Transformer Temperature Monitoring | Dynamic Ratings

Transformer Temperature Monitoring and Cooling Control

Why is transformer temperature monitoring so important?

Temperature is a basic part of transformer monitoring. Temperature affects everything. If it is too high, it may indicate a fault that subsequently could further damage multiple components of the transformer if left unchecked.

Measure temperature in several areas of the transformer:

Winding insulation is very sensitive to temperature and moisture. Therefore, monitoring the winding hot spot is key because the winding hot spot directly impacts the aging rate of the transformer. For every 6˚C increase in winding temperature above the rated maximum of 110˚C, the transformer aging rate roughly doubles!

Power Transformer Image

Why Online Monitoring?

Transformer Temperature Monitoring

There are three key advantages to using online electronic temperature monitors (ETMs):

  1. Reduce maintenance costs and outages. These days, utilities face challenges such as reduced staffing, budget constraints and loss of industry knowledge. Condition-based monitoring is part of the solution that helps create more efficient teams.
  2. Replace outdated technology with modern IED’s that communicate remotely. Remote communication means less trips to site.
  3. Safely optimize load to take transformers to their operating limits without compromising life expectancy or reliability.


Three Ways to Measure Transformer Temperature

Traditional Temperature Gauge

1. Traditional Temperature Gauges

Oil Temperature Indicator (OTI) and Winding Temperature Indicator (WTI) Gauge technology was developed in the 1940s and was used to control the cooling system and provide indication of issues to give the transformer thermal protection. They are sufficiently rugged to be used for protection purposes if the recommended maintenance and/or calibration verification is carried out regularly.

However, there are some limitations to this method. Regular on-site maintenance must be scheduled to ensure relative accuracy. WTIs are prone to mechanical damage which may result in an inaccurate temperature reading which leads to inefficient cooling and tripping control. For a deeper understanding, download this white paper.

2. Electronic Temperature Monitors

Electronic Temperature Monitors (ETMs) can replace up to three analog gauges and consolidate temperature data and cooling control into a single point of communication. They provide better measurement accuracy than traditional gauges by using transformer design information to calculate winding hot spot temperature both during load fluctuations and steady conditions. ETMs connect to multiple bushing CTs so that all three phases are monitored with one device to identify which one is truly the hottest. Additionally, they calculate the insulation loss of life which helps utilities manage the estimated life of the transformer.

ETMs have electronic communication which provides the data to utilities remotely. It keeps detailed records of data so you are able to download history at any time. ETMs come in robust fail-safe designs to provide additional reliability beyond what a gauge can offer.

Fiber Optic Temperature Probe

3. Direct Measurement with Fiber Optic Temperature Probes

Fiber Optic Temperature Probes provide real-time direct winding monitoring. They help validate that the thermal model is truly doing what it is supposed to do. Probes are installed in the winding in a modified spacer. The tip is the measurement point which sends a light pulse signal down the fiber. The probe connects to a Transformer Monitor which communicates direct winding data back to the utility. The monitor also has the ability to model multiple windings.

This option is great for critical transformers such as a bulk power transformer because the cost of adding fiber optic temperature probes is reasonable.

Introducing Electronic Temperature Monitors

B100 H2

B100 Electronic Temperature Monitor

The B100 Series Electronic Temperature Monitor (ETM) is a complete monitoring solution for any transformer. The B100 ETM is an analog gauge replacement that provides accurate indication of problems inside the transformer via fault gas detection.


Fiber Optic Temperature Module

Fiber optic temperature modules bundle with the E3 or C50 Transformer Monitors, allowing an automatic comparison of the real-time measured temperatures to the advanced analytics calculations. Accuracy is everything!


Additional Resources


Improved Transformer Temperature Monitoring.


The essentials of transformer monitoring.


Three methods for measuring winding hot spot temperature.