[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)
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FIGURE 2.6.25 Standard two-element connection.<br />
FIGURE 2.6.27 00-V-class slipover CT installed on high-voltage bushing with ground shield. Note the ground lead<br />
going back through the CT window to avoid shorted electrical turn. (Photo courtesy of Kuhlman <strong>Electric</strong> Corp.)<br />
FIGURE 2.6.26 Three-element connection.<br />
the core material should have a square B-H characteristic, which will minimize the notches. There are<br />
several connection schemes that can eliminate the notches but will increase the overall cost.<br />
A simple approach could be the three-element connection (Figure 2.6.26), where the third element<br />
acts as a smoothing choke to the notches. This also increases overall frequency response by providing ac<br />
coupling between the primary and the output circuits.<br />
Finally, there is the Hall-effect device. This solid-state chip is inserted into the gap of an iron core<br />
whose area is much larger than the device itself. The core has no secondary winding. The Hall effect<br />
requires a low-voltage dc source to power and provides an output proportional to the dc primary current<br />
and the flux linked to the core and gap.<br />
2.6.4.12 Slipover CT Installations<br />
CTs are often installed on existing systems as power requirements increase. One of the most common<br />
retrofit installations is the application of a window-type or external slipover CT, which is mounted over<br />
the high-voltage bushing of a power transformer or circuit breaker (Figure 2.6.27). To maintain the<br />
integrity of the insulation system, adequate strike clearances must be observed. It is also important to<br />
protect the CT from a high-voltage flashover to ground. This can be done by placing the CT below the<br />
ground plane or bushing flange. When using an external slipover CT, a ground shield can be placed on<br />
top of the unit and connected to ground. When grounding the shield, it is important that the lead is<br />
routed such that it does not make a shorted electrical turn around the CT.<br />
A slipover, when used internally, is called a bushing CT or BCT. Its insulation is not suitable for direct<br />
outdoor use and requires protection from the weather. In high-voltage switchgear, BCTs are fitted over<br />
the high-voltage bushing in a similar manner as described above, but they are enclosed inside a metallic<br />
FIGURE 2.6.28 High-voltage circuit breaker with BCTs mounted underneath a metallic cover. (Photo courtesy of<br />
Kuhlman <strong>Electric</strong> Corp.)<br />
cover that protects them from the weather while providing a ground shield (Figure 2.6.28). The BCT can<br />
also be inside the apparatus, mounted off of the high-voltage bushing, either above or submerged in the<br />
oil. Either way, special materials must be used for exposure to oil.<br />
2.6.4.13 Combination Metering Units<br />
The last major assembly is the combination metering unit, which consists of a single VT and a single CT<br />
element within one common housing. These are typically available in 15-kV class and up. In most cases<br />
this is more economical than using single-housed elements and can save space. With two primary<br />
terminals, it mimics the conventional wound-type CT, thus simplifying the installation. The H1 (line)<br />
terminal is a common junction point for both the CT and VT elements. The H2 (load) terminal completes<br />
the current loop. The VT element is connected from the H1 terminal to ground. All secondary terminals<br />
are isolated and located inside the secondary-terminal box.<br />
Optical metering units are available for use in high-voltage substations. They utilize the same principles<br />
as previously discussed, with the current and voltage transducers housed in a common structure.<br />
2.6.4.14 Primary Metering Units<br />
Primary metering units are single CT and VT elements assembled on a common bracket for pole-mounted<br />
installations or inside a pad-mount compartment for ground installations. They are typically molded<br />
© 2004 by CRC Press LLC<br />
© 2004 by CRC Press LLC