The Waukesha UZD is a load tap changer used in Waukesha power transformers. It uses a transfer switch, selector switch, and reversing switch design with an oil-quenched arcing chamber. The defining mechanical feature of the UZD is a roller arcing contact at the transfer switch — rather than a sliding or fixed arcing tip, the arcing contact rolls along the stationary contact surface during the interruption sequence. This roller design distributes arcing wear over a broader contact surface than a fixed-tip arcing contact, and inspecting the roller contact requires assessing the wear pattern across the full roller travel range rather than measuring a single arcing tip depth.
The roller arcing contact is the primary wear component on the UZD. Inspect the roller for wear at the contact surface, checking for flat spots, surface erosion, and any evidence of the roller binding on its axle. A roller that has developed a flat spot from contact wear will not roll freely during the interruption sequence — it will slide across the stationary contact surface, concentrating wear at the flat spot location and producing an arcing pattern similar to a fixed-tip contact. Inspect the roller axle condition and verify that the roller spins freely by hand before reinstalling.
The stationary contact surface at the transfer switch must be inspected for the wear track left by roller arcing contact travel. The wear track will show as a zone of surface erosion and carbonization along the path the roller follows. Measure the surface profile depth at the deepest point in the wear track and compare to the manufacturer’s minimum remaining material specification. Even if the roller arcing contact itself is within wear limits, a deeply eroded stationary contact surface changes the contact engagement geometry and must be replaced.
The main moving contact assembly carries load current after the roller arcing contact completes the interruption sequence. Inspect the main moving contact for surface wear and verify that contact pressure against the stationary contact surface is adequate. An assembly with insufficient contact pressure produces elevated contact resistance at the transfer switch position — elevated contact resistance at the dwell position while the arcing contact is serviceable points specifically to the main moving contact assembly or the stationary contact surface at the main contact zone.
The reversing switch on the UZD uses a moving contact assembly that engages stationary contacts in the raise, lower, and neutral positions. An upgrade moving contact assembly is available for the UZD reversing switch that improves current-carrying capacity and wear resistance compared to the original design. If the existing reversing moving contact assembly is at or near wear limits and the transformer is expected to remain in service for many more years, the upgrade assembly is worth evaluating as a maintenance option. Inspect the existing reversing switch contacts — raise, lower, and neutral stationary contacts — and note the wear condition as part of the maintenance record.
The current collector assembly routes current from the tap winding connections through the LTC mechanism. It includes a bushing that passes through the LTC compartment wall. Inspect the collector assembly for contact surface condition and verify that the bushing shows no signs of insulation deterioration or tracking. The bushing is the primary insulation element between the current-carrying collector and the grounded LTC compartment structure — any evidence of surface tracking or degradation on the bushing insulation must be addressed before the transformer is returned to service.
UZD maintenance follows the manufacturer’s operation-count intervals with calendar intervals governed byNERC PRC-005For utility applications. Sample and test LTC oil at each maintenance interval for dielectric strength, moisture, and dissolved gas. The roller arcing contact design produces a distinctive dissolved gas signature — ethylene is the primary gas produced by arcing in oil. Trending ethylene levels between maintenance intervals provides a direct indicator of cumulative arcing energy and can identify when contact wear is accelerating above the baseline rate.
Southern Switch stocks UZD contact components including the roller arcing contact and reversing switch upgrade assembly. Contact us with your transformer nameplate and operation count for a quote and lead time.