How do Oil-Immersed Transformers handle overloads
Nov 10, 2025
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Oil-immersed transformers handle overloads
primarily through their cooling system, which involves circulating oil to absorb and dissipate heat. While they have a higher thermal tolerance than dry-type transformers and can handle short-term overloads, prolonged or severe overloads will cause damage by increasing temperatures to dangerous levels, degrading insulation, and potentially leading to failure. Their ability to handle overloads depends on factors like the duration of the overload and the transformer's design, but a key limit is the winding's hot-spot temperature.
How they handle overloads
Cooling system: The transformer oil absorbs heat from the core and windings and circulates to radiators where the heat is released into the surrounding air. This continuous cycle helps manage temperature rises, but the effectiveness is limited during an overload.
Built-in thermal capacity: Oil-immersed transformers are designed to operate above their rated power for short periods due to superior cooling performance and the use of high-temperature-resistant insulating materials and oils.
Short-term overload: According to standards like IEC 60076-7, they can operate at 1.5× rated current for up to 30 minutes or 2× rated current for about 5 minutes, provided temperature monitoring is in place.
Long-term overload: For extended periods, strict temperature limits must be observed, such as keeping the top oil temperature below 105℃ and the winding hot-spot temperature below 98℃(according to IEEE C57.91).
Risks and limitations
Insulation degradation: Consistent operation at higher-than-rated loads generates more heat, which can damage the winding insulation over time, eventually causing short circuits and other failures.
Inaccurate temperature readings: The oil temperature is not a reliable indicator of the winding temperature because the windings heat up much faster than the oil. Therefore, the winding hot-spot temperature is the critical factor to monitor.
Observable signs of overload: If a transformer is overloaded, signs include unusual noises (like a loud hum or crackling), increased vibration, and visible signs of overheating, which can eventually lead to darkened oil or a burnt odor.
GNEE Oil Immersed Transformer Product Parameters
| Items | Item |
| Production Descriptions | Oil-Immersed Transformer |
| Reference standards | IEC 60076-11 |
| Capacity | 50-2500kVA or Customized |
| Frequency | 60 / 50Hz |
| Voltage (HV) | 22KV or customized |
| Voltage (LV) | 0.4KV or customized |
| Impedance | 4% |
| Connections (Vector Group) | Dyn11 |
| Tapping Range | ±2×2.5% |
| Core materials | Silicon Steel |
| Winding materials | Aluminum or copper |
| Lightning Impulse Withstand Voltage | IEC standard |
| AC withstand voltage | IEC standard |
| Temperature Rise class | H-class |
| Temperature Rise | 125K |
| Cooling method | AN/AF |
| Delivery Capacity | 300 units per month |
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What is the permissible overload percent for oil immersed transformers?
Normal Short-Term Overload Limits:
55°C rise oil-immersed transformers: Up to 30% overload for limited durations. 65°C rise oil-immersed transformers: Up to 15% overload for short periods.
What are the advantages of oil immersed transformers?
Oil-immersed transformers have several advantages over dry-type transformers. Firstly, they are more efficient, as the oil helps to dissipate heat and reduces energy losses. Secondly, they can operate at higher temperatures, which makes them suitable for high-temperature applications.
What happens if I overload a transformer?
Overload is when a transformer is subjected to voltages and/or currents that exceed its design specifications. During overloading conditions, excess heat will cause the insulation system to break down, resulting in decreased life expectancy of the transformer.
company profile
GNEE TRANSFORMER has strong production capacity and an efficient marketing team.
Our transformer products include 500kV, 400kV, 330kV, 220kV, and 110kV ultra-high voltage transformers, various dry-type transformers/oil immersed transformers/amorphous alloy transformers of 35kV and below, new energy transformers, and prefabricated substations,
Export series: single-phase transformer, three-phase transformer, cylindrical transformer, isolation transformer, control transformer, voltage regulator and other products, And various specifications of special transformers such as grounding transformers, arc suppression coils, Bitcoin transformers, prefabricated cabins, reactors, electric furnace transformers, rectifier transformers, mining transformers, marine transformers, etc.
GNEE TRANSFORMER has successively passed ISO9001, ISO14001, ISO45001, SA8000 system certification. At the same time, 10kV and 35KV level distribution transformers and box type substations have obtained interna- tional Electrotechnical Commission IEC certification, US UL certification, German TUV certification, EU CE certification, Dutch KEMA certification, and GOST certification. Our product sales cover the global market, including 72 countries and regions such as the United States, Europe, Canada, Mexico, etc.
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types of transformer from GNEE
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GNEE Transformer Order specification
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Demander: |
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Project technical leader: |
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Mailbox: |
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Transformer type selection |
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□Dry transformer {□SC(B) Epoxy resin □SC(B) Non-encapsulated} □American Box(ZGS) {□Terminal type(-Z-) □Ring network type(-H-)} □European Box change(YBM){□ Dry change □Oil change} |
□ Oil-immersed transformer{□ Fully sealed(-M-) □Oil-bearing pillow} □Amorphous alloy transformer{□Dry amorphous □ Oil amorphous} □ Special transformer{Rectifier, Furnace, Mine, Etc} |
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Transformer basic data selection
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Country of use: |
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Phase number: |
□Three-phase(S conventional) □ Monophase □Other( ) |
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Frequency: |
□50Hz(conventional) □60Hz □ Other( ) |
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Transformer oil: |
□#25(conventional) □#45(minimum temperature below-25℃) |
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Rated capacity: |
□( )kVA Conditions of use □Outdoor type □Indoor type |
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Altitude: |
□Less than 1000m(conventional) □1000-2000m □3000m以上 over |
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Rated voltage(High voltage) |
□6kV □10kV □20kV □35kV □66kV □110kV □220kV □Other( ) |
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Rated voltage(Low pressure) |
□0.4kV □0.66kV □10kV □10.5kV □35kV □其他 Other( ) |
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Connected group |
□Dyn11(conventional) □Yyn0 □YNd11 □li0(Single-phase) □Other( ) |
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Impedance(%): |
□4% □4.5% □5.5% □6% □6.5% □7.5% □8% □Other( ) |
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Voltage regulation mode |
□Voltage regulation without excitation(conventional) □On-load voltage regulation(Z) |
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Voltage regulation range |
□±2*2.5% □±5% □±3*2.5% □Other( ) |
Transformer accessory selection
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□Fan □Temperature control{□ Common □With 485 computer interface} □Dry case{□ Ordinary steel sheet □Aluminum alloy » Stainless steel} □Protection level of dry case(□IP20(Conventional) □IP( )} □Dry case height requirements{□None □Requirements[ ]} □ Cable out mode of the dry variable shell {□ Up-and-out □ Up-and-down □Down-in/DOWN-out □Down-in/UP-out □Down-in/DOWN-out □Down-in/ side-out [□Left □ Right-out]} |
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Note: 1. (Dry change requirements s 100kVA transformer does not include fans, such as fans, need to be charged 2. (Oil changes <630kVA transformer does not include temperature control, gas relay, if necessary, need to increase the fee) |
Other special requirements(additional charges apply below)
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Dry transformer |
□Shell and bottom plate{□Ordinary □Fully sealed} □Travel switch □Magnetic lo □Zero sequence transformer □Copper bar to the low voltage cabinet □Protective mesh □Lighting □100kVA or less Fan □Shock absorber |
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Oil-immersed transformer |
□No.45 oil for non-low temperature areas □Designated brand on-load voltage regulator □400kVA and below low voltage outlet OABC phase with wiring terminals □630kVA or below requires temperature control □800kVA or below Low voltage outlet phase 0 requires wiring terminals □630kVA and below must be equipped with gas relay |
GNEE TRANSFORMER Industrial Layout
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Europe: Bulgaria, North Macedonia, Finland, Norway, Ukraine, Greece, Hungary, Italy, United Kingdom
South America: Argentina, Paraguay, Ecuador, Colombia, Guyana, Venezuela, Uruguay, Chile
Middle East: The Middle East region
Oceania: Australia, Papua New Guinea. Fiji