- Temperature Indicator Definition: A temperature indicator in transformers is defined as a precision device used for protection, temperature indication, and cooling control.
- Oil Temperature Indicator (OTI): OTI measures the top oil temperature using a sensing bulb and liquid expansion to drive a pointer indicating temperature.
- Winding Temperature Indicator (WTI): WTI measures winding temperatures using a sensing bulb heated by a coil, reflecting the current through the transformer winding.
- Remote Temperature Indicator (RTI): RTI uses a potentiometer as a transmitter to relay temperature data to a remote repeater.
- Winding Temperature Indicator Working Principle: The WTI operates by indirectly measuring winding temperature through a heated sensing bulb, with the heater coil powered by a current transformer.
Temperature Indicator of Transformer
A transformer temperature indicator is a local measuring instrument for a power transformer. Depending on its fitted contacts and outputs, it can indicate temperature and provide cooling-control signals. It may also supply an input to the protection of transformer. The indicator itself is not the complete protection or tripping system.
Mechanical oil and winding indicators respond to temperature through a bulb, capillary and bellows system. Their readings follow the sensor’s thermal response and should not be described as instantaneous.
- A resettable maximum pointer can record the highest indicated oil or simulated winding temperature since it was last reset.
- An adjustable contact can initiate a high-temperature alarm at the specified setpoint.
- A separate protection circuit may use an indicator contact to trip the associated circuit breakers at an approved high-temperature setpoint.
- Adjustable contacts can control the cooling system of transformer. Each fan or pump stage starts at its setpoint and stops after temperature falls through the specified differential.
Types of Temperature Indicator of Transformer
A transformer may use a local oil-temperature indicator, a simulated winding-temperature indicator and one or more remote outputs. The arrangement on a power transformer depends on its monitoring and cooling scheme.
The three functions described here are:
- Oil temperature indicator (OTI)
- Winding temperature indicator (WTI)
- Remote temperature indicator (RTI)
Construction of Temperature Indicator of Transformer
In a classic mechanical indicator, a sensing bulb sits in an oil-filled thermometer pocket in the transformer tank cover or wall. The pocket uses compatible transformer oil for heat transfer to the bulb. A sealed capillary connects the bulb to a bellows or Bourdon mechanism in the indicator case. Some designs add ambient compensation. The mechanism drives the pointer, adjustable switches and any remote output. The number and type of switches vary by model; modern instruments commonly use independent mechanical contacts rather than mercury switches. Assigned contacts may control cooling stages, alarms or a separate trip circuit.
The dial of an oil temperature indicator may include a resettable maximum pointer. The main pointer pushes it upwards as temperature rises, but it remains at the highest indicated temperature when the main pointer falls. It records maximum temperature rather than temperature rise above ambient. Reset it according to the operating procedure.
Oil Temperature Indicator or OTI
An oil temperature indicator measures temperature at its thermometer pocket, normally arranged to represent top-oil temperature. Its switches and outputs can support cooling control and alarms. A separate transformer protection scheme may use a configured contact.
Operating Principle of Oil Temperature Indicator
In a liquid-filled capillary instrument, heating expands the sealed fluid in the sensing bulb and capillary system. The resulting pressure moves a bellows or Bourdon element. A linkage converts that motion into pointer movement and operates configured contacts or remote outputs. The indicated value represents the temperature at the bulb after the sensor and mechanism respond; it is not a direct measurement of every point in the tank.
Winding Temperature Indicator or WTI
A traditional winding temperature indicator estimates the hot-spot temperature of the winding represented by its current-transformer input and calibration. Separate channels or indicators are required where more than one winding temperature must be represented. The WTI can provide local indication and cooling-control contacts. Configured alarm or trip contacts can feed separate circuits.
Operating Principle of Winding Temperature Indicator
A conventional WTI is a thermal-image instrument based on an oil-temperature measurement plus simulated winding rise. A heater near the sensing element is supplied from a bushing current transformers circuit through a matching unit. The heater input follows load current, while calibration sets the required winding-to-oil temperature gradient from transformer heat-run data. The indicated value is therefore an estimate of winding hot-spot temperature, not a direct measurement inside the winding. Modern transformers can also use embedded fibre-optic probes for direct winding hot-spot measurement.
Remote Temperature Indicator
Legacy remote systems may use a precision potentiometer linked to the local pointer as a transmitter. The receiver reads the resulting resistance position directly, or a resistance transducer converts it to a current signal for a repeater. Current instruments may instead provide Pt100, 4-20 mA, voltage or digital outputs. The remote display and monitoring system must be configured for the fitted output type and range.
