6
Negative reactance
principle
If the software switch SGF1/2=1 and parallel
operation has been selected, the regulator starts
operating according to the modified negative
reactance principle. By comparing the actual
phase-shift measured with the set load phase-
shift value, and measuring the current, the com-
pensating value U
ci
can be calculated. The com-
pensating value U
ci
affects the regulator con-
trol voltage U
p
in such a way, that a stable volt-
age control of parallel operating transformers is
obtained once the reactive load is relatively sta-
ble.
The compensating value U
ci
is obtained from
the following expression:
U
ci
= x x U
n
I
ci
= circulating current [kA]
I
nt
= rated current of the power trans-
former [kA]
U
n
= rated phase-to-phase voltage of the
power transformer
stability = percentage setting value
If the parallel operating transformers have dif-
ferent rated currents, the regulator stability set-
ting value should be proportional to the rated
currents, i.e. the higher the rated current the
higher the stability setting value.
I
ci
stability
I
nt
100
Minimizing
circulating current
If the software switch SGF1/2=0 and parallel
operation has been selected, the regulator starts
operating according to the minimizing circu-
lating current principle. Each voltage regulat-
ing module transmits its own current and phase-
shift values to the other modules operating in
parallel. Then the modules calculate the total
value of the busbar current and the phase-shift
and compare it with its own values measured.
This calculation gives the circulating current to
be minimized. The same formula as for the nega-
tive reactance principle is used for calculating
the compensating value U
ci
. In this case, the
same stability setting value can be set for the
separate regulators, since the regulators take into
account differences in the transformers’s rated
currents.
Remote/local control If the software switch SGF1/3=1, the module
can be switched between remote and local con-
trol via the external control input BLOCK.
When the external control input is energized,
the regulator mode and operation is controlled
remotely using the serial communication param-
eters I6, I7, I9, I10 and V152. When the exter-
nal control input is deenergized, local push-but-
ton control is enabled. Control of regulator
mode and operation via external control inputs
MAN’, AUTO’, RAISE’, LOWER’ and RSV is
always enabled.
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