Voltage Sensorless Synchronization of a Grid-Side Converter Based on a Discontinuous Operating Mode and a Sliding Mode Observer

Recent advances in the control of grid-side converters (GSCs) aim to improve system reliability by eliminating the need for voltage sensors. Effective control without voltage sensors requires both synchronization before the GSC is connected to the grid and continuous operation afterwards. To meet these two requirements, a novel voltage sensorless control method is proposed using a robust and reliable estimation structure with a sliding-mode observer (SMO) and a double generalized second-order integrator with frequency-locked loop (DSOGI-FLL). The main objective of sensorless start-up of GSC is to suppress the inrush current that occurs when the grid and GSC voltages are not well synchronized. To achieve this, the discontinuous operation of the GSC is exploited by forcing controlled short circuits at the GSC terminals, resulting in a discontinuous current between the grid and the GSC. Therefore, the proposed estimation structure is analyzed and adapted to operate properly in discontinuous operation. Thus, a successful synchronization, i.e. an estimation of the grid angle and amplitude, is ensured based on the sampled counterpart of the discontinuous current. After synchronization and with only minor modifications to the estimation structure, the proposed method also enables stable operation in continuous mode, which implies standard voltage-oriented control (VOC) with pulse-width modulation (PWM). Consequently, the whole process of voltage sensorless control of the GSC is performed using a single estimation structure, which is inherently robust to disturbances such as voltage distortion due to the specific characteristics of SMO and DSOGI-FLL. The proposed method is validated by simulations and experimental tests.