Distance protection (ANSI 21)

Distance protection (ANSI 21) — impedance-based line protection

Distance protection (ANSI 21) measures the impedance between the relay location and a fault, and provides fast, selective tripping based on line impedance and zone reach. It is the primary protection for MV and HV transmission lines where simple overcurrent does not provide adequate selectivity, and for sub-transmission lines feeding industrial plants.

SYMAP^® Y-D is the dedicated distance protection variant supporting multiple zones, directional logic, power-swing blocking and zone coordination with adjacent relays. SYMAP^® Compact+ (+F3 or +F4 with the 21 option) applies where distance is one of several functions in a compact bay.

Typical applications

MV and HV transmission lines

Overhead and cable MV/HV lines with quadrilateral or mho characteristic. Up to 6 zones, forward and reverse, with project-specific zone reach and timing per coordination study.

Sub-transmission feeding industrial plants

Sub-transmission lines feeding refineries, steel mills and other industrial plants. Distance with power-swing blocking, switch-onto-fault and out-of-step logic.

Tee-tap and parallel-line configurations

Tee-tap lines with infeed from multiple sources, parallel lines with cross-coupling. Specialised zone reach and directional logic per system study.

Cable lines and mixed overhead/cable

Distance protection on cable lines and mixed overhead/cable circuits, with cable-specific impedance settings and reach correction.

Backup protection for line differential

Distance as backup to primary line-differential protection (87LD). Coordinated trip times, communication-aided tripping schemes (POTT, DCB, DCUB).

HIMAP-D retrofit

For legacy HIMAP-D distance panels, a project-specific retrofit is coordinated with HQ Hamburg, with the same panel cut-out and a modern distance algorithm.

Compact+ or classic SYMAP^®?

Classic SYMAP^® Y-D is engineered for distance protection of complex MV and HV lines with multiple zones and coordination with adjacent relays. SYMAP^® Compact+ (+F3 or +F4 with 21 OPTION) suits applications where distance is one of several functions in a compact feeder or connection bay.

Products covering this application

Need detailed technical specifications?

For exact protection function coverage, settings ranges, ANSI device numbers, CT/VT specifications, communication protocols and configuration support, contact our team to receive the relevant datasheet and engineering proposal.

Request datasheet Engineering inquiry

Distance protection (ANSI 21)