Environmentally Friendly Transformers for America’s Largest Reservoir

by Christine Rüth

Siemens will deliver 14 power transformers to the Glen Canyon Dam power plant at Lake Powell in Arizona. A special feature of the transformers is that they contain a biodegradable insulating fluid.

In the near future, new transformers will replace units that were installed when the Glen Canyon Dam was built over 50 years ago. The power plant’s operator, the U.S. Bureau of Reclamation, is taking advantage of the equipment upgrade to switch to the use of transformers that contain esters as an insulating fluid and liquid coolant. Although mineral oils or silicone oils are normally used for this purpose, they are environmentally damaging and easily flammable. Esters, on the other hand, are biodegradable and have a high flash point. As a result, ester-filled transformers pose a much smaller environmental risk to the national recreation area along the Colorado River, where the power plant is located.

Specially Designed Transformers
Lake Powell has the largest surface area of any reservoir in the United States.The power plant in the approximately 200-meter-high dam has been generating around five billion kilowatt-hours of electricity every year since 1964. The plant’s eight turbines drive generators with a combined capacity of 1.3 gigawatts. Step-up transformers convert this electricity to the extremely high voltages needed for the transmission grid. The turbines and generators were upgraded in recent years as part of a modernization project. Now it’s time for the transformers to be replaced. The 14 new, ester-insulated transformers are rated at 125 megavolt-amperes (MVA) and have output voltages of either 345 or 230 kilovolts (kV).

Siemens has been producing power transformers that contain alternative insulating fluids since 2004, and the company is a world leader in this field. It is still the only supplier of such systems with a maximum voltage of 420 kV. Transformers have to be specially designed for the insulating fluids they contain, because esters’ properties differ greatly from those of conventional transformer oils. For example, esters are more viscous than conventional fluids, which means they dissipate heat more slowly. Such differences affect the cooling system’s design. Esters also behave differently than conventional oils with regard to their electric strength, i.e. their ability to prevent flashovers up to a certain voltage. The windings of the transformers and the design of the associated electric insulation have to be conceived in line with these properties.

Siemens operates a specialized research department that investigates and tests the thermal and electrical properties of esters and other insulating fluids. Among other things, this unit has researched esters for use in transformers for high-voltage direct-current (HVDC) transmission. Although ester-based technology is not yet being used for this purpose, Siemens could fill HVDC transformers rated at up to 500 kV with esters.