Polarity
Polarity marks designate the relative
instantaneous directions of currents. At the same instant that the
primary current is entering the marked primary terminal, the
corresponding secondary current is leaving the similarly marked
secondary terminal, having undergone a magnitude change within the
transformer (see Figure 1).
Figure 1 - Current Transformer Polarity
The primary H1 and secondary X1
terminals are marked with white dots, or with a ± symbol, or with H1
and X 1. As illustrated in Figure 1, the marked secondary conductor
can be considered a continuation of the marked primary line as far as
instantaneous current flow is concerned.
Connections
CTs are usually connected on
three-phase circuits in one of three ways, as follows:
a) Wye connection.
In the wye connection, a CT is placed
in each phase with timeovercurrent relays (Device 51) placed in
either two or three CT secondaries to detect phase faults. On
grounded four-wire systems, a time-overcurrent relay (Device 51N) in
the CT common wire known as a residually connected relay detects any
ground fault or neutral load currents. If neutral load currents are
not to be detected by the Device 51N relay as ground-fault currents,
a fourth CT is placed in the neutral conductor to cancel the neutral
load currents. Secondary currents are in phase with primary currents
(see Figure 2).
Figure 2 - Wye connected CT
b)Vee connection.
A vee connection is basically a wye
with one leg omitted, using only two CTs. Applied as shown in Figure
3, this connection detects three-phase and phase-to-phase faults. A
zero-sequence CT (window or bushing) and a ground overcurrent relay
(Device 50GS or Device 51GS) are required to detect ground-fault
currents. All three-phase conductors and the neutral (if present)
shall pass through the CT.
Figure 3 - Vee or open delta connected CT
c)Delta connection.
A delta connection uses three CTs with
the secondaries connected in delta before the connections are made to
the relays. The delta connection shown in Figure 4 is typically
used for power transformer differential relay protection schemes
where the power transformer has delta-wye-connected windings.
Figure 4 - Delta connected CT
The Cts on the delta side are connected
in wye, and the CTs on the wye side are connected in delta. The delta connection
is also used for overcurrent protection of grounding transformers
where filtering out the third-harmonic currents is desirable.
When connected in delta, the current in
the relays is equal to times the CT secondary current. This fact
should be considered when selecting the primary ratings of CTs and
the secondary device ratings of delta connected CTs
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