Advanced Type-1c FLL for Enhancing Converters Synchronization During Frequency Drift

This paper introduces a novel out-loop compensation scheme with a selective filtering stage for Type-1 Frequency Locked Loop (FLL) to obtain the same features of Type-2 FLL in response to frequency drift. The proposed scheme can eliminate the phase angle error during frequency drift without compromising its benefits of the low order control system. The performance of the proposed Type1-c FLL is evaluated with alternatively employed Delayed Signal Cancellation (DSC), Low Pass Filter (LPF) with selective harmonics filtering and multiple second-order generalized integrator (MSOGI) as a pre-filtering stage under the various grid disturbances. The effects of each filtering scheme on the dynamic response and harmonics mitigation are identified. As each filter has a distinct advantage in response to each grid disturbance, a selective control approach is used to insert the most suitable filter based on the severity of the grid disturbance to achieve the best performance. A comprehensive study is conducted to demonstrate the superior performance of the proposed schemes with both simulation and experimental validation. In addition, the potential application of the proposed Type-1c FLL for enhancing the islanded microgrid operation has been presented and verified.