System Modeling and Stability Analysis of Single-Phase Transformerless UPQC Integrated Input Grid Voltage Regulation

This article extends the conventional features of a transformerless unified power quality conditioner (TL-UPQC). An enhanced control methodology is presented to allow exchanging reactive power between the system and the grid to provide input grid voltage regulation. Therefore, both load side and input grid-side voltages are regulated with one converter. In this regard, a phase angle is created between the input current and the input voltage. Thereby, the system behavior as a capacitive or an inductive reactance is controlled. An additional ac voltage control loop has been designed. The inner current loop has been reformed to receive reference information from two outer voltage loops. The enhanced control strategy takes action based on local information collected by the TL-UPQC with no requirements of additional sensor circuits. Since the conventional functions of the TL-UPQC system have been extended, aspects related to system modeling and control design should be developed. The small-signal model that characterizes the dynamics of the power stage and the controller are presented. Design guidelines considering grid impedance to achieve a desired performance are developed. A 500 VA / 120 V, 60 Hz prototype has been built to verify the models and the overall system performance. Steady-state and transient experimental results are presented and discussed.