the frequency effect on the saturated reactor. It passes
maintain voltage Vr constant. If there is any deviation
the same buck current at a particular line voltage at any
in generator voltage from its normal value, the system
frequency between 57 and 63 Hz.
will make corrections until the three-phase voltages,
1-2-3, are the values that will produce normal voltage
Reactive Compensation
Vr.
Correct phase sequence of the connections of the
When ac generators are operated in parallel,
potential unit to the generator leads is required for
division of the load between machines is a function of
correct functioning of this network. If the connections
the governors of the prime movers. The division of the
are reversed, for example, by interchanging the two
reactive load (kVA) is a function of the regulators, which
leads from the secondary teaser winding, the voltage,
increase or decrease the excitation of the generators.
7-0, would be subtracted from the voltage, 0-6, instead
The division of the reactive load (kVA) between
of added to it. The voltage, Vr, impressed on the
generators (when operated in parallel) is accomplished
regulator would then be approximately one-fifth the
by a compensating potentiometer, P, and a current
required value. Thus, the regulator in attempting to go
transformer, CT, provided for each machine (fig. 8-9,
to the ceiling voltage would overexcite the generator to
view A). The rheostat is connected in series with the
abnormal levels.
teaser leg of the T-connected potential transformer
secondary. The current transformer is connected in the
Frequency Compensation
B phase of the generator. Its secondary is connected
across one side of the potentiometer.
The reactance of the saturated reactor (fig. 8-8)
increases as the frequency increases. Thus, an increase
The generator voltage, 1-2-3, feeds the primary of
in frequency from 60 to 63 Hz at normal 100 percent
the T-connected potential transformer (fig. 8-9, view A).
voltage would decrease the buck current. The boost
The line current, Ib, of phase B in which the current
current would predominate so the regulator would tend
transformer is connected, is in phase with the
to hold a higher voltage. A frequency lower than 60 Hz
line-to-neutral voltage at unity power factor. Ib is at 90
would have the opposite effect. This would tend to
to the voltage, 2-3 (fig. 8-9, view C). At any other power
increase the buck current so it would predominate. The
factor, current Ib swings out of phase with the
regulator would then tend to hold a lower voltage.
line-to-neutral voltage depending on lag or lead
conditions.
Therefore, a voltage regulator system using a
saturated reactor must have a means to compensate for
The secondary voltage, 7-6 (fig. 8-9, view B), which
frequency changes. Frequency compensation is
is the resultant output voltage of the three-phase
provided by an inductor, L1, and a capacitor, C 1, in
response network, is in phase with the line voltage, 2-3,
parallel with each other. They are across the resistance
and is the voltage impressed on the saturated reactor. At
portion of the positive phase sequence network used for
unity power factor, current Ib produces a voltage, 6-8,
three-phase response (fig. 8-8). The values of the
across the compensating rheostat, P, which is 90
inductor and capacitor are such that at 60 Hz they
out-of-phase with voltage 7-6 (fig. 8-9, view C). The
provide a resonant parallel circuit. This acts like a high
voltage 6-8 (IbRp) is the compensating voltage. The
resistance. The other components of the system are
voltage 7-8 (Vr) is now impressed on the saturated
adjusted so this resistance has no effect on the action of
reactor. The regulator tends to hold the voltage
the regulator at normal frequency.
proportional to 7-8.
When the frequency increases above 60 Hz, the
When two duplicate generators, A and B, are
parallel circuit has a capacitive effect. This raises the
operating in parallel at rated power factor, the
apparent voltage "seen" by the saturated reactor. his
causes it to pass as much buck current on normal voltage
"seen" by the saturated reactors of both regulators will
at the higher frequency as at normal frequency.
also be equal if the following conditions exist:
When the frequency decreases below 60 Hz, the
1. The field currents are balanced (made equal)
parallel circuit has an inductive effect. This lowers the
2. The compensating rheostats are set at the same
apparent voltage as "seen" by the saturated reactor. This
value of resistance
causes it to pass as much buck current at normal voltages
3. The governors are set for equal division of the
at the lower frequency than it would at normal
kilowatt load
frequent y. Thus, the parallel circuit compensates for
8-15
