[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 2.6.17 (c) Zero-sequence scheme with all three-phase leads going through the window of the CT. This
connection, as well as the residual connection in Figure 2.6.17a, will cancel out the positive- and negative-sequence
currents leaving only the zero-sequence current to the 50G device. Sometimes the ground or neutral lead will be
included. The diagram on the right shows sheathed cable. It is important that one ground point go back through
the window to avoid the possibility of a shorted electrical turn via the ground path.
TABLE 2.6.9 Total Burden on CT in Fault Conditions
CT Secondary Connections Three-Phase or Ph-to-Ph Fault Phase-to-GND Fault
FIGURE 2.6.18 Bar-type CT. (Photo courtesy of Kuhlman Electric Corp.)
Wye-connected at CT R S + R L + Z R R S + 2R L + Z R
Wye-connected at switchboard R S + 2R L + Z R R S + 2R L + Z R
Delta-connected at CT R S + 2R L + 3Z R R S + 2R L + 2Z R
Delta-connected at switchboard R S + 3R L + 3Z R R S + 2R L + 2Z R
Note: R S = CT secondary-winding resistance + CT lead resistance, ohms; R L = one-way circuit lead resistance, ohms;
Z R = relay impedance in secondary current path, ohms
The wye-connected secondary circuit is the most commonly used. The CT will reproduce positive-,
negative-, and zero-sequence elements as they occur in the primary circuit. In the delta-connected
secondary, the zero-sequence components are filtered and left to circulate in the delta. This is a common
scheme for differential protection of delta-wye transformer. A general rule of thumb is to connect the
CT secondaries in wye when they are on the delta winding of a transformer and, conversely, connect the
secondaries in delta when they are on the wye winding of the transformer. See Table 2.6.9.
2.6.4.7 Construction
There are four major types of CT: window-type CT, which includes bushing-type (BCT); bar-type CT;
split-core-type CT; and wound-type CT, the latter having both a primary and secondary winding. There
is ongoing development and limited use of optical-type current transformers (OCT), which rely on the
principles of light deflection.
2.6.4.7.1 Window-Type CT
The window-type CT is the simplest form of instrument transformer. It is considered to be an incomplete
transformer assembly, since it consists only of a secondary winding wound on its core. The most common
type is that wound on a toroidal core. The secondary winding is fully distributed around the periphery
of the core. In special cases when taps are employed, they are distributed such that any connection made
would utilize the entire core periphery. Windings in this manner ensure optimum flux linkage and
distribution. Coupling is almost impervious to primary-conductor position, provided that the return
path is sufficiently distanced from the outer periphery of the secondary winding (see Figure 2.6.5a). The
effects of stray flux are negligible, thus making this type of winding a low-reactance design. The primary
winding in most cases is a single conductor centrally located in the window. A common application is
to position the CT over a high-voltage bushing, hence the name BCT. Nearly all window-type CTs
manufactured today are rated 600-V class. In practice, they are intended to be used over insulated
conductors when the conductor voltage exceeds 600 V. It is common practice to utilize a 600-V-class
window type in conjunction with air space between the window and the conductor on higher-voltage
systems. Such use may be seen in isolated-phase bus compartments. There are some window types that
can be rated for higher voltages as stand-alone units, for use with an integral high-voltage stress shield,
FIGURE 2.6.19 Split-core type CT with secondary winding on three legs of core. (Photo courtesy of Kuhlman
Electric Corp.)
or for use with an insulating sleeve or tube made of porcelain or some polymeric material. Window-type
CTs generally have a round window opening but are also available with rectangular openings. This is
sometimes provided to fit a specific bus arrangement found in the rear of switchgear panels or on drawout-type
circuit breakers. This type of CT is used for general-purpose monitoring, revenue metering
and billing, and protective relaying.
2.6.4.7.2 Bar-Type CT
The bar-type CT is, for all practical purposes, a window-type CT with a primary bar inserted straight
through the window (Figure 2.6.18). This bar assembly can be permanently attached or held in place
with brackets. Either way, the primary conductor is a single turn through the window, fully insulated
from the secondary winding. The bar must be sized to handle the continuous current to be passed
through it, and it must be mechanically secured to handle high-level short-circuit currents without
incurring damage. Uses are the same as the window-type.
2.6.4.7.3 Split-Core Type
The split-core type CT is a special case of window-type CT. Its winding and core construction is such that
it can hinge open, or totally separate into two parts (Figure 2.6.19). This arrangement is ideal to use in
cases where the primary conductor cannot be opened or broken. However, because of this cut, the winding
is not fully distributed (see Figure 2.6.5b). Often only 50% of the effective magnetic path is used. Use of
this type should be with discretion, since this construction results in higher-than-normal errors. There is
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC