On-LoadTap-Changer

On-Site Workshop on

TransformerOn-LoadTap-ChangersErection, Commissioning and Service issues:What preventive maintenance measures can be used forimprovement in life performance of tap-changer?

What are the selectionparameters of a tap-changer?�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�Application�are some of the major parameters for selection�of a tap-changer.

What are the advantages ofvacuum type OLTC?New generation tap changers utilize vacuum technology.Unlike traditional solutions, electrical arc quenching takesplace in a vacuum environment and not in oil. Thissubstantially reduces contact wear, thereby increasing life ofcontacts. Also arcing does not pollute the insulating oil as thearcing takes place in vacuum bottles.

The benefits of using thevacuum tap-changer are:�Lower life cycle costs�Less maintenance needs�Increased time in operation because of a radical�Reduction in contact wear�Routine inspection takes place in a cleaner environment

which reduces downtime.�Reduced sensitivity to moisture in the insulating oil due to the

absence of polluting by products such as carbon.�Switching characteristics of the tap-changer does not depend

on the status of the insulating oil.�Reliable due to redundant contact system.�Fully interchangeable with existing installations.�Possible to use alternative insulating fluids.

Whether OLTC conservator canbe at a different heightcompared to main conservatorof transformer?Many utilities have a practice of keeping height differencebetween main conservator and OLTC conservator. With thispractice, OLTC conservator is kept near to OLTC at a reducedheight than the main conservator. A pressure differencedepending upon the height difference appears on OLTC diverterand all sealing joints of OLTC for its life-time when conservatorsare placed at different heights, which OLTC is supposed towithstand (design pressure of diverter cylinder and such sealingjoint is higher). To avoid pressure differential, OLTC conservatormay be placed at the same height as of main conservator. In casethe OLTC conservator is at a lower height, any possible leakageinto the OLTC diverter switch, through sealing gaskets will leadto flow of oil from main tank to OLTC which will eventually lead toover flow of OLTC conservator indicating leakage in OLTC, a

blessing in disguise! The choice of location of OLTC conservatorshould be left to manufacturer's standard practice.

What is the basic differencebetween Transition Resistanceand Reactor type OLTC?The difference depends on which one is used as transitionimpedance to control the circulating current during the tapchange operation i.e. in Resistance type OLTC, a resistor withsuitable ohmic values are used as transition impedance where inthe reactor type OLTC a preventive auto transformer will be usedas transition impedance.

What is the need for Tie inResistors?In a Reversing / Coarse-fine tapping arrangement, during the tapselector change over operation, the tap winding will temporarilybe disconnected from the main winding and the potential of thetap winding floats. The floating potential of tap winding isdetermined by the voltages of the adjacent winding and as wellas the coupling capacitance to these windings and to the groundparts. In such a case, discharge between the opening andclosing contacts occur during the operation of change-overselector. In order to avoid the difficulties in regard to thedielectric stress and formation of gases which could arise if thevalues reach the admissible limits, one of the methods toovercome the problem is to use Tie in Resistor.

Does the OLTC used in the HVDCnetwork transformer needsspecial care?Yes, the OLTC needs a special care because the transformer inthe HVDC station including the OLTC is subjected to windingcurrents that more resemble square waves than sinusoidalwaves due to the effect of converter system. These squarewaves have a large impact on the switching stress which has tobe controlled by diverter switch.

Does the Buchholz relay and Oilsurge relay work in a sameprinciple?No. Buchholz relay and Oil surge relay works in a differentprinciple. Buchholz relay is gas operated relay which is actuatedby the gases flowing through and accumulated in Buchholz relay.Oil surge relay is actuated when the flow rate of oil exceeds thepreset value.

What preventive maintenancemeasures can be used forimprovement in life erformanceof tap-changer?Relevant OLTC manufacturer's documents should be referredfor the maintenance schedule. Transformer oil in diverterswitch is contaminated by electrical arc during OLTCoperation and therefore depending upon number of operationsone has to carefully decide on period of oil sample testing.Filtration of oil or replacement is to be decided based on testresults. BDV, PPM Tan~delta and sludge content value shall beguiding factor to take decision. Tap-changer oil can befiltered with the use of on line filtration device when shutdownis not possible.

We can:Train your engineers and technicians on every aspect ofPreventive Maintenance, Routine Maintenance and Servicing ofyour OLTCs irrespective of their make or present condition. Ourinstructors have decades of hands-on working experience andthey’ve also undergone training in the most advanced OLTC plantin the world. We’re also equipped with stocks of spares to meetyour immediate requirements and can ourselves undertakingservicing and repair work.

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