Siemens 7SD5221-5PB99-2HM7 Protection Relay Unit

Module Feature

7SD5221-5PB99-2HM7 belongs to Siemens SIPROTEC 4 7SD5 series digital line differential protection relay.

It supports two-terminal up to six-terminal line differential protection, and integrates distance protection function in one device. It adopts fiber-optic communication for peer-to-peer data exchange between relays, trip action within 10~15ms, with built-in fault locator function. It can realize overload, phase-to-phase short circuit, earth fault, power swing blocking, inrush restraint and other protection logic. Equipped with local text display panel, supports IEC61850, Modbus, IEC 60870-5-103 communication protocols. This series has been phased out, successor model is 7SD82/7SD85/7SD86, widely stocked as spare for legacy substation line protection cabinets.

Working principle

The relay collects CT current sampling data at each terminal of the protected line, exchanges current data through optical fiber channel.

The internal CPU calculates differential current; when the differential value exceeds the setting threshold, it sends trip command to circuit breaker. The distance protection serves as backup protection. It continuously uploads electrical measurement values, alarm signals and fault event records to SCADA system.

Onboard components

Main CPU processing board, analog sampling module, fiber communication transceiver, human-machine display panel, binary input/output relay module, DC power supply unit, status indicator group, rear terminal block.

LED indicators

Front LEDs indicate power normal, protection running, trip action, channel fault and self-diagnosis alarm status.

Hardware self‑diagnosis

Built-in supervision detects CT circuit abnormality, fiber communication interruption, internal hardware error, power supply failure and program exception. Fault codes and disturbance records can be checked on local panel or through SIPROTEC software.

Programming software

SIPROTEC DIGSI 4 commissioning software, used for protection parameter setting, communication mapping, fault wave record download and relay calibration.

Electrical & Process Parameters

Model:7SD5221-5PB99-2HM7 Brand:Siemens Device type:Multi-terminal line differential and distance protection relay Function:High-speed main protection for overhead and cable transmission lines

Mechanical & Structure

Mounting:Standard 19-inch panel flush mounting Protection class:IP40 front panel Housing:Metal reinforced molded protection case

Environmental

Operating temperature:-25℃ ~ +70℃ Storage temperature:-40℃ ~ +85℃ Ambient humidity:5~95% non‑condensing

Certification:CE, IEC 60255, UL

Status:Discontinued legacy Siemens power protection spare

Installation

Install in protection cabinet, complete CT wiring, fiber channel and DC control power connection.

Set differential protection parameters, communication address and time synchronization, perform channel test and protection logic test before energizing.

Interface

CT/PT analog input terminals, binary DI/DO terminal block, fiber communication port, 24/110/220VDC auxiliary power, serial and Ethernet communication port.

Diagnostic

Differential protection false alarm: CT polarity wrong, channel delay mismatch or sampling data abnormal.

Cannot communicate between relays: Fiber broken, optical module damage or network parameter mismatch.

Protection fails to trip: Protection blocking condition active, setting value unreasonable or output relay circuit failure.

Product Brief

Siemens 7SD5221-5PB99-2HM7 protection relay, SIPROTEC 7SD5 line differential relay spare for substation overhead and cable line protection cabinet replacement.