A digital sampling rate synchronization scheme for fully digital relay protection

The application of IEC61850 and non-conventional instrument transformers (NCIT) brings a new era in the development of fully digital protection system in which the protection IED receives sampled data from different digital data sources (DDS). The mismatch in sampling rate between the protection IED and the DDS requires sampling rate synchronization (SRS) to make sure the protection and other algorithms can run correctly. In this paper, a digital SRS scheme based on oversampling interpolator and quasi-continuous FIR interpolator is proposed, in which the SRS requirement in a protection IED can be realized. A poly-phase implementation structure for the oversampling interpolator is designed and the Lagrange interpolation algorithm is chosen to approximate the impulse response of the quasi-continuous FIR interpolator considering the reduction in error and computation. Design examples are given to demonstrate the effectiveness of the proposed approach.