A transformer-less unified power quality conditioner with fast dynamic control

A single-phase transformer-less unified power quality conditioner (TL-UPQC) is presented. Apart from having no isolation transformer, the proposed structure utilizes four switching devices only, forming two half-bridge voltage-source inverters - one connected in parallel with the load and another one connected in series with the ac mains. The two inverters share the same dc link. The parallel inverter, which is controlled by a hysteresis current controller, is used to shape the current drawn from the ac mains and regulate the dc-link voltage. The series inverter, which is controlled by a boundary controller with second-order switching surface, is used to regulate the steady-state load voltage and provide voltage sag/swell ride-through. A dc-link capacitor voltage balancing control that coordinates the operations of the hysteresis and boundary controllers is designed. Modeling, design, and analysis of the whole system will be given. A 1 kVA, 110 V, 60 Hz prototype has been built and evaluated on a setup with a nonlinear load. The steady-state and transient responses under a voltage sag will be given. Experimental results are favorably compared with the theoretical predictions and the performance of other UPQCs.