[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

FIGURE 2.7.22 Equalizer winding incorporated into the main coil of a regulator.
FIGURE 2.7.21 Tap-changer interruption envelope.
Figure 2.7.21 shows a typical tap-changer interruption envelope. Load current (I L ), tap voltage,
reactor circulating current (I C ), and power factor (PF) are key variables affecting a tap changer’s
interrupting ability and its contact life. A circulating current (I C ), caused by the two contacts being
at different positions (reactor energized with 1 1 / 4% tap voltage), is limited by the reactive impedance
of this circuit. Two opposing requirements must be kept in mind when designing the amount of
reactance for the value of the circulating current. First, the circulating current must not be excessive;
second, the variation of reactance during the switching cycle should not be so large as to introduce
undesirable fluctuations in the line voltage. The reactor has an iron core with gaps in the magnetic
circuit to set this magnetizing circulating current between 25 and 60% of full-load current, thus
providing an equitable compromise between no-load and load conditions. The value of this circulating
current also has a decided effect on switching ability and contact life. The ideal reactor, from
an arcing standpoint, would be one that has a closed magnetic circuit at no-load with an air gap that
would increase in direct proportion to increase in load.
The voltage at the center tap is 5/8%, one-half of the 1 1 / 4% tap voltage of the series winding taps.
Some regulators, depending upon the rating, use an additional winding (called an equalizer winding) in
the bridging reactor circuit. The equalizer winding is a 5/8% voltage winding on the same magnetic
circuit (core) as the shunt and series winding. The equalizer winding is connected into the reactor circuit
opposite in polarity to the tap voltage. This is done so that the reactor is excited at 5/8% of line voltage
on both the symmetrical and bridging positions. Figure 2.7.22 shows an equalizer winding incorporated
into the main coil of a regulator.
Voltage regulators are designed and manufactured in two basic constructions, defined by IEEE standards
as Type A and Type B. Type A step-voltage regulators have the primary circuit (source voltage)
connected directly to the shunt winding of the regulator. The series winding is connected to the load
side of the regulator and, by adjusting taps, changes the output voltage. With Type A construction, the
core excitation varies with the source voltage because the shunt winding is connected across the primary
circuit. A separate voltage transformer is used to provide voltage for the tap-changer and control. The
FIGURE 2.7.23 Voltage regulation on the load side (Type A).
maximum range of regulation of the “raise” side equals the maximum range of regulation of the “lower”
side, with 10% being the minimum amount of regulation. See the schematic diagram in Figure 2.7.23.
Type B step-voltage regulators are constructed so that the primary circuit (source voltage) is applied
by way of taps to the series winding of the regulator, which is connected to the source side of the regulator.
With Type B construction, the core excitation is constant, since the shunt winding is connected across
the regulated circuit. A control winding located on the same core as the series and shunt windings is
used to provide voltage for the tap-changer and control. The maximum range of regulation of the “raise”
side is higher than the maximum range of regulation of the “lower” side, with 10% being the minimum
amount of regulation on the “raise” side. See the schematic diagram in Figure 2.7.24.
Usually the choice of Type A or Type B is that of the supplier. However, the user can specify that an
identical regulator design be provided if the application can anticipate the need to parallel with another
unit in the same substation. Paralleling of regulators that are not of identical design can cause excessive
circulating current between them. This is true even for short-term operation during switching, when the
units are placed on the same numerical tap position and the control function is disabled. Caution: Any
paralleling of step-voltage regulators (as described in this chapter) that may operate, even momentarily,
on differing tap positions requires the inclusion of supplemental system reactance to avoid excessive
circulating current during operation.
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC