GSU lease transformer. Courtesy of Siemens Energy.
GSU lease transformer.

Fast-Tracking Transformer Replacement

Oct. 24, 2023
Prolonged heatwaves and record breaking high temperatures are stressing the power grid’s most critical elements.

This has been one hot summer, and it’s extended into fall. September has seen continuous record setting high temperatures and unprecedented electricity consumption. The Electric Reliability Council of Texas (ERCOT) reported a peak load of 82,705 megawatts (MW) in September, but it’s more complicated. Epic customer loads resulted in ERCOT issuing an Energy Emergency Alert 2, which meant there was less than 1,750 MW of reserve power.

ERCOT hasn’t been alone facing this freaky weather. Extreme temperatures are occurring throughout the northern hemisphere, and they are taking their toll on everyone including the power grid. Unparalleled temperatures diminish the capability of power generation including renewables. They also reduce transmission lines and distribution circuits efficiencies by raising conductor resistance. Heat related equipment failures can result in outages, and transformers are one of the grid’s most vulnerable devices.

The Weakest Link    

According to DOE (Department of Energy), more than 90% of consumed power passes through high-voltage transformers. That makes them one of the most critical elements on the power grid, but they are also very susceptible to prolonged heatwaves. Keeping it simple, transformers produce a lot of internal heat as they step-up or step-down voltage.

Doing that when ambient temperatures are high adds can be a problem. Transformers are designed with sophisticated cooling systems to dissipate the internal buildup of heat. The cooling system design usually takes advantage of the overnight drop in ambient temperatures to catch up with dissipating heat. It’s a rest period when loads drop, and the ambient temperatures are cooler.

With extended heatwaves, it just stays hot. There isn’t an overnight temperature drop, and the customer’s cooling loads remain high. In other words, there is no overnight relief for heat-stressed transformers. Thanks to global warming these heatwaves are expected to continue while increasing in frequency and intensity. This double whammy is expect to increase transformer failures and associated outages.

If the failure is a distribution transformer, the outage will affect a few customers for a few hours. But as the size of the transformer increases so does the customer number and the duration of the outage. Large power transformers (LPTs), units above 100 megavolt amperes (MVA), would affect large numbers of customers and the power outage could extend into weeks. The effect of one LPT is a major concern, but numerous failures would be catastrophic.

Experts are concerned that our power grid is aging, and it gets more disturbing when they talk about the age of the grid’s LPTs. DOE estimates the average age of these LPTs is  40 years. That’s an average age, which implies that about half of these transformers are over 40 years of age. What makes this more concerning is the expected operational life of an LPT is also 40 years. That infers about half the transformers on the grid have reached or exceeded their life expectancy. It’s not a good feeling especially when we consider that continuously stressing the elderly is not a great idea. It’s also upsetting because LPTs are critical to the nation’s power grid.  

Standardization vs Customization    

DOE has said the loss of critical LPTs could disrupt electricity services to large parts of the country for unacceptable periods of time. They proposed stakeholders evaluate the establishment of a “Strategic Transformer Reserve” for LPTs. There were also proposals encouraging utilities to get together and standardize their transformer specifications for the purpose of sharing LPTs. Recently, there have been several mutual assistance programs started to share equipment such as STEP (Spare Transformer Equipment Program), SPAREConnect, and RESTORE (Regional Equipment Sharing for Transmission Outage Restoration).

Mobile transformers are another option for speeding up reaction times. They have proven to be essential for utilities when it comes to fast response to transformer outages, but their size and weight have been a limiting factor when it comes to deployment. That’s changing, however, with the adaptation of modern materials and advanced designs. The marketplace for mobile transformers is growing as are the number of suppliers. Delta Star, Siemens Energy, GE, Hitachi Energy, and Southern States are a few of the companies providing mobile transformers.

Speeding up Deployment    

Siemens Energy has been attracting attention with their innovative approaches for mobile transformer applications called the “Multi-Voltage Pretact Mobile Resilience Transformer.” According to Siemens Energy, these mobile units are designed for, “ease of transportation, flexibility to system conditions, and minimization of installation time.” By using single-phase transformers the mobile units are compact, which minimizes transportation limitations. Once onsite they can be configured into three-phase units with cabling, quick-connect cooler piping, and conservator tanks.

In addition to mobile transformers Siemens Energy offers another option for utilities and renewable generation facilities. Siemens Energy’s brochure leads off by saying, “Stored in the U.S. – available on demand.” That refers to a GSU (generator step-up unit) that can be leased. Talking with Siemens Energy U.S.’s Scott Gray, Technical Support Manager, and David Calitz, Transformer Specialist Engineer, highlighted some interesting facts about the GSUs and the leasing program.

Gray started off the discussion saying, “The idea of leasing a smaller (70 MVA) mobile transformer isn’t a new one, but taking a multi-voltage Pretact 250 MVA GSU transformer, and making it available as a leased unit was a pioneering step for Siemens Energy. The leasable GSU was delivered to the U.S. in 2019. Shortly after, Siemens Energy began evaluations with multiple customers, working together closely studying the units, the ratings, and the application. Then in 2021, a GSU at a combined cycle plant in Ohio failed.”

Gray continued, “It was determined that the rapid response leased GSU was a compatible replacement for this failed GSU. The Pretact Mobile Resilience Transformer was leased and deployed. The easy-to-install Pretact technology resulted in the facility being returned to service in less than two months, which was much faster than the typical replacement scenario.”

Calitz explained, “The GSU transformer application utilizes three single-phase modular units for compactness with plug-and-play bushings and cable connections for operational flexibility. It also takes advantage of high temperature insulation material (aramids), to make the units as lightweight as possible. Each single-phase unit is rated 83.3 MVA, which allows the mobile unit to be configured to replace GSUs up to 250 MVA. In the voltage category, they have multi-voltage ratings on both the high and low voltage terminals for more flexibility. Siemens Energy has also made the units environmentally friendly by filling them with biodegradable synthetic ester insulating fluids. It reduces environmental contamination risks during transport and operation.”

Gray pointed out, “Siemens Energy saw a need for leased transformers to fill the gap for users without spare units. The current solution is a single unit, but Siemens Energy is evaluating an enhanced portfolio to serve customer needs. The leased GSU was initially deployed for 10 months beginning in 2021. Just as soon as that deployment ended, another customer leased the unit. Today, the unit is on its third deployment, and it has been in service continuously since 2021. The product has proven to be a valuable resource for the lessees. The current solution is a single unit, but Siemens Energy is evaluating the program to determine if more leasable units should be built and offered to the marketplace.”


When it comes to fast replacement of an LPTs, it’s all about flexibility. In early 2022, the prototype “flexible LPT” was commissioned and placed in service. This first-of-its-kind transformer is funded by DOE’s TRAC (Transformer Resilience and Advanced Components) program and being managed by NETL (National Energy Technology Laboratory). The “flexible LPT” was designed and built by GE and Prolec GE.

It’s designed to be adaptable for a wide range of voltage ratios and impedance levels. This flexibility can cut down on manufacturing costs and time. It also does away with the need for customized, one-of-a-kind LPTs. The protype was installed at a Cooperative Energy substation in Columbia, Mississippi where it’s operation is being evaluated.

There is no easy solution to this issue. Experts agree, however, on one thing when it comes to LPTs and the growing risk of failure, it’s important to perform a risk analysis on each of the LPTs on the grid. Stakeholders should determine the criticality of each unit, its health, and how fast it can be replaced. Is there a spare available, or what about a parallel LPT, and how much of the load can it be expected to pick up?

LPTs are assets when they operate correctly, but quickly become a liability when they aren’t. Keeping them in the asset column has been complicated by climate-change-induced extreme events. To battle global warming we need to take advantage available. Being able to lease an LPT is an innovative step in the right direction as is the “flexible LPT” project. Standardization of LPTs is another step that should be taken advantage of to make sharing units between utilities easier. It’s an ambitious task, but it’s necessary for grid resiliency!

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