Fig. 1

Rebuilt/Retrofit Transformers Lower the Cost of Factory Upgrades

Rush Island Energy Center’s use of retrofitted transformers offers insights for any plant seeking to increase capacity on a budget.

When the call comes from upper management to produce more energy or increase capacity, plant managers face the challenge of upgrading the infrastructure, and upgraded power transformers often represent the biggest ticket item. Adding to the expense is the fact that such change-outs usually require completely reworking the connections to and from the transformer. When spatial constraints that require rebuilding a new enclosure or pad mount are factored in, the prospect of increasing capacity can explode into a budget-busting endeavor.

In response, many plant managers have discovered the advantages of “new” retrofit transformers. These fully customized power transformers provide the benefits of increased power capacity while duplicating the form and fit of the originals. The option of a perfectly matched, plug-and-play transformer holds the potential for capacity upgrades that meet regulations, timelines, and budgets.

The experience of one major utility shows that plant managers can successfully balance the demand for upgrades against cost constraints by retrofitting.

In a retrofit, engineers must ensure all bus bars lineup with customer’s electrical connections to reduce installation time and complexity.

Since “first fire” over 40 years ago, management at Ameren Missouri’s Rush Island Energy Center in Festus, Mo. has run a tight ship. The plant’s two coal-fired generating units have often scored one and two in the nation for the lowest NOx produced by units without selective catalytic reduction, while producing 1,242 MW of electricity.

Here, the impetus for a transformer upgrade stemmed from a larger project that required more power for added in-house load. “We were upgrading our bottom ash subsystem, changing from a wet sluicing approach with bottom-ash clinker grinders to a submerged flight conveyor, and the existing auxiliary boiler stood in the way of the conveyor,” explains Herb Fischer, consulting engineer, Rush Island Technical Support. “So, we demolished the old aux’ boiler and put in a new one.”

The jump from 75 to the 250 horsepower needed for a forced draft fan on that boiler drove the need for the larger transformers. The load study called for upgrading the supply transformers from 1,000 kVA to 1,300 kVA.

On the face of it, ordering bigger transformers with larger secondary windings would seem simple enough, except that Fischer and his team faced the difficult task of fitting the new transformers into an existing cabinet with no leeway for extra girth. Rebuilding the enclosure and reworking the terminations would have pushed back the project deadline and led to a large cost overrun.

“There were several components involved in making a decision for the replacements, and cost and fit were high on the list,” continues Fischer. “For those reasons, we had to have a supplier that could handle this type of custom work, and having used ELSCO Transformers before, we figured that was the route to go. They built a number of units for us that continue to operate in our boiler circulation pump application.”

The project took place at a double-ended unit substation within the energy center. Only four days were allotted for complete removal of the old transformers and installation of the two 6,900 V to 480 V replacements.

“The enclosure was going to stay so the dimensions had to fit, and the transformers had to match up with the high voltage connection and the low voltage bus bar,” explains Fischer. “Not having to perform any field modifications was critical, so only a duplicate retrofit would do.”

Retrofit transformers represent a large cost savings for plant managers seeking to reduce capital expenses primarily because of the plug-and-play advantage of a perfect fit. Yet, without proper manufacturing controls, performance and reliability could end up sacrificed.

When it comes to the material used in the windings of a well-manufactured retrofit, copper is a superior conductor to aluminum because copper offers less resistance—and hence, less heat. Even the geometry of the windings makes a difference, with round-wound holding the advantage.

Attention to detail also pays dividends in the retrofit manufacturing process. Hand wiring yields the highest quality. Additionally, deburring the copper conductor helps ensure the insulation won’t be punctured and create a short-circuit.

Even insulation plays a major part in ensuring reliability. As daily temperatures can reach 200°C in a dry transformer, skimping on insulation can lead to disasters. Glastic fiberglass insulation or Nomex provide significantly greater protection from fires and short circuits than paper insulation.

A plug-and-play retrofit power transformer saves money by reusing existing cabinets and hardware.

“Ameren wanted top quality,” said Alan Ober, VP of engineering and manufacturing for ELSCO Transformers. “The dry-type unit we made for them incorporated all the characteristics of a well-manufactured retrofit so that we could give them a three-year warranty, where one year is more common.”

According to a DoE study, Large Power Transformers and the U.S. Electric Grid, “The average lead time between a customer’s large power transformer order and the date of delivery ranged from five to 12 months for domestic producers.” That would be too long for this project. Aside from having to meet the project’s stage gates, the window for an outage must compete with other projects. Miss it, and rescheduling can delay implementation for weeks and quickly inflate costs as contractors sit idle. For the Rush Island transformer upgrade, the window was Columbus Day weekend of 2017. Fortunately, retrofitting let them complete the job ahead of schedule

“We already had received the drawings, so it was easy on our part to build the transformers to spec and get them shipped out quickly,” explains Ober.

“An electrical contractor did the work and had no installation issues whatsoever,” adds Fischer. “ELSCO placed the bus bars where they needed to be and provided cabling to reach the existing terminations. It fit like a glove.”

After implementation, Rush Island ordered two more units that also had to be delivered on a short timeline, this time with different voltages.

To date, the utility reports it has had no issues with any of the transformers and that everything is functioning to specifications.

If you have any questions regarding upgrading transformers or electrical equipment, please feel free to call an application engineer from Electric Service Co. (ELSCO) at (800) 232-9002 or click here.

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