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 3.2.2.1 Secondary Circuit. Potential transformers are classified by the type
of design of the secondary circuit.
Single secondary: In general, standard rating potential transformers,
q
15 KV class and below, are supplied with a single electrical secondary
circuit designed for 120 volts.
Tapped secondary: There are applications where it is desirable to
q
have two or more values of secondary potential available from the same
secondary winding. This is particularly true where it is desirable to
connect the secondary winding in delta or wye, for three-phase
circuits, and obtain the same secondary voltage.
For such applications, the' transformer secondary is rated at 120 volts
with a tap at 69.3 volts.
Double secondary: Unlike current transformers, double or
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multisecondary potential,transformers have individual secondary
electrical circuits on the common magnetic core. These windings may
or may not be tapped. Each secondary is affected by the burden
conditions on the other secondaries.
3.2.3 Transformer Accuracy. Because of the function they perform, all
instrument transformers must be classified as to accuracy. The two most
important specifications for these transformers are ratio accuracy and
phase-angle error. Phase-angle errors should be less than 50 minutes for use
with analyzers of the 99 percent accuracy class and 10 min. or less when used
with instruments having accuracies of 99.75 percent or better. It is noted
that control transformers are not suitable for metering since their voltage
can be off by 10 percent.
3.2.4 Instrument Transformer Polarity. When instrument transformers are used
the current or voltage, the phase position or direction of flow of current is
of no consequence; connection to the secondary terminals may be reversed
without changing indication of the instrument. When instrument transformers
are used with watthour meters, in which the operation depends on the
interaction of both current and voltage, the direction of current in the
primary and secondary windings must be known. This is indicated by marking
one primary and one secondary terminal with a distinctive POLARITY MARKER.
When current is flowing toward the transformer in the marked primary lead, it
is flowing away from the transformer in the marked secondary lead.
3.2.5 Grounding. It is always desirable to ground a transformer rated meter
and also to ground secondaries of both the current transformer and the
potential transformer. This avoids danger of high electrostatic voltages that
might otherwise be present as a result of the capacitance (condenser effect)
of the potential-transformer windings. This grounding practice also minimizes
hazard from high voltages reaching the secondary circuit as a result of
insulation breakdown due to lightning surges or other abnormal circuit
conditions.
9-30
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