Viva Mexiko – Retrofit at a furnace transformer

Replacement of a de-energized tap-changer with an on-load tap-changer

Metaloides, S.A. de C. V., the only Mexican manufacturer of calcium carbide is located in Puebla, to the southeast of Mexico City. In order to supply its electric arc furnace, the company operates an old 20 MVA, 20 kV furnace transformer from 1967 which is installed on the floor above the furnace.

Originally manufactured by Ferranti Packard, it was refurbished by the Ohio Transformer company in 1998 at which time the original de-energized tap-changer was replaced by an RMV-II-1500S from Reinhausen Manufacturing (RM). This is a modified RMV-II-1500 without vacuum interrupters and without a reactor as transition impedance, which is why the tap changer can only be operated as a de-energized tap-changer, meaning without load. For this reason, Metaloides has only used the tap changer in one tap position, as a tap change operation requires about 10 minutes. However, due to an energy saving program planned for 2015, more frequent changes will be necessary in the future. The solution suggested by RM was the replacement of the de-energized tap-changer with an on-load tap-changer.

The initially obvious option of using a complete RMV-II-1500 was examined but rejected, in particular due to the high expenditures involved in the installation of a new reactor in the transformer tank. It was therefore decided that a high-speed resistor-type tap-changer would be used. The challenge: For this purpose an auxiliary tank for the new tap changer had to be designed for installation onto the transformer tank. Luckily, the responsible project team of the Reinhausen Group could fall back on positive experiences with auxiliary tanks from projects already carried out in South Africa. The VACUTAP® VV III was ultimately chosen. Metaloides was impressed by MR’s solution competence and placed the order for the retrofit.

Now things got specific: following the on-site inspection by the responsible specialist from Reinhausen Mexico (RMX), the technical formulation of the offer could begin. The retrofit experts of RM in Humboldt worked closely together with the Asset Management department in Regensburg with additional support from the MR Technology department. Everything was ready to go in November: representatives from RM, RMX and MR were all on site and carried out the replacement work together. Thanks to the exceptional cooperation of the international MR team, the tap changer retrofit could be handled as a turnkey project in which MR was also responsible for all adjustment work on the transformer, as well as for the oil handling and all electrical tests.

Calcium carbide is produced at temperatures of up to 2,200°C in so-called melting reduction furnaces. Switching off the furnace transformer therefore meant a production stop and the retrofit work had to be carried out in the shortest time possible. For this reason, the VV tap changer was already installed on trial into the prefabricated auxiliary tank and all necessary lead connections were prepared at RM in Humboldt. This ensured a smooth and efficient replacement before all of the required materials were delivered to Puebla. After the RMV-S was removed from the transformer in Puebla, the auxiliary tank could be mounted by means of an adapter plate and the VV tap changer could then be installed as planned. The most strenuous work was then carried out, namely to lay the new connecting leads in very little available space.


Following the successful tap-changer attachment, the motor-drive unit was mounted and the important interlock of the drive was commissioned for the star-delta reconnection of the VV tap changer. Finally, the transformer was filled with oil under vacuum conditions. The retrofit specialists naturally also mounted the transformer accessories, such as a new MPreC® pressure relief device and a maintenance-free MTraB® dehydrating breather.

Upon completion of the replacement work, the transformer was successfully tested and approved by the Reinhausen team for operation. Since December 2014, calcium carbide production has again been possible and around 15,000 tons have been produced per year.