[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

trend analyses, and prognoses. Monitoring application is a judgment of transformer size and importance
and of maintenance and equipment costs.
FIGURE 3.3.22 Overvoltage protection devices arranged within the arcing switch.
Nonlinear resistors are solutions for high-power transformers and for all transformers where the
service conditions would cause the spark gaps to respond frequently.
In the early stages, silicon-carbide (SiC) elements were installed. The specific characteristics of
this material did not allow full-range application. When high-power zinc-oxide (ZnO) varistors
came on the market, the application of these elements for overvoltage protection was studied in
detail with good results. For more than 20 years, ZnO varistors have solely been in use.
3.3.7 Maintenance of Load Tap Changers
LTC maintenance is the basis for the regulating transformer’s high level of reliability. The background
for maintenance recommendations is as follows:
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. 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 the type of LTC, the LTC rated through-current, the
field experience, and the 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.
The recommended maintenance intervals for an individual LTC type are given in the operating and
inspection manuals available for each LTC type.
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,
3.3.8 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. This overhaul should be
performed by specialists from the LTC manufacturer in order to avoid any risk when judging the condition
of the LTC components, when deciding which components have to be replaced, and with regard to the
disassembly and the reassembly as well as the cleaning of insulation material.
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.
3.3.9 Future Aspects
For the time being, no alternative to regulating transformers is expected. The LTC will therefore continue
to play an essential part in the optimum operation of electrical networks and industrial processes in the
foreseeable future.
With regard to the future of LTC systems, one can say that a static LTC, without any mechanical system
and consisting only of power electronics, leads to extremely uneconomical solutions, and this will not
change in the near future. Therefore, the mechanical LTC will still be used.
Conventional LTC technology has reached a very high level and is capable of meeting most requirements
of the transformer manufacturer. This applies to the complete voltage and power fields of today,
which will probably remain unchanged in the foreseeable future. It is very unlikely that, due to new
impulses given to development, greater power and higher voltages will be required.
Today the main concern goes to service behavior as well as reliability of LTCs and how to keep this
reliability at a consistently high level during the regulating transformer’s life cycle.
At the present time and for the foreseeable future, the proper implementation of the vacuum switching
technology in LTCs provides the best formula of quality, reliability, and economy achievable toward a
maintenance-free design. The vacuum switching technology entirely eliminates the need for an on-line
filtration system and offers reduced down-times with increased availability of the transformer and
simplified maintenance logistics. All this translates into substantial savings for the end-user. Consequently,
today’s design concepts of LTCs — resistance- and reactance-type LTCs — are based more and more on
vacuum interrupters. The vacuum switching technology — used in OLTCs — is in fact “state of the art”
design today and tomorrow.
Another target of development is the insulation and cooling media with low or no flammability for
regulating transformers, mainly relevant in the field of medium-power transformers (