Fast and Stable Synchronization Between the Grid and Generator by Virtual Coordinates and Feed-Forward Compensation in Grid-Tied Uninterruptible Power Supply System

For synchronization between the grid voltage and generator voltage, we propose a phase-locked loop (PLL) algorithm with virtual rotational reference frame and feed-forward compensation. It is possible to accurately estimate the phase angle through a virtual coordinate system that uses the phase angle information of the grid instead of the phase angle information of the generator. In addition, to eliminate the effect of harmonic components, which make it difficult to calculate the exact phase angle and to alleviate the very sensitive problem to external noise, a feed-forward compensation algorithm is applied. To verify the feasibility of the proposed approach, PSIM simulations and experiments are performed on prototypes. In particular, in order to utilize the emergency generator as a demand resource, the existing stand-alone emergency generator system is changed to a grid-tied uninterruptible power supply structure and verified through a 280kW actual load test for four operating modes. With the proposed virtual coordinates and feed-forward compensation, fast and stable transfer switching is possible. The error of the phase angle estimation is excellent as 0.02°, and it has a 20% lower time-constant for phase angle estimation compared to the conventional method at the moment of phase transfer when 5% of the 5th harmonic component is included in the grid voltage. Consequently, it can be confirmed that inrush current do not occur in all operating modes.