IL
44-751-1
Instruction
Leaflet
Page 9 November, 2001
Electrostatic Shielding and Insulation
Electrical spacing requirements between the tap decks and the tank wall and between the tap
decks and adjacent phases or other metallic objects is a complex subject. When designing the
transformer, required electrical clearance distances should be calculated between phase decks
and between phase decks and ground or other parts of the transformer. Three levels of shielding
and insulation are available for the tap decks.
1. Unshielded: Tap decks do not have any electrostatic shields. This configuration is typically
used for applications, which have a impulse voltage withstand requirement of 200 kV or less.
See Figure 8.
2. Level 1 Electrostatic Shielding: A cast-aluminum electrostatic shield ring is placed between
the tap changer deck and the transformer tank wall. This ring is referred to as the “outer
shield”. The ring is bolted to an aluminum mounting plate, which is, in turn, bolted to the
worm gear housing. The ring portion of each shield is insulated with a layer of crepe paper
wrap. This configuration is, typically, used for applications, which require a impulse voltage
withstand of 650 kV or less. See Figure 9.
3. Level 2 Electrostatic Shielding: Cast-aluminum, electrostatic shield rings are mounted on both
the tank wall side and the coil side of each tap deck. Both the inner and outer shield rings
are bolted to the shield mounting plate, which was described above. The outer ring portion of
each shield is a continuous ring. The inner shield consists of two half-circle rings. See
Figure 10. In addition to the aluminum rings, a pressboard disk is placed in front of the
aluminum mounting plate and a pressboard box shield is added to the tank wall side of each
tap deck. This box shield is attached to the transformer support structure with cotton tape.
The pressboard disk is attached to the aluminum mounting plate with cotton tape. This
configuration is, typically, used for applications which require a impulse voltage withstand
greater than 650 kV; the upper limit of impulse withstand in this configuration is determined
by the electrical clearances designed into the transformer.
Figure 8. DTW without any shielding.
Figure 9. DTW with shield on tank side.
Figure 10. DTW with both inner and outer
shielding. NOTE: only one of the inner shields
are assembled in this figure.
