Procedimiento técnico para el mantenimiento de transformadores de media tensión

In the industrial zones of Torreón, Saltillo, and Piedras Negras, where summer temperatures easily exceed 40 °C and the electrical demand of manufacturing plants operates at its limit, the transformer is the heart of the operation. A failure in this equipment not only represents a blackout; it means halted production lines, contractual penalties with automotive or metal-mechanical clients, and, in the worst-case scenario, a catastrophic incident within the plant.

Industrial medium voltage transformer maintenance is not an optional expense or a superficial cleaning task; it is a high-precision engineering procedure that requires qualified personnel, calibrated test equipment, and strict adherence to Mexican safety standards. In this technical guide, we will address, step by step, the professional protocol for intervening, testing, and certifying these critical assets of your electrical infrastructure.

At SINE Mexico, with over two decades of experience in the Industrial Electricitysector, we know that the key to operational continuity lies in scientific prevention. Below, we present the detailed technical procedure applied by our field engineers to guarantee the reliability of your power equipment.

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Table of Contents

• Introduction to medium voltage transformer maintenance
• Applicable regulations in Mexico (NOM-001-SEDE and CFE guidelines)
• Types of maintenance: Preventive, Predictive, and Corrective
• Step-by-step technical procedure for preventive maintenance
  • 1. Planning, clearance, and safe de-energization
  • 2. Visual inspection, general cleaning, and re-tightening of connections
  • 3. Physical, chemical, and electrical tests on dielectric oil
  • 4. Critical electrical tests (Megger, TTR, and Power Factor)
• Advanced diagnostic tests and infrared thermography
• Industrial safety measures and essential Personal Protective Equipment (PPE)
• Benefits of a periodic maintenance plan for industry in Mexico
• [Common errors in Mexican industry](#errores-comunes-en-la-industria mexicana)
• Real case: Rescue of a 1500 kVA transformer in Saltillo, Coahuila
• Professional checklist for transformer maintenance
• Frequently Asked Questions
• Conclusion and delivery of the technical maintenance report

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Introduction to medium voltage transformer maintenance

The medium voltage transformer (typically in ranges of 13.2 kV, 23 kV, or 34.5 kV in Mexican distribution networks) is equipment designed to operate continuously under severe environmental conditions. However, factors such as thermal overload, natural degradation of insulating paper, humidity, and contamination of dielectric oil drastically reduce its useful life if not intervened in time.

Industrial medium voltage transformer maintenance aims primarily to evaluate the condition of the solid and liquid insulation of the equipment. The insulation of an oil-filled transformer is a binary system: cellulose paper and mineral oil. While the oil can be filtered, dehydrated, or changed, the cellulose paper cannot be replaced without completely rebuilding the transformer. Therefore, timely diagnosis of the presence of water or combustible gases in the oil is vital to prevent catastrophic failure.

A rigorous maintenance program not only protects the physical asset but also ensures supply stability, preventing voltage variations that could damage sensitive machinery in the plant and guaranteeing compliance with safety requirements demanded by insurers and federal authorities.

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Applicable regulations in Mexico (NOM-001-SEDE and CFE guidelines)

In Mexico, the operation and maintenance of electrical substations and transformers are strictly regulated. Ignoring these regulations can result in severe fines from the Ministry of Labor and Social Welfare (STPS), loss of insurance coverage in case of an incident, or rejection of the signing of the UVIE verification report.

The main standards governing these procedures are:

1. NOM-001-SEDE-2012 (Electrical Installations - Utilization): Defines safety distances, grounding requirements, and installation conditions for transformers and their protections within the NOM 001 SEDE 2012 substations.
2. NOM-029-STPS-2011 (Maintenance of electrical installations in workplaces): Establishes mandatory safety conditions for performing maintenance work in substations, including the obligation to have written procedures, authorized personnel, and the use of specific personal protective equipment against electrical arc.
3. NMX-J-116-ANCE-2017 and NMX-J-308-ANCE-2020: Mexican standards that specify the quality parameters, sampling, and physical, chemical, and electrical tests that dielectric oil must meet (both new and in operation).

Standard / Norm	Main Focus	Application in Maintenance
NOM-001-SEDE-2012	Safety in electrical design and installation.	Verification of safety distances, physical grounds, and bushing connections.
NOM-029-STPS-2011	Safety in maintenance execution.	Clearance, lockout/tagout (LOTO) protocols, and use of dielectric PPE.
NMX-J-308-ANCE	Dielectric mineral oils for transformers.	Limit parameters for dielectric strength, interfacial tension, color, and moisture.
IEEE C57.104	Guide for the interpretation of dissolved gases.	Diagnosis of internal faults (hot spots, arcs) through gas chromatography.

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Types of maintenance: Preventive, Predictive, and Corrective

To structure an efficient asset management strategy, it is necessary to classify interventions into three well-defined areas:

Preventive Maintenance

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