Mexican Standards (NMX) are voluntary standards prepared and endorsed by the Mexican Government and National Industry. They are the result of a consensus among the different industry parties, such as users, manufacturers, academia, as well as all related chambers and associations. The Mexican Standard NMX-J-284-ANCE-2018 covers the Specifications for Power Transformers and Autotransformers, establishing, in this way, a “parallel path” for all manufacturers to cover aspects of reliability, safety and performance of their products; while, at the same time, considering the specific needs of the national industry.

In this article we mention the main changes that the Standard has undergone in its most recent edition, and how these changes contribute to a better specification of transformers.

The first change is in the application field. The application field is amended considering that instrument transformers, mining transformers, and mobile transformers are not within the scope of this standard, so it is required separately to establish specific detailed specifications for these other products, and the second one are environmental guidelines. The minimum ambient temperature is now -10°C so that most of the national territory can be represented, which results in an optimization in the use of materials. 

In order for the standard to be in compliance with the trends in international standardization, in addition to meeting certain requirements of the National Power Industry (i.e. CFE), the following dielectric requirements have been adjusted:

• The 69 kV and 85 kV power transformer voltage levels, Class I, have been changed to Class II. This means carrying out the induced voltage test with partial discharge measurement, routine impulse test, in addition to performing other tests and meeting additional requirements.
• For each rated voltage, Basic Insulation Level (BIL) options haven been eliminated, leaving only the highest magnitude option. This in order to comply with the national practice.
• The maximum limit for the level of partial discharges, which are measured during the induced voltage test, has been reduced from 500 pC to 300 pC, also in order to comply with National Practices and International Standardization Trends.

The NMX-J-284-ANCE-2018 Standard has been revised and updated. It now contains significant changes with respect to its previous edition of 2012. These changes specify a more reliable, safe and with improved power transformer. For this review, two main philosophies were considered: being in compliance with industry practices based in the specifications of the main client; and following trends of international standardization.