Durable and efficient, oil-immersed transformers deliver powerful energy for your electricity!
01 General
1.1 Project Description
20 MVA power transformer was delivered to South Africa in 2025. The rated power of the transformer is 20 MVA with ONAN/ONAF cooling. The primary voltage is 66 kV with ±6*1.67% tapping range (OLTC), the secondary voltage is 11 kV, they formed a vector group of YNd11.
The OLTC is a Y-connection model with a gas relay on the switch and a trip contact on the gas relay. The 20 MVA power supply transformer is equipped with a lightning arrester bracket to protect the equipment from lightning shocks. Equipped with winding thermometer with alarm trip contact and current transformer required for winding thermometer. Oil level thermometer with alarm trip contact and oil level meter with low oil level alarm. In addition, the terminal box and other places are equipped with heaters, and the low voltage side is equipped with a low voltage cable box to ensure normal operation in low temperature environment. Auxiliary power supply control voltage 110V, switch drive motor power supply voltage 220V, heater power supply voltage 220V.
1.2 Technical Specification
20 MVA power supply transformer specifications type and data sheet
| Delivered to | South Africa |
| Year | 2025 |
| Type | Oil immersed power transformer |
| Standard | IEC60076 |
| Rated Power | 20 MVA |
| Frequency | 50HZ |
| Phase | 3 |
| Cooling Type | ONAN/ONAF |
| Primary Voltage | 66 kV |
| Secondary Voltage | 11 kV |
| Winding Material | Copper |
| Angular displacement | YNd11 |
| Impedance | 8% |
| Tap Changer | OLTC |
| Tapping Range | ±6*1.67% |
| No Load Loss | 20(±10%)kW |
| On Load Loss | 105(±10%)kW |
| Accessories | Standard Configuration |
1.3 Drawings
20 MVA power supply transformer diagram drawing and size.
02 Manufacturing
2.1 Core
The core features a laminated structure, comprised of multiple thin steel sheets stacked together. The thickness of each sheet is precisely calculated to ensure optimal loss reduction while withstanding fluctuating work magnetic flux. The core is designed as a closed magnetic circuit, ensuring that magnetic flux can efficiently circulate within the core, thereby improving transformer operating efficiency. A closed magnetic circuit helps minimize leakage magnetic losses and enhances overall performance.
2.2 Winding
 | The continuous winding refers to the coils within the winding being wound without interruption or segmentation. This structure allows the current to form a stable magnetic field within the winding, reducing electrical losses and the risk of faults caused by broken wires or connection points. Due to the continuous design, it enables more uniform current distribution, minimizing the formation of hot spots and enhancing the overall efficiency of the transformer. |
2.3 Tank
| Shape the cut steel plates through bending and folding to create the tank’s outer shell structure. Use MIG or TIG welding techniques to permanently weld the components, controlling weld quality to prevent future leaks. Clean the welded tank to remove oxides and oil residues. Apply a corrosion-resistant primer and topcoat to the tank’s exterior to improve resistance to corrosion and weathering. Drill necessary holes on the tank to install the inlet, outlet, breather, oil level gauge, thermometer, and other accessories to ensure functionality. Install support structures and connectors to ensure the tank’s stability during operation. |  |
2.4 Final Assembly
 | Winding Manufacture and Installation: Install the low voltage winding on the core first, followed by the high voltage winding. Body Assembly: Place the core and winding into the transformer tank and fix them in position. Install tap changers and other internal accessories. Vacuuming and Oil Filling: Remove air from the coils and the tank by vacuuming to create a vacuum state. Fill the tank with transformer oil to ensure insulation and aid cooling. Accessory Installation: Install external accessories such as coolers, oil conservators, gas relays, and pressure relief valves. |
03 Testing
1. Measurement of dissolved gasses in dielectric liquid from each separate oil compartment except diverter switch compartment
2. Measurement of voltage ratio and check of phase displacement
3. Measurement of winding resistance
4. Check Of Core And Frame Insulation For Liquid Immersed Transformers With Core Or Frame Insulation
5. Measurement Of D.C Insulation Resistance Between Each Winding To Earth And Between Windings
6. Determination of capacitances windings to earth and between windings
7. Applied Voltage Test (AV)
8. Measurement of No-load Loss and Current
9. Induced Voltage Withstand Test
10. Measurement of Short-circuit Impedance and Load Loss
11. Measurement of dissolved gasses in dielectric liquid from each separate oil compartment except diverter switch compartment
12. Leak Testing With Pressure For Liquid-Immersed Transformers (Tightness Test)
04 Packing and Shipping
05 Site And Summary
Thank you for taking the time to learn about our power supply transformer. As a core component in power transmission and distribution, our products are renowned for their exceptional performance, reliable stability, and efficient insulation and cooling capabilities. Whether in industrial sectors or commercial environments, oil-immersed transformers can meet diverse power demands, ensuring the safe and efficient operation of systems. Choosing us means choosing advanced technology, professional support, and continuous customer care. We look forward to partnering with you to enhance the reliability and efficiency of your power systems. Thank you for your attention and trust!
