Waukesha UZD Load Tap Changer Maintenance Guide

The Waukesha UZD is a load tap changer used in Waukesha power transformers. It is a high-speed resistance-bridging tap changer: transition resistors limit the circulating current while two adjacent taps are bridged, and the make-and-break arcing takes place in oil. The switching is done by a tap selector, which carries the arcing and the main current-carrying contacts on one moving arm, and a reversing change-over selector for plus and minus operation. The defining mechanical feature is that the arcing is carried on rolling contacts rather than a sliding or fixed arcing tip. The rollers distribute arcing wear over a broader surface, so inspection means assessing the wear pattern across the full roller travel rather than measuring a single arcing tip depth.

Roller arcing contact

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 arcing contact surface on the tap selector 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.

Main moving contact assembly

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 tap position. Elevated resistance measured while the arcing contacts are still serviceable points specifically to the main current-carrying contact or its stationary contact surface.

Reversing switch and upgrade option

The reversing change-over selector on the UZD is a single-pole, double-throw switch. Its moving contact wipes between two stationary reversing contacts, and the change-over happens at a point in the cycle where the contact is not carrying current. 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 moving contact and the two stationary reversing contacts, and note the wear condition as part of the maintenance record.

Current collector assembly

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. In the UZD the cast epoxy phase moldings, with copper rods embedded in them, act as the bushings that insulate the live current-carrying path from the grounded compartment structure. Any surface tracking or degradation on that insulation must be addressed before the transformer is returned to service.

Maintenance intervals

Waukesha sets the UZD inspection interval by operation count, not a calendar figure. The instruction book says to estimate the contact life, which should never be assumed higher than 500,000 operations, and divide it by five to set the interval between inspections. Heavy-cycling units reach that count sooner. (PRC-005 covers protection systems, not LTCs; see our PRC-005 guide.) Sample and test LTC oil at each maintenance interval for dielectric strength, moisture, and dissolved gas. Because the arcing happens in oil, acetylene and hydrogen are the gases to watch. A rising acetylene trend that outpaces the operation count is the signal that arcing wear is accelerating and the unit should be opened for inspection.