Analysis and research of digitized sampling characteristics in smart substations

Digitized sampling adopts decentralized, independent sampling mode, which introduces a new problem of data synchronization to secondary system of substations, especially for relay protections. This paper analyzes the time delay of every sections in sampling and the causes of non-synchronized sampling data. It states two general applied data synchronization method in smart substations at present, one method is based on external synchronizing signal while the other is based on interpolation method for resampling. The paper also compares two important indexes of digital sampling, rated delay time and phase error. Then the paper presents two methods, steady-state indirect method and time-scale direct method, to measure sampling delay and in turn to solve the problems of data synchronization. The test data of practical project shows that both two methods are able to achieve microsecond precision, but time-scale direct method is more accurate and stable. At last, the paper gives some precautions and recommendations for application of digital sampling in smart substations.