[James_H._Harlow]_Electric_Power_Transformer_Engin(BookSee.org)

Substation A
Substation B
FIGURE 3.7.7A Step-voltage regulator in routine application.
Substation A
FIGURE 3.7.7B Step-voltage regulator in reverse power-flow application.
52A
52B
52A
52B
Open
L A1
L B1
L A1
L B1
or often up to three different percentages, using different taps on the fooler transformer. Having the
sensed voltage boosted by, say, 5% without changing the voltage set point of the control will cause the
control to run the tap position down by 5% voltage, thus accomplishing the desired voltage reduction.
The percentage of three reduction steps, most commonly 2.5, 5.0, and 7.5%, is preestablished by the
design of the fooler transformer.
Digital controls do the same function much more conveniently, more accurately, and more quickly.
The voltage reduction applicable to steps 1, 2, and 3 are individually programmed and, upon implementation,
effectively lower the voltage set point. Control based on the new set point is implemented without
intentional time delay, reverting to panel time delay after the voltage reduction has been implemented.
Most often, controls provide for three steps of voltage reduction, each step individually programmed up
to 10.0%.
3.7.6.3 Reverse Power Flow
Voltage regulators as used on distribution feeders are sometimes subjected to reverse power flow due to
system switching. A common practice is to install the distribution system per the illustration of
Figure 3.7.7. Normally, step-voltage regulators A and B both usually “see” forward power flow, as in
Figure 3.7.7a. Line switching, such as might be necessary due to service required in Substation B, causes
Feeder B to be served from Substation A and the Regulator B to operate in reverse power flow mode, as
in Figure 3.7.7b. Were special precautions not taken, Regulator B would operate incorrectly, inevitably
running to a “raise” or “lower” tap limit. This would have occurred because the regulator would continue
to attempt to operate based on knowledge of the voltage at its load terminal, but now that terminal is,
in fact, the source. Tap-changer operation does nothing to correct that voltage, but it severely disturbs
the voltage to loads L B1 between Regulator B and Substation B.
The remedy is to operate Regulator B based on the voltage at its usual source (now the load) terminal.
Analog controls have been provided to automatically switch between the load and source terminals upon
FPF
V S V L
Reg A
V S
FPF
V L
Reg B
FPF
V S
V S
Reg A
RPF
Reg B
V L
V L
L A2
L B2
L A2
L B2
recognition of reverse power flow, but the expense of doing so, involving in most cases the addition of
a new VT, seldom justified the avoidance of simply manually switching off automatic operation during
the planned power-reversal event. Digital controls accomplish this feature automatically with no additional
hardware. These controls calculate the voltage at the normal source terminal using knowledge of
the measured load terminal voltage, the recognized tap position plus, in some cases, an approximation
of the internal regulation of the regulator. Accomplishing voltage regulation with Regulator B during
reverse power-flow operation in this manner adds only about 0.5% to the error of the control vis a vis
having a supplemental VT for the application.
The apparatus and procedures defined above for step-voltage regulators are not correct for most
transformer applications where reverse power flow can occur. The basic difference is that the feederregulator
application remains a radial system after the line switching is complete. Reverse power in
transformers is more likely to occur on a system where the reverse power is due to a remote generator
that is operating continuously in parallel with the utility. The proper operation of the LTC in this case
must be evaluated for the system. Perhaps the preferred operation would be to control the LTC so as to
minimize the var exchange between the systems. Some systems are simply operated with the LTC control
turned off of automatic operation during reverse power flow so that the LTC says fixed on position until
forward power flow resumes.
3.7.7 Introduction to Control for Parallel Operation of LTC Transformers
and Step-Voltage Regulators
For a variety of reasons, it may be desirable to operate LTC transformers or regulators in parallel with
each other. This may be done simply to add additional load-handling capability to an existing overloaded
transformer, or it may be by initial design to afford additional system reliability, anticipating that there
may be a failure of one transformer.
Most common paralleling schemes have the end objective of having the load tap changers operate on
the same, or on nearly the same, tap position at all times. For more-complex schemes, this may not be
the objective. A knowledge of the system is required to assess the merits of the various techniques.
3.7.7.1 The Need for Special Control Considerations
To understand why special control consideration needs to be given to paralleling, consider two LTC
transformers operating in parallel, i.e., the primary and secondary of the transformers are bused together
as in Figure 3.7.8. If the transformers are identical, they will evenly divide the load between them at all
times while they are operating on the same tap position. Consider that the voltage at the secondary bus
goes out of band. Even if the controls are set the same, they are never truly identical, and one will
command tap-change operation before the other. Later, when the voltage again changes so as to require
a second voltage correction, the same control that operated more quickly the first time would be expected
to do so again. This can continue indefinitely, with one LTC doing all of the operation. As the tap positions
of the LTC transformers digress, the current that circulates in the substation increases. This current simply
circulates around the loop formed by the buses and the two transformers doing no useful work, but it
causes an increase in losses, perhaps causing one or both of the transformers to overheat. To put the
matter in perspective, consider the case of a distribution substation where there are two 5/8%-step, 15-
MVA LTC transformers of 8.7% impedance in parallel. The secondary voltage is 12.47 kV. For a tap
difference of only one step, a circulating current of 25 A flows in the substation LV bus. This current
magnitude increases linearly with tap-position difference.
In the illustration above for LTC transformers, the circulating current was limited by the impedances
of the transformers. It is very important to recognize that the same procedure cannot be done with stepvoltage
regulators, as the impedance of a regulator is very low at even the extreme tap positions, and it
may be essentially zero at the neutral tap position. In this case, if one regulator is on neutral and the
other moves to position 1R, the circulating current will be expected to be sufficiently large to cause
catastrophic failure of the regulators. The user is cautioned: Step-voltage regulators can only be operated
in parallel when there is adequate supplemental impedance included in the current loop. This is most
© 2004 by CRC Press LLC
© 2004 by CRC Press LLC