Testing of mixed sensor technologies in transformer differential protection

In protection and control of power system, variety of acquisition chains may co-exist. One of the important and vulnerable schemes is differential protection application where different acquisition chains may affect protection relay. Moreover, interoperability of conventional copper wire and IEC 61850 with various chain acquisition may complicate the operation. This paper presents a test bed for interoperability studies of LPIT, standalone merging unit, and conventional analogue sensor measurements. Special laboratory tests are performed where one side of a transformer differential setup comes from an analogue CT (A) and the other side from either analogue (A), digital (SMU) and digitized Rogowski sensor (RS) measurements. The combination of SMU-A and RS-A shows phase shifts around 0.8 degrees and 5.4 degrees respectively. The observed phase shifts had limited influence on the differential protection, but the transient response of the RS gave increased trip delays, scatter and significant reduced sensitivity in some cases.