Three-phase differential protection relay
7PG21 Siemens Energy

The 7PG21 is a three phase unit differential protection used to detect in zone phase and earth faults. The relay is applied to overhead line and underground cable circuits as well as shorter circuits such as interconnectors.
Relays (and associated CTs) are installed at each end of the protected circuit and connected together with dedicated metallic pilot wires.
This established protection system provides a simple, robust protection of proven performance facilitating straightforward installation, commissioning and maintenance.
Relays have 3 current inputs and are housed in E6 cases.
Additional relays and components can be added to the basic protection to provide:
Increased insulation level:
Pilot Wire Supervision: 'Send' end and 'receive' end relays are used. A small dc current is injected onto the pilots at the send end, measurement of this current at the receive end confirms the integrity of the pilot circuit
Overcurrent guard (check) functionality 7PG217 B69
Intertripping/unstabilising