Why Does Temperature Affect Tan Delta?
The dissipation factor (tan δ) of an insulation material represents the ratio of active power losses to the capacitive reactive power—or equivalently, the ratio of resistive current to capacitive current in the equivalent parallel circuit, as shown below.
Both the dielectric losses and the capacitive behavior of an insulation material depend on its intrinsic properties, including various polarization and conduction mechanisms. Since these mechanisms are governed by the motion and behavior of the atoms and molecules that make up the material—and because these behaviors change with temperature—the dissipation factor also becomes temperature-dependent.
However, this dependency is not straightforward or linear, because different polarization mechanisms respond differently across temperature ranges. These variations affect both the polarization losses and the dielectric capacitance of the insulation, leading to complex and material-specific changes in the dissipation factor with temperature.
The Importance of Temperature Correction in Tan Delta Testing
Standards such as IEC 60137 and IEEE C57.152 specify absolute limits for the dissipation factor (tan δ) of equipment like bushings and power transformers. It is important to note that these limits are defined at a reference temperature of 20 °C.
If a piece of equipment is tested at a temperature other than 20 °C, the measured dissipation factor must be corrected to the reference temperature using appropriate temperature correction factors. Without this correction, the results cannot be accurately compared to the standard limits.
Another key application of temperature correction is in trend analysis, since dissipation factor testing is fundamentally a comparative method based on historical values. This means that when an asset is tested today, its results must be compared not only to absolute limits, but also to previous test records. For such comparisons to be meaningful, the tests must either be performed at the same temperature or all measurements must be corrected to 20 °C using appropriate correction factors before comparison.
Temperature Correction Factors for Dissipation Factor (Tan Delta)
One of the key references for dissipation factor testing is the IEEE Standard C57.12.90. In its 2006 edition, this standard provided general temperature correction factors for the dissipation factor of power transformers. As a result, most tan delta test equipment manufacturers used these default correction factors in their devices.
However, in the 2010 revision of the standard, these correction factors were removed. The reason for this change is the wide range of insulation materials with varying properties used by different manufacturers in power transformers. Due to this variability, the dielectric behavior of transformers from different manufacturers can differ significantly.
This means a single, universal correction factor cannot be reliably applied to all transformers. The standard now emphasizes that the appropriate temperature correction factor should be obtained directly from the equipment manufacturer.
The correction factors that were previously included in the older version of the standard are shown in the table below, and the correction is applied using the following equation. However, as noted above, these factors are no longer recommended for use according to the updated standard, and customized correction data from the manufacturer should be used instead.
Table1: Temperature Correction Factor for Transformer Dissipation Factor
| Correction Factor | Temperature (℃) |
| 1.25 | 10 |
| 1 | 20 |
| 0.8 | 30 |
| 0.64 | 40 |
| 0.51 | 50 |
| 0.41 | 60 |
| 0.33 | 70 |
How Moisture Changes Tan Delta Behavior
The dissipation factor (tan δ) is influenced by both the temperature and the moisture content of the insulation. Moisture impacts the insulation in two ways: it increases conductivity and, due to the presence of dipolar water molecules, enhances polarization losses.
A critical point when discussing temperature correction factors is: For what moisture level was the correction factor derived?
The following graph—sourced from ABB, Bushing Diagnostics and Conditioning—illustrates how the tan δ of an OIP (Oil-Impregnated Paper) bushing changes with temperature at different internal moisture levels.
As shown, at low moisture levels, the dissipation factor initially decreases with temperature before increasing again. However, as moisture content rises, the slope of tan δ versus temperature becomes steeper. In other words, the higher the internal moisture content of a bushing, the larger the correction factor needed to refer the measurement to 20 °C.
Yet, there is a particularly important point in this graph that deserves attention and will be discussed next.
Practical Tips for Avoiding Temperature Correction Errors
As seen in Figure 2, the variation of tan δ with temperature is dependent on the internal moisture level of the insulation. A key insight from the graph is that when the moisture content inside a bushing is low, the dissipation factor does not change significantly with temperature. For example, when the moisture content is 0.5%, the correction factor from 60 °C to 20 °C is only about 1.02.
This observation can be very useful in practice. If a bushing in a high-voltage substation is tested immediately after shutdown—say, at 60 °C—and then tested again later at 20 °C, both measurements should be quite similar if the insulation is dry. A large difference between the two would be unexpected and may indicate insulation aging or degradation due to moisture.
Therefore, as a practical diagnostic approach, measuring the dissipation factor of suspect bushings at two different temperatures can help assess their condition. A significant discrepancy between the two values may signal moisture ingress or insulation deterioration.
As a final recommendation: whenever possible, perform periodic tan δ tests within a consistent temperature range—preferably between 20 °C and 30 °C. This minimizes the need for correction factors and allows for easier comparison of current results with historical data and manufacturer specifications.
