Westinghouse UTT Upgrade Series: Part 3 of 3

Upgrading your UTT, UTT-A or UTT-A70 to the UTT-B and RMT-1 Style Transfer Switches

In this white paper, we will discuss possible upgrades for the transfer switch assembly on the Westinghouse UTT Series load tap changer (LTC). These upgrades address areas of the transfer switch that are prone to failure due to overheating and carbon buildup and are applicable to the UTT, UTT-A, UTT-A70 and UTT-B models. Form, fit, and function of these upgrades are of the highest quality available.

Each phase of a 3-phase UTT LTC contains two transfer switch assemblies. Figure 1 below contains 3-D models of individual phase panels for each of the different UTT Series LTC models. Red arrows indicate areas where overheating and coking typically occur. These arrows also help visually identify areas where differences exist in the individual model designs. Additionally, the area of the phase panel that includes the transfer switches has been identified in a red rectangle.

Figure 2 below depicts the transfer switch portion of a UTT phase panel. The stationary and moving contact portion of the transfer switch assemblies are circled for ease of identification. You will note that two transfer switch contact assemblies are included for each phase, one on either side of each phase panel.  When standing in front of the open inspection door, the front transfer contact assembly on each phase is readily visible on the right hand side of each phase panel and can be easily inspected. The second transfer contact assembly for each phase is more difficult to inspect, as it is in the back of the compartment and is most often obscured from sight by the leads. In order to complete a thorough inspection, care must be taken to inspect all six transfer switch contact assemblies.

By upgrading the transfer switch contact assemblies of the UTT, UTT-A and UTT-A70 to the UTT-B design, the risk of overheating and carbon buildup in the stationary contact areas is fundamentally reduced. Figure 3 below depicts the transfer switch stationary contact assembly of a UTT. While slight differences exist between the UTT, UTT-A and UTT-A70 designs, the UTT design is sufficient to represent all three for the purpose of illustrating the improvements incorporated into the UTT-B design. Figure 4 shows the improved transfer switch stationary contact assembly of the UTT-B. The transfer switch stationary contact assembly design utilized in the UTT-B is the same design that was applied in the RMT-1. When comparing the two transfer switch stationary contact assemblies presented in Figures 3 and 4 below, the following design enhancements are readily evident:

  • Cross sectional area of the stationary contact fingers is significantly larger in the
  • UTT-B design, thereby resulting in a sizable increase in current carrying capability
  • Stationary contact plates of the UTT-B design are substantially larger, yielding more surface area immersed in oil; as a result, the amount of heat dissipated in the oil is much greater in the UTT-B design
  • A braided copper jumper has been incorporated into the UTT-B design, allowing the primary flow of current to be from the finger contact into the plate thus bypassing the static spring-loaded joint of the finger contact
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