Testing considerations for line current differential schemes

Testing line current differential (87L) schemes is more complicated than testing directional comparison schemes. The latter exchange simple on/off (permission or block) signals that can be conveniently checked at the output of a given relay and conveniently forced at the input to a given relay in the scheme. Line current differential schemes exchange synchronized current values that cannot be easily intercepted or forced at the 87L communications port of the relay. Moreover, the communications channel is an integral part of the 87L scheme, and its characteristic can impact some fundamental aspects of the scheme, such as sensitivity and speed. Therefore, it is preferable that the actual channel be in use when testing the scheme. Line current differential schemes that apply external time sources for synchronization need to be tested with time sources as well. Often, 87L schemes are tested with multiple crews dispatched to two or more line terminals using either test sets synchronized via the Global Positioning System or other methods of controlling the relationship and timing between current sources at the line terminals. This paper presents several approaches to testing 87L schemes. These approaches include verification of relay hardware, firmware and settings, channel performance, and, if used, external time sources. The paper also describes features of line current differential relays and multiplexers to aid testing.