Page 80 - Electrical Equipment Handbook _ Troubleshooting and Maintenance

TRANSFORMER COMPONENTS AND MAINTENANCE

TRANSFORMER COMPONENTS AND MAINTENANCE 4.9

Current Transformers
Current transformers (CTs) are auxiliary transformers used normally for metering or oper-
ation of auxiliary equipment and relays. They could be located inside or outside the tank
assembly.

Bushings
A bushing is an insulated conductor passing through a cover of a piece of enclosed electri-
cal equipment. The bushing carries the current through it and seals the opening against
weather or oil pressure. It also supports the leads and provides protection against flashover
due to overvoltage or heavy contamination (Fig. 4.5).
A bushing has a voltage less than 35 kV and consists of a current-carrying conductor, an
epoxy or porcelain insulator, and a mechanical assembly to hold it all together. It also has
terminals on the top and bottom for connection. The bushing is filled with oil for cooling.

Tap Changers
Many transformers have on-load or off-load tap changers. They change the effective num-
ber of the windings in the transformer to maintain the secondary voltage constant.

Insulation
Figure 4.6 shows the main components of a power transformer and the major uses of Kraft
paper. The winding insulation is normally made of paper. Cellulose board is also used as
internal insulation in liquid-filled transformers. Cellulose insulation is impregnable with
the insulating fluid of the transformer. This maintains a uniform dielectric stress through-
out the transformer.

TYPES AND FEATURES OF INSULATION

1. Low-density calendered board, available in flat and formed parts in thickness up to
0.188 in.
2. Laminated low-density calendered board using dextrin resin.
3. High-density precompressed board designed for high mechanical strength and dimen-
sional stability. The dimensional stability is important for the long life of the trans-
former. It ensures that the windings do not loosen over time. This insulation is used for
space ducts in the transformer.
4. Laminated high-density boards using polyester resin. This material is typically used for
clamping plates at each end of the transformer windings. It is completely impregnable
with oil despite its high density. This ensures freedom from partial discharge.

Aramid is another type of insulation used in power transformers. It is chemically simi-
lar to nylon. It is completely impregnable with oil. It is generally known as Nomex. It has
high strength, and it can withstand an operating temperature of 170°C without deteriora-
tion (cellulose insulation can only withstand 105°C). However, Nomex is very expensive

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