Oil-filled transformer
The oil-immersed transformer is a static electrical machine that transfers electrical energy between circuits through inductively coupled conductors, using insulating oil as a dielectric and cooling medium. The variation of current in the primary winding induces a changing magnetic field that generates an alternating voltage in the secondary winding. For an ideal transformer, the relationship between voltages and currents follows a law of proportionality based on the number of turns.
U₁ / U₂ = N₁ / N₂ = I₂ / I₁
| Variable | Symbol | Unit |
|---|---|---|
| Electric voltage (potential) | U | V |
| Number of winding turns | N | dimensionless |
| Electric current | I | A |
In a typical example, a transformer with 500 primary turns, 3000 secondary turns, and a primary alternating voltage of 230 V will generate a secondary voltage of 1380 V (1,38 kV).
Standard Specifications
Section titled “Standard Specifications”The standard specifications of oil-immersed transformers are mainly defined by their apparent power rating in kVA and the full-load current they can supply at different voltage levels. The following tables present the values for common single-phase and three-phase configurations in industrial and distribution applications.
Single-Phase Transformer
Section titled “Single-Phase Transformer”| Apparent Power (kVA) | Full Load Current 120 V (A) | Full Load Current 240 V (A) | Full Load Current 480 V (A) |
|---|---|---|---|
| 0,05 kVA | 0,42 A / 0,42 A | 0,21 A / 0,21 A | 0,10 A / 0,10 A |
| 0,075 kVA | 0,63 A / 0,63 A | 0,31 A / 0,31 A | 0,16 A / 0,16 A |
| 0,1 kVA | 0,83 A / 0,83 A | 0,42 A / 0,42 A | 0,21 A / 0,21 A |
| 0,15 kVA | 1,25 A / 1,25 A | 0,63 A / 0,63 A | 0,31 A / 0,31 A |
| 0,25 kVA | 2,08 A / 2,08 A | 1,04 A / 1,04 A | 0,52 A / 0,52 A |
| 0,5 kVA | 4,17 A / 4,17 A | 2,08 A / 2,08 A | 1,04 A / 1,04 A |
| 0,75 kVA | 6,25 A / 6,25 A | 3,13 A / 3,13 A | 1,56 A / 1,56 A |
| 1 kVA | 8,33 A / 8,33 A | 4,17 A / 4,17 A | 2,08 A / 2,08 A |
| 1,5 kVA | 12,5 A / 12,5 A | 6,25 A / 6,25 A | 3,13 A / 3,13 A |
| 2 kVA | 16,7 A / 16,7 A | 8,33 A / 8,33 A | 4,17 A / 4,17 A |
| 3 kVA | 25 A / 25 A | 12,5 A / 12,5 A | 6,3 A / 6,3 A |
| 5 kVA | 41,7 A / 41,7 A | 20,8 A / 20,8 A | 10,4 A / 10,4 A |
| 7,5 kVA | 62,5 A / 62,5 A | 31,3 A / 31,3 A | 15,6 A / 15,6 A |
| 10 kVA | 83,3 A / 83,3 A | 41,7 A / 41,7 A | 20,8 A / 20,8 A |
| 15 kVA | 125 A / 125 A | 62,5 A / 62,5 A | 31,3 A / 31,3 A |
| 25 kVA | 208 A / 208 A | 104,2 A / 104,2 A | 52,1 A / 52,1 A |
| 37,5 kVA | 313 A / 313 A | 156,3 A / 156,3 A | 78,1 A / 78,1 A |
| 50 kVA | 417 A / 417 A | 208,3 A / 208,3 A | 104,2 A / 104,2 A |
| 75 kVA | 625 A / 625 A | 313 A / 313 A | 156 A / 156 A |
| 100 kVA | 833 A / 833 A | 417 A / 417 A | 208 A / 208 A |
| 167 kVA | 1392 A / 1392 A | 696 A / 696 A | 348 A / 348 A |
| 200 kVA | 1667 A / 1667 A | 833 A / 833 A | 417 A / 417 A |
| 250 kVA | 2083 A / 2083 A | 1042 A / 1042 A | 521 A / 521 A |
Three-Phase Transformer
Section titled “Three-Phase Transformer”| Apparent Power (kVA) | Full Load Current 240 V (A) | Full Load Current 480 V (A) |
|---|---|---|
| 3 kVA | 7,2 A / 7,2 A | 3,6 A / 3,6 A |
| 6 kVA | 14,4 A / 14,4 A | 7,2 A / 7,2 A |
| 9 kVA | 21,7 A / 21,7 A | 10,8 A / 10,8 A |
| 15 kVA | 36,1 A / 36,1 A | 18,0 A / 18,0 A |
| 22 kVA | 53,0 A / 53,0 A | 26,5 A / 26,5 A |
| 30 kVA | 72,2 A / 72,2 A | 36,1 A / 36,1 A |
| 45 kVA | 108 A / 108 A | 54,1 A / 54,1 A |
| 75 kVA | 180 A / 180 A | 90,2 A / 90,2 A |
| 112,5 kVA | 271 A / 271 A | 135 A / 135 A |
| 150 kVA | 361 A / 361 A | 180 A / 180 A |
| 225 kVA | 541 A / 541 A | 271 A / 271 A |
| 300 kVA | 722 A / 722 A | 361 A / 361 A |
| 500 kVA | 1203 A / 1203 A | 601 A / 601 A |
| 750 kVA | 1806 A / 1806 A | 903 A / 903 A |
Insulating Oil Properties
Section titled “Insulating Oil Properties”Insulating oil for transformers is a fluid stable at high temperatures with excellent dielectric properties, whose main function is to electrically insulate, suppress partial discharges and arcs, and act as a coolant. It must maintain high dielectric strength, thermal conductivity, and chemical stability for extended periods at demanding operating temperatures.
| Property | Typical Value |
|---|---|
| Minimum dielectric strength (breakdown voltage) | 30 kV RMS |
| Flash point | > 140 °C / 284 °F |
| Pour point | < −40 °C / −40 °F |
| Specific resistance at 27 °C | 1500 × 10¹² ohm·cm |
| Specific resistance at 90 °C | 35 × 10¹² ohm·cm |
| Maximum oil temperature under overload | 90 °C / 194 °F |
There are two main types of transformer oil: paraffinic-base oil and naphthenic-base oil. Naphthenic oil oxidizes more quickly, but the oxidation products (sludge) are more soluble and do not settle at the bottom of the tank, maintaining oil circulation and cooling system effectiveness. Paraffinic oil oxidizes more slowly, but its sludge is insoluble and accumulates at the bottom, potentially obstructing circulation and reducing cooling capacity. Despite this disadvantage, paraffinic oil is commonly used in many countries due to its high availability.
Cooling System
Section titled “Cooling System”Cooling in oil-immersed transformers is achieved by heat transfer from the core and windings to the oil, and from the oil to the environment through the tank surface. Small distribution transformers dissipate heat directly through the tank walls. Larger power transformers incorporate external radiators through which the oil circulates by natural convection.
In transformers with capacities in the thousands of kVA, the cooling system can be reinforced with forced-air cooling fans mounted on the radiators, oil circulation pumps, and even oil–water heat exchangers for high-power applications. Transformers without an expansion tank (conservator) are usually equipped with sudden pressure relays, while those with a conservator incorporate gas detector relays such as the Buchholz relay, capable of detecting slow gas accumulations or rapid pressure increases and tripping the circuit protection switch.
Environmental and Service Conditions
Section titled “Environmental and Service Conditions”The ambient temperature range for continuous operation of a typical oil-immersed transformer is between −25 °C and 40 °C (−13 °F to 104 °F) [VERIFY]. Power transformers undergo prolonged drying processes using electrical self-heating, vacuum application, or a combination of both methods, with the aim of completely removing water vapor from the interior before filling with insulating oil. This procedure prevents the formation of partial discharges (corona) and subsequent dielectric breakdown under load.
The oil level is typically monitored using a magnetic oil level gauge (MOG), and system tightness must be ensured to prevent the entry of atmospheric oxygen and moisture, factors that rapidly degrade the insulating properties of mineral oil and accelerate the oxidation of the winding cellulosic insulation.
Typical Applications
Section titled “Typical Applications”Oil-immersed transformers are used in power distribution and transmission substations, industrial plants, generation stations, railway traction systems, renewable energy installations (wind and solar photovoltaic farms), and in any application requiring voltage transformation with power ratings from a few kVA to hundreds of MVA. Due to the high dielectric and thermal capacity of the oil, this technology dominates the power transformer market compared to dry-type transformers in outdoor and indoor applications where insulation and heat dissipation requirements are critical.
When fire safety regulations require the use of less flammable liquids indoors, synthetic or natural esters with fire points above 300 °C / 572 °F may be used, or dry-type transformers without insulating liquid may be chosen.
Frequently Asked Questions (FAQ)
Section titled “Frequently Asked Questions (FAQ)”What is the minimum acceptable dielectric strength for transformer oil according to field tests?
Section titled “What is the minimum acceptable dielectric strength for transformer oil according to field tests?”The minimum dielectric strength of transformer oil, measured as breakdown voltage in a spark gap with electrode spacing of 2.5 mm, is considered acceptable from 30 kV.
What is the full-load current of a three-phase 500 kVA transformer at 480 V?
Section titled “What is the full-load current of a three-phase 500 kVA transformer at 480 V?”A three-phase 500 kVA transformer at 480 V delivers a full-load current of 601 A.
What maximum temperature can transformer oil reach under overload conditions?
Section titled “What maximum temperature can transformer oil reach under overload conditions?”The oil temperature under full load conditions can reach up to 90 °C, and the specific resistance of the oil at that temperature must remain at least 35 × 10¹² ohm·cm to ensure safe operation.
What is the flash point of typical mineral transformer oil?
Section titled “What is the flash point of typical mineral transformer oil?”The flash point of mineral transformer oil is above 140 °C / 284 °F.
What secondary voltage is obtained with 500 primary turns, 3000 secondary turns, and 230 V supply?
Section titled “What secondary voltage is obtained with 500 primary turns, 3000 secondary turns, and 230 V supply?”Applying the transformation ratio, the resulting secondary voltage is 1380 V (1,38 kV).
At what temperature is the specific resistance of insulating oil measured to verify its quality?
Section titled “At what temperature is the specific resistance of insulating oil measured to verify its quality?”The specific resistance is measured at 27 °C and 90 °C, must reach minimum values of 1500 × 10¹² ohm·cm and 35 × 10¹² ohm·cm respectively.
References
Section titled “References”- engineeringtoolbox.com: https://www.engineeringtoolbox.com/transformer-d_1398.html
- electrical4u.com: https://www.electrical4u.com/transformer-insulating-oil-and-types-of-transformer-oil/