Line differential protection (87LD) — absolute selectivity for transmission lines
Line differential protection (87LD) compares the current entering and leaving a transmission line through a dedicated communication link between the two line ends. It provides absolute selectivity (the protected zone is exactly between the two CTs), fast tripping for internal faults and high stability for external faults. The communication link is typically optical fiber with up to 10 km direct reach without repeaters.
SYMAP® Y-LD is the dedicated line-differential variant with built-in L-Diff optical link. SYMAP® Compact+ with the 87LD option applies where line differential is one of several functions in a compact bay and the communication infrastructure allows.
