The oil-immersed low-loss power transformer is the standard power distribution equipment for three-phase 50 Hz systems up to the 35 kV class. Its design combines a grain-oriented silicon steel core, robust windings and a fully sealed oil system to achieve low no-load loss, low noise and maintenance-free operation. This article walks through the key design elements and presents the technical parameters of the 35 kV on-load tap-changing series from 2000 to 40000 kVA.
Core Design and Loss Control
The core is built from high-permeability grain-oriented silicon steel sheet, which aligns the magnetic domains in the rolling direction and reduces hysteresis loss compared with non-oriented steel. The core is stacked flat with a step-lap multistage joint design, which lowers the magnetic resistance at the joints and further reduces no-load loss and noise. Core steel is specified according to standards such as GB/T 2521 or IEC 60404-8-7, and the finished product is processed by vacuum oil injection so that no air remains in the insulation system.
Winding Structure
The high-voltage winding uses a multi-layer cylindrical construction, which improves the voltage distribution under lightning and switching impulses. The low-voltage winding uses a cylindrical or spiral construction, giving high mechanical strength, good ampere-turn balance and strong short-circuit withstand capability. The active part is fitted with positioning structures so that it cannot shift during transport, and all fasteners are secured with locking nuts so that they do not loosen during long-term operation, meeting the requirement that the core never needs to be lifted for re-tightening.
Tank, Sealing and Protection
The transformer tank is fully sealed: the insulating oil and the insulation system never contact atmospheric air, so the oil does not absorb moisture or oxidise. Under normal service no oil change is needed, which cuts maintenance cost and extends service life. The tank is equipped with standard protection and monitoring devices - a pressure relief valve, a signal thermometer and a gas relay, also called a Buchholz relay - installed to the applicable standards to ensure safe operation. The external surfaces are degreased, derusted and phosphated before a primer and finish coat are applied, so the transformer can be specified for metallurgical, petrochemical and wet or dirty environments.
Performance Characteristics
Low no-load loss and low noise thanks to the grain-oriented core and step-lap joints.
Good short-circuit withstand from the robust low-voltage winding structure.
Fully sealed oil system: no oil change in normal service, low maintenance.
Compact footprint and reduced installation area.
Stable and reliable performance in continuous utility duty.
Technical Parameters: 35 kV Class, On-Load Tap-Changing
The table lists the main ratings of the 35 kV class series with on-load voltage regulation. The high voltage is 35 or 38.5 kV with a tap range of plus or minus 3 by 2.5%; the low voltage is 6.3, 6.6 or 10.5 kV; and the vector groups are Yd11 and YNd11.
| Capacity (kVA) | HV (kV) | LV (kV) | Vector group | No-load loss (kW) | Load loss (kW) | No-load current (%) | Impedance (%) |
|---|---|---|---|---|---|---|---|
| 2000 | 35 | 6.3, 10.5 | Yd11 | 2.30 | 19.2 | 0.5 | 6.5 |
| 2500 | 35 | 6.3, 10.5 | Yd11 | 2.72 | 20.6 | 0.5 | 6.5 |
| 3150 | 35, 38.5 | 6.3, 10.5 | Yd11 | 3.23 | 24.7 | 0.5 | 7.0 |
| 4000 | 35, 38.5 | 6.3, 10.5 | Yd11 | 3.87 | 29.1 | 0.5 | 7.0 |
| 5000 | 35, 38.5 | 6.3, 10.5 | Yd11 | 4.64 | 34.2 | 0.5 | 7.0 |
| 6300 | 35, 38.5 | 6.3, 10.5 | Yd11 | 5.63 | 36.7 | 0.68 | 8 |
| 8000 | 35, 38.5 | 6.3, 6.6, 10.5 | YNd11 | 7.87 | 40.6 | 0.6 | 8 |
| 10000 | 35, 38.5 | 6.3, 6.6, 10.5 | YNd11 | 9.28 | 48.0 | 0.6 | 8 |
| 12500 | 35, 38.5 | 6.3, 6.6, 10.5 | YNd11 | 10.9 | 56.8 | 0.56 | 8 |
| 16000 | 35, 38.5 | 6.3, 6.6, 10.5 | YNd11 | 13.1 | 70.3 | 0.54 | 8 |
| 20000 | 35, 38.5 | 6.3, 6.6, 10.5 | YNd11 | 15.5 | 82.7 | 0.5 | 8 |
| 25000 | 35, 38.5 | 6.3, 6.6, 10.5 | YNd11 | 18.3 | 97.8 | 0.5 | 10 |
| 31500 | 35, 38.5 | 6.3, 6.6, 10.5 | YNd11 | 21.8 | 116.0 | 0.5 | 10 |
| 40000 | 35, 38.5 | 6.3, 6.6, 10.5 | YNd11 | 26.2 | 139.0 | 0.4 | 10 |
The values follow the manufacturer standard series; for a specific project, the guaranteed losses, impedance and accessory list are confirmed in the technical agreement, and the transformer is tested according to IEC 60076-1 and GB 1094.1.
Scope of Application
This series is suitable for three-phase 50 Hz power systems at 35 kV and below, serving as the main transforming equipment of small and medium substations, and for industrial and agricultural power distribution, power and lighting loads.
FAQ
Why is low no-load loss important?
No-load loss is incurred 24 hours a day regardless of load. A lower value means lower energy cost and a reduced capitalised cost of losses over the transformer service life.
What does fully sealed mean?
The oil and insulation are isolated from atmospheric air, so the oil does not absorb moisture or oxidise. Under normal operation no oil change is required.
What is the difference between on-load and off-load tap changing?
An on-load tap changer switches taps while the transformer is energised, allowing voltage regulation under load. An off-load tap changer requires the transformer to be de-energised before changing taps.
Which standards apply?
IEC 60076-1 and the Chinese GB 1094 series cover rating and testing. GB/T 6451 defines technical parameters and requirements for oil-immersed power transformers in China.
Why is grain-oriented silicon steel used for the core?
The oriented grain structure reduces hysteresis loss in the rolling direction, which directly lowers no-load loss and noise compared with non-oriented steel.
What routine maintenance is needed?
With the sealed design, maintenance is limited to periodic inspection of the gas relay, thermometer, oil level indicator and pressure relief device, plus oil quality checks at long intervals.