Major rehabilitation of a 12.5 MVA power transformer

Project Summary

• Equipment: Three-Phase Oil-Immersed Transformer.
• Capacity: 12.5 MVA.
• Make: ALSTOM.

1. The Challenge (The Problem)

The equipment was in a critical condition that put the client's operational continuity at risk. The main alert was the tripping of the Buchholz relay due to unusual internal gas generation, pointing to a serious thermal fault.

Main findings after untanking:

• Hot Spots: Defective joints (cold solder) at the secondary winding taps and OLTC nodes.
• Critical Degradation: Crepe paper insulation carbonized by localized overheating.
• Technical Obsolescence: The original design prevented core-to-tank insulation resistance measurement, making preventive monitoring difficult.

2. Our Solution (Technical Intervention)

The Costel engineering team carried out an in-depth corrective and preventive maintenance protocol to return the equipment to its nominal design parameters:

• Rebuilding of Connections: The defective joints were removed and the secondary and primary taps were re-soldered, reinforced with L-shaped copper busbars to maximize conductivity and heat dissipation.
• Insulation Renewal: Complete replacement of the degraded paper with high-quality 55H crepe paper.
• Design Improvement: Modification of the core grounding system and fabrication of an external metal box to allow future measurements without internal intervention.
• Fluid Treatment: Application of high vacuum (0.8 mBar) to remove moisture and gases, followed by filling with treated dielectric oil.
• Industrial Finish: Mechanical cleaning and application of a double coat of white RAL polyurethane enamel.

3. Results and Testing (Guaranteed Success)

To validate the intervention, the transformer underwent a rigorous battery of tests under the NVC 3172:95 standard:

• Electrical Tests: Megger (5 kV), turns ratio (TTR) at all 33 tap changer positions, induced and applied voltage.
• Leak Tightness: 24-hour pressure test with no leaks detected.
• Certification: Physical and chemical analysis of the oil (Protocol #144449) with satisfactory results.

Conclusion: Thanks to Costel's specialized intervention, the 12.5 MVA transformer has returned to service with an extended service life, guaranteeing the stability of the power supply.

Asset Technical Data