[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

2.7.4 Theory
A step-voltage regulator is a tapped autotransformer. To understand how a regulator operates, one can
start by comparing it with a two-winding transformer.
Figure 2.7.12 is a basic diagram of a transformer with a 10:1 turns ratio. If the primary winding or
V source has 1000 V applied, the secondary winding or V load will have an output of 100 V (10%). These two
independent windings can be connected so that their voltages aid or oppose one another. A voltmeter
connected across the output terminals measures either the sum of the two voltages or the difference
between them. The transformer becomes an autotransformer with the ability to raise (Figure 2.7.13) or
lower (Figure 2.7.14) the primary or system voltage by 10%. This construction is similar to a “Type A”
single-phase step-voltage regulator, as described later in thissection.
+
VSOURCE = 1000
+
VLOAD= 100
-
-
FIGURE 2.7.15 Tap-changer position indicator.
10 : 1
FIGURE 2.7.12 Transformer with 10:1 turns ratio.
+
-
+
VSOURCE = 1000 VLOAD = 1100
-
FIGURE 2.7.13 Step-up autotransformer.
FIGURE 2.7.14 Step-down autotransformer.
+
-
+
VSOURCE = 1000 VLOAD = 900
-
In a voltage regulator, the equivalent of the high-voltage winding in a two-winding transformer would
be referred to as the shunt winding. The low-voltage winding would be referred to as the series winding.
The series winding is connected to the shunt winding in order to boost or buck the applied or primary
voltage approximately 10%. The polarity of its connection to the shunt winding is accomplished by the
use of a reversing switch on the internal motor-driven tap changer. Eight approximately 1 1 / 4 % taps are
added to the series winding to provide small voltage-adjustment increments. To go even further to provide
fine voltage adjustment, such as 16 approximate 5/8% tap steps, a center tapped bridging reactor (preventive
autotransformer) — used in conjunction with two movable contacts on the motorized tap changer
— is utilized. In all, there are 33 positions that include neutral, 16 lower positions (1L, 2L, 3L, 4L, etc.),
and 16 raise positions (1R, 2R, 3R, 4R, etc.). Figure 2.7.15 illustrates a dial that indicates the arrangement
of the tap positions.
It is common practice to have the tap changer located in the same compartment as the core and coil.
Dielectric practices consider the oil and insulation degradation due to the arc by-products. Figure 2.7.16
shows a typical load-break tap changer used in a single-phase step-voltage regulator.
The process of moving from one voltage-regulator tap to the adjacent voltage-regulator tap consists
of closing the circuit at one tap before opening the circuit at the other tap. The movable tap-changer
contacts move through stationary taps alternating in eight bridging and eight nonbridging (symmetrical)
positions. Figure 2.7.17 shows the two movable tap-changer contacts on a symmetrical position, with
the center tap of the reactor at the same potential. This is the case at tap position N (neutral) and all
evenly numbered tap positions.
An asymmetrical position, as shown in Figure 2.7.18, is realized when one tap connection is open
before transferring the load to the adjacent tap. At this juncture, all of the load current flows through
one-half of the reactor, magnetizing the reactor, and the reactance voltage is introduced into the circuit
for about 25 to 30 msec during the tap change.
Figure 2.7.19 shows the movable contacts in a bridging position; voltage change is one-half the 1 1 / 4 %
tap voltage of the series winding because of its center tap and movable contacts located on adjacent
stationary contacts. This is the case at all oddly numbered tap positions.
Voltage phasor relations shown in Figure 2.7.20 represent a tap change. In this figure, S–SL represents
source or unregulated voltage, and sections of the series winding are represented by S–TAP 1 and
TAP 1–TAP 2. In operating the tap changer from TAP 1 to TAP 2, the load or regulated voltage has the
following successive values:
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC