ABB RELION REX640, Technical Manual, Page: 1706 / 1960

It depends on the load characteristics how much the load is reduced as the voltage
drops. For instance, purely resistive loads are proportional to the square of the
voltage, whereas motor drives based on frequency controllers may draw constant
power despite small voltage changes.
The status of the
RSV
input can be read from the
RSV
input data.
9.10.5.6 Automatic voltage regulation of parallel transformers (Auto parallel)
A circulating current may occur between transformers if two or more transformers
with slightly different ratios are energized in parallel. This is due to the unbalanced
short circuit impedances of the parallel transformers. To avoid such currents, the
tap changers of the transformers should be adjusted to achieve equilibrium. If
the transformers are assumed identical, the tap (voltage) steps and tap positions
should also match. In this case, the Master/Follower principle can be used. However,
unequally rated transformers with different tap steps can be connected in parallel
and these configurations can also be managed by the tap changer control function.
For these configurations, the Minimizing Circulating Current (MCC) or Negative
Reactance Principle (NRP) should be used. MCC and NRP are also suitable for
identical transformers.
The circulating current, which is almost purely inductive, is defined as negative if
it flows towards the transformer. U
ci
in
Equation 373
is positive and the control
voltage U
p
rises as a result to the
RAISE_OWN
output signal activation if the
circulating current level is sufficient ( Equation 377 and Equation 379 ) and the
other parameters remain the same. As a result, the voltage rise should diminish
the circulating current.
LDC equation and parallel connection
The additional challenge in the parallel connection regarding LDC is to know the
total current which flows through the parallel transformers.
In the Master/Follower mode, it is easier to know the total current than in other
parallel modes since the transformers are assumed to have identical ratings, that
is, the total current (I
L
in Equation 375 ) is obtained by multiplying the measured
load current (the average of the currents I_A, I_B and I_C of the connected
own transformer) by the number of parallel transformers. OL5ATCC can internally
conclude the number of parallel transformers from the connected tap changer
position inputs. However, if there is no connected position information from
the other parallel transformers, the correct number of the parallel transformers,
excluding the own transformer, needs to be set with the Parallel trafos setting.
In the MCC mode, the horizontal communication transfers the information from the
measured load currents between the regulators so that the total current needed in
the line drop compensation can be summed accurately. Here, I
L
is defined to be the
phasor sum of all the parallel power transformer currents. The currents from other
transformers must be fed via the
TR_I_AMPL
and
TR_I_ANGL
inputs of the OLGAPC
function.
In the NRP mode, the parallel transformers have different ratings and there is no
communication between the regulators. Therefore, when setting
Line drop V React
and
Line drop V Ris, the I
CT_n1
used in the equation should be the sum of the rated
currents of all the transformers operating in parallel. Here, I
L
is also defined as the
average of the currents (I_A, I_B and I_C). The calculated line drop compensation
value can be read from the monitored data LDC.
Control functions 1MRS759142 F
1706 REX640
Technical Manual