THE OLD ADAGE THAT WHAT COMES AROUND GOES AROUND TENDS TO BE TRUE when you work around a power utility long enough. It's easy to criticize a problem when you're not responsible for the solution. Well, that was the case when I worked on a substation maintenance crew trying to help the Vermont Electric Power Co. Inc. (VELCO; Rutland, Vermont, U.S.) perform emergency installs of its so-called “Transportable” 50-MVA transformer. Years later, I found myself working at VELCO and responsible for overseeing installations of this transformer. This time, I was on the receiving end of the criticism and determined to turn the Transportable into a true mobile unit.

VELCO was incorporated in 1956, becoming the first transmission company founded in the United States. The company was formed by the Vermont distribution utilities to deliver and account for a new influx of power from the St. Lawrence hydro project in upstate New York. VELCO designs, plans, constructs, owns, operates and maintains the high-voltage (13.8-kV to 345-kV) transmission in Vermont.

In 1970, VELCO owned approximately 20 power transformers that fed their owner's subtransmission system. The 30-MVA, 40-MVA, 50-MVA and 56-MVA units stepped down the 115 kV to either 34.5 kV or 46 kV depending on interconnected companies. With most transformers being in the 10- to 15-year-old range, it was deemed prudent to purchase a spare unit that could easily be placed into service at any of the VELCO substations with voltage ratings of 115 kV to 34.5 kV and 46 kV.

The unit needed to be 50 MVA and easy to transport. General Electric-Pittsfield (Pittsfield, Massachusetts, U.S.) was awarded the contract to design and construct the transformer. Delivered on Oct. 1, 1974, the unit was commissioned for testing, leaving Vermont utilities feeling they had a reliable backup transformer.

The transformer sat ready for deployment in the center of the state until it was needed in May 1996, when a 50-MVA power transformer failed in Middlebury, Vermont. Crews were rallied to ready the Transportable to replace the critically damaged unit. The Transportable was dismantled, transported, reassembled, commissioned and put into service while the permanent unit was being repaired. After experiencing the extensive amount of work the Transportable entailed, the men who moved the unit renamed it the “Barely Transportable.”

On Labor Day weekend in September 2001, VELCO suffered the failure of another critical 50-MVA unit, again requiring the service of the Barely Transportable. The crew responsible for the emergency move was experienced and had moved the unit enough to be intimately familiar with the process.

The crew was pressured with an in-service deadline of the Tuesday morning following Labor Day, before customers suffered loss of electric service. The unit was pressed into service just in the nick of time, but not after many tense moments caused by transportation issues regarding permits, stubborn butterfly valves and trapped air in the gas-accumulator gage. The unit was successfully commissioned, but it was decided after this experience that the risk involved in putting the unit in service was too great and improvements had to be made.

The failed unit went to the repair shop, was rewound and returned to VELCO nine months later. During this time, a conceptual design was being developed to transform the Barely Transportable into a true mobile unit. The design gained the support of the VELCO board of directors and the concept became a project. Weidmann Technical Services Inc. (St. Johnsbury, Vermont) was awarded the contract to design and manage the project.

As the design began to take shape, there were requirements and restrictions on dimensions, weight and bus work configurations that were needed to accommodate installation of the unit in a substation with limited access.

The Vermont Department of Transportation was contacted for advice. To eliminate the need for engineering surveys and 14-day waiting periods for permits, the unit needed to weigh less than 150,000 lb (68,039 kg) for the truck and trailer combined, be less than 14 ft (4 m) tall and no wider than 12.5 ft (3.8 m). The required dimensions were easy to meet, but the weight posed a particular problem. To ensure quick mobilization, the unit was designed to weigh just under the 150,000-lb limit. Careful weight calculations left VELCO with a completed unit that, when combined with the truck to haul it, weighs in at approximately 148,000 lb (67,132 kg).

Maneuverability and high-voltage connections proved to be the biggest challenges. Limited space, tight access roads and other physical obstacles needed to be thought through. Drawings of each substation were reviewed with the turning radius of the trailer in mind. The resulting requirement was a trailer that could be kept as short as possible for maneuverability.

With the maneuverability issue solved, focus turned to the high-voltage electrical connections. A 115-kV bus was required to extend the connections from the H bushings and surge arresters to the rear of the trailer to allow the unit to be backed into place, either under or up to an existing 115-kV bus. To eliminate the need for portable switching devices and relay packages, the unit needed to be attached to the same points as the failed unit. The 115-kV connections could be attached and shielded if the unit could be backed into place, but the side-mounted X bushings were the challenge. Rather than build emergency low-voltage bus work, it was decided to use 46-kV cables to make the connection to the existing low-voltage bus. The 100-ft-long (30-m) cables are heavy, but manageable with cable trailers and some strong men to put them in place.

Other upgrades VELCO requested included a fresh coat of white paint to match the trailer, the replacement of all the original GE bushings while the unit was drained and the processing of the oil while it was pumped.

As soon as VELCO was comfortable the unit could be designed to be safely and easily put into place at all potential locations, the project was turned over to Weidmann Technical Services to complete.

An initial feasibility study determined that the transformer was a good candidate for a mobility upgrade by virtue of its dimensional and weight characteristics with respect to the desired shipping requirements. Following this, the first step in the design process was to gather specific data for the existing transformer in its current configuration. This was achieved by an on-site visit to document dimensional information as well as nameplate data from the pumps, fans and oil coolers. The factory transformer assembly drawings were unavailable. However, the transformer outline drawing provided the bulk of the critical data that was needed and the dimensions collected during the on-site visit filled in the blanks.

The next step in the process was to investigate various design solutions by creating layout drawings that would upgrade the mobility of the transformer without decreasing its performance. Some of the design issues were:

• Relocate and re-pipe the cooling equipment to be within the shipping “window” such that it can be permanently fixed to the trailer and still be functional.

• Customize the trailer to accommodate the main tank base support members and provide supports at the new cooler location.

• Verify that overall weight and axle-loading distribution are within acceptable limits.

• Maintain air-strike clearances.

• Design insulator supports at the rear of the trailer for the rigid high-voltage line leads.

• Provide a shipping location for the oil conservator tank.

Of course, there were many details that required some attention but the primary issues are discussed below.