Temperature Signal Conversion in Transformer Monitoring Systems

The Role of Temperature Transmitters in Protection and Monitoring

In modern protection and control systems, accurate temperature measurement is essential to assess transformer health and prevent thermal faults. Parameters such as core temperature, winding temperature, and oil temperature form the foundation of transformer “tri-temperature” monitoring.

The STT Series temperature transmitters from ODES are engineered for this purpose. Designed to meet IEEE and IEC wiring practices, they provide three-way isolation—between the auxiliary power, input, and output circuits—ensuring reliable, interference-free signal conversion from RTD (Pt100) sensors to standardized analog outputs.

Wiring Method and Terminal Assignments

Each STT module provides a clear terminal layout for power, sensor input, and signal output, supporting simplified installation and verification.

The transmitter input is a standard three-wire Pt100 RTD, with one analog output channel. Two standard signal types are available:

• When the output is 0–5 V, the load resistance must be ≥ 10 kΩ.

• When the output is 4–20 mA, the load resistance must be ≤ 500 Ω.

1. Power Supply Terminals

• Terminal 1 (+) — auxiliary supply positive

• Terminal 2 (−) — auxiliary supply negative

The STT-A1 model accepts AC/DC wide-range auxiliary power, while the STT-C1 version operates on 24 VDC nominal.

2. Sensor Input Terminals (Pt100, three-wire connection)

• Terminals 21, 22 — identical leads of the Pt100 sensor

• Terminal 23 — single lead of the Pt100 sensor

This wiring method minimizes lead resistance imbalance, maintaining accuracy even in longer cable runs.

3. Output Signal Terminals

• Terminal 41 (+) — analog output positive

• Terminal 42 (−) — analog output negative

The transmitter provides one of two standard analog outputs:

• 4–20 mA DC (load ≤ 500 Ω)

• 0–5 V DC (load ≥ 10 kΩ)

These output standards allow direct interfacing with protective relays, SCADA, and measurement-control systems following IEEE C37 and IEC 61850 frameworks.

Input-Output Configuration Examples

These configurations support transformer applications ranging from oil-immersed units to dry-type designs, ensuring that analog outputs remain linear and stable across the entire temperature range.

Engineering Advantages

• Three-way electrical isolation — separates auxiliary supply, input, and output circuits to eliminate ground potential interference.

• High accuracy and stability — optimized for long-term measurement in high-EMC substation environments.

• Wide compatibility — outputs directly interface with most DCS, PLC, and protection devices without signal conditioning.

• Compact rail-mount form factor — simplifies installation in retrofit and new-build panels alike.

Application Example

In an oil-immersed transformer installation, three Pt100 sensors—core, winding, and oil—feed the STT transmitters. Each channel converts temperature into a 4–20 mA analog signal, transmitted to the substation’s protection and control system. The isolated design ensures accurate readings despite ground loops and high transient interference typical in substation environments.

The STT Series Temperature Transmitters provide a reliable link between field-level temperature sensing and secondary protection systems. Their IEEE-compliant wiring, high accuracy, and robust isolation make them a preferred choice for transformer monitoring, industrial control, and any environment where temperature signals must remain stable, standardized, and interference-free.

Ensure that your transformer temperature monitoring meets both protection accuracy and system integration standards.

• Request detailed wiring diagrams and input-output configuration tables for the STT Series.

• Discuss with our engineering team how to standardize temperature measurement across your protection systems.

• Build monitoring architectures that are reliable, safe, and aligned with IEEE and IEC best practices.

Contact: 📩 tonyzhang@odes-electric.com

Or visit: 🌐 https://www.odes-electric.com/

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