Current (MCC) or Negative Reactance Principle (NRP) should be used. The MCC
and NRP principles 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 89 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 92 and Equation 94) 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 the line drop
compensation 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
injected
in
Equation 91) is obtained by multiplying the measured
load current (the average of the secondary currents I_A, I_B and I_C of the
connected own transformer) with the number of parallel transformers. OLATCC
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
injected
is defined
to be the phasor sum of all the parallel power transformer secondary-side currents.
The currents from other transformers must be fed via the TRx_I_AMPL and
TRx_I_ANGL inputs.
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
injected
is also defined as
the average of the connected secondary currents (I_A, I_B and I_C). The calculated
line drop compensation value can be read from the monitored data LDC.
9.5.10.1 Master/Follower principle M/F
The Master/Follower (M/F) operation principle is suitable for power transformers
with identical ratings and step voltages. One voltage regulator (master) measures
and controls and the other regulators (followers) follow the master, that is, all the
tap changers connected in parallel are synchronized. This parallel operation is
obtained by connecting the FLLWx_CTL output of the master to the corresponding
input TAPCHG_FLLW of the followers via a horizontal GOOSE communication.
1YHT530004D05 D Section 9
Control functions
615 series 737
Technical Manual
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