ABB DTW Application, Installation & Selection Manual

Type DTW
De-energized Tap Changer
Application, Installation &
Selection Guide

IL 44-751-1

Instruction Leaflet

Page 1 November,2001

IL 44-751-1
Instruction Leaflet
Page 2 November, 2001

Scope

This guide contains general information for ordering and installing the type DTW de-energized tap
changer. This guide does not describe all possible contingencies, which may arise during
installation, operation, or maintenance of the tap changer, nor does it describe all details and
variations of the equipment. If you require additional information regarding the installation and
the operation or maintenance of your equipment, contact the local representative of ABB.

Safety Definitions

Safety notations are intended to alert personnel of possible conditions, which may cause personal
injury, property damage, or even death. They have been inserted into this instructional text prior
to the step in which the condition is cited.

The safety notations are headed by one of three hazard intensity levels, which are defined as
follows:

1. DANGER- immediate hazard that will result in severe personal injury, property damage, or
death.

2. WARNING- hazard or unsafe practice that could result in personal injury, property damage,
or death.

3. CAUTION- hazard or unsafe practice, which could result in minor personal injury, or property
damage.

Introduction

The type DTW de-energized tap changer is one of a family of ABB tap changers for power
transformers. A de-energized tap changer is a switch, which is connected to the winding taps of
the transformer. When the tap changer is moved from one position to another, the amount of the
tap winding connected into the circuit is changed. This permits the adjustment of the voltage ratio
of the transformer to best suit the voltage requirements at the transformer’s installation site. The
de-energized tap changer is usually installed into the high voltage circuit of the transformer and,
most of the time, it is used to adjust the primary voltage of the transformer within a 10 percent
range in 5 steps.

D A N G E R

The type DTW tap changer is a de-energized tap changer, and must, be operated only
when the transformer is completely de-energized. This type of tap changer must never be
operated when the transformer is energized. Operation when the transformer is energized
is dangerous and will result in severe personal injury, property damage, or death.

The type DTW tap changer is a modular, bridging-type (zigzag) tap changer with external
operating mechanism. It is most commonly available as a 5-position device. Other
configurations are available or can be designed; for these, please contact ABB. The type DTW
tap changer is made up of individual tap decks (one or two per phase). The phase tap decks are
connected to a common set of horizontal operating shafts through individual gear sets. The tap
changer is mounted in front of each phase coil, parallel to the side of the transformer tank. The
tap changer is supported by an insulating framework, which is part of and attached to the
transformer superstructure. See Figure 1 for a general view of a type DTW tap changer
installation.
The ability to locate the type DTW tap changer on the side of each phase coil permits very direct
tap lead routing with a minimum of bends.

IL 44-751-1
Instruction Leaflet
Page 3 November, 2001

Figure 1: Typical installation of the type DTW tap changer

All materials used in the construction of the type DTW tap changer have been chosen for their
superior electrical properties, mechanical strength, temperature extremes capability, corrosion
resistance, transformer oil compatibility, and light weight.

IL 44-751-1
Instruction Leaflet
Page 4 November, 2001

Ratings

The type DTW tap changer is available in one basic current rating and is offered with several
different electrostatic shielding and insulation options. Table 1 presents the various rating options.

Table 1. DTW Ratings

Test/ Parameter Rated Value Ref. Dimension

on Tap Changer

Voltage Class

Phase to ground and phase to phase:

Lightning Impulse
60 Hz. Power Frequency

Between Adjacent contacts:

Lightning Impulse
60 Hz. Power Frequency

Short circuit current

Rated Current

1

See Figure 2

2

Based on a steady state contact temperature rise of 15°C at a continuous current equal to 120%

of rated current.

3

The dielectric withstand from tap deck to ground and from phase to phase is very much

2

dependent on the electrical clearance distances which are maintained by the transformer design.
See also Electrostatic Shielding and Insulation (page 9).

See note 3 N/A

See note 3 N/A

220 kV

70 kV

(A-B, B-C, C-D,
D-E, E-F, F-A)

10,000 A r.m.s. N/A

850 A N/A

1

Figure 2.

IL 44-751-1
Instruction Leaflet
Page 5 November, 2001

Construction Details and Features

Basic Information

The DTW, de-energized tap changer, is constructed from six major components: 1) Tap deck, 2)
Worm gear set, 3) Moving contacts, 4) Stationary contacts, 5) Mounting post and 6) Drive shaft
and External operating mechanism. These individual components (except number 6) are shown
in Figure 3 and Figure 4.

Copper Mounting

5

Post

Stationary

4

Contact

Silver/Copper
Contact Button

Moving Contact

3

Assembly

1

Tap Deck
Plate

Contact Drive
Shaft

Figure 3: Contact structure

Figure 4: Worm gear set, 2), and ball end fittings

IL 44-751-1
Instruction Leaflet
Page 6 November, 2001

Tap Deck

The tap deck is a flat insulating plate. This plate serves as the base to which all of the other tap
changer components are bolted. The main deck contains six through holes, which are used to
bolt the tap changer to the transformer framework. The flat insulating plate is machined from high
strength, electrical grade, low power factor Micarta. This structure carries all of the mechanical
loads created by the weight of the tap leads and the thrust and weight of the drive shafts.

Worm Gear Set

This gear set is mounted on the main deck’s outboard face. Its function is to convert the rotary
action of the driving shaft 90 degrees in order to turn the moving contacts from position to
position. This gear set is contained within a cast metal housing. The function of the housing is to
position and hold the gear set and to provide a smooth, rounded corner enclosure in order to
minimize electrical stress concentration. The nature of the gearing is such that one full turn of the
drive shaft rotates the moving contacts from one set of stationary contacts to the next.

Contacts

The stationary contacts are machined from copper flat stock and are 0.25 inches (6.4 mm) thick.
The leading edges are chamfered to permit the moving contacts to easily slide up into position.
These contacts are tin-plated. The stationary contacts are bolted to copper mounting posts,
which in turn, are bolted to the phase deck. Two bolts are used for each contact. The bolt heads
and nuts are shielded with special, toriodially-shaped, washers to minimized electrical stress
concentration.

The copper mounting posts serve as the connection point for the tap leads. The tap leads from
the coil should be terminated with a two-hole flat connector. The flat connector is bolted to the
mounting post with two 0.375 (9.5 mm) bolt sets. The mounting posts “capture” the bolt head so
that only the nut has to be turned to tighten the joint.

The moving contact set is a “pincer” type of contact; the spring loaded contact plates slide over
the stationary contacts. See Figure 5. Each contact plate is made from copper flat stock. The
copper plates are tipped with semi-spherical, silver-copper alloy contact buttons. This alloy
contact button assures a positive, low friction, low resistance current path at all times, even when
taps are changed very infrequently. The sliding action of the contact button over the chamfered
stationary contact plate creates a good wiping action during the tap change which further
guarantees a solid, low resistance electrical contact. Contact pressure is precisely controlled by
two coil springs, which are set and calibrated at the factory to provide uniform and consistent
contact pressure. The combination of contoured interface surfaces between the moving and
stationary contacts and the accurately calibrated spring loaded pincer contacts minimizes the
force needed to drive the tap changer from one position to another. The pincer type of contact
takes advantage of the magnetic attractive forces created during through faults or current surges.
These additional compression forces yield high fault current withstand capability by preventing
contact bounce and arcing during faults. These features permit the tap changer to have a
relatively low driving torque and, yet maintain a high through fault current withstand capability.