A novel PLL based algorithm for seamless transfer from autonomous to grid mode for utility interactive converters

In this paper, a novel algorithm for the seamless transfer of a three phase utility interactive converter from an autonomous mode to a grid connected mode is proposed. The proposed technique does not make use of any grid detecting and synchronising infrastructure. In this scheme, the angular deviation of the synchronously rotating working frame of the converter with respect to the grid frame is estimated at the instant of transfer. This error is added as a feedback term to the phase locked loop (PLL) to the dynamics of the PLL and enhance the speed of alignment to the grid frame. A hysteresis control scheme based on the d and q axes components of the line currents, initiates the enhancement action. Further, this idea is extended to develop a new scheme for self synchronization of parallel converters without any dedicated infrastructure. The proposed scheme aligns the working frames of the parallel converters to a common working frame which makes the synchronisation possible. A power reference reset control along with the proposed algorithm also ensures the load sharing feature of the parallel converters. The proposed technique is validated by adequate simulation and experimental test results.