Transformer Uprating with Nomex® Insulation Yields Big Benefits
After a 9-MVA mobile transformer failed during scheduled maintenance of a municipal substation’s transformer, Niagara Mohawk Power Corporation (NIMO) decided to turn failure into success by opting for a transformer uprating with DuPont™ Nomex® brand paper and pressboard. This transformer uprating strategy provided a sound economical alternative to a repair in kind or the purchase of a new, larger unit—and delivered added benefits.
NIMO awarded the contract to Southwest Electric Company of Oklahoma City, Oklahoma, a leader in the electrical service industry for more than 60 years.
Because a significant transformer uprating was required—9 MVA to 12 MVA—Southwest Electric Company used a hybrid insulation system. The cooler areas in the transformer were insulated with traditional cellulose material while Nomex® paper and pressboard were used in the hot areas. Specifically, Nomex® insulation was used for conductor wrap, radial spacers and axial sticks, resulting in a unit with a 95°C average winding rise.
Increased capacity and versatility: By uprating from 9 MVA to 12 MVA, NIMO increased not only the mobile unit’s nameplate capacity but also its versatility. After the transformer uprating with Nomex® paper and pressboard, this mobile unit is better able to cover a more complete array of emergency and scheduled maintenance situations.
Improved reliability: Its remarkable tensile strength means that Nomex® insulation is exceptionally resistant to tearing. In addition, transformer windings insulated with Nomex® remain tight and far more resistant to ordinary and extraordinary stresses because of the exceptional compression characteristics of Nomex®.
Longer service life: The superior mechanical toughness of Nomex® paper and pressboard enables windings to withstand the most severe mechanical shocks, a common occurrence for mobile units that are hauled and jostled over all types of terrain. And, at typical operating temperatures of liquid-immersed transformers, Nomex® will not become brittle. Even at temperatures up to 200°C, Nomex® has an infinitesimal thermal degradation factor which makes it a far superior material than cellulose.
Materials Selected and Why
Based on prior experience, NIMO understood the integral role that Nomex® paper and pressboard plays in transformer uprating. Likewise, Southwest Electric Company, which had already redesigned several units with Nomex® insulation, was well aware of the benefits, especially the increased versatility and added reliability it provides at a lower cost. The fact that Nomex® insulation maintains its integrity under harsh conditions was also important because these mobile units are subjected to extreme mechanical forces.