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