With the increasing installation of digital substations utilizing IEC61850 process bus technology, operating the protection and control elements on sampled analog values (SAVs) is becoming more practical. This paper is aimed at understanding the impact of using SAVs to operate protection and control elements specifically for line current differential element even in hybrid cases where one end is conventional analog values acquisition and other end is a SAV acquisition. In conventional relays the analog signals from CTs and VTs to the relay travel at the speed close to that of light, i.e., almost instantaneously. As a result, the current and voltage measurements are naturally time aligned at the time of analog to digital (A/D) conversion at all line terminals. As such, the synchronization of relays at two or more terminals, which remains the most crucial aspect of the successful operation of 87L element, is inherent and guaranteed.
However, the SAVs travel over an Ethernet network which has inherent jitter and delay (which could be variable) associated with it. For successful operation of any protection element, it needs to be ensured that all the current and/or voltage samples that it uses are time-aligned to each other. In case of 87L element the compensation for the network delay is critical. Improper delay compensation would result in erroneous differential current.
This paper discusses the physical layout of the SAV system, the mechanism of alignment of the SAVs from various streams, applying compensation of network delay to SAVs. This paper also captures unique capabilities of SAV-based systems. Thereafter, this paper presents the various topologies of SAV-based 87L system that are implemented in the field along with the assumptions and considerations of each topology. Additionally, few theoretical topologies for SAV-based 87L are also discussed. The various enabling and limiting factor such topologies are also captured in this paper. Field examples of the application and performance of the line current differential element for both hybrid and SAV-to-SAV systems are given, with SAV and clocking impairments scenarios.