ABB REG670 - Page 591

To achieve a more correct regulation, an adjustment to a value of j2 slightly less than
-90° (2 – 4° less) can be made.
The effect of changing power factor of the load will be that j2 will no longer be close
to -90° resulting in U
L
being smaller or greater than U
B
if the ratio Rline/Xline is not
adjusted.
Figure 294 shows an example of this where the settings of Rline and Xline for j = 11°
from figure 293 has been applied with a different value of j (j = 30°).
U
B
jI
T
*Xline
U
L
Xline
Rline
Zline
I
T
I
T
*Rline
j=30
0
j
1
j
2
DU
j
1
=11
0
-90
0
=-79
0
en06000630.vsd
IEC06000630 V1 EN
Figure 294: Transformer with reverse reactance regulation poorly adjusted to the
power factor
As can be seen in figure 294, the change of power factor has resulted in an increase of
j2 which in turn causes the magnitude of U
L
to be greater than U
B
. It can also be noted
that an increase in the load current aggravates the situation, as does also an increase in
the setting of Zline (Rline and Xline).
Apparently the ratio Rline/Xline according to equation 278, 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 Xline gives the sensitivity of the parallel regulation. If Xline is set too
low, the 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 Xline
can cause a hunting situation as the transformers will then be prone to over react on
deviations from the target value.
There is no rule for the setting of Xline such that an optimal balance between control
response and 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. Xline equal
1MRK 502 071-UEN - Section 15
Control
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062 585
Application manual