[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

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