Control of transformerless T-type DVR using multiple delayed signal cancellation PLL under unbalanced and distorted grid condition

Abstract

In the near future, the power system network may experience severe voltage distortions due to association of renewable energy sources (RES), and the large penetration of power electronic based loads into the microgrid. To protect sensitive loads from these distorted and unbalanced grid conditions, the commonly used dynamic voltage restorer (DVR) requires effective control algorithms. The effective control algorithm comprises a controller, and accurate estimation of grid phase angle/frequency, and magnitude, thereby the reference DVR voltage is accurately estimated. The synchronous reference frame based phased locked loop (SRF-PLL) is commonly used for phase angle estimation. However, during unbalanced, distorted, and dc-offset conditions, the performance of SRF-PLL is unsatisfactory. To improve its performance, it is cascaded with prefilters such as dual second order generalized integrator (DSOGI), cascaded delayed signal cancellation (CDSC), and multiple delayed signal cancellation (MDSC). Among these prefilters, MDSC has superior performance under all grid conditions with less delay time and memory requirement. In addition to an effective control algorithm, the DVR system requires high power density converters. The power density of the system is increased using transformerless T-type converter. In this paper, the effectiveness of MDSC-PLL for the transformerless T-type converter based DVR is evaluated in MATLAB/Simulink for various grid conditions such as voltage sag/swell, unbalance voltage sag/swell, harmonics, and dc-offset.