In July 2015, the US Bureau of Reclamation awarded a US$37M contract to Yellowstone Electric Co from Billings, Montana to replace 50-year-old single-phase transformers at the Glen Canyon hydro power plant. The plant is located at the toe of the dam which, at a height of 221m, is the second highest concrete-arch dam in the US. While eight generators are powered by 245,000hp turbines and generate 5BkWh of power annually for 5.8M customers.

Having reached the end of their service life, replacement of the 12 transformers at Glen Canyon provided the Bureau of Reclamation and Yellowstone Electric the opportunity to ensure sustained power quality and delivery; as well as utilising environmentally-friendly ester-insulated technology – a first for Reclamation for transformers of this size filled with natural oils.

In late 2015, Siemens was named as the transformer technology provider for the project and has been contracted to deliver 14 transformers for the hydroelectric dam, 12 for immediate operation and two spares. The ester fluid transformers will be supplied by the company’s factory in Linz, Austria, and are rated at 125MVA (ten units designed for 345-kV and four units for 230-kV). They will connect Glen Canyon’s generation units to the transmission grid and increase transmission power capacity to provide stable and reliable electricity to customers. Siemens will begin delivering the transformers in late 2017 and the units are expected to be installed and begin operation in 2018 and 2019.

"Yellowstone Electric is very proud to have been selected by the U.S. Bureau of Reclamation to execute this project with Siemens. Together, Yellowstone and Siemens are partnered to provide the Bureau with the latest in environmental technology to solve a potential problem that has existed on the Colorado River since 1964," said George Haddenhorst, vice president of Yellowstone Electric Co. "This project will showcase how environmental concerns can be eliminated using the best currently available technology, while at the same time, upgrading system capacity and reliability."

"Hydroelectric generation is our nation’s largest producer of renewable power, so we need to ensure the systems that support this integral resource are operating efficiently and reliably while keeping environmental and sustainability concerns at the top of our minds," said Richard Boyd, vice president of Siemens Transformers US. "Our goal in working with the Bureau of Reclamation and Yellowstone Electric Co was to address these issues, and we’re proud that our new ester fluid transformers will enhance the integral power infrastructure for the dam and do so in the most environmentally conscious way possible."

Ester insulation

Sustainability and biodegradability are significant concerns for any modern power operator. Ester-insulated power transformers, derived from alternative oils, offer significant environmental benefits to the petroleum-based mineral counterparts typically used in power generator step-up (GSU) transformers. The ester fluid is fully biodegradable, and operates at a much higher flash point which results in a lower fire risk

As seen below, ester fluid has a fire point which is double that of mineral oil. Natural ester fluid is classified by Underwriters Laboratories and FM Global as a "less flammable fluid", and is also classified as a K Class fluid (low-fire hazard liquid.) With these improved fire safety parameters, installation of the transformers is simplified with respect to fire suppression systems or wall clearances.

  • Ester (K-fluids): Flash point [oC] min. 260; Fire point [oC] min. 300
  • Mineral Oil: Flash point [oC] min. 150; Fire point [oC] min. 170

In addition to its fire-resistant properties, ester fluids can dissipate in water quickly if necessary. The natural ester liquids, since they are made from plant seed oils, are processed from their naturally saturated condition (chemically stable but high viscosity) into single, double and triple unsaturated fatty acids (lower viscosity but unstable in oxidation). Therefore, the fluid remains stable for electrical operation in the sealed transformer but quickly breaks down (oxidises) if there is spillage into the external environment (see chart below.) It can be seen that ester fluids will dissipate in the environment in a matter of a few days, whereas mineral oil would have essentially unlimited residency if spilled into waterways.

Research has shown that biodegradation of esters is much better compared to mineral oil or silicon liquids, and flash and fire point analyses demonstrate the value of alternative insulating liquids.

Siemens began designing and manufacturing distribution transformers more than 30 years ago and small power transformer units were insulated with ester oil soon after. As rating grew larger, in addition to synthetic ester research, the development of natural ester-filled transformers was extended to larger MVA ratings. Along with the higher flash point, these transformers allow for a smaller footprint, eliminating rock, grating and support steel along with a shorter bus duct length. In some cases cost savings with ester-filled transformers can reach close to US$400,000 compared to traditional transformer technology. The life span can also be longer as the polymerisation of cellulose happens more slowly when using ester instead of mineral oil.

Ester oil transformer technology is becoming more common in the US but it already has a significant footprint in the global power marketplace. For example, Siemens partnered with the Letsi hydro power plant in Sweden to use ester-filled transformers as replacement of 40-year-old oil-filled transformers to improve reliability and ensure environmentally-friendly operation.

Transformer technology expansion

Siemens’ transformer technology spans much further than environmental benefits. The company is continuing to advance with a portfolio of solutions to prevent failures, protect assets and react in cases of emergency to address resiliency and power security challenges for the US market. The solutions include:

  • New mobile resiliency transformers that allow utilities to react rapidly and replace units within days or energise within a few hours rather than weeks in times of major substation events like extreme weather.
  • Bullet-resistant transformer protection systems which can be installed on new or existing units and help protect the power grid against physical security threats.
  • Transformers specifically designed for geomagnetically-induced current (GIC) resistance can protect the power grid against solar storms that can result in widespread blackouts.

Finally, a broad portfolio of preventive service measures offered by Siemens assists transformer operators in predicting potential failures and taking precautions to minimise operational risks. Monitoring, condition assessment, retrofitting and repair can enhance grid resiliency and advance transformer investment and technology into the future