor voltages could be decreased and superior modulation degrees could be obtained. Besides, dc-link voltage could be greatly reduced by decreasing the magnitude of active capacitor voltages. Correspondingly, power balance control is proposed for the coordinate operations of VSCs and the regulation of dc-link voltage. In this regard, active power is embedded in voltage and current compensating to improve the flexibility of operation even without an external bi-directional dc power supply. Simulation platform and laboratorial prototype are established to validate the effectiveness of the proposed TC-UPQC.