RTD protection 38 and 49: bearings and windings

What RTD protection covers

An RTD (resistance temperature detector) is a sensor whose resistance changes with temperature. Installed inside the machine, it lets the relay measure the actual heat where the damage happens:

• Stator windings. Insulation ages faster the hotter it runs. Sustained overheating shortens the life of the motor or generator.
• Bearings. A bearing that loses lubrication or wears out heats up before it fails. Detecting it in time keeps the shaft from being damaged.
• Cooling air, oil and ambient, as references for the thermal model.

In IEEE C37.2, number 38 is the bearing protective device: it operates on excessive bearing temperature or other abnormal mechanical conditions that would end in bearing failure. The 49 is the machine or transformer thermal relay. The stator RTDs feed the measured part of 49; the part calculated from the current is explained in thermal overload 49.

It is not protection against electrical faults: it reacts to an abnormal condition that develops over minutes or hours.

Where it is applied

In transformers, oil temperature and the winding thermal image have their own indicators, described in transformer temperature indicators and in mechanical protection 63 and 26.

Measuring principle

The most widely used sensor is the IEC 60751 Pt100: platinum with 100 Ω at 0 °C and a resistance that rises almost linearly with temperature. Some relays also accept nickel or copper sensors; the type is selected in the relay.

• Three-wire connection. The relay measures the lead resistance with the third wire and subtracts it, assuming the conductors are identical. With two wires, the cable resistance adds to the sensor resistance and shows up as temperature.
• RTD module. The inputs can be in the relay itself or in an external module near the motor, connected to the relay by optical fiber, which shortens the sensor wiring.
• Groups. The relay groups the RTDs by location: stator, bearings, ambient and others. Two RTDs per phase in the stator and one per bearing are common.

The characteristic

Each RTD or group has two temperature thresholds, each with its time delay:

• Alarm, above the normal operating temperature.
• Trip, below the limit of the thermal class of the insulation or of the bearing.

It is drawn on a temperature versus time plane: the reading rises, crosses the alarm and, if it keeps rising, crosses the trip.

Voting. Tripping can require two or more RTDs of the group to exceed the threshold. That way a faulty sensor does not take the motor out.

Sensor failure. An open-circuit reading (very high resistance) or a short-circuit reading (very low) is declared an RTD failure: it alarms and takes that sensor out of the protection.

Thermal image correction. The relay compares the hottest stator temperature with the one its 49 model calculates and uses the higher one. If cooling fails, the RTD sees it even with normal current. With the ambient RTD, some relays also adjust the thermal limits.

Setting criteria

• Stator: according to the thermal class of the insulation (IEC 60034-1, NEMA MG 1) and the motor manufacturer’s data, with the actual operating temperature as the reference for the alarm.
• Bearings: according to the bearing type and the lubricant, with the machine manufacturer’s values.
• Time delays: short compared with the machine thermal time constant, long enough to filter noise.
• Voting: for tripping, in groups with several sensors.
• Sensor type matching the one installed.

Interaction with other functions

• 49: the measured temperature corrects the thermal image calculated from the current.
• 66 and restart blocking: a high thermal image, corrected by the RTDs, delays the next start.
• Locked rotor 51LR and prolonged starting 48: the stator RTDs respond slowly and do not protect the rotor during starting; 48 and 51LR are there for that.
• Vibration (39): vibration monitoring is another function, with other sensors; both warn of the same bearing damage by different paths.
• Outputs: alarms go to the process control system; the trip goes to the motor contactor or breaker.

How each manufacturer implements it

• ABB Relion 615: the REM615 measures the RTDs with the XRGGIO130 node and uses the multipurpose analog protection MAPGAPC for the temperature thresholds.
• SEL: motor relays with internal RTD inputs or external modules connected by fiber; it models the thermal information with its own node.
• GE Vernova (Multilin), Siemens SIPROTEC and Schneider (MiCOM, Easergy): RTD inputs in the relay or in external units, with groups, voting and thermal image correction.

The practical differences are the number of inputs, the sensor types accepted, how they are grouped and voted, and how the temperature enters the 49 model.

IEC 61850 logical node

IEC 61850-7-4 has the STMP node, for temperature supervision. One instance is created per sensor (represented by the TTMP node) and it publishes the temperature (Tmp), the alarm (Alm) and the trip (Trip), with their thresholds. The 49 thermal image is modeled with PTTR.

In practice, many manufacturers use generic or proprietary nodes for the RTD inputs. Before integrating the signals into a control system it is worth reviewing the relay data model. The trip follows the chain to PTRC and the breaker (XCBR).

How it is tested

In general, the following are verified:

• The reading of each input, simulating the sensor with a known resistance and comparing with the IEC 60751 table.
• The alarm and trip thresholds and their time delays.
• Voting: a single high RTD gives an alarm and not a trip.
• Detection of an open and a shorted sensor.
• The correction of the 49 thermal image.

On site, with the motor cold after a long shutdown, all RTDs must read a temperature close to ambient. We test the relay with secondary injection and the motor with motor testing, in Colombia, or elsewhere on request.

Typical mistakes

• Wrong sensor type selected in the relay: the reading is offset over the whole range.
• Two-wire connection with long cables, which adds false temperature.
• RTD cables run next to power cables, or with the shield wrongly grounded: noisy readings.
• Trip by a single RTD without voting, which takes the motor out because of a damaged sensor.
• Sensor failure configured as a trip.
• Thresholds copied from another machine, without checking the thermal class or the bearing type.
• Ambient RTD in the sun or near a heat source.

We configure your motor relay

Does your relay show odd temperatures, or does it trip on an RTD? We review the configuration of the sensors, groups, voting and thermal correction remotely, worldwide. Tell us the make and model through remote relay configuration or contact us.

Related guides

• Thermal overload 49
• Locked rotor 51LR and prolonged starting 48

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations (devices 38, 39 and 49).
• IEEE C37.96, Guide for AC Motor Protection.
• IEEE C37.102, Guide for AC Generator Protection.
• IEC 60751, Industrial platinum resistance thermometers and platinum temperature sensors.
• IEC 60034-1, Rotating electrical machines – Part 1: Rating and performance: thermal classes and temperature limits.
• IEC 61850-7-4, Compatible logical node classes and data object classes: STMP, TTMP and PTTR nodes.
• ABB, REM615 Application Manual: RTD inputs and multipurpose analog protection.

Frequently asked questions