ABB rex521chhplh05d Rex521 Feeder Protection Relay

ABB REX521 is a microprocessor-based feeder protection relay for medium-voltage distribution networks, providing protection, control, measurement, and supervision in a single compact unit. It belongs to ABB’s Relion family of protection relays and is designed to guard MV feeders, transformer feeders, and bus sections by applying configurable protection blocks and control logic that coordinate with upstream and downstream devices. The rex521chhplh05d designation refers to a specific unit within the REX521 family, aligned with the H05 variant configuration, commonly used in protection schemes requiring high-sensitivity fault detection and robust interlocking. Core protection functions typically include phase overcurrent protection and earth-fault protection, with additional capabilities such as residual voltage protection and synchro-check to ensure safe breaker operation before a parallel connection is made. Circuit-breaker failure protection, start/stop interlocking, and remote indication/control are supported through configurable input/output groups, enabling tailored protection and control for feeders and substations. For monitoring and analysis, the REX521 includes a disturbance recorder to log events around faults and a power-quality monitor to track key electrical parameters, supporting post-event analysis and performance verification. In practice, the relay is used to protect incoming and outgoing feeders, coordinating with adjacent protection to minimize nuisance trips and improve substation reliability, with ABB documentation detailing standard configurations across the H01–H08 range for different protection philosophies.