20 MVA substation power transformer-33/6.65 kV | South Africa 2025
Delivery Country: South Africa 2025
Capacity: 20MVA
Voltage: 33/6.65 kV
Feature: four wheels (10-ton)
01 General
Project Description
A South African customer ordered a 20MVA-33/6.65kV step down power transformer in May 2025. Our customer is supplying transformers, small substations and power cables to local and international mines. This transformer is also one of the main transformers customized for mine substations. South Africa is one of the most developed mining countries in the world, and there is a large demand for transformers for mines.
This step down power transformer adopts IEC 60076-1:2011 standard. It adopts the ONAN cooling method. Wiring group: YNyn0, suitable for a complex grounding environment. The winding material is copper, and the copper loss is lower. It is rated at 33kV on the primary side and 6.65kV on the secondary side. Equipped with an on-load tap changer (OLTC) with a ±10 × 1% regulation range, it allows flexible voltage adjustment under load, ensuring a stable power supply to mining equipment. The substation power transformer achieves excellent energy performance, with a no-load loss of 13.65 kW, load loss at 75°C of 132.8 kW, and a total loss limited to 72 kW. To improve mobility, especially at remote mining sites, the transformer is designed with heavy-duty wheels, a feature that significantly simplifies on-site operations.
Why It’s Ideal for Mining Applications
In mining operations, a reliable and efficient power supply is essential to ensure both safety and productivity. This substation power transformer is perfectly suited to typical mining power distribution scenarios:
Surface Substations: Steps down 33kV from the utility grid or captive power stations to 6.6kV, the standard voltage for mining equipment.
Underground Power Delivery: Transmits 6.6kV power through cables to underground systems – an ideal voltage for long-distance transmission with low losses.
High-Power Equipment Operation: Powers essential heavy-duty mining machinery such as ball mills, hoists, ventilation fans, and dewatering pumps.
- Technical Data Sheet
- Outline Drawing
- Nameplate
| Delivered to | South Africa |
| Year | 2025 |
| Type | Oil immersed power transformer |
| Standard | IEC 60076-1:2011 |
| Rated Power | 20MVA |
| Frequency | 50HZ |
| Phase | 3 |
| Cooling Type | ONAN |
| Primary Voltage | 33 KV |
| Secondary Voltage | 6.65 KV |
| Winding Material | copper |
| Angular displacement | YNyn0 |
| Impedance | 10% |
| Tap Changer | OLTC |
| Tapping Range | ±10*1 |
| No Load Loss | 13.65 kW |
| Load loss at 75℃ | 132.8 kW |
| Total loss | 72 kW |
2.1 Core
The substation power transformer features a three-phase three-limb core, shown in the image as a technician removes the upper yoke. The core is made of cold-rolled grain-oriented silicon steel, which offers excellent magnetic properties along the rolling direction-ideal for transformer applications. To further reduce no-load loss, fully mitered (step-lap) joints are used to align the magnetic flux with the steel’s rolling direction, enhancing overall efficiency.
2.2 Winding
The YNyn0 power transformer adopts a continuous disc winding structure for both high-voltage and low-voltage windings. The continuous disc winding offers superior axial cooling performance and is especially suitable for high-current, high-capacity applications. Each winding undergoes strict insulation treatment, layer-by-layer drying, and dimensional control to ensure consistency and insulation strength.
2.3 Tank
The 20 MVA power transformer oil tank must undergo a pressure leak test before leaving the factory. All welds, flange joints, and bushing bases are designed to ensure long-term sealing reliability. Detachable plate-type radiators are adopted, and four 10-ton wheels are installed at the bottom for easy on-site movement. A manhole is provided on the side of the tank to facilitate internal inspection and maintenance of the equipment.
2.4 Final Assembly
1. The active part of the transformer in the picture has been assembled, and the tester is performing routine electrical performance tests.
2. The active part is being lifted into the tank.
3. Install other parts: oil temperature indicator, pressure relief valve, oil level indicator, gas relay, breather, earthing terminal, terminal box, Wheels(10T).
4. Pour insulating oil into the transformer tank to fill the internal space of the transformer and ensure that the oil level meets the requirements.
5. Due to the large area occupied by the radiator, it will be assembled before the test and then disassembled and packed for transportation after the test.
03 Testing
Test Procedure
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. Check of ratio and polarity of built-in current transformers
4. Measurement of Winding Resistance
5. Measurement of Core Insulation Resistance
6. Applied Voltage Test
7. Measurement of No-Load Loss and Current
8. Test on On-load tap changers – Operation test
9. Induced Voltage Test
10. Measurement of Short-circuit Impedance and Load Loss
11. Insulation of auxiliary wiring (AuxW)
12. Leak Testing with Pressure for Liquid-Immersed Transformers
04 Packing and Shipping
05 Site and Summary
This 20MVA-33/6.65kV step-down power transformer is made for hard mining work. It is strong, efficient, and easy to move. With IEC 60076-1:2011 compliance, optimized copper windings, and an advanced OLTC regulation system. It gives stable and flexible power for mining. The tank is strong, the radiators can be removed, and the wheels are heavy-duty. This makes it easy to use at remote sites.
It has low losses, high efficiency, and is easy to handle. This shows our goal to give mining customers equipment that meets standards and keeps work safe and productive. Mining in South Africa is growing fast. We are proud to support our partners with tailored power solutions that keep operations running smoothly today and for the future.
