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FAQ on OLTC
Design and Engineering:
1. What are the selection parameters of a tapchanger?
2. What are the advantages of vacuum type OLTC?
3. Whether OLTC conservator can be at a different height compared to main conservator of
transformer?
4. What is the basic difference between Transition Resistance and Reactor type OLTC?
5. What is the need for Tie in Resistors?
6. Does the OLTC used in the HVDC network transformer needs special care?
7. Does the Buchholz relay and Oil surge relay works in a same principle?
Erection, Commissioning and Service issues:
1. What preventive maintenance measures can be used for improvement in life performance of
tapchanger?
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Design and Engineering:
1. What are the selection parameters of a tapchanger?
MVA rating of transformer, voltage class, Vector group, Location of tapping, Rated current and step
voltage, number of steps, winding configuration, voltage variation, overloading requirements and
application are some of the major parameters for selection of a tapchanger.
2. What are the advantages of vacuum type OLTC?
New generation tap changers utilize vacuum technology. Unlike traditional solutions, electrical arc
quenching takes place in a vacuum environment and not in oil. This substantially reduces contact
wear, thereby increasing life of contacts. Also arcing does not pollute the insulating oil as the arcing
takes place in vacuum bottles. The benefits of using the vacuum tap-changer are:
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Lower life cycle costs, less maintenance needs, and increased time in operation because of
a radical reduction in contact wear.
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Routine inspection takes place in a cleaner environment which reduces downtime.
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Reduced sensitivity to moisture in the insulating oil due to the absence of polluting by
products such as carbon.
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Switching characteristics of the tap-changer does not depend on the status of the
insulating oil.
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Reliable due to redundant contact system.
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Fully interchangeable with existing installations.
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Possible to use alternative insulating fluids.
3. Whether OLTC conservator can be at a different height compared to main conservator of
transformer?
Many utilities have a practice of keeping height difference between main conservator and OLTC
conservator.
With this practice OLTC conservator is kept near to OLTC at reduced height than the main
conservator. A pressure difference depending upon the height difference appears on OLTC diverter
and all sealing joints of OLTC for its life time when conservators are placed at different heights,
which OLTC is supposed to withstand (design pressure of diverter cylinder and such sealing joint is
higher) To avoid pressure differential, OLTC conservator may be placed at the same height as of
main conservator. In case the OLTC conservator is at a lower height, any possible leakage into the
OLTC diverter switch, through sealing gaskets will lead to flow of oil from main tank to OLTC which
will eventually lead to over flow of OLTC conservator indicating leakage in OLTC, a blessing in
disguise!
The choice of location of OLTC conservator should be left to manufacturer’s standard practice.
4. What is the basic difference between Transition Resistance and Reactor type OLTC?
The difference depends on which one is used as transition impedance to control the circulating
current during the tap change operation i.e. in Resistance type OLTC, a resistor with suitable ohmic
values are used as transition impedance where in the reactor type OLTC a preventive auto
transformer will be used as transition impedance.
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5. What is the need for Tie in Resistors?
In a Reversing / Coarse-fine tapping arrangement, during the tap selector change over operation
the tap winding will temporarily disconnected from the main winding and the potential of the tap
winding floats. The floating potential of tap winding is determined by the voltages of the adjacent
winding and as well as the coupling capacitance to these windings and to the ground parts.
In such a case, discharge between the opening and closing contacts occur during the operation of
change-over selector. In order to avoid the difficulties in regard to the dielectric stress and
formation of gases which could arise if the values reach the admissible limits, one of the methods
to overcome the problem is to use Tie in Resistor.
6. Does the OLTC used in the HVDC network transformer needs special care?
Yes, the OLTC needs a special care because the transformer in the HVDC station including the OLTC
is subjected to winding currents that more resemble square waves than sinusoidal waves due to
the effect of converter system. These square waves have a large impact on the switching stress
which has to be controlled by diverter switch.
7. Does the Buchholz relay and Oil surge relay works in a same principle?
No, Buchholz relay and Oil surge relay works in a different principle. Buchholz relay is gas operated
relay which is actuated by the gases flowing through and accumulated in buchholz relay. Oil surge
relay is actuated when the flow rate of oil exceeds the preset value.
Erection and Commissioning:
1. What preventive maintenance measures can be used for improvement in life performance of
tapchanger?
Relevant OLTC manufacturer’s documents should be referred for the maintenance schedule.
Transformer oil in diverter switch is contaminated by electrical arc during OLTC operation and
therefore depending upon number of operations one has to carefully decide on period of oil
sample testing. Filtration of oil or replacement is to be decided based on test results. BDV, PPM
Tandelta and sludge content value shall be guiding factor to take decision. Tapchanger oil can be
filtered with the use of on line filtration device when shutdown is not possible.
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