22 MVA 3 Phase Power Transformer-33/11 KV | South Africa 2025

High-performance 20/22 MVA transformers for South Africa, built for extreme altitudes and temperatures with stable, efficient, and reliable operation

01 General

1.1 Project Description

In 2025, two 20/22 MVA, 33/11 kV three-phase power transformers were delivered to South Africa for a substation project at an altitude of around 1,800 meters. The site conditions are not exactly mild-ambient temperatures range from -10°C to +40°C, and the reduced air density at this elevation makes heat dissipation less effective. Over time, that also means more thermal cycling, more mechanical stress. These factors aren’t secondary-they shape the design from the beginning.

Each unit is rated at 20/22 MVA, operating at 20 MVA under ONAN and increasing to 22 MVA when ONAF is activated, providing a margin for higher load demand. The voltage ratio is 33 kV / 11 kV with a Dyn11 vector group-a standard configuration, but one that ensures stable three-phase performance and reliable grounding in substation applications.

Under these conditions, some design choices carry more weight. Copper windings are used to improve current capacity and withstand short-circuit forces, while the impedance is set at 14.29% to limit fault current levels. In practice, the transformer can withstand 21 kA for 3 seconds on the HV side and up to 18 kA peak on the LV side during parallel operation. Not just nominal values-these are necessary margins for real grid conditions.

Voltage regulation is managed by a side-mounted vacuum OLTC with a ±8 × 1.25% range, allowing smooth adjustment under load without interruption. On the insulation side, mineral oil with a dielectric strength of ≥80 kV is used together with an optimized insulation structure. At higher altitudes, air insulation becomes less effective, so the design compensates through oil insulation and external clearances-keeping the overall insulation margin where it needs to be.

1.2 Technical Specification

22 MVA 3 phase power transformer specification and data sheet

1.3 Drawings

22 MVA 3 phase power transformer diagram and nameplate

02 Manufacturing

2.1 Core

The 20/22 MVA 3-phase power transformer core is made according to ISO 60076. The silicon steel sheets in the core meet the standard for electrical and magnetic performance. They give low core losses and reliable operation. High-quality core reduces energy loss as heat. Low core loss prevents overheating, extends service life, and reduces cooling system needs.

2.2 Winding

The coils are constructed from high-quality copper. They follow strict design requirements. The high-voltage Delta winding can withstand high phase-to-phase short-circuit currents. It has strong mechanical strength. The low-voltage star winding has a grounded neutral. The copper coils are tightly wound with controlled layer pressure. This ensures even current and good heat dissipation. The coils give reliable performance and long service life.

2.3 Tank

The step down power transformer has a one-piece oil conservator combining the switch and main tank compartments, separated by a partition with the switch mounted on the side. The tank passes a 30 kPa seal test for 24 h with no collapse. All materials and dimensions meet drawings, with deviations under 1 mm. Steel plates follow national standards and are free of deformation, rust, peeling, or dents. Welds are straight, smooth, uniform, and free of defects.

2.4 IP55 Terminal box

The terminal box is made of a 3mm thick steel plate and has IP55 protection, ensuring a strong structure and resistance to dust and water. It is equipped with winding and oil temperature indicators for real-time monitoring. A 230V heater is provided to prevent moisture, which is important even in South Africa, where many areas are dry, but highlands, rainy seasons, or coastal regions can cause condensation. An LED lamp turns on when the door is open, offering convenient inspection.

2.5 On-Load Tap Changer

The power supply transformer has a vacuum-type On-Load Tap Changer. It has 17 tap positions with 1.25% step regulation. The voltage range is 29,700/11,000 V (-10%) to 36,300/11,000 V (+10%). It gives precise voltage control during load changes. The OLTC works in manual and automatic mode. The OLTC supports both manual and automatic modes, can operate in parallel with other equipment, and offers remote control capabilities.

2.6 Final Assembly

After the transformer core and windings are assembled, the active part is placed in the drying oven. After drying, it is lifted and installed into the oil tank. This 3 phase power transformer has many accessories in addition to standard ones, such as the on-load tap changer, oil temperature indicator, winding temperature indicator, Buchholz relay, fan, and one-piece oil conservator. Additional accessories include the breather, high-voltage lightning arresters with supports, and 15 current transformers. After all accessories are installed, the assembly process is completed with oil filling and sealing, electrical connections and tests, and inspection of appearance and markings.

03 Testing

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 Testing Standard and Routine Test

Applied Standard

• IEC 60076-1:2011 Power Transformers – Part 1: General
• IEC 60076-3:2013 Power Transformers – Part 3: Insulation levels, dielectric tests and external clearances in air
• IEC 60076-7:2018 Power Transformers – Part 7: Loading guide for mineral-oil-immersed power transformers
• Other refer to technical regulation

1. Check of Core and Frame Insulation for Liquid Immersed Transformers with Core or Frame Insulation
2. Check of Voltage Ratio and Vector Group
3. Current Transformer Measurement
4. Measurement of Winding Resistance
5. Measurement of No-load Losses and No-load Current
6. Induced Voltage Withstand Test
7. Measurement of Impedance Voltage and Load Losses
8. On-load Tap Changers – Operation Test
9. Applied Voltage Test (AV)
10. Auxiliary Wiring Insulation (AuxW)
11. Seal Test
12. Oil Test

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04 Packing and Shipping

4.1 Packing

4.2 Shipping

05 Site and Summary