A Fast Recovery Technique for Grid-Connected Converters After Short Dips Using a Hybrid Structure PLL

Repeated faults in an industrial grid have affected a steel manufacturing plant and resulted in frequent tripping of all medium voltage converters. The reason for the trips was a synchronization fault due to a severe short duration voltage dip. Converters need a relatively long time to resume its normal operation after dip recovery due to the capacitors precharging process. If the capacitor's discharging is prevented, the converter can resume its normal operation after being synchronized. The synchronization process is performed by a phase locked loop type SRF-PLL with a very low bandwidth and low-pass in-loop filter, which delays the recovery process. In this paper, a solution for the aforementioned trip is presented by proposing two different synchronization hybrid schemes. The first scheme is assuming the converter already lost the synchronization, and a ride-through sequence is implemented to prevent the capacitor from discharging. The second scheme is proposed to prevent the synchronization fault and to recover the converter quickly after dip ends. Both schemes are implemented using a single-input fuzzy logic controller to obtain a very fast response. The performance of the proposed schemes is validated through simulation and a prototype experimental setup.