ABB REX521 REX521GHHPSH07G Medium‑Voltage Feeder Protection and Control Relay

Product Description

The REX521, ordering code REX521GHHPSH07G, is a legacy digital feeder protection relay manufactured by ABB for medium‑voltage distribution networks. It adopts a multi‑processor hardware platform with conventional CT and VT analog inputs, local LCD HMI, binary inputs and relay trip outputs. The conformal coated circuit board is built for continuous operation in substation switchgear with temperature variation, dust and heavy electromagnetic interference. This specific ordering configuration defines I/O layout, auxiliary power supply type and communication options. It performs protection, measurement, breaker control and event recording and is widely used as spare replacement hardware for existing distribution substations and industrial power switchgear.

Product Functions

• Three‑stage phase overcurrent, directional overcurrent, three‑stage earth‑fault and thermal overload protection for cables and overhead feeders
• Overvoltage, undervoltage, negative‑sequence current and frequency protection; optional auto‑reclosing and circuit‑breaker failure protection
• Measurement of three‑phase current, voltage, active/reactive power, energy and power‑quality parameters
• Circuit breaker local and remote control, interlock logic, binary signal monitoring and alarm contact output
• Event logging, fault value recording and disturbance waveform capture for post‑fault analysis
• Serial communication via RS485, supporting IEC60870‑5‑103 and Modbus RTU to connect with SCADA and substation automation systems
• Onboard hardware self‑diagnosis, watchdog supervision and front‑panel LED status and fault indication

Application Scenarios

• Utility distribution substations, protection for incoming busbar and outgoing medium‑voltage feeders
• Industrial plants, chemical, mining and pulp‑and‑paper internal MV power distribution switchgear
• Medium‑voltage large AC motor feeder protection and control for factory drive systems
• Backup protection for distribution transformers and backup relay of high‑voltage line protection devices
• Radial, resonant‑earthed, solidly‑earthed and resistance‑earthed distribution network protection
• Spare parts and maintenance retrofit for aging ABB REX521 relay installations

Common Problems

• Relay fails to power on：auxiliary power supply voltage out of range, loose terminal wiring or internal power circuit aging
• False tripping or failure to trip：incorrect protection setting groups, CT/VT polarity reversal or damaged analog input channels
• Serial communication intermittent dropout：RS485 bus termination resistor missing, baud‑rate mismatch, cable damage or ground‑loop interference
• LCD display blank or button unresponsive：front‑panel connector loose, HMI hardware failure or firmware crash
• Event and fault records lost after power interruption：onboard backup battery depleted or flash memory degradation
• Self‑diagnosis persistent fault alarm：dust accumulation inside housing, poor cabinet ventilation and long‑term overheating
• Engineering software cannot establish connection：incompatible setting tool version, wrong communication address or damaged serial transceiver chip