Westinghouse URT Load Tap Changer Maintenance Guide

The Westinghouse URT is a reactance-type load tap changer, using a preventive autotransformer to limit circulating current, found on power transformers throughout the utility fleet. Like all in-tank LTCs, it operates in oil and its contacts wear with every tap change. The arcing contacts absorb the brunt of that wear, but the reversing switch, drive mechanism, and oil condition all require attention on the same maintenance cycle.

Maintenance intervals

Westinghouse based the URT inspection schedule on two factors, time and operation count. The instruction book calls for a full inspection of the tap changer at the end of the first year of service, then at least every fifth year. Where the unit taps infrequently, time is the dominant factor and the five-year cycle governs. Where it taps frequently, operation count governs: the first inspection should come at or before 40,000 operations, and subsequent intervals are set by the contact life remaining at that inspection but should never exceed 500,000 operations or five years.

The instruction book for the specific URT model governs. URT variants differ in contact geometry and wear limits, and the nameplate or instruction book number is the correct starting point for any maintenance interval or specification question. If the instruction book is not on site, it should be located before any internal inspection is attempted.

Oil testing runs on a separate track. Sample the LTC compartment oil annually on active units and compare dielectric strength, moisture content, and acid number to Westinghouse specifications. A unit that taps frequently may need oil service well before the contacts reach their wear limit.

URT contact design

The URT is a reactance-transition changer built from selector switches, which pick the tap, and spring-loaded transfer switches, which do the switching, with a preventive autotransformer limiting the circulating current while two taps are briefly bridged. Each transfer switch carries a main blade and an arcing contact. On a tap change the main blade opens first, then the arcing contact breaks the current in oil, so the arcing duty falls on the arcing contact and not on the main current-carrying blade. A separate reversing switch sets tap direction and operates far less often.

The preventive autotransformer must maintain winding integrity across its service life. An open or shorted preventive-autotransformer winding changes the circulating current and the interruption duty on the arcing contacts in ways that accelerate wear. If contact wear appears inconsistent with the known operation count, check the preventive autotransformer and its connections before assuming the contacts alone are the cause.

Contact inspection

With the LTC compartment drained, inspect the arcing contacts for erosion. Westinghouse calls for replacement when the arcing material has eroded to 1/16 inch thickness at any point on the surface of either the moving or the stationary arcing contact. Contact worn to that limit must be replaced regardless of where the unit falls in its maintenance calendar, because running worn arcing contacts increases the risk of a failed interruption or a contact weld on the next operation.

Look at the contact surface morphology beyond just thickness. Normal arcing contact wear produces a relatively uniform erosion crater. Uneven wear across the contact face, deep localized pitting, or evidence of material splash onto adjacent surfaces indicates abnormal arcing, possibly caused by a timing problem in the drive sequence or a preventive-autotransformer issue. These conditions warrant investigation before simply replacing contacts and returning the unit to service.

Inspect main contacts for surface condition and spring-loaded contact pressure. A main contact that is not seating with adequate pressure shows elevated resistance on a milliohm measurement after reassembly. Measure contact resistance on each phase before closing the compartment; a reading significantly above baseline for that contact assembly indicates an alignment or spring problem that needs to be resolved before the unit goes back into service.

The reversing switch changes tap direction and operates far less frequently than the arcing contacts, but it must be inspected at every maintenance visit. Reversing switch contacts that are pitted or have insufficient contact pressure will cause high resistance or arcing on the relatively infrequent operations they do perform, conditions that are harder to catch because they happen so rarely.

Drive mechanism

The URT transfer switch is spring driven, with the spring charged by the drive motor and released to snap the switch through its travel. Aside from replacing the arcing contacts, the main service item Westinghouse calls out is adjustment of the brake that controls that snap action, and a supplemental leaflet covers the brake adjustment. Inspect the mechanism for adequate lubrication at all pivot points and the spring guide, gear tooth condition, and correct latch engagement. Old lubricant dries into a varnish that adds friction throughout the mechanism. Clean all surfaces thoroughly before applying fresh lubricant of the type specified in the instruction book, and use the reducer lubricant grade and fill quantity the instruction book specifies.

Time the tap change operation from motor start to completion and verify it falls within specification. A slow tap change extends the bridging interval, which increases the energy the arcing contacts must absorb on each operation. If timing is slow after cleaning and lubrication, the spring has likely taken a permanent set and should be replaced.

Verify motor operation, brush condition on brush-type motors, and correct control voltage at the motor terminals. A motor that struggles to charge the spring or takes longer than specified to complete the charge cycle is a maintenance item, not a monitoring item, deferred motor work translates directly into mechanism timing problems.

Oil service

The URT LTC compartment is isolated from the main transformer tank. Do not mix the oils and do not use the main tank oil specification interchangeably with the LTC compartment. LTC compartment oil is managed and evaluated separately from main tank oil; keep the two systems strictly separate.

Sample oil before draining at each inspection. The sample documents the condition of the oil going out and gives you trending data across maintenance cycles. Dark or heavily carbon-loaded oil means the compartment walls and contact assembly should be cleaned before the new charge of oil goes in. Introducing fresh oil into a carbon-fouled compartment shortens the service life of the new oil immediately.

DGA sampling of LTC oil alongside the routine dielectric and moisture tests provides early warning of abnormal arcing beyond the normal tap change signature. Elevated acetylene in the LTC compartment oil, particularly when it is rising between samples, is a sign of an arc problem that warrants inspection before the next scheduled maintenance cycle.

High-ampere applications

The URT was applied across a range of transformer ratings, including high-current applications where the contact assembly carries substantially higher load current than standard ratings. On high-ampere units, contact wear accelerates and oil degradation from I²R heating at the contact interface is more pronounced. High-ampere contact kits use heavier contact material and larger current-carrying cross-sections to manage the additional thermal load.

Verify which contact kit applies to the specific unit before ordering replacement contacts. A standard-ampere contact kit installed on a high-ampere application will wear faster than specified and may not provide adequate current-carrying capacity under full load.