Insulation Performance and Temperature Rise Test of 1000 kVA Oil-Immersed Transformer
In power distribution, the reliability of a transformer is established by testing. For a 1000 kVA oil-immersed transformer, two properties determine the service life and safety of the unit: the integrity of the insulation system and the temperature rise under rated load. Both are verified by standard tests, principally those defined in IEC 60076, and both must be reviewed carefully when a transformer is specified for outdoor or industrial service.
Why Insulation and Temperature Rise Testing Matters
The insulation system of an oil-immersed transformer combines liquid insulation (transformer oil) and solid insulation (cellulose paper and pressboard). It must withstand not only the continuous operating voltage but also transient over-voltages from lightning and switching surges. Temperature rise testing, in turn, proves that the transformer can carry its rated load without the insulation degrading prematurely: the heat generated by load loss in the windings and no-load loss in the core must be dissipated by the cooling system so that the hot-spot temperature stays within the limits that protect the paper insulation.
The Insulation System of a 1000 kVA Oil-Immersed Transformer
The oil performs two duties: it insulates the windings from each other and from the tank, and it carries heat to the cooling surface. Solid insulation (paper, pressboard and other cellulose products) provides the mechanical and dielectric structure of the winding insulation. Two manufacturing details are critical to insulation quality:
High-vacuum oil filling removes air and moisture from the insulation before the oil is admitted, eliminating air pockets that could initiate partial discharge.
Precision winding controls the tension and layer spacing of the conductors so that the paper insulation is applied evenly and the winding is mechanically stable.
High-purity mineral oil with high dielectric strength provides a robust barrier against electrical breakdown, and the oil's oxidation stability keeps the insulation performance stable over many years of service.
Temperature Rise Testing per IEC 60076
During the temperature rise test, the transformer is loaded with its total loss (no-load plus load loss) so that steady-state temperatures are reached, and the top-oil temperature rise and the average winding temperature rise are measured. IEC 60076 sets the typical limits:
Top-oil temperature rise: 60 K above ambient.
Average winding temperature rise: 65 K above ambient.
These limits correspond to a design hot-spot temperature that keeps the cellulose insulation from becoming brittle, which is the main aging mechanism of the insulation system. A transformer that stays within these limits during the test will provide the expected service life at its rated load. For higher ambient temperatures, the permissible rise must be reduced accordingly, or the cooling surface enlarged.
Components That Control Thermal Performance
To pass the temperature rise test, several parts of the transformer must be designed as a system:
Radiators and cooling fins: their surface area is calculated from the total loss and the ambient temperature, and they are the primary heat rejection path in ONAN (Oil Natural, Air Natural) cooling.
Transformer oil: the heat-transfer medium; its viscosity and thermal properties determine the natural circulation rate.
The tank: designed to promote the thermosiphon effect, where hot oil rises and cool oil returns, maintaining continuous circulation through the radiators.
Gaskets and seals: high-temperature-resistant materials prevent leaks during the expansion and contraction cycles of outdoor service.
Typical Technical Specifications
| Technical Parameter | Specification Value |
|---|---|
| Rated capacity | 1000 kVA |
| Type | Three phase, oil immersed, step-down |
| High voltage (HV) | 6 kV, 10 kV, 11 kV, 22 kV, 33 kV |
| Low voltage (LV) | 0.4 kV, 0.415 kV, 0.48 kV |
| Insulation level | LI 75 / AC 28 (standard 10 kV class) |
| Top-oil temperature rise | ≤ 60 K |
| Winding temperature rise | ≤ 65 K |
| Cooling method | ONAN |
| Oil weight | Approx. 550–650 kg |
| Total weight | Approx. 2450 kg |
Advanced Insulation Options
Material advances allow the insulation system to be pushed further without compromising safety. Thermally upgraded paper can operate at slightly higher temperatures for the same aging rate, which increases the overload capability of the unit. For installations with high fire-safety or environmental requirements, ester fluids (including natural and synthetic esters) are an alternative to mineral oil: they have a significantly higher flash point and are biodegradable, which makes them suitable for environmentally sensitive locations while enhancing the insulation life of the transformer.
FAQ
What is the temperature rise limit for a 1000 kVA oil immersed transformer?
Per IEC 60076, the typical limits are 60 K for top-oil temperature rise and 65 K for average winding temperature rise above the reference ambient temperature. The transformer design must keep the measured rises within these values at rated load.
Why is the temperature rise test important?
The test verifies that the cooling system can remove the heat generated by the total losses at rated load. If the temperature rise is too high, the paper insulation ages rapidly and the transformer life is shortened.
What is the insulation level LI 75 / AC 28?
It is the standard insulation level for a 10 kV class transformer: LI 75 means the winding withstands a 75 kV peak lightning impulse voltage, and AC 28 means it withstands 28 kV power-frequency test voltage. The level is defined in IEC 60076-3.
How does the ONAN cooling method work?
ONAN stands for Oil Natural, Air Natural: the oil circulates naturally by convection (thermosiphon) through the tank and radiators, where heat is removed by natural air flow. No pumps or fans are used, so the cooling is simple and reliable.
Can ester fluid replace mineral oil in a 1000 kVA transformer?
Yes. Ester fluids are used as an alternative to mineral oil where fire safety or environmental protection is critical. They have a higher flash point and are biodegradable, but the transformer design must account for their different cooling and dielectric properties.
What routine tests are performed on a 1000 kVA oil immersed transformer?
Routine tests per IEC 60076 include winding resistance measurement, turns ratio and vector group verification, no-load loss and no-load current, load loss and impedance voltage, separate-source and induced AC withstand tests, and oil dielectric tests. Type tests additionally cover temperature rise and lightning impulse.