SEL-710-5 Motor Protection Relay

Overview

The SEL-710-5 Motor Protection Relay from Schweitzer Engineering Laboratories protects and controls low- and medium-voltage three-phase induction and synchronous motors, including motors fed from variable frequency drives. At its core is a patented thermal model that tracks stator and rotor heating during starting and running, supported by overcurrent, unbalance, load-loss, voltage, power and frequency elements. Optional cards add motor differential protection, synchronous motor field protection and excitation control, arc-flash detection and low-energy analog sensor inputs.

Seven Star LLC supplies the SEL-710-5 to oil and gas producers, refineries, water and desalination utilities, mining and cement plants and EPC contractors in Oman and the GCC, with engineering support in Muscat. Typical duties are pumps, compressors, fans and conveyors where unplanned motor trips or burnt windings carry a high production cost.

Motor Thermal Protection Principle

Motor heating depends on load current, on negative-sequence current from voltage unbalance and on the number and duration of starts. The SEL-710-5 thermal element calculates the percent thermal capacity used (TCU) for both the stator and the rotor, and the relay trips when either reaches 100 percent. Because rotor resistance and heat generation change with slip, the relay dynamically calculates motor slip and uses it in the rotor model. Calculating rotor TCU correctly reduces the waiting time between starts and gives the motor more time to reach rated speed before tripping, which matters for high-inertia loads such as large fans and centrifugal compressors.

The same SEL-710-5 model covers locked rotor starts, running overload, unbalance heating and repeated or frequent starting. Starting and thermal data are kept in nonvolatile memory, so a loss of relay power does not reset the thermal state and allow an unsafe restart. A Virtual Speed Switch can back up locked rotor protection without a physical speed sensor, and a Coast Time setting recognises that motors cool faster while coasting, shortening the lockout before the next start is permitted.

Engineering Design of the SEL-710-5

Inputs are defined by the Slot Z card: conventional 1 A or 5 A phase CTs with voltage inputs, or LEA versions using Rogowski coil or low-power CT inputs and voltage sensor inputs through RJ45 connectors. Slot E options include a combined four-channel arc-flash and three-channel differential card, an eight-channel arc-flash card, and a synchronous motor card that adds three differential current channels together with field voltage and current measurement. Further slots accept I/O cards, a 10-input internal RTD card and communications cards, and the relay can also read as many as 12 RTDs from an external SEL-2600 module.

Operators use a two-line LCD or an optional five-inch, 800 × 480 color touchscreen, with eight programmable pushbuttons for start, stop and reset functions. Built-in control logic supports star-delta starting, two-speed motors and forward/reverse starting, and 32 local and 32 remote control bits are available for operator schemes. Settings are made in ACSELERATOR QuickSet SEL-5030 software, and a built-in web server displays metering, event data and Sequential Events Recorder records and accepts firmware upgrades.

Key Features

Motor protection engineers select the SEL-710-5 for these functions:

• Patented slip-dependent thermal model for stator and rotor, with starts-per-hour, time-between-starts and antibackspin timers.
• Locked rotor, load jam, load loss, current unbalance, phase reversal and breaker or contactor failure protection.
• Phase, negative-sequence and residual-ground instantaneous and inverse-time overcurrent elements, plus incipient cable fault detection.
• Optional motor current differential protection through the arc-flash/differential or synchronous motor cards.
• Synchronous motor starting control with loss-of-field, out-of-step, pull-out, field resistance, field voltage and field current protection.
• Optional four- or eight-channel fiber-optic arc-flash detection with overcurrent supervision for motor control centres and switchgear.
• Broken rotor bar detection, vibration monitoring, motor start reports and start trending for condition-based maintenance.
• Up to 10 internal or 12 external RTDs and PTC thermistor input for winding and bearing temperature protection.

Communications

Modbus RTU, MIRRORED BITS and a fiber-optic serial port are standard. Optional Ethernet ports add IEC 61850 Edition 2, Modbus TCP/IP, DNP3 serial and LAN/WAN, EtherNet/IP, IEC 60870-5-103, SNTP and IEEE 1588 PTP, with PRP and RSTP for redundant networks, while DeviceNet is available on a card. This makes it straightforward to bring SEL-710-5 motor status, trips and thermal capacity data into a DCS, PLC or electrical control system.

Industrial Applications

In Oman, the SEL-710-5 is suited to crude oil transfer and water injection pumps in the interior oil fields, gas compressor drives, refinery and petrochemical process pumps in Sohar and Duqm, and high-pressure pumps in reverse osmosis desalination plants along the coast. The –40 to +85 °C IEC performance rating suits hot motor control centre rooms, and UL Class I, Division 2 and ATEX hazardous-location certifications support installation near classified process areas.

For synchronous motors on large compressors and refiners, the SEL-710-5 synchronous motor card combines field protection with power factor or reactive power control, keeping excitation supervision within the protection relay. Asset teams use broken rotor bar detection and motor start trending to identify degradation before a failure. Seven Star LLC can configure the SEL-710-5 to the motor rating, CT and VT secondaries, RTD count, starter type and protocol, and supply the matching data sheet for project approval.

Related SEL relays include the SEL-751 feeder protection relay and SEL-787 transformer protection relay.

Technical Specifications

The table below summarises SEL-710-5 data from the current SEL-710-5 data sheet. Options depend on the model ordered.

How to Order

To receive an SEL-710-5 quotation, send Seven Star LLC the motor nameplate data, starting method, CT and VT ratios, RTD and arc-flash requirements, auxiliary supply and preferred protocol. Our Muscat team will confirm the configuration, check delivery to plants in Oman and the GCC and supply the SEL-710-5 data sheet. Other motor and feeder protection products on our site include the Schneider Electric TeSys LRD thermal overload relay, the ABB REF615 feeder protection relay and the Schneider Electric Vigirex RH197M ground fault relay.