An improved control strategy of 3P4W DVR systems under unbalanced and distorted voltage conditions

This paper proposes an enhanced nonlinear control technique based on a combination of feedback linearization (FBL) and resonant regulators as tracking controllers for three-phase four-wire (3P4W) dynamic voltage restorers (DVR) under unbalanced and distorted source voltages. A nonlinear model of the DVR system with a PWM voltage-source inverter (VSI) and output LC filters in the distribution system containing nonlinear loads is firstly linearized, which then the proportional integral plus resonant (PI + R) regulators are utilized for tracking controllers. In this control scheme, the voltage references of the controlled variables expressed in the d-q-0 synchronous reference frame (SRF) for the DVR system are generated based on an in-phase compensation strategy, which results in the appearance of the second and multiple-sixth order components in the DVR voltage references under the unbalanced and distorted voltages conditions. With the combination of FBL technique and the tracking controllers based on resonant regulators, the DVR system provides an excellent control performance in the transient and steady states for compensating the load voltages with almost zero steady-state errors. The proposed method is validated by the simulation and experimental results, which show that the DVR with the suggested control scheme restores the load voltages at a low THD (total harmonic distortion) value of about 2.3% compared to 7.7%, 6.2%, and 5.4% by the typical PI, PR controllers, and conventional FBL technique, respectively.