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KVCG202/EN M/H11
Note, the minimum operating voltage of the opto inputs is >35 V and so the maximum
limiting series lead resistance for a single opto input is 2000 ohms.
In general master-follower schemes are not suited for parallel control of transformers
which have dissimilar tap step increments or number of taps. Such transformer groups
require each transformer to be individually controlled within the group, described as
category b.
Most master-follower schemes suffer from the disadvantage that following the loss of one
transformer on fault, either the voltage control is lost completely (loss of the master) or
the LDC setting is increased to twice the required value (loss of the follower). Also, many
of the control circuits are complex and rely on satisfactory operation of numerous
electrical contacts in step correcting switches, out of step relays etc. and many of the
older schemes are unreliable and expensive to maintain.
4.7.2
Instability of individually controlled parallel transformers
Where two or more transformers are operated in parallel by their individual VRR's then it
is inevitable that one transformer may operate earlier than the other transformers in the
group.
This will result in a disparity of tappings between transformers. The busbar
voltage will change only by the percentage change in transformer ratios divided by the
number of transformers in parallel. This may be sufficient to correct the voltage and the
VRR's on the other transformers will then reset without operating.
A tapping disparity creates a circulating current, Ic, between the transformers through the
busbars. The circulating current is limited by the impedance of the one transformer plus
the effective parallel impedance of the remaining transformers in the group. As the
transformer impedances are almost entirely reactive, the circulating current will also be
reactive. Hence, each transformer in a parallel group sees a nominal load current
component Ic which is leading in one transformer and lagging in the others, relative to the
IL component, which is of a predominantly higher power factor.
The effect of the circulating current is to increase the I
hence the operating temperature of the transformers. For a small tap disparity, one or
two taps apart, it can be shown that both these effects are negligible. A large tap
disparity can give rise to a circulating current in the transformers which exceeds the full
load ratings of the transformers. This effectively sets a limit to the allowable difference
between the tap positions of the transformers.
tapchangers must always be kept perfectly in step but in practice, this is rarely necessary.
4.7.2.1
Runaway
A situation that must be avoided is where tapchangers run to their opposite limits. For this
situation the losses discussed in the previous section would certainly be excessive but,
more importantly, voltage control would be completely lost. Unfortunately, the basic VRR
with or without LDC will not ensure that parallel transformers are kept in step. In fact if
basic VRR's were applied separately to two parallel transformers it would soon lead to
runaway and it is important to understand how it would occur.
Even if the systems on each transformer appeared to be identical, component tolerances
would cause one VRR to operate before the other. Say, for example that as the load
increased and the busbar voltage dropped, VRR2 tapped first to raise the busbar voltage.
VRR1 which would have been just about to tap, would see that the voltage was now back
within limits and so reset itself without tapping. The tap positions of the two transformers
would now differ by one step. The problem is that if the load increased further, the
process would be repeated, VRR2 would always be the first to operate.
compounding the problem, if the load decreased VRR1 could be the first to tap to lower
the busbar voltage.
transformer tapchangers would diverge and the circulating currents would become
excessive.
Voltage control would also be lost when the maximum range of the
tapchangers was reached. If line drop compensation were in use, the situation would be
worse still, in that runaway would occur even without the load changing and therefore
even more quickly, see 'Effect of Circulating Current on LDC' below.
Clearly, the VRR's for paralleled transformers must be modified in some way in order to
prevent runaway and so to limit circulating currents. Three techniques are widely used:
Thus, as the load varied naturally throughout the day, the two
2
R transformer copper losses and
There is a temptation to think that
Technical Manual
KVGC202
Also,
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