Multilevel Limit Cycle Oscillator for Synchronizing Grid Connected Inverters in Fault Ride-Through Applications

Grid synchronization algorithms are important in control systems of power electronic converters. These algorithms must ensure proper synchronization even in the case of disturbances such as voltage-sags and phase-jumps, which are common in high-voltage and low-voltage ride-through applications. Therefore, the development of robust synchronization algorithms in these situations is of interest. In this article, a nonlinear synchronization algorithm suitable for application in grid-following inverters operating under fault ride-through conditions is proposed. This algorithm is called multilevel limit cycle oscillator frequency-locked loop (MLCO-FLL), and it is used as a prefilter for a synchronous reference frame phase-locked loop (SRF-PLL). This algorithm is robust against phase-jumps and variations in the voltage, both representing important features for fault ride-through applications. Also, it automatically detects the operation at different voltage levels without state machines or logic conditions. These features might be of interest in other research fields, where the detection of orbits is important. A comparison with other linear and nonlinear synchronization algorithms showed that MLCO-FLL features important advances. Moreover, in future, it is possible to improve its performance once its characteristics and parameters are examined in detail. Simulations and experimental results obtained from a 15 kW inverter connected to a grid-emulator were used to validate the MLCO-FLL.