A novel control method of a VSC without AC line voltage sensors

The original algorithm to estimate position of line voltage essential for operation of a regenerative three-phase pulsewidth-modulation-controlled voltage-source converter (VSC) is presented. The proportional-plus-integral current regulator in the d axis of the VSC current control is modified to obtain the angle error signal. The angle error signal drives the observer (similar in structure to a phase-locked loop) which provides position or angle of line voltages. To achieve a fast and reliable lock with a utility system with minimal duration of transients, initial line voltage position is estimated first. The initial line voltage position estimation is done by measuring the amount of current change when zero-state voltage vectors were applied at the output of the inverter for defined time intervals. After that, continuous position estimation by observer is enabled. To estimate the magnitude of line voltage, another observer in the q axis is used. The observer is implemented in synchronous frame of reference and is driven by q-axis current error. Theoretical results are experimentally verified.