Comparative Study of the Second-Order and Third-Order Systems for the Synchronization System under Distorted Grid Conditions

This paper presents our research on the performance of the grid synchronization system in a grid-connected inverter under distorted grid conditions. The generalized second-order and the third-order synchronization systems which are widely implemented in the inverter applications are compared in detail. Note that these synchronization systems are derived based on the transfer function approach and are able to generate two orthogonal outputs which are basically sine and cosine signals in the steady-state. It is interesting to see that the second-order system can give the same performance as the third-order system in terms of harmonic injection if its poles are designed properly. The simulation and experimental results are presented to confirm that these generalized systems are able to generate two perfectly orthogonal outputs to determine the grid information, i.e. voltage magnitude and phase angle. In addition, the performance of the generalized second-order and third-order systems under the distorted grid conditions are discussed. It is verified later that the second-order system is able to remove harmonics and accurately detect the grid information. In other words, the second-order system has similar to the third-order system if it is carefully designed.