Customization of the trailer was a joint effort between Weidmann and the trailer manufacturer, Talbert Manufacturing Inc. The trailer design began as a standard 55-ton four-axle trailer and was modified to suit this application. For instance, Talbert designed the main weight-bearing members of the trailer to align precisely with the four large steel transformer tank base supports. The transformer was later welded directly to these members. Supports for the relocated oil coolers and conservator tank were designed by Weidmann and were welded to the customized trailer by Talbert. The trailer has a removable fourth axel for increased maneuverability and a full hydraulic system for operating the tongue of the trailer that can either be run off the truck's hydraulic system or from a “pony motor” located on board the trailer. The trailer was purchased through a local dealer so that the dealer would be familiar with the unit when future service work was required.

Cooler relocation was the most significant design challenge. Relocating and re-piping the coolers requires knowledge of the oil-flow rate characteristics of the pumps and coolers. To minimize additional head losses, often referred to as pressure drop, resulting from the “remote” cooler location at the rear of the trailer, the piping had to be designed appropriately. Weidmann acquired the head-flow curves for the existing pumps. This, along with Weidmann's knowledge of the head losses associated with the General Electric cooler design, enabled suitable pipe sizes to be selected to minimize these losses. If the piping was improperly designed, and head losses became significant, the coolers might lose efficiency, and thus, the transformer would no longer behave thermally as it did in its original configuration. The concern of running rigid piping the length of the trailer was solved with the utilization of rubber expansion joints precisely located in the piping system.

The elliptical conservator tank (atmoseal) is located outside the shipping window while in its operational position. It extends beyond both the shipping height and shipping width profile. Several ideas were considered to eliminate the need to remount this conservator tank each time the transformer is commissioned. One concept was to convert the transformer to a nitrogen blanket system and thereby eliminate the need for a conservator tank. This was not possible because of the lack of sufficient would-be gas space available above the top oil outlets at the top of the tank wall. A second option was to relocate the tank and raise it by means of an onboard mechanical or hydraulic system each time the transformer was commissioned. This was determined to be impractical due to costs and other design considerations, such as shipping forces. Ultimately, it was decided to simply provide a shipping location at the rear tag of the trailer, that being the removable fourth axle. The conservator tank installation is a fairly simple procedure and doesn't require that oil be removed, so the best solution in this case was the most simplistic one. While remounting of the conservator tank is still required, it is no longer necessary for VELCO to provide separate trucking for this component, as had been the case prior to this project.

The manufacturing process began with the trailer production at Talbert's factory. The trailer was custom built per detail drawings provided by Weidmann. Once the customized trailer was fully assembled and painted, it was shipped to VELCO's substation where the transformer was being stored.

VELCO provided a crane and personnel to place the transformer in the precise location as specified by Weidmann. The transformer was then permanently welded to the trailer. The transformer and trailer assembly, along with all the coolers and other loose components were shipped to a transformer service shop, Transformer Service Inc. (TSI; Concord, New Hampshire, U.S.). Under the direction of Weidmann, TSI painted the transformer and performed final assembly of the coolers and related piping while VELCO installed the station service power equipment.

To enhance the portability of the transformer and reduce the cycle time for emergency restoration, the mobile transformer was equipped with onboard station service. This station service was designed by Weidmann-ACTI according to VELCO's specific requirements.

The primary three-phase power source was the 13.8-kV tertiary voltage terminals from the mobile transformer. Passing through current-limiting fuses, these lines fed two main circuits: the breaker panel and the protection cabinet. The 13.8-kV three-phase feed passed through a pole-mount-type distribution transformer to be stepped-down to 240-V/120-V four-wire, which feeds the station service electrical equipment. All cabinets were made of stainless steel and built to NEMA 4X specifications.

One important concern in the physical layout of the equipment was insulating the live components from ground potential. To this end, the outdoor cable connecting the mobile transformer tertiary voltage terminals to the distribution and potential transformers was a shielded conductor, suitably insulated for 15-kV use with a PVC jacket. These cable runs were then enclosed in rigid steel conduit. Also, all cable terminations were insulated with polymeric 15-kV class-insulating sleeves for outdoor use. Furthermore, the bushings of the distribution and potential transformers were insulated. This was accomplished using 15-kV class elastomeric insulating bushing “boots.” Finally, all 240-V/120-V external wiring runs were enclosed in rigid stainless-steel conduit.

The transformer is truly mobile now, capable of being pressed into service at a moment's notice. VELCO estimates the installation time to be less than 12 hours now, excluding travel time. When the mobile unit is called upon, the substation crew responsible for the work will have considerable more confidence in the equipment and their ability to restore power in a timely fashion. In addition, redesigning and refurbishing the unit was done at about one-third the cost of a new transportable with the same options.

Jeffery Wright is the manager of safety and asset maintenance for VELCO. He obtained an associates degree in accounting from Southern New Hampshire University before joining Central Vermont Public Service Co. in 1989 as a substation electrician. In 1997, he transferred to VELCO, working as a system operator until 1999, at which point he assumed his current position of managing the maintenance of VELCO's transmission assets. He is currently responsible for all substation and transmission maintenance activities, right-of-way management, facilities and fleet maintenance, as well as for the health and safety of the VELCO workforce. jwright@velco.com