Application, Selection, and Design of Three-Winding Transformers in LNG and Petrochemical Facilities

Transformers are used in power systems to transform (step-down) voltages while maintaining the same frequency. Electrical engineers designing the power systems for a liquefied natural gas (LNG) or petrochemical facilities typically use two-winding transformers to feed multiple process units in redundant distribution schemes throughout the facility. This method is repeated further to derive different voltage levels to feed process loads. As the industry moves toward larger production facilities, which increase the power demand, multiple large two-winding transformers are utilized. This approach leads to an increase in substation footprint, switchgear equipment, power feeders, site works, etc. This article focuses on an alternative design using three-winding transformers for LNG and petrochemical facilities. The input and output of a three-winding transformer generally consist of one primary (H) input winding and two output windings defined as secondary (X) and tertiary (Y). The three-winding transformers are connected in a redundant configuration to feed multiple process units. In this article, different types of three-winding configurations and construction (radial or axial) methods are reviewed. In addition, this article addresses the application of tap changers, overall system performance, defining impedance parameters, and specification requirements.