Winding Hot Spot Temperature refers to the specific area within a transformer that has the highest temperature.

Advanced Power Technologies is focused on creating technically advanced warnings for when your transformer is having a problem. The most important being Temperature. But why is temperature so important?

It all comes down to one factor and that is dielectric integrity of the power transformer’s insulation system. Dielectric integrity is affected by the ambient temperature as well as the transformer’s in-tank temperatures.

While the dielectric integrity of all solid insulation material is vitally important, the insulation material surrounding the winding conductors, typically known as Kraft paper, undergoes accelerated degradation at elevated temperatures. The process is called de-polymerization which results in a breakdown of the polymer chains that in-turn reduce the paper insulation’s mechanical strength and its dielectric strength. Dielectric strength is an insulating material’s ability to prevent inadvertent current to flow from one conductor to another. The higher the temperature the faster the rate of de-polymerization. The rate of de-polymerization is a function of temperature that can be predicted by the Arrhenius equation that may be found in the loading guides for power transformers by both IEEE and IEC Standards.

While most power transformers will function without issue even with compromised paper insulation, the danger is from a “through fault” that deforms the winding and reduces the distance between the turns of the windings. If the insulation is dielectrically compromised this will lead to a flash-over that may result in a catastrophic failure of the power transformer.

What Temperatures are Important for Winding Hot Spot Temperature Monitoring?

1. Ambient temperature: This is the temperature of the air surrounding the transformer and if the basis for the temperature of insulating liquid and the transformer’s winding, core, and structural components.
2. Liquid temperature: this is the temperature in the main tank or OLTC tank in liquid-immersed transformers. The main tank liquid temperature is referred to as: Top Oil Temperature.
3. Hottest Spot Winding temperature: There is only one hottest winding hot spot temperature in any transformer. There may be other hot spot temperatures, but these are referred to as inadvertent hot spots. An example is a conductor connected to a winding that is not tight that may heat up at higher loading levels.

How is Oil Temperature Measured?

Figure 1  ̶  Mechanical Oil Temperature Indicator                                            Figure 2  ̶  Electronic Temperature Monitor

Figure 1 above illustrate a mechanical Oil Temperature Indicator. Temperature measurement gas or liquid filled bulb known as Bourdon tube where the gas or liquid expands as a function of temperature. The tube entering in the assemblies above, press on a bellow that moves the dial. Figure 2 above illustrates our Total ECLIPSE monitor that uses a Resistive Temperature Device (RTD) to sense temperature. It is known as an Electronic Temperature Monitor (ETM).

How is Winding Hot Spot Temperature Measured?

There are multiple methods used for winding hot spot temperature monitoring: Simulated, Calculated and Direct Measurement. The instruments in Figure 3 and 4 are examples of the hardware used to measure the winding hotspot temperature:

Figure 3  ̶  Mechanical Winding Temperature Indicator                                        Figure 4  ̶  ETM using RTD Top OIL and CT 

Figure 5 – Fiber Temperature Probe      

Simulated Winding Hotspot indicators:

Figure 3 above illustrates a mechanical Winding Temperature Indicator (WTI). It uses the same temperature measurement technology of an Oil Temperature Indicator, but the temperature sensing bulb is inserted into a thermometer well surrounded by a heater powered by a current balance transformer connected directly to the transformer’s bushing CT. As load current increases, the temperature rise of the heater is added to the temperature of the insulating liquid. The circuitry used varies from one transformer manufacture to another, but includes the elements noted and will provide the correct Delta Hot Spot gradient shown below for Calculated Winding Hot Spot Temperature Monitoring.

Calculated Winding Hotspot indicators:

The calculated method for winding temperature indication uses two measured parameters: Top Oil Temperature and Load Current from one or all the windings on the transformer using the equation:

Figures 4 illustrates the Total ECLIPSE Transformer Monitor using Advanced Power Technologies’ calibration-free RTD top oil temperature measurement probe and our auxiliary split-core CT.

Figure 5 illustrates Advanced Power Technologies fiber optic temperature probe featuring advanced phosphor sensing technology that connects to our ECLIPSE product line.  Our Total ECLIPSE product has the capacity to interface with up to 16 phosphor tipped fiber optic temperature probes.

Advantages and Disadvantages or Mechanical OTI and WTI Versus Advanced Power Technologies’ Electronic Temperature Monitors: