The actual winding connections are
shown in a diagram with each winding and its taps labeled. A set of
tables then specifies the voltage ratings, ampere ratings, and the
connections for all the available taps. For transformers with load
tap changing equipment, the connection diagrams and the accompanying
tables are quite extensive.
The connection diagram usually also
gives the general physical layout of the transformer, showing the
placement of the bushings and the locations of current transformers
(CTs) and a schematic representation of the load tap changing
equipment, including the preventative autotransformer, moving
contacts, arcing contacts, transfer switch, and reversing switch.
A portion of an actual nameplate that
shows the winding connection diagram is illustrated in Figure 7.2.
The nameplate depicted is rather interesting. The transformer has a
load tap changer.
From the connection diagram we see that
the buried tertiary is also a tapped winding that supplies a
buck/boost voltage to the secondary windings through auxiliary
transformers connected between the tertiary and the secondary.
Therefore, the tertiary simultaneously
provides four important functions:
1. It provides a path for third
harmonic currents.
2. It helps stabilize voltages in the
Y-Y primary-secondary connection.
3. It provides a grounding bank action
by providing a path for zero sequence currents.
4. It provides the necessary voltage
taps for regulating the low-side voltage.
The only function that the buried
tertiary cannot perform is to supply an external load. The voltage
rating of the buried tertiary is not given because it cannot be
connected to a system voltage, but one corner of the Δ connection is
grounded internally.
This grounding is done so the winding
potential voltage does not ‘‘float’’ because of capacitive
coupling to the other windings. Without this ground connection,
capacitively induced voltages are indeterminate and could be large
enough to cause insulation damage.
The voltage taps for the primary and
secondary are shown on the connection diagram and on the winding
rating tables in Figure 7.2. These also specify which terminal
numbers and letters are connected for each tap.
This transformer has a total of 14
current transformers that are used for metering, protective relaying,
and other purposes. Note the CTs marked ‘‘LDC’’ and ‘‘WDG.
TEMP.’’ The term LDC stands for line drop compensation. The LDC
CT supplies metered line current to a compensating device in the
voltage regulator controls.
The compensating device effectively
moves the voltage control point into the system connected to the
secondary winding. The CT labeled WDG.TEMP supplies current to the
winding temperature gauges.
These gauges use a heating element
surrounding a temperature probe mounted in the top oil in order to
mimic the winding temperature. The ratios of these CTs would be shown
on an actual nameplate, but this information is not shown in Figure
7.2.
Just below the connection diagram is a
layout sketch showing the physical locations of the bushings, the
load tap changing compartment and the operating handle for the tap
changer at deenergized conditions. The load tap changer is
represented schematically in the connection diagram.
Note the terminals labeled P1, P2, and
P3. These terminals correspond to the connections to the preventative
autotransformer. The two series arcing contacts per phase that are in
series with the movable contacts are shown as well.
FIGURE 7.2 Part of a transformer’s
nameplate showing the voltage ratings, MVA ratings, percent
impedances, connection diagram, physical layout, vector diagram, tap
connections, CT connections, and BIL ratings.
