Everything You Need to Know About Transformer Oil and Winding Temperature Indicators

Oil and winding temperature are critical parameters that are measured in power and distribution transformers. Reliable and accurate temperature measurement ensures a longer life for your transformer and is crucial to maintaining the overall health of the asset. Temperature indicators are most commonly utilized for the indication of top oil or winding hotspot temperatures which are both critical parameters to measure within a transformer. They are commonly referred to in the industry as Oil Temperature Indicators (OTI) and Winding Temperature Indicators (WTI). Electrical utilities often utilize oil and winding temperature indicators to provide alarm and control signals that are used to activate cooling control systems on a transformer. Maintaining proper cooling controls can also extend the lifetime of the transformer past the typical life expectancy.

There are a variety of different types of oil and winding temperature indicators (OTI/WTI) on the market based on the size and functionality required on power transformers. Indicators can be purely mechanical or have some integrated contacts and electronics to provide the additional alarming capability.

Remote Winding Temperature Indicators

Remote mount OTI / WTI units are used for larger transformers where due to their size, the top oil well or pocket is not accessible from the ground, making it hard to see the temperature dial.  The temperature sensing bulb is mounted in a pocket that is generally difficult to reach and the body of the indicator is mounted at eye level in another location on the transformer.  A capillary connects the temperature sensing bulb and the body of the OTI / WTI indicator which includes the measurement dial.  This capillary is filled with a fluid that expands as the temperature rises and moves an indication pointer on the face of the dial.  The mechanism that drives the pointer is one of two technologies typically used for this application, either bellows-type or bourdon spring technology.  Each has their own advantages and have been around for many decades.  They both accomplish the same end goal of driving the temperature indication pointer.

Direct Mount Oil & Winding Temperature Indicators

Direct mount OTI/WTI are generally chosen for smaller, lower voltage transformers where the temperature indicator can be submerged directly into the insulating oil.  These units could be easily read by the user at eye level in the position where they are mounted.  These temperature indicators are made using bi-metal technology.   The principal behind them is the use of two dissimilar metals that expand at different rates to drive the indicating pointer on the dial.

Winding Gradient in Power Transformers

Measuring top oil temperature in a transformer gives a good indication of the overall operating state of the transformer. Top oil generally carries the highest temperature profile in the transformer other than measuring the direct winding for temperature hot spots. While top oil is a good indication of the highest temperature within the transformer tank, the overall temperature of the oil changes very slowly as it is an excellent insulator and has a large thermal mass. Simulating the winding temperature will give a more accurate idea of the temperature is inside the windings versus the top oil temperature. Transformer windings are where the heat is generated and therefore you will find the highest temperatures. Increased temperatures in the windings lead to accelerated aging and can signal a breakdown in the insulation or indicate a fault condition.

Based on the style or technology of the winding temperature indicator that is being used in your power transformer, the winding temperature can be simulated using the Current Transformer (CT) Current in various ways. This includes simulation inside the device itself, through the use of a heated well or thermal plate. Simulated winding temperature schemes are excellent because they can also be retrofit onto older transformers, not just installed at the time of manufacture like other hot spot solutions such as fiber optics.

The combination of oil temp (top and bottom) plus direct winding can provide a highly accurate thermal model for any power transformer. This provides the foundation for calculating the load profile for the transformer. It is estimated that each 6-8 degree incremental increase in temperature approximately doubles the rate of life decay of the power transformer. This will dramatically cut short the estimated lifespan of the transformer if thermal runaway is allowed to occur.


Qualitrol Power Transformer Oil and Winding Temperature Indicators

Figure 1. Aging Accelerator Factor IEEE, Fundamental Principals of Transformer Thermal Loading and Protection; Joe Perez, ERL Phase Power Technologies, Winnipeg, MB, R3Y 1G4

[IEEE std. C57.91.1995 / IEC 60076-7:2018]  use Arrhenius formulas for deterioration of organic insulation material with time and temperature

Winding Temperature Sensors

In addition to traditional winding temperature indicators, the winding temperature can also be measured using a resistance temperature device (RTD) or through fiber optic winding temperature sensors.

Some of the most commonly used resistance temperature devices (RTD) are Pt100 Sensors or Pt100 Sensor simulation. These sensors can be used with the winding simulation methods mentioned above or with a power transformer monitor.

Fiber optic winding temperature sensors are an alternative option over traditional winding temperature indicators and are inserted into windings at the time of power transformer manufacturing. This is one of the most accurate ways to measure winding temperature in real-time.


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