How Transformer Tap Changer Control
Works?
The tap-changing mechanism is usually
motor-driven and can be controlled manually and automatically. In the
automatic mode, the output voltage of the transformer is compared to
a reference voltage and a raise/lower signal is sent to the tap
changer motor when the output voltage falls outside a specified band,
called a dead band.
The dead band must not be smaller than
the voltage between taps; otherwise, it will ‘‘hunt’’
endlessly and burn up the tap changer. The dead band must not be too
wide, however, because the purpose of voltage regulation will be
defeated.
Ordinarily, the dead band is set to a
voltage between two and three tap increments. With the tap voltage
typically around 1% of the nominal secondary voltage, this provides
regulation within a 2–3% range.
If two or more transformers with load
tap changers are connected in parallel, then it is important that all
transformers operate at the same transformer turns ratio; otherwise,
excessive circulating KVAR results. The way this is implemented is to
have the tap changer on one of the parallel transformers control
voltage as the lead tap changer, with the other tap changers as the
followers.
Circuitry is installed on the followers
to sense the direction of reactive power flow through each
transformer. If too much reactive power is flowing from the primary
to the secondary, then its secondary taps are lowered.
If too much reactive power is flowing
from the secondary to the primary, then secondary taps are raised.
Appropriate dead bands are established to prevent the LTCs from
hunting and to limit interactions among the tap changer controls.
One such control scheme, called a
load-balancing method, is depicted in Figure 6.16 for three
transformers with voltage regulators supplying a common load bus.
FIGURE 6.16 A load-balancing control
scheme for three parallel transformers with load tap changers.
If the three transformer impedances
equal and if the transformers are set on the proper tap positions,
the transformer secondary currents will all be in phase with the load
current and there will be no current unbalance.
If one or more transformer is set on
the wrong tap, circulating currents will flow in all three
transformers. The principle of operation of the load-balancing method
is to separate each of the transformer secondary currents into a load
current component and a circulating-current component. The
transformer secondary currents flow through current transformers,
labeled CT 1, CT 2, and CT 3 in Figure 6.16.
The currents at the CT secondaries
split into two paths at each of the CT secondary windings. Path 1 (to
the right) goes through a set of auxiliary transformers, labeled CT
4, CT 5, and CT 6. The secondaries of the auxiliary CTs are connected
in series, forcing the currents in all three primary windings equal
one another, each being one-third of the total load current. By
default, the unbalance-current components must flow in path 2 to the
left.
Each of the circulating-current
components (also called unbalanced current components) flows through
an inductive reactance element, called a paralleling reactor. The
paralleling reactors are labeled jX in Figure 6.16. In general, the
unbalance-current components of the three transformers are unequal.
The voltages developed across the
paralleling reactors are added to the sensed voltages at the
secondary windings of the main transformers, which are used to
control the movement of the tap changers.
If transformer 1 is on a higher tap
position than transformer 2 or transformer 3, the unbalanced currents
flowing through the parallel reactors increase the sensed voltage at
transformer 1 and reduce the sensed voltages at transformers 2 and 3.
This causes transformer 1 to lower its
taps and transformers 2 and 3 to raise their taps. If transformer 1
is on a lower tap position than transformers 2 and 3, the unbalanced
currents flowing through the parallel reactors decrease the sensed
voltage at transformer 1 and increase the sensed voltages at
transformers 2 and 3. This causes transformer 1 to raise its taps and
transformers 2 and 3 to lower their taps.
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