[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 2.6.21 15-kV wound-type CT cast in epoxy resin. (Photo courtesy of Kuhlman Electric Corp.)
FIGURE 2.6.24 Linear-coupler equivalent circuit.
fields, large eddy currents flow in the housings, producing high temperatures. The shield can also be
internal as an integral part of the secondary winding. This technique eliminates the eddy-current
problems. Both techniques are effective in protecting the core from stray flux, but neither will make
it immune from it.
FIGURE 2.6.22 High-voltage wound-type CT in combination steel tank, oil, and porcelain construction. (Photo
courtesy of Kuhlman Electric Corp.)
FIGURE 2.6.23 00-V, indoor-class auxiliary CT frame shell-type laminated “EI” core. (Photo courtesy of Kuhlman
Electric Corp.)
2.6.4.10 Linear Coupler
The linear coupler (LC), or Rogowski coil, is a current transformer that utilizes a nonmagnetic core,
e.g., wood, plastic, or paper, which acts only as a form for the winding. It is typically a window-type
construction. With an air core, the magnetizing components of error have been removed, thus offering
linear response and no possibility of saturation. They do not produce secondary currents that would
be provided by an ideal CT. If the magnetizing impedance approached infinity, the secondary current
would approach the ideal, i S = –i P (N P /N S ), as seen in Figure 2.6.24. The low permeability of the core
prevents a high-magnetizing inductance, thus providing considerable divergence of performance from
that of a conventional CT. Consequently, protective equipment must be designed to present essentially
infinite impedance to the LC and operate as mutual inductors. Closely matching the LC impedance
will provide maximum power transfer to the device. LC outputs are typically defined by V S /I P , e.g., 5
V per 1000 A. Since the load is high impedance, the LC can be safely open-circuited, unlike the
conventional iron-core CT.
Because coupling is important and there is no iron to direct the flux, the window size is made as small
as possible to accommodate the primary conductor. Positioning is critical, and the return conductor and
adjacent phases must be far enough away from the outer diameter of the LC so that stray flux is not
introduced into the winding.
2.6.4.11 Direct-Current Transformer
The basic direct-current transformer (DCT) utilizes two coils, referred to as elements, that require external
ac (alternating current) excitation (Figure 2.6.25). The elements are window-type CTs that fit over the
dc bus. The elements are connected in opposition such that the instantaneous ac polarity of one is always
in opposition to the other. The ac flux in one element opposes the dc flux in the primary bus and
desaturates the core, while in the other element the ac flux aids the dc flux and further saturates the core.
This cycle is repeated during the other (opposite polarity) half of the ac cycle. The need to rectify is due
to the square-wave output. The direction of current flow in the primary is not important.
Proximity of the return conductor may have an effect on accuracy due to local saturation. For best
results, all external influences should be kept to a minimum.
The output waveform contains a commutation notch at each half-cycle of the applied exciting voltage.
These notches contribute to the errors and can interfere with the operation of fast-acting devices. Ideally,
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC