Thermal Design Considerations for Transformer Bushings
- Jun 10, 2025
- 2 min read
Transformer bushings are crucial components that ensure the safe transfer of electrical energy between transformer winding leads and transmission line. While their primary function is to provide reliable electric insulation, effective thermal management is equally essential to ensure operational longevity and performance. Effective thermal management means maintaining the hot spot of temperature within limits related to thermal class of insulation. Bushings with simultaneous application of electrical stress and thermal stress, thus the thermal stability needs to be maintained within limits to prevent faster insulation ageing & prevent insulation degradation.
Heat Source in Bushings
The main sources of heat generation in bushings are:
I²R losses due to current passing through Conductor (either Draw Lead or Draw Rod or Stem Type) passing through Bushing.
Dielectric heating due to dielectric losses (on account of Tan Delta).
Ambient conditions such as temperature, solar radiation, proximity to nearby components at higher temperature, i.e., transformer tank.
Key Considerations for Thermal Design
IEC 60137:2017 defines maximum values of temperature and temperature rise above ambient air for spring contacts, screwed contacts, terminal to be connected to external conductors by screws or bolts, and current carrying parts in contact with the insulation based on the thermal class of insulation.
Current Density: Current density needs to be maintained within limits used based on the chosen conductor material. There are two kinds of current densities which need to be ensured to be in limits, namely "Cross Sectional Current Density" & "Surface Current Density”.
Thermal Class of Insulation: The temperature limits of metal parts in contact with insulating material under normal operating conditions are- 105 °C for OIP (Thermal Class A) and 120 °C for RIP (Thermal Class E). Based on the thermal class of insulation, the maximum hot spot temperatures are also defined.
Monitoring in Service
Infrared (IR) thermography is a non-contact diagnostic technique that identifies hot spots in bushings due to resistive losses, loose connections, or insulation degradation (refer to Figure 1). An IR camera measures the heat emitted from the bushing, revealing abnormal temperature patterns. Hot spots typically indicate resistive heating from internal faults or poor connections. While IR thermography cannot diagnose all insulation-related issues, it is effective when used with other tests. For example, a combination of an elevated Tan Delta value and a hot spot suggests insulation deterioration, calls for further investigation.
Figure 1. Infrared Imaging of Transformer bushing.
Thermal design is crucial in guaranteeing transformer bushing reliability. Proper evaluation of heat sources, current density, and ambient conditions can considerably increase bushing life.