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On-load tap changers

The on-load tap changer (OLTC) is usedto change the tapping connection of thetransformer winding while the trans-former is energized. The tap changercan be designed as a single unit for sin-gle and three-phase applications withone common neutral point. Dependingon the three-phase rating, it mightrequire three separate units, each havingits own insulated phases. Tap changerscan be located either inside the trans-former main tank or outside in its owncompartment.

Switching from one position to anoth-er has to be performed through animpedance to avoid a short circuitbetween two steps of the regulatingwinding. The transition impedance canbe either a resistor or a reactor. ABB-type tap changers are of the resistortype.

On-load tap changer design

ABB's active component construction can beeither of the diverter or selector type switchingprinciple.

The diverter type is built of two separate sections with the diverter part having its owncompartment while the tap-selector shares oil with the transformer. The tap-selector is mountedbelow the diverter switch housing and has twomoving arms with one carrying the current andthe other free to pre-select the next tap position.The diverter switch moves from left to right andtransfers current to the other tap position (selectorpre-selection) via the transition resistors. All arc-ing takes place from within the diverter compart-ment.

The complete unit is suspended from thetransformer tank cover, and ABB utilizes this sys-tem for its in-tank Type-UC tap changer.

Author: Douglas. M. GetsonDocument updated: 2002-11-06e-mail: douglas.m.getson@de.abb.com

Electrical principle for the diverter type on-load tap changer(red: diverter)(blue: selector)

Type UC tap changerincluding BUL motor drive

Active part mounting of type UC tap changer.

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4.3

The selector type on-load tap changer combinesthe diverter switch and tap selector into one sim-ple switch. The system consists of one fixed andone moving contact unit. Each fixed contact hastwo contact paths on each side, one for the mainmoving contact and another for the switchingcontacts.

The drive system can be built as one rigidunit rotated by a common drive shaft. ABB utilis-es this system for its on-tank Type UZ, and in-tank Type UB tap changers. The Type BUF motordrive comes as an integral part of the Type UZ tapchanger and requires no additional transformerwall space. This allows for quick and easy installa-tion while lowering the cost for field service. OurType BUE and BUL motor drives come separatefrom the tap changer and operate with the TypeUB and UC tap changers.

All tap changers are manufactured and testedin accordance with industry standards such as IEC214 and IEEE C57.13.131. The following rou-tine tests are performed: dielectric testing of TypeUZ phase moldings, transition resistor measure-ments, transition time between operations,mechanical 500-operation tests, and helium sealtests.

Electrical principle for the selector type on-loadtap changer (diverter/selector combination)

Type UZ (incl. BUF motor drive)

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Type UZ (Main Door Open)

Type UB (ActiveComponents)

Field service of type UC tap changer

Easy maintenance

Tap-changers have been manufactured by ABBsince 1910 and more than 30,000 of today’sType UC/UZ/UB are in service. The hallmarkhas always been quick and easy maintenance,without requiring special skills or special tools.One example is that maintenance on the TypeUC and UZ tap changers is possible in any tapposition without dismantling the shaft system.

Tap Changer Service

Tap changer service consists of two parts,inspection and overhaul.

The annual inspection, which can be car-ried out while the transformer is in service,includes checking the motor drive mechanismand recording the number of tap change opera-tions.

Overhaul requires that the transformer istaken out of service and is normally requiredafter seven years of service (five years for TypeUB) or every 80,000 – 100,000 operations,whichever comes first.

Extending Intervals betweenOverhauls

The time-based overhaul criteria is normallydecided by the network application. By assess-ing the dielectric integrity of the insulating oilfor deterioration, contamination and moisture,the time-based overhaul intervals for the TypeUC can be extended to every 15 years and innumber of operations to 150,000 – 200,000operations, provided that an oil sample is takenprior to commissioning the tap changer andthen every fourth year. Oil analysis should bein accordance with IEC 60 422.