Maintenance of transformer load tap changers (LTC)

Regulating transformer’s level of reliability

LTC maintenance is the basis for the regulating transformer’s high level of reliability. The background for maintenance recommendations is as follows:

Oil: For LTCs where oil is used for arc-quenching, the arcing at the arcing switch or arcing tap switch contacts causes contact erosion and carbonization of the arcing switch oil. The degree of contamination depends upon the operating current of the LTC, the number of operations, and to some degree the quality of the insulating oil.

Vacuum interrupters:  For LTCs using vacuum interrupters for arc-quenching, contact life of the vacuum interrupters and the mechanically stressed parts of the device are the key indicators for the maintenance recommendations. The overall performance of vacuum-type LTCs leads more and more toward maintenance-free LTC designs.

Maintenance and inspection intervals depend on:

1. Type of LTC,
2. LTC rated through-current,
3. Field experience, and
4. Individual operating conditions.

They are suggested as periodical measures with respect to a certain number of operations or after a certain operating time, whichever comes first.

Normally, maintenance of an LTC can be performed within a few hours by qualified and experienced personnel, provided that it has been properly planned and organized. In countries with tropical or subtropical climate, the humidity must also be taken into consideration. In some countries, customers decide to start maintenance work only if the relative humidity is less than 75%.

Economical factors are taken more and more into consideration by users of large power transformers in distribution networks when assessing the operating parameters for cost-intensive operating equipment.

While users are aiming at cost reduction for transformer maintenance, they are also demanding higher system reliability. Besides the new generation of LTCs with vacuum switching technology, modern supervisory concepts on LTCs (LTC monitoring) offer a solution for the control of these divergent development tendencies.

Today a few products are on the market that differ significantly in their performance.

A state-of-the-art LTC on-line monitoring system should include an early-fault-detection function and information on condition-based maintenance, which requires an expert-system of the LTC manufacturer. The data processing and visualization should provide information about status-signal messages, trend analyses, and prognoses.

Monitoring application is a judgment of transformer size and importance and of maintenance and equipment costs.

Refurbishment/Replacement of Old LTC Types

With regard to system planning of power utilities, the lifetime of regulating transformers is normally assumed to be 25 to 30 years.

The actual lifetime is, however, much longer. Due to economic aspects and aging networks, as well as the requirement to improve reliability, refurbishment/replacement is becoming a major policy issue for utility companies.

Refurbishment includes a complete overhaul of the regulating transformer plus other improvements regarding loading capability, an increase in insulation levels, a decrease in noise levels, and the possible replacement of the bushings and of the LTC or a complete overhaul of the LTC.

The replacement of an old risky LTC (for which neither maintenance work nor spare parts are available) by a new LTC may economically be justified, compared with the expenses for a new regulating transformer, even if the transformer design has to be modified for that reason.

The manufacturer of the new LTC must, of course, guarantee maintenance work and spare parts for the foreseeable future.

Reference // The electric power engineering handbook – L.L. Grigsby (Purchase hardcover book from Amazon)