INNO fibre optic temperature sensors ,temperature monitoring systems.
A Winding Temperature Indicator (WTI) is a transformer-mounted or panel-mounted instrument that displays and manages the estimated (or directly measured) hot-spot temperature of the transformer windings. It is one of the most critical protection devices on a power transformer, providing real-time temperature display, alarm and trip contacts, and automatic cooling control to prevent insulation damage and extend equipment life. This guide explains how the transformer winding temperature indicator works, its temperature range, alarm and trip settings, technical specifications, and how it compares with the Oil Temperature Indicator (OTI).
Table of Contents
- What is a Winding Temperature Indicator (WTI)?
- Function of a Winding Temperature Indicator in a Transformer
- How Does a Winding Temperature Indicator Work?
- Transformer Winding Temperature Gauge Range and Typical Range
- Winding Temperature Gauge Range vs Oil Temperature Gauge Range
- Winding Temperature Indicator Specifications (BWR2)
- Dial Type vs Digital vs Fiber Optic Winding Temperature Indicator
- How to Install a Winding Temperature Indicator
- Common Problems and Solutions
- Relevant International Standards
- Real-World Application Case
- Customer Reviews
- Frequently Asked Questions
1. What is a Winding Temperature Indicator (WTI)?
1.1 Definition
A Winding Temperature Indicator (WTI) is a dial-type or digital instrument installed on oil-immersed power transformers to display and manage the estimated hot-spot temperature of the transformer windings. Unlike an Oil Temperature Indicator, which measures actual oil temperature directly, a conventional WTI reproduces the winding hot-spot using a thermal image (thermal replica) technique.
1.2 Measurement Method
A conventional WTI takes the top oil temperature as its base reading and adds a load-proportional temperature rise generated by an internal heating element fed from a current transformer (CT) on the winding circuit, so that the displayed value tracks the winding hot-spot rather than the oil alone.
1.3 Why It Matters
The winding hot-spot is the single hottest point inside the transformer and the location where insulation ages fastest. Monitoring it with a WTI protects the insulation system, supports automated cooling, and provides early warning before overload damage occurs.
2. Function of a Winding Temperature Indicator in a Transformer
2.1 Temperature Display
The WTI provides a continuous, at-a-glance reading of the estimated or measured winding hot-spot temperature for operators and control rooms.
2.2 Alarm and Trip
Built-in switching contacts trigger alarms when winding temperature thresholds are reached and trip the transformer offline if a critical limit is exceeded, protecting the insulation from accelerated aging or failure.
2.3 Cooling Control
The indicator’s contacts automatically start cooling fans and oil pumps (ONAF/OFAF stages) in staged sequence, using winding temperature rather than oil temperature as the more safety-critical control input.
2.4 Diagnostics and Asset Management
Trends in winding temperature support loading decisions, insulation life expectancy modeling, and predictive maintenance scheduling, since consistently elevated readings may prompt load reduction or cooling system upgrades.
3. How Does a Winding Temperature Indicator Work?
3.1 Thermal Image (Thermal Replica) Principle
The traditional WTI operates on the thermal image principle: a temperature-sensing bulb in the top-oil pocket provides the base oil reading, while a small heating element inside the WTI body — energized by current proportional to the transformer’s load, via a dedicated CT — adds a simulated temperature rise on top of the oil reading. The combined value is displayed as the estimated winding hot-spot temperature.
3.2 CT-Driven Heating Element
The heating coil current is normally fed from a CT on one phase of the winding circuit, with the CT ratio and heater gradient calibrated to match the transformer’s factory heat-run test data so the simulated rise matches the actual design temperature gradient.
3.3 Dial / Capillary Mechanism
As with an OTI, the sensing bulb is connected by a capillary tube to a Bourdon or bellows element inside the indicator body. As the combined oil-plus-simulated temperature rises, the fluid expands, moving the pointer across the dial and operating the micro-switch contacts.
3.4 Modern Digital and Fiber Optic Methods
Digital WTIs use Pt100 RTD inputs combined with a load-current input, programmable setpoints, and 4–20 mA or communication outputs. For the highest accuracy, armored fluorescent fiber optic sensors embedded directly inside the winding insulation provide a true measured hot-spot value, removing the accuracy limitations of the thermal-image simulation.
4. Transformer Winding Temperature Gauge Range and Typical Range
The transformer winding temperature gauge range is defined by international standards and manufacturer heat-run data. The table below shows the typical range, alarm and trip settings for mineral-oil-immersed power transformers (per IEC 60076-7, 40°C ambient baseline).
| Operating Condition | Winding Hot-Spot Temperature | Remarks |
|---|---|---|
| Normal (rated continuous load) | Below 110°C | Maximum continuous winding hot-spot temperature |
| Typical operating range | 65°C – 110°C | Varies with load and ambient |
| Alarm setting | 105°C – 120°C | Operator corrective action / cooling stage activation |
| Trip setting | 115°C – 160°C | Protective disconnection (varies by manufacturer) |
| Emergency / short-term overload | Up to 130°C – 140°C | Short duration only; accelerates insulation aging |
| Differential over top oil | 10°C – 25°C above top oil | Reflects the simulated load-proportional rise |
5. Winding Temperature Gauge Range vs Oil Temperature Gauge Range
A common question is the difference between the winding temperature gauge range vs oil temperature gauge range. The WTI estimates (or, with fiber optic sensors, directly measures) the winding hot-spot, while the Oil Temperature Indicator (OTI) measures the actual top oil temperature directly, which always runs cooler than the winding hot-spot. The comparison table below summarizes the differences.
| Parameter | Winding Temperature Indicator (WTI) | Oil Temperature Indicator (OTI) |
|---|---|---|
| Measured Object | Winding hot-spot temperature | Top oil temperature |
| Measurement Method | Top oil + CT-driven heater simulation (thermal image), or direct fiber optic sensing | Direct sensing in oil |
| Normal Range | 65°C – 110°C | 55°C – 85°C |
| Alarm Setting | 105°C – 120°C | 80°C – 85°C |
| Trip Setting | 115°C – 160°C | 95°C – 100°C |
| Primary Purpose | Insulation life & overload protection | Cooling control & oil protection |
5.1 Why Both Are Needed
The OTI and WTI work together to provide staged protection: the oil gauge manages cooling activation and oil health trending, while the winding gauge protects the insulation that ultimately determines transformer service life.
5.2 Modern Trend
Traditional WTI relies on a thermal image simulation calibrated against factory heat-run data. Direct fiber optic winding sensors now replace this estimation with true measured hot-spot temperature, closing the accuracy gap between WTI and OTI readings.
6. Winding Temperature Indicator Specifications (BWR2)
The following are typical specifications of the BWR2 Transformer Winding Temperature Indicator. Please confirm exact values against your selected model.
| Specification | Parameter |
|---|---|
| Model | BWR2 (Winding Temperature Indicator / WTI) |
| Working Principle | Thermal image (additional temperature rise) method |
| Measuring Range | 0°C – 150°C (0°C – 160°C option available) |
| Accuracy Class | ±1.5% FS (Class 1.5) |
| Dial Diameter | Φ150 mm |
| Switching Contacts | 4 independent micro-switches, expandable to 6 (alarm / trip / cooling stages) |
| Contact Capacity | AC 220V, ≤5A |
| Sensing Bulb | Φ15 mm, insertion depth ≥150 mm |
| Mounting Thread | M27×2 mm or M33×2 mm |
| Capillary Length | 6 m standard, customizable up to 26 m |
| CT Input (heater circuit) | Current transformer signal, adjustable via internal rheostat |
| Output Signal | Pt100 RTD / 4–20 mA (optional) |
| Protection Class | IP55 |
| Ambient Operating Temperature | -40°C to +55°C |
7. Dial Type vs Digital vs Fiber Optic Winding Temperature Indicator
The table below compares the three main winding temperature indicator technologies for transformer applications.
| Feature | Dial Type (Thermal Image) | Digital (RTD + CT) | Fiber Optic |
|---|---|---|---|
| Measurement Type | Indirect (simulated) | Indirect (simulated, digital) | Direct (measured) |
| Typical Accuracy | ±5°C – 8°C (simulation error) | ±5°C (simulation error) | ±0.1°C – 1°C (direct) |
| EMI Immunity | Good (mechanical) | Requires shielding | Fully immune |
| Response Time | Slow (thermal lag) | Slow (thermal lag) | Fast, sub-second to a few seconds |
| Output / Communication | Contacts only | 4–20 mA, Modbus | Multi-point, digital |
| Maintenance | Periodic CT/heater calibration | Periodic calibration | Maintenance-free |
| Best Application | Standard winding monitoring | SCADA-integrated winding monitoring | Critical / HV transformers, new-build or retrofit |
For standard winding hot-spot monitoring and cooling control, a dial-type or digital WTI such as the BWR2 is the practical, cost-effective choice. For direct winding hot-spot measurement in critical or high-voltage transformers, fluorescent fiber optic sensing embedded in the winding insulation is recommended.
8. How to Install a Winding Temperature Indicator
8.1 Thermometer Pocket and Bulb Installation
Insert the sensing bulb into the oil-filled thermometer pocket at the top of the transformer tank, with an insertion depth of at least 150 mm to ensure the base oil reading is accurate.
8.2 CT Wiring
Connect the WTI’s internal heating coil to a dedicated current transformer on the winding circuit, typically taken from one phase of the LV winding, and adjust the CT ratio or internal rheostat so the simulated temperature rise matches the transformer’s factory heat-run curve.
8.3 Capillary Routing
Route the capillary tube without sharp bends or kinks, secure it along the tank to prevent vibration damage, and avoid excess coiling near hot surfaces.
8.4 Calibration and Setpoint Configuration
Configure alarm, trip, and cooling activation setpoints according to the transformer nameplate and applicable standards, then verify operation during commissioning by simulating load current through the CT input.
9. Common Problems and Solutions
9.1 Winding Temperature Reads Too High Under Normal Load
Likely causes: incorrect CT ratio or heater calibration, blocked cooling fans, high ambient temperature, or sustained overload. Action: verify CT wiring and heater gradient settings, and check cooling system operation against nameplate load.
9.2 Oil and Winding Temperature Readings Are Inconsistent
Likely causes: heater/CT miscalibration, oil circulation pump failure, or temperature stratification at low load. Action: recalibrate the CT ratio and gradient, check pump operation, and for critical units consider direct fiber optic winding sensors.
9.3 Cooling Fans Do Not Start at the Setpoint
Likely causes: faulty contact, wiring fault, or incorrect setpoint. Action: test the contact by simulating an over-temperature condition and verify wiring continuity to the fan contactor.
9.4 Reading Stable but Inaccurate (Drift)
Likely causes: bulb fluid degradation, CT ratio error, or aging heater element. Regular calibration is recommended, ideally every few years, to avoid misjudgment caused by CT ratio drift. Fiber optic sensors are immune to this type of calibration drift since they measure the hot-spot directly.
10. Relevant International Standards
| Standard | Scope |
|---|---|
| IEC 60076-7 | Loading guide for oil-immersed transformers; defines hot-spot temperature limits and thermal image method |
| IEC 60076-2 | Temperature rise limits for liquid-immersed transformers |
| IEEE C57.91 | North American loading guide and thermal model for oil-immersed transformers |
| JB/T 8450 | Chinese national standard for transformer winding and oil temperature controllers |
11. Real-World Application Case
At a 110 kV substation, the winding temperature indicator displayed a normal simulated hot-spot reading of 98°C during summer peak load, well within the 105°C alarm threshold. However, a subsequent fiber optic verification during a scheduled outage measured an actual hot-spot of 112°C at one localized winding position, a discrepancy exceeding 10°C that pointed to a CT ratio calibration error accumulated over years of service. Correcting the CT calibration and tightening the alarm setpoint prevented a potential undetected overload condition. This case illustrates why periodic verification of thermal-image WTI readings against a direct measurement method is recommended for aging installations.
12. Customer Reviews
| Customer | Application | Rating | Comment |
|---|---|---|---|
| Power Utility Engineer | 110 kV substation | ★★★★★ | Reliable trip contacts and straightforward CT calibration. Easy commissioning. |
| Transformer OEM | Distribution transformers | ★★★★★ | Stable thermal-image performance and customizable capillary length. Great value. |
| Maintenance Manager | Industrial plant | ★★★★☆ | Solid build quality and clear dial display. Cooling control works well. |
| Substation Operator | Wind farm step-up transformer | ★★★★★ | Reliable alarm function and low maintenance over years of service. |
13. Frequently Asked Questions
13.1 What does a winding temperature indicator measure?
A conventional WTI displays the estimated hot-spot temperature of the transformer winding, calculated by adding a load-proportional simulated rise to the measured top oil temperature. Advanced WTIs using fiber optic sensors measure the hot-spot directly.
13.2 What is the difference between WTI and OTI?
The WTI estimates (or directly measures with fiber optic sensors) the winding hot-spot temperature, while the OTI measures the actual top oil temperature directly using an immersion sensor.
13.3 What is the normal winding temperature range in a transformer?
Typically 65°C–110°C under normal load, with alarm settings commonly configured between 105°C and 120°C depending on the manufacturer and insulation class.
13.4 What is the trip setting for transformer winding temperature?
The winding temperature trip is usually set between 115°C and 160°C depending on manufacturer and transformer design, with alarm typically set 10–15°C lower.
13.5 How does the CT-driven heater in a WTI work?
Current from a dedicated current transformer on the winding circuit feeds a small heating element inside the WTI. This heater is adjusted via an internal rheostat so its output matches the winding’s actual load-dependent temperature rise above the oil.
13.6 Dial type vs digital vs fiber optic WTI — which is better?
Dial type is robust and cost-effective for standard monitoring; digital offers SCADA integration and tighter setpoint control. Fiber optic delivers true direct hot-spot measurement and is best for critical, high-EMI, or high-voltage transformers.
For a customized transformer Winding Temperature Indicator solution, contact FJINNO at web@fjinno.net or WhatsApp +8613599070393, or view the BWR2 Transformer Winding Temperature Indicator.
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