ABB Relion 670 Series Applications Manual page 374

Section 13
Control
Figure 176
figure 175
ANSI06000630 V1 EN-US
Figure 176: Transformer with reverse reactance regulation poorly adjusted to the power
As can be seen in figure 176, the change of power factor has resulted in an increase of j2 which in
turn causes the magnitude of V
load current aggravates the situation, as does also an increase in the setting of Zline (
Xline ).
Apparently the ratio Rline/Xline according to equation 129, that is the value of j1 must be set with
respect to the power factor, also meaning that the reverse reactance method should not be
applied to systems with varying power factor.
The setting of
transformers will not pull together and a run away tap situation will occur. On the other hand, a
high setting will keep the transformers strongly together with no, or only a small difference in tap
position, but the voltage regulation as such will be more sensitive to a deviation from the
anticipated power factor. A too high setting of
transformers will then be prone to over react on deviations from the target value.
There is no rule for the setting of
susceptibility to changing power factor is achieved. One way of determining the setting is by trial
and error. This can be done by setting e.g.
then observe how the parallel control behaves during a couple of days, and then tune it as
required. It shall be emphasized that a quick response of the regulation that quickly pulls the
transformer tap changers into equal positions, not necessarily corresponds to the optimal setting.
This kind of response is easily achieved by setting a high
the disadvantage is then a high susceptibility to changing power factor.
A combination of line voltage drop compensation and parallel control with the negative reactance
method is possible to do simply by adding the required
separately to get the combined impedance. However, the line drop impedance has a tendency to
drive the tap changers apart, which means that the reverse reactance impedance normally needs
to be increased.
368
shows an example of this where the settings of
has been applied with a different value of j (j = 30°).
j =11 0 -90 0 =-79 0
1
Rline
j
1
Xline Zline
j=30 0
I
T
factor
to be greater than V
L
Xline gives the sensitivity of the parallel regulation. If Xline is set too low, the
Xline such that an optimal balance between control response and
Rline and Xline for j = 11° from
DV
V
B
. It can also be noted that an increase in the
B
Xline can cause a hunting situation as the
Xline equal to half of the transformer reactance, and
Xline value, as was discussed above, and
Rline values and the required Xline values
1MRK 511 358-UUS A
I *Rline
V
T
L
jI *Xline
T
j
2
en06000630_ansi.vsd
Rline and
Application manual